Fundamental limitations of the polarization purity of x rays - Schulze, K. S. (APL Photonics, 2018)
For a few years, x-ray polarimeters have been discussed and even used as a key method for the investigation of fundamental physical questions, from quantum electrodynamics to solid state physics. However, the sensitivity of optical instruments is limited. In the case of x-ray polarimeters, this limitation is connected with the polarization purity. This article quantifies two fundamental effects which lead to a limited polarization purity and, thus, to a limited sensitivity: the divergence of the source and multiple-wave diffraction inside the polarizer crystals. A comparison shows that the current best polarization purities realized in the x-ray range are limited by these effects. The quantitative knowledge of their influence, however, can improve the purity by two orders of magnitude in future polarimetric experiments.
hij:1, public:1,

Investigating the influence of incident laser wavelength and polarization on particle acceleration and terahertz generation - Woldegeorgis, A. H.; Beleites, B.; Ronneberger, F.; Grosse, R.; Gopal, A. (Physical Review E, 2018)
The interaction of a high-power laser pulse with a thin foil can generate energetic, broadband terahertz radiation. Here, we report an experimental investigation on the influence of incident laser polarization and wavelength on the terahertz emission and maximum proton energy from the target rear surface. For similar incident laser intensities, the characteristics of the particle beams and the terahertz radiation show a wavelength dependence. The results fit well with the established scaling laws for the terahertz yield and the maximum proton energy as a function of the incident laser irradiance.
hij:1, public:1,

Ultrafast thulium fiber laser system emitting more than 1 kW of average power - Gaida, C.; Gebhardt, M.; Heuermann, T.; Stutzki, F.; Jauregui, C.; Limpert, J. (Optics Letters, 2018)
In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.
hij:1, public:1,

A nozzle for high-density supersonic gas jets at elevated temperatures - Heyl, C. M.; Schoun, S. B.; Porat, G.; Green, H.; Ye, J. (Review of Scientific Instruments, 2018)
We present the development of a gas nozzle providing high-density gas at elevated temperaturesinside a vacuum environment. Fused silica is used as the nozzle material to allow the placement ofthe nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk ofsputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases thegas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction regionbetween consecutive laser shots. The nozzle accommodates a 50μm opening hole from which asupersonic gas jet emerges. Heater wires are used to bring the nozzle temperature up to 730 °C, whilea cooling unit ensures that the nozzle mount and the glued nozzle-to-mount connection is kept at atemperature below 50 °C. The presented nozzle design is used for high-order harmonic generationin hot gases using gas backing pressures of up to 124 bars.
hij:1, public:1,

A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV - Behm, K. T.; Cole, J. M.; Joglekar, A. S.; Gerstmayr, E.; Wood, J. C.; Baird, C. D.; Blackburn, T. G.; Duff, M.; Harvey, C.; Ilderton, A.; Kuschel, S.; Mangles, S. P. D.; et al. (Review of Scientific Instruments, 2018)
We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.
hij:1, public:1,

Diagnostics of polarization purity of x rays by means of Rayleigh scattering - Surzhykov, A.; Yerokhin, V. A.; Fritzsche, S.; Volotka, A. V. (Physical Review A, 2018)
Synchrotron radiation is commonly known to be completely linearly polarized when observed in the orbital plane of the synchrotron motion. Under actual experimental conditions, however, the degree of polarization of the synchrotron radiation may be lower than the ideal 100%. We demonstrate that even tiny impurities of polarization of the incident radiation can drastically affect the polarization of the elastically scattered light. We propose to use this effect as a precision tool for the diagnostics of the polarization purity of the synchrotron radiation. Two variants of the diagnostics method are proposed. The first one is based on the polarization measurements of the scattered radiation and relies on theoretical calculations of the transition amplitudes. The second one involves simultaneous measurements of the polarization and the cross sections of the scattered radiation and is independent of theoretical amplitudes.
hij:1, public:1,

First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target - Jahn, D.; Schumacher, D.; Brabetz, C.; Ding, J.; Weih, S.; Kroll, F.; Brack, F.; Schramm, U.; Blazevic, A.; Roth, M. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018)
In the last two decades, the generation of intense ion beams based on laser-driven sources has become an extensively investigated field. The LIGHT collaboration combines a laser-driven intense ion source with conventional accelerator technology based on the expertise of laser, plasma and accelerator physicists. Our collaboration has installed a laser-driven multi-MeV ion beamline at the GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung delivering intense proton bunches in the subnanosecond regime. We investigate possible applications for this beamline, especially in this report we focus on the imaging capabilities. We report on our proton beam homogenization and on first imaging results of a solid target.
hij:1, public:1,

Ground-state g factor of middle-Z boronlike ions - Agababaev, V. A.; Glazov, D. A.; Volotka, A. V.; Zinenko, D. V.; Shabaev, V. M.; Plunien, G. (Journal of Physics: Conference Series, 2018)
Theoretical calculations of the interelectronic-interaction and QED corrections to the g factor of the ground state of boronlike ions are presented. The first-order interelectronic-interaction and the self-energy corrections are evaluated within the rigorous QED approach in the effective screening potential. The second-order interelectronic interaction is considered within the Breit approximation. The nuclear recoil effect is also taken into account. The results for the ground-state g factor of boronlike ions in the range Z = 10-20 are presented and compared to the previous calculations.
hij:1, public:1,

High magnetic fields for fundamental physics - Battesti, R.; Beard, J.; B{\"{o}}ser, S.; Bruyant, N.; Budker, D.; Crooker, S. A.; Daw, E. J.; Flambaum, V. V.; Inada, T.; Irastorza, I. G.; Karbstein, F.; Kim, D. L.; et al. (Physics Reports, 2018)
Various fundamental-physics experiments such as measurement of the magnetic birefringence of the vacuum, searches for ultralight dark-matter particles (e.g., axions), and precision spectroscopy of complex systems (including exotic atoms containing antimatter constituents) are enabled by high-field magnets. We give an overview of current and future experiments and discuss the state-of-the-art DC- and pulsed-magnet technologies and prospects for future developments.
hij:1, public:1,

High-power frequency comb at 2 μm wavelength emitted by a Tm-doped fiber laser system - Gaida, C.; Heuermann, T.; Gebhardt, M.; Shestaev, E.; Butler, T. P.; Gerz, D.; Lilienfein, N.; Sulzer, P.; Fischer, M.; Holzwarth, R.; Leitenstorfer, A.; Pupeza, I.; et al. (Optics Letters, 2018)
We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30 fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz–30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.
hij:1, public:1,

Microcalorimeters for X-Ray Spectroscopy of Highly Charged Ions at Storage Rings - Kraft-Bermuth, S.; Hengstler, D.; Egelhof, P.; Enss, C.; Fleischmann, A.; Keller, M.; St{\"{o}}hlker, T. (Atoms, 2018)
X-ray spectroscopy of highly charged heavy ions is an important tool for the investigation of many topics in atomic physics. Such highly charged ions, in particular hydrogen-like uranium, are investigated at heavy ion storage rings, where high charge states can be produced in large quantities, stored for long times and cooled to low momentum spread of the ion beam. One prominent example is the determination of the 1s Lamb Shift in hydrogen-like heavy ions, which has been investigated at the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research. Due to the large electron binding energies, the energies of the corresponding photon transitions are located in the X-ray regime. To determine the transition energies with high accuracy, highly resolving X-ray spectrometers are needed. One concept of such spectrometers is the concept of microcalorimeters, which, in contrast to semiconductor detectors, uses the detection of heat rather than charge to detect energy. Such detectors have been developed and successfully applied in experiments at the ESR. For experiments at the Facility for Antiproton and Ion Research (FAIR), the Stored Particles and Atoms Collaboration (SPARC) pursues the development of new microcalorimeter concepts and larger detector arrays. Next to fundamental investigations on quantum electrodynamics such as the 1s Lamb Shift or electron–electron interactions in two- and three-electron systems, X-ray spectroscopy may be extended towards nuclear physics investigations like the determination of nuclear charge radii.
hij:1, public:1,

Multi-MV/cm longitudinally polarized terahertz pulses from laser–thin foil interaction - Woldegeorgis, A.; Kurihara, T.; Almassarani, M.; Beleites, B.; Grosse, R.; Ronneberger, F.; Gopal, A. (Optica, 2018)
Longitudinally polarized terahertz radiation offers access to the elementary excitations and particles that cannot be addressed by transverse waves. While transverse electric fields exceeding 1 MV/cm are widely utilized for nonlinear terahertz spectroscopy, longitudinally polarized terahertz waves at this field strength are yet to be realized. In this paper, we experimentally demonstrate that by focusing radially polarized terahertz fields generated from laser–thin metallic foil interaction, longitudinally polarized terahertz with record-breaking field strength above 1.5 MV/cm can be obtained. Furthermore, we also traced the evolution of the geometric phase of the longitudinal component as it propagates through focus. A novel scheme based on noncollinear electro-optic detection has been utilized to unambiguously measure the polarization states. Our result will scale up the nonlinear spectroscopy of solid materials and particle acceleration experiments where on-axis polarization plays a crucial role.
hij:1, public:1,

Progress on experiments towards LWFA-driven transverse gradient undulator-based FELs - Bernhard, A.; Rodr{\'{i}}guez, V. A.; Kuschel, S.; Leier, M.; Peiffer, P.; S{\"{a}vert, A. and Schwab, M. and Werner, W. and Widmann, C. and Will, A. and M{\"{u}}ller, A.-S. and Kaluza, M. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018)
Free Electron Lasers (FEL) are commonly regarded as the potential key application of laser wakefield accelerators (LWFA). It has been found that electron bunches exiting from state-of-the-art LWFAs exhibit a normalized 6-dimensional beam brightness comparable to those in conventional linear accelerators. Effectively exploiting this beneficial beam property for LWFA-based FELs is challenging due to the extreme initial conditions particularly in terms of beam divergence and energy spread. Several different approaches for capturing, reshaping and matching LWFA beams to suited undulators, such as bunch decompression or transverse-gradient undulator schemes, are currently being explored. In this article the transverse gradient undulator concept will be discussed with a focus on recent experimental achievements.
hij:1, public:1,

Simulation studies on generation, handling and transport of laser-accelerated carbon ions - Ding, J.; Schumacher, D.; Jahn, D.; Blazevic, A.; Roth, M. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018)
To this day the interaction of high-intensity lasers with matter is considered to be a possible candidate for next generation particle accelerators. Within the LIGHT collaboration crucial work for the merging of a high-intensity laser driven ion source with conventional accelerator technology has been done in the past years. The simulation studies we report about are an important step in providing short and intense mid-Z heavy ion beams for future applications.
hij:1, public:1,

Very-long distance propagation of high-energy laser pulse in air - Wang, J.; Yu, W.; Yu, M. Y.; Rykovanov, S.; Ju, J.; Luan, S.; Li, K.; Leng, Y.; Li, R.; Sheng, Z.-M. (Physics of Plasmas, 2018)
Long distance propagation of an energetic laser pulse with intensity slightly below that for multi-photon ionization in air is considered analytically, by noting that in the process, it is mainly the peak region of the pulse that interacts with the air molecules. Similar to that of much shorter femtosecond laser pulses of similar intensity, the affected air becomes slightly ionized and self-consistently forms a co-propagating thin and low-density plasma filament along the axis. It is found that a hundred-Joule-level laser pulse with a relatively large spot radius and pulse duration can propagate (also in the form of a self-consistent filament) tens of kilometers through the atmosphere. Such laser propagation properties should have applications in many areas.
hij:1, public:1,

Watt-scale super-octave mid-infrared intrapulse difference frequency generation - Gaida, C.; Gebhardt, M.; Heuermann, T.; Stutzki, F.; Jauregui, C.; Antonio-Lopez, J.; Schulzgen, A.; Amezcua-Correa, R.; T{\"{u}}nnermann, A.; Pupeza, I.; Limpert, J. (Light: Science { &} Applications, 2018)
The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.
hij:1, public:1,

Analytical solutions for nonlinear Thomson scattering including radiation reaction - Ruijter, M.; Kharin, V. Y.; Rykovanov, S. G. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2018)
Analytical solutions for the emitted nonlinear Thomson scattering spectrum with radiation reaction (RR) included are provided for a single electron colliding with a high intensity laser pulse. Further expressions are derived for the peak intensity for a given harmonic order and the downshift of the frequency when RR is included. Controlling the spectrum with shaping of the laser pulse frequency (chirp) has been investigated. It is shown that chirping of the laser pulse gives a distinct fingerprint of the effect of RR in the spectrum.
hij:1, public:1,

BTZ dilatonic black holes coupled to Maxwell and Born-Infeld electrodynamics - Panah, B.; Hendi, S.; Panahiyan, S.; Hassaine, M. (Physical Review D, 2018)
Motivated by the string theory corrections in the low-energy limit of both gauge and gravity sides, we consider three-dimensional black holes in the presence of dilatonic gravity and the Born-Infeld nonlinear electromagnetic field. We find that geometric behavior of the solutions is similar to the behavior of the hyperscaling violation metric, asymptotically. We also investigate thermodynamics of the solutions and show that the generalization to dilatonic gravity introduces novel properties into thermodynamics of the black holes which were absent in the Einstein gravity. Furthermore, we explore the possibility of tuning out part of the dilatonic effects using the Born-Infeld generalization.
hij:1, public:1,

Controlling the Self-Injection Threshold in Laser Wakefield Accelerators - Kuschel, S.; Schwab, M. B.; Yeung, M.; Hollatz, D.; Seidel, A.; Ziegler, W.; S{\"{a}vert, A. and Kaluza, M.C. and Zepf, M. (Physical Review Letters, 2018)
Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as ∼50 μm and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.
hij:1, public:1,

Discrete worldline instantons - Schneider, C.; Torgrimsson, G.; Sch{\"{u}}tzhold, R. (Physical Review D, 2018)
The semiclassical approximation of the worldline path integral is a powerful tool to study nonperturbative electron-positron pair creation in spacetime-dependent background fields. Finding solutions of the classical equations of motion, i.e., worldline instantons, is possible analytically only in special cases, and a numerical treatment is nontrivial as well. We introduce a completely general numerical approach based on an approximate evaluation of the discretized path integral that easily and robustly gives the full semiclassical pair production rate in nontrivial multidimensional fields, and apply it to some example cases.
hij:1, public:1,

Elastic scattering of twisted electrons by diatomic molecules - Maiorova, A. V.; Fritzsche, S.; M{\"{u}}ller, R. A.; Surzhykov, A. (Physical Review A, 2018)
The elastic scattering of twisted electrons by diatomic molecules is studied within the framework of the nonrelativistic first Born approximation. In this process, the coherent interaction of incident electrons with two molecular centers may cause interference patterns in the angular distributions of outgoing particles. We investigate how this Young-type interference is influenced by the complex internal structure of twisted beams. In particular, we show that the corkscrewlike phase front and the inhomogeneous intensity profile of the incident beam can strongly modify the angular distribution of electrons, scattered off a single well-localized molecule. For the collision with a macroscopic target, composed of randomly distributed but aligned molecules, the angular-differential cross section may reveal valuable information about the transverse and longitudinal momenta of twisted states. To illustrate the difference between the scattering of twisted and plane-wave beams for both single-molecule and macroscopic-target scenarios, detailed calculations have been performed for a H2 target.
hij:1, public:1,

Enzymatic Bioweathering and Metal Mobilization From Black Slate by the Basidiomycete Schizophyllum commune - Kirtzel, J.; Madhavan, S.; Wielsch, N.; Blinne, A.; Hupfer, Y.; Linde, J.; Krause, K.; Svato{{s}}, A.; Kothe, E. (Frontiers in microbiology, 2018)
Schizophyllum commune is a filamentous basidiomycete causing white-rot in many wood species with the help of a broad range of enzymes including multicopper oxidases such as laccases and laccase-like oxidases. Since these enzymes exhibit a broad substrate range, their ability to oxidatively degrade black slate was investigated. Both haploid monokaryotic, and mated dikaryotic strains were able to grow on black slate rich in organic carbon as sole carbon source. On defined media, only the monokaryon showed growth promotion by addition of slate. At the same time, metals were released from the slate and, after reaching a threshold concentration, inhibited further growth of the fungus. The proteome during decomposition of the black slate showed induction of proteins potentially involved in rock degradation and stress resistance, and the gene for laccase-like oxidase mco2 was up-regulated. Specifically in the dikaryon, the laccase gene lcc1 was induced, while lcc2 as well as mco1, mco3, and mco4 expression levels remained similar. Spectrophotometric analysis revealed that both life forms were able to degrade the rock and produce smaller particles.
hij:1, public:1,

First Ionization Potentials of Fm, Md, No, and Lr: Verification of Filling-Up of 5f Electrons and Confirmation of the Actinide Series - Sato, T.; Asai, M.; Borschevsky, A.; Beerwerth, R.; Kaneya, Y.; Makii, H.; Mitsukai, A.; Nagame, Y.; Osa, A.; Toyoshima, A.; Tsukada, K.; Sakama, M.; et al. (Journal of the American Chemical Society, 2018)
We report the first ionization potentials (IP1) of the heavy actinides, fermium (Fm, atomic number Z = 100), mendelevium (Md, Z = 101), nobelium (No, Z = 102), and lawrencium (Lr, Z = 103), determined using a method based on a surface ionization process coupled to an online mass separation technique in an atom-at-a-time regime. The measured IP1 values agree well with those predicted by state-of-the-art relativistic calculations performed alongside the present measurements. Similar to the well-established behavior for the lanthanides, the IP1 values of the heavy actinides up to No increase with filling up the 5f orbital, while that of Lr is the lowest among the actinides. These results clearly demonstrate that the 5f orbital is fully filled at No with the [Rn]5f147s2 configuration and that Lr has a weakly bound electron outside the No core. In analogy to the lanthanide series, the present results unequivocally verify that the actinide series ends with Lr.
hij:1, public:1,

Polarization evolution in single-ring antiresonant hollow-core fibers - Jayakumar, N.; Sollapur, R.; Hoffmann, A.; Grigorova, T.; Hartung, A.; Schwuchow, A.; Bierlich, J.; Kobelke, J.; Schmidt, M. A.; Spielmann, C. (Applied Optics, 2018)
Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.
hij:1, public:1,

Selectivity of the Br 3d⁻1 Auger decays in HBr - Palaudoux, J.; Kaneyasu, T.; Andric, L.; Carniato, S.; Gamblin, G.; Penent, F.; Hikosaka, Y.; Shigemasa, E.; Ito, K.; Fritzsche, S.; Kukk, E.; Sheinerman, S.; et al. (Physical Review A, 2018)
The Auger decay of the spin-orbit and molecular-field split Br 3d−1 core holes in HBr is investigated, both by a photoelectron–Auger-electron coincidence experiment and by ab initio calculations based on the one-center approximation. The branching ratios for the Auger decay of the five different core-hole states to the 4p(σ,π)−2 dicationic final states are determined. Experimental and theoretical data are in good agreement and conform to results for the 4pπ−2 final states from a previous analysis of the high-resolution conventional Auger-electron spectrum. The branching ratios for the Br 3d−1 Auger decay to the 4p(σ,π)−2 with Σ symmetry follow the propensity rule of L2,3VV Auger decay stating that the oriented core holes decay preferentially by involving a valence electron from an orbital with the same spatial orientation. For the M4,5VV decay in HBr this propensity rule has to be supplemented by the requirement that the Auger-electron channel and the other valence orbital have the same preferential orientation. We also probe the influence of the Auger kinetic energy on the distortion of the photoline caused by the postcollision interaction effect. For small kinetic energies, differences between experimental results and theoretical predictions are identified.
hij:1, public:1,

Strong-field polarizability-enhanced dissociative ionization - Yue, L.; Wustelt, P.; Sayler, A.; Oppermann, F.; Lein, M.; Paulus, G.; Gr{\"{a}fe, S. (Physical Review A, 2018)
We investigate dissociative single and double ionization of HeH^+ induced by intense femtosecond laser pulses. By employing a semiclassical model with nuclear trajectories moving on field-dressed surfaces and ionization events treated as stochastical jumps, we identify a strong-field mechanism wherein the molecules dynamically align along the laser polarization axis and stretch towards a critical internuclear distance before dissociative ionization. As the tunnel-ionization rate is larger for larger internuclear distances and for aligned samples, ionization is enhanced. The strong dynamical rotation originates from the anisotropy of the internuclear distance-dependent polarizability tensor, which features a maximum at certain internuclear distances. Good qualitative agreement with our experimental observations is found. Finally, we investigate under which experimental conditions isotope effects of different isotopologues of HeH^+ can be observed.
hij:1, public:1,

Table-top nanoscale coherent imaging with XUV light - Rothhardt, J.; Tadesse, G.; Eschen, W.; Limpert, J. (Journal of Optics, 2018)
Modern laser-based XUV light sources provide very high photon fluxes which have previously only been available at large scale facilities. This allows high-performance XUV nanoscale imaging to be implemented in a table-top manner, and thus qualifies XUV imaging as a novel imaging technique complementing electron and visible-light microscopy. This article presents the current state-of-the-art in table-top XUV light sources and matched coherent imaging schemes. Selected experiments demonstrate the unique capabilities of XUV imaging{–}namely, nanoscale (sub-20 nm) resolution, single shot imaging, imaging of extended samples and 3D imaging of µm-sized objects. In addition, future prospects will be discussed, including scaling to few-nm resolution, extension to the soft x-ray spectral region, chemically-specific imaging at absorption edges and time-resolved imaging on femtosecond time-scales.
hij:1, public:1,

Calibration and cross-laboratory implementation of scintillating screens for electron bunch charge determination - Kurz, T.; Couperus, J.; Kr{\"{a}mer, J. and Ding, H. and Kuschel, S. and K{\"{o}}hler, A. and Zarini, O. and Hollatz, D. and Schinkel, D. and D’Arcy, R. and Schwinkendorf, J.-P. and Osterhoff, J. and Irman, A. and Schramm, U. and Karsch, S. (Review of Scientific Instruments, 2018)
We revise the calibration of scintillating screens commonly used to detect relativistic electron beams with low average current, e.g., from laser-plasma accelerators, based on new and expanded measurements that include higher charge density and different types of screens than previous work. Electron peak charge densities up to 10 nC/mm2 were provided by focused picosecond-long electron beams delivered by the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf. At low charge densities, a linear scintillation response was found, followed by the onset of saturation in the range of nC/mm2. The absolute calibration factor (photons/sr/pC) in this linear regime was measured to be almost a factor of 2 lower than that reported by Buck et al. retrospectively implying a higher charge in the charge measurements performed with the former calibration. A good agreement was found with the results provided by Glinec et al.. Furthermore long-term irradiation tests with an integrated dose of approximately 50 nC/mm2 indicate a significant decrease of the scintillation efficiency over time. Finally, in order to enable the transfer of the absolute calibration between laboratories, a new constant reference light source has been developed.
hij:1, public:1,

Chemical-vapor deposited ultra-fast diamond detectors for temporal measurements of ion bunches - Jahn, D.; Tr{\"{a}ger, M. and Kis, M. and Brabetz, C. and Schumacher, D. and Blazevic, A. and Ciobanu, M. and Pomorski, M. and Bonnes, U. and Busold, S. and Kroll, F. and Brack, F.-E. and Schramm, U. and Roth, M. (Review of Scientific Instruments, 2018)
This article reports on the development of thin diamond detectors and their characterization for their application in temporal profile measurements of subnanosecond ion bunches. Two types of diamonds were used: a 20 μm thin polycrystalline chemical vapor deposited (CVD) diamond and a membrane with a thickness of (5 ± 1) μm etched out of a single crystal (sc) CVD diamond. The combination of a small detector electrode and an impedance matched signal outlet leads to excellent time response properties with a signal pulse resolution (FWHM) of τ = (113 ± 11) ps. Such a fast diamond detector is a perfect device for the time of flight measurements of MeV ions with bunch durations in the subnanosecond regime. The scCVD diamond membrane detector was successfully implemented within the framework of the laser ion generation handling and transport project, in which ion beams are accelerated via a laser-driven source and shaped with conventional accelerator technology. The detector was used to measure subnanosecond proton bunches with an intensity of 108 protons per bunch.
hij:1, public:1,

Coherent Beam Combination of Ultrafast Fiber Lasers - Klenke, A.; M{\"{u}}ller, M.; Stark, H.; Kienel, M.; Jauregui, C.; T{\"{u}}nnermann, A.; Limpert, J. (IEEE Journal of Selected Topics in Quantum Electronics, 2018)
The performance of fiber laser systems has drastically increased over recent decades which has opened up new industrial and scientific applications for this technology. However, currently a number of physical effects prevents further power scaling. Coherent combination of beams from multiple emitters has been established as a power scaling technique beyond these limitations. It is possible to increase the average power and, for pulsed laser systems, also parameters such as the pulse energy and the peak power. To realize such laser systems, various aspects have to be taken into account which include beam combination elements, stabilization systems and the output parameters of the individual amplifiers. After an introduction to the topic, various ways of implementing coherent beam combination for ultrashort pulses are explored. Besides the spatial combination of beams, the combination of pulses in time will also be discussed. Recent experimental results will be presented, including multi-dimensional (i.e. spatial and temporal) combination. Finally, an outlook on possible further developments is given, focused on scaling the number of combinable beams and pulses.
hij:1, public:1,

Hard X-ray generation from ZnO nanowire targets in a non-relativistic regime of laser-solid interactions - Samsonova, Z.; H{\"{o}}fer, S.; Hollinger, R.; K{\"{a}mpfer, T. and Uschmann, I. and R{\"{o}}der, R. and Trefflich, L. and Rosmej, O. and F{\"{o}}rster, E. and Ronning, C. and Kartashov, D. and Spielmann, C. (Applied Sciences, 2018)
We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5E16 W/cm^2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance the development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in high energy density (HED) physics.
hij:1, public:1,

Judd–Ofelt analysis and experimental spectroscopic study of erbium doped phosphate glasses - Lachheb, R.; Herrmann, A.; Assadi, A.; Reiter, J.; K{\"{o}}rner, J.; Hein, J.; Russel, C.; Ma{\^{a}}lej, R.; Damak, K. (Journal of Luminescence, 2018)
A series of phosphate glasses including two compositions that are similar to commercial laser glasses and 3 new compositions doped with 2 × 10^20 Er3+/cm3 were prepared by using the classical melt quenching technique. The new glass compositions show much better glass forming properties than the commercially available glasses, lower molecular weights and lower optical basicities which are expected to be advantageous for their luminescence and laser properties. From the UV–vis–NIR absorption spectra, detailed Judd–Ofelt analyses were conducted and the radiative properties of the luminescent levels of Er3+ in these host materials were calculated. In fact all three compositions show longer calculated luminescence lifetimes than the compositions that are based on commercially available laser glasses. The absorption and the emission cross sections, the luminescence lifetimes and the quantum efficiency at 1530 nm were investigated. LiZnLaAPF glass can be suggested as a good host to generate efficient lasing action at 1530 nm. The variation of the Judd–Ofelt intensity parameters Ω2, Ω4 and Ω6 is discussed with respect to the glass compositions and their properties. For this, the calculated Ω2, Ω4 and Ω6 values are compared to the results of numerous publications on Er3+ doped phosphate glasses. From this data a correlation with the symmetry at the local rare earth site (Ω2) and with the theoretical optical basicity (Ω6) of the glass composition can be assumed.
hij:1, public:1,

Observation of anomalous side-scattering in laser wakefield accelerators - Krushelnick, K.; Dangor, A. E.; Kaluza, M.; Mangles, S. P. D.; Murphy, C. D.; Najmudin, Z.; Thomas, A. G. R. (Laser and Particle Beams, 2018)
High-intensity femtosecond laser–plasma interaction experiments were performed to investigate laser–plasma wakefield acceleration in the “bubble” regime. Using a 15 TW laser pulse, the emission of side-scattered radiation was spectrally and spatially resolved and was consequently used to diagnose the evolution of the laser pulse during the acceleration process. Side-scattered emission was observed immediately before wavebreaking at a frequency of ωL + 1.7ωp (where ωL is the laser frequency and ωp is the background plasma frequency). This emission may result from scattering of laser light by large amplitude plasma oscillations generated in the shell of the wakefield “bubble” and which occurs immediately prior to the wavebreaking/injection process. The observed variation of the frequency of scattered light with electron density agrees with theoretical estimates.
hij:1, public:1,

Testing atomic wave functions in the nuclear vicinity: The hyperfine structure with empirically deduced nuclear and quantum electrodynamic effects - Ginges, J. S. M.; Volotka, A. V. (Physical Review A, 2018)
Calculations of the magnetic hyperfine structure rely on the input of nuclear properties{–}nuclear magnetic moments and nuclear magnetization distributions{–}as well as quantum electrodynamic radiative corrections for high-accuracy evaluation in heavy atoms. The uncertainties associated with assumed values of these properties limit the accuracy of hyperfine calculations. For example, for the heavy alkali-metal atoms Cs and Fr, these uncertainties may amount collectively to almost 1% or 2%, respectively. In this paper, we propose a method for removing the dependence of hyperfine structure calculations on assumed values of nuclear magnetic moments and nuclear magnetization distributions by determining these effects empirically from measurements of the hyperfine structure for high states. The method is valid for s, p1/2, and p3/2 states of alkali-metal atoms and alkali-metal-like ions. We have shown that for s states, the dependence on QED effects may also be removed to high accuracy. The ability to probe the electronic wave functions, through hyperfine comparisons, with significantly increased accuracy is important for the analysis of atomic parity violation measurements, and it may enable the accuracy of atomic parity violation calculations to be improved. More broadly, it paves the way for further development of high-precision atomic many-body methods.
hij:1, public:1,

Thermal fluctuations of charged black holes in gravity’s rainbow - Upadhyay, S.; Hendi, S.; Panahiyan, S.; Panah, B. (Progress of Theoretical and Experimental Physics, 2018)
Quantum fluctuation effects have an irrefutable role in high-energy physics. Such fluctuation can be often regarded as a correction of the infrared (IR) limit. In this paper, the effects of the first-order correction of entropy, caused by thermal fluctuation, on the thermodynamics of charged black holes in gravity’s rainbow will be discussed. It will be shown that such correction has profound contributions to the high-energy limit of thermodynamical quantities and the stability conditions of black holes, and, interestingly, has no effect on thermodynamical phase transitions. The coupling between gravity’s rainbow and the first-order correction will be addressed. In addition, the measurement of entropy as a function of fluctuation of temperature will be covered, and it will be shown that the de Sitter case enforces an upper limit on the values of temperature and produces cyclic-like diagrams, while for the anti-de Sitter case, a lower limit on the entropy is provided; although for special cases a cyclic-like behavior could be observed, no upper or lower limit exists for the temperature. In addition, a comparison between non-correction and correction-included cases on the thermodynamical properties of solutions will also be discussed and the effects of the first-order correction will be highlighted. It will be shown that the first-order correction provides solutions with larger classes of thermal stability conditions, which may result in the existence of a larger number of thermodynamical structures for the black holes.
hij:1, public:1,

Thulium-doped nonlinear fiber amplifier delivering 50 fs pulses at 20 W of average power - Heuermann, T.; Gaida, C.; Gebhardt, M.; Limpert, J. (Optics Letters, 2018)
In this Letter, we present an optimized nonlinear amplification scheme in the 2 µm wavelength region. This laser source delivers 50 fs pulses at an 80 MHz repetition rate with exceptional temporal pulse quality and 20 W of average output power. According to predictions from numerical simulations, it is experimentally confirmed that dispersion management is crucial to prevent the growth of side pulses and an increase of the energy content in a temporal pedestal surrounding the self-compressed pulse. Based on these results, we discuss guidelines to ensure high temporal pulse quality from nonlinear femtosecond fiber amplifiers in the anomalous dispersion regime.
hij:1, public:1,

Vacuum birefringence in the head-on collision of x-ray free-electron laser and optical high-intensity laser pulses - Karbstein, F. (Physical Review D, 2018)
The focus of this article is on providing compact analytical expressions for the differential number of polarization-flipped signal photons constituting the signal of vacuum birefringence in the head-on collision of x-ray free electron (XFEL) and optical high-intensity laser pulses. Our results allow for unprecedented insights into the scaling of the effect with the waists and pulse durations of both laser beams, the Rayleigh range of the high-intensity beam, as well as transverse and longitudinal offsets. They account for the decay of the differential number of signal photons in the far field as a function of the azimuthal angle measured relative to the beam axis of the probe beam in the forward direction, typically neglected by conventional approximations. Moreover, they even allow us to extract an analytical expression for the angular divergence of the perpendicularly polarized signal photons. We expect our formulas to be very useful for the planning and optimization of experimental scenarios aiming at the detection of vacuum birefringence in XFEL/high-intensity laser setups, such as the one put forward at the Helmholtz International Beamline for Extreme Fields at the European XFEL.
hij:1, public:1,

Above-threshold ionization by few-cycle Bessel pulses carrying orbital angular momentum - B{\"{o}}ning, B.; Paufler, W.; Fritzsche, S. (Physical Review A, 2018)
We investigate theoretically the above-threshold ionization (ATI) of localized atomic targets by intense few-cycle Bessel pulses that carry orbital angular momentum (OAM), known also as twisted light. More specifically, we use the strong-field approximation (SFA) to compute the photoelectron energy spectra. While for plane-wave laser pulses the outgoing photoelectron is typically described by Volkov states within the SFA, no equivalent is known for an electron in a twisted laser field. Here, we therefore introduce a local dipole approximation for the (continuum) state of the photoelectron that is justified for few-cycle pulses. Based on this approximation, we demonstrate that the photoelectrons can also be emitted into the propagation direction of the pulse. When measured in propagation direction, moreover, we show that the magnitude of the ATI peaks depend on the opening angle and the (projection of) total angular momentum of the Bessel pulse.
hij:1, public:1,

Continuous every-single-shot carrier-envelope phase measurement and control at 100 kHz - Hoff, D.; Furch, F. J.; Witting, T.; R{\"{u}}hle, K.; Adolph, D.; Sayler, A. M.; Vrakking, M. J. J.; Paulus, G.G.; Schulz, C. P. (Optics Letters, 2018)
With the emergence of high-repetition-rate few-cycle laser pulse amplifiers aimed at investigating ultrafast dynamics in atomic, molecular, and solid-state science, the need for ever faster carrier-envelope phase (CEP) detection and control has arisen. Here we demonstrate a high-speed, continuous, every-single-shot measurement and fast feedback scheme based on a stereo above-threshold ionization time-of-flight spectrometer capable of detecting the CEP and pulse duration at a repetition rate of up to 400 kHz. This scheme is applied to a 100 kHz optical parametric chirped pulse amplification few-cycle laser system, demonstrating improved CEP stabilization and allowing for CEP tagging.
hij:1, public:1,

Controlling quantum random walk with a step-dependent coin - Panahiyan, S.; Fritzsche, S. (New Journal of Physics, 2018)
We report on the possibility of controlling quantum random walks (QWs) with a step-dependent coin (SDC). The coin is characterized by a (single) rotation angle. Considering different rotation angles, one can find diverse probability distributions for this walk including: complete localization, Gaussian and asymmetric likes. In addition, we explore the entropy of walk in two contexts; for probability density distributions over position space and walker's internal degrees of freedom space (coin space). We show that entropy of position space can decrease for a SDC with the step-number, quite in contrast to a walk with step-independent coin (SIC). For entropy of coin space, a damped oscillation is found for walk with SIC while for a SDC case, the behavior of entropy depends on rotation angle. In general, we demonstrate that quantum walks with simple initiatives may exhibit a quite complex and varying behavior if SDCs are applied. This provides the possibility of controlling QW with a SDC.
hij:1, public:1,

Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases - Saule, T.; H{\"{o}}gner, M.; Lilienfein, N.; de Vries, O.; Pl{\"{o}}tner, M.; Yakovlev, V.; Karpowicz, N.; Limpert, J.; Pupeza, I. (APL Photonics, 2018)
Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-μJ input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25–50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular.
hij:1, public:1,

Diagnostics for plasma-based electron accelerators - Downer, M. C.; Zgadzaj, R.; Debus, A.; Schramm, U.; Kaluza, M.C. (Review Modern Physics, 2018)
Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (rf) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron bunches can emerge with smaller normalized transverse emittance (εn<0.1 mm mrad) and shorter duration (τb∼1 fs) than bunches from rf linacs. Single-shot diagnostics are reviewed that determine such small εn and τb noninvasively and with high resolution from wide-bandwidth spectral measurement of electromagnetic radiation the electrons emit: εn from x rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; τb from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of ∼1 fs bunches can also be measured by sampling individual cycles of a copropagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here a new generation of laboratory diagnostics is reviewed that yield snapshots, or even movies, of laser- and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. Spatiotemporal resolution limits of these imaging techniques are discussed, along with insight into plasma-based acceleration physics that has emerged from analyzing the images and comparing them to simulated plasma structures.
hij:1, public:1,

Elastic scattering of twisted electrons by an atomic target: Going beyond the Born approximation - Kosheleva, V. P.; Zaytsev, V. A.; Surzhykov, A.; Shabaev, V. M.; St{\"{o}}hlker, T. (Physical Review A, 2018)
The elastic scattering of twisted electrons by neutral atoms is studied within the fully relativistic framework. The electron-atom interaction is taken into account in all orders, thus allowing us to explore high-order effects beyond the first Born approximation. To illustrate these effects, detailed calculations of the total and differential cross sections as well as the degree of polarization of scattered electrons are performed. Together with the analysis of the effects beyond the first Born approximation, we discuss the influence of the kinematic parameters of the incident twisted electrons on the angular and polarization properties of the scattered electrons.
hij:1, public:1,

Energy enhancement of the target surface electron by using a 200 TW sub-picosecond laser - Mao, J. Y.; Rosmej, O.; Ma, Y.; Li, M. H.; Aurand, B.; Gaertner, F.; Wang, W. M.; Urbancic, J.; Schoenlein, A.; Zielbauer, B.; Eisenbarth, U.; Bagnoud, V.; et al. (Optics Letters, 2018)
One order of magnitude energy enhancement of the target surface electron beams with central energy at 11.5 MeV is achieved by using a 200 TW, 500 fs laser at an incident angle of 72° with a prepulse intensity ratio of 5×10−6. The experimental results demonstrate the scalability of the acceleration process to high electron energy with a longer (sub-picosecond) laser pulse duration and a higher laser energy (120 J). The total charge of the beam is 400±20 pC(E>2.7 MeV). Such a high orientation and mono-energetic electron jet would be a good method to solve the problem of the large beam divergence in fast ignition schemes and to increase the laser energy deposition on the target core.
hij:1, public:1,

Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction - Rosmej, O. N.; Samsonova, Z.; H{\"{o}}fer, S.; Kartashov, D.; Arda, C.; Khaghani, D.; Schoenlein, A.; Z{\"{a}hter, S. and Hoffmann, A. and Loetzsch, R. and Saevert, A. and Uschmann, I. and Povarnitsyn, M. E. and Andreev, N. E. and Pugachev, L. P. and Kaluza, M.C. and Spielmann, C. (Physics of Plasmas, 2018)
We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 10^23 cm^−3 electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.
hij:1, public:1,

Heteronuclear Limit of Strong-Field Ionization: Fragmentation of HeH⁺ by Intense Ultrashort Laser Pulses - Wustelt, P.; Oppermann, F.; Yue, L.; M{\"{o}}ller, M.; St{\"{o}}hlker, T.; Lein, M.; Gr{\"{a}fe, S. and Paulus, G. and Sayler, A. (Physical Review Letters, 2018)
The laser-induced fragmentation dynamics of this most fundamental polar molecule HeH+ are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for nonionizing dissociation, single ionization, and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems leading to a more general picture of strong-field molecular dynamics and facilitating interpolation to systems between the two extreme cases.
hij:1, public:1,

Hyperfine interaction with the ²²⁹Th nucleus and its low-lying isomeric state - M{\"{u}}ller, R.; Maiorova, A.; Fritzsche, S.; Volotka, A.; Beerwerth, R.; Glowacki, P.; Thielking, J.; Meier, D.-M.; Okhapkin, M.; Peik, E.; Surzhykov, A. (Physical Review A, 2018)
The thorium nucleus with a mass number A=229 has attracted much interest because its extremely low-lying first excited isomeric state at about 8 eV opens the possibility for the development of a nuclear clock. Both the energy of this state as well as the nuclear magnetic dipole and electric quadrupole moment of the 229mTh isomer are subjects of intense research. The latter can be determined by investigating the hyperfine structure of thorium atoms or ions. Due to its electronic structure and the long lifetime of the nuclear isomeric state, Th2+ is especially suitable for such kinds of studies. In this Rapid Communication, we present a combined experimental and theoretical investigation of the hyperfine structure of the 229Th^(2+) ion in the nuclear ground state, where a good agreement between theory and experiment is found. For the nuclear excited state we use our calculations in combination with recent measurements to obtain the nuclear dipole moment of the isomeric state μ_iso=−0.35 μN, which is in contradiction to the theoretically predicted value of μ_iso=−0.076 μN.
hij:1, public:1,

Measurement of the Dispersion of an Antiresonant Hollow Core Fiber - Grigorova, T.; Sollapur, R.; Hoffmann, A.; Hartung, A.; Schwuchow, A.; Bierlich, J.; Kobelke, J.; Schmidt, M. A.; Spielmann, C. (IEEE Photonics Journal, 2018)
Due to unique properties, antiresonant hollow core fibers have found widespread use in various fields of science and application. Particular regarding applications that involve ultrashort pulses, precise knowledge of group velocity dispersion is essential to understand the underlying physics and to optimize device performance. Here we report on the successful measurement of the spectral distribution of the group velocity dispersion of the fundamental mode of an antiresonant hollow core fiber in close proximity to and away from a strong strand resonance. The results show the variations of the hundreds of fs^2/cm near the resonance region, whereas the dispersion is identical to that of a perfect cylindrical waveguide away from the resonance in accordance with a literature. An additional zero dispersion wavelength that is not present in the case of a capillary was experimentally verified. The possibility to tune dispersion via strand resonances opens up a novel pathway towards engineering pulse dispersion, with applications in fields such as nonlinear science and pulse propagation management.
hij:1, public:1,

Particle-in-cell simulations of laser–plasma interactions at solid densities and relativistic intensities: the role of atomic processes - Wu, D.; He, X. T.; Yu, W.; Fritzsche, S. (High Power Laser Science and Engineering, 2018)
Direct numerical simulation of intense laser–solid interactions is still of great challenges, because of the many coupled atomic and plasma processes, such as ionization dynamics, collision among charged particles and collective electromagnetic fields, to name just a few. Here, we develop a new particle-in-cell (PIC) simulation code, which enables us to calculate laser–solid interactions in a more realistic way. This code is able to cover almost ’ the coupled physical processes. As an application of the new code, the generation and transport of energetic electrons in front of and within the solid target when irradiated by intense laser beams are studied. For the considered case, in which laser intensity is 10^20 W/cm2 and pre-plasma scale length in front of the solid is 10 μm, several quantitative conclusions are drawn: (i) the collisional damping (although it is very weak) can significantly affect the energetic electrons generation in front of the target, (ii) the Bremsstrahlung radiation will be enhanced by 2–3 times when the solid is dramatically heated and ionized, (iii) the -off’ electron energy is lowered by an amount of 25% when both collision damping and Bremsstrahlung radiations are included, and (iv) the resistive electromagnetic fields due to Ohmic heating play nonignorable roles and must be taken into account in such interactions.
hij:1, public:1,

Radiation reaction studies in an all-optical set-up: experimental limitations - Samarin, G. M.; Zepf, M.; Sarri, G. (Journal of Modern Optics, 2018)
The recent development of ultra-high intensity laser facilities is finally opening up the possibility of studying high-field quantum electrodynamics in the laboratory. Arguably, one of the central phenomena in this area is that of quantum radiation reaction experienced by an ultra-relativistic electron beam as it propagates through the tight focus of a laser beam. In this paper, we discuss the major experimental challenges that are to be faced in order to extract meaningful and quantitative information from this class of experiments using existing and near-term laser facilities.
hij:1, public:1,

Relativistic calculations of x-ray transition energies and isotope shifts in heavy atoms - Tupitsyn, I. I.; Zubova, N. A.; Shabaev, V. M.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2018)
X-ray transition energies and isotope shifts in heavy atoms are evaluated. The energy levels with vacancies in the inner shells are calculated within the approximation of the average of a nonrelativistic configuration employing the Dirac-Fock-Sturm method. The obtained results are compared with other configuration-interaction theoretical calculations and with experimental data.
hij:1, public:1,

Annular beam driven high harmonic generation for high flux coherent XUV and soft X-ray radiation - Klas, R.; Kirsche, A.; Tschernajew, M.; Rothhardt, J.; Limpert, J. (Optics Express, 2018)
Separation of the high average power driving laser beam from the generated XUV to soft-X-ray radiation poses great challenges in collinear HHG setups due to the losses and the limited power handling capabilities of the typically used separating optics. This paper demonstrates the potential of driving HHG with annular beams, which allow for a straightforward and power scalable separation via a simple pinhole, resulting in a measured driving laser suppression of 5⋅10−3. The approach is characterized by an enormous flexibility as it can be applied to a broad range of input parameters and generated photon energies. Phase matching aspects are analyzed in detail and an HHG conversion efficiency that is only 27% lower than using a Gaussian beam under identical conditions is demonstrated, revealing the viability of the annular beam approach for high flux coherent short-wavelength sources and high
hij:1, public:1,

Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser - Poder, K.; Tamburini, M.; Sarri, G.; Piazza, A. D.; Kuschel, S.; Baird, C.; Behm, K.; Bohlen, S.; Cole, J.; Corvan, D.; Duff, M.; Gerstmayr, E.; et al. (Physical Review X, 2018)
The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself{–}the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of 4×10²⁰ W/cm²). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.
hij:1, public:1,

Making pions with laser light - Schumaker, W.; Liang, T.; Clarke, R.; Cole, J. M.; Grittani, G.; Kuschel, S.; Mangles, S. P. D.; Najmudin, Z.; Poder, K.; Sarri, G.; Symes, D.; Thomas, A. G. R.; et al. (New Journal of Physics, 2018)
The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using 'all optical' methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (²⁷Mg) which is a signature for pion (π⁺) production and subsequent muon decay. Using a 300 TW laser pulse, we have measured the generation of 150 ± 50 pions per shot. We also show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the ²⁷Mg measurement as background via the (n, p) process.
hij:1, public:1,

Maximum Elliptical Dichroism in Atomic Two-Photon Ionization - Hofbrucker, J.; Volotka, A.; Fritzsche, S. (Physical Review Letters, 2018)
Elliptical dichroism is known in atomic photoionization as the difference in the photoelectron angular distributions produced in nonlinear ionization of atoms by left- and right-handed elliptically polarized light. We theoretically demonstrate that the maximum dichroism |ΔED|=1 always appears in two-photon ionization of any atom if the photon energy is tuned in so that the electron emission is dominantly determined by two intermediate resonances. We propose the two-photon ionization of atomic helium in order to demonstrate this remarkable phenomenon. The maximum elliptical dichroism could be used as a sensitive tool for analyzing the polarization state of photon beams produced by free-electron lasers.
hij:1, public:1,

Phase-shift evolution of the thermally-induced refractive index grating in high-power fiber laser systems induced by pump-power variations - Stihler, C.; Jauregui, C.; T{\"{u}}nnermann, A.; Limpert, J. (Optics Express, 2018)
A phase shift between the modal interference pattern and the thermally-induced refractive index grating is most likely the ultimate trigger for the damaging effect of transverse mode instabilities (TMI) in high-power fiber laser systems. By using comprehensive simulations, the creation and evolution of a thermally-induced phase shift is explained and illustrated in detail. It is shown that such a phase shift can be induced by a variation of the pump power. The gained knowledge about the generation and evolution of the phase shift will allow for the development of new mitigation strategies for TMI.
hij:1, public:1,

Scaling ultrashort laser pulse induced glass modifications for cleaving applications - Bergner, K.; M{\"{u}}ller, M.; Klas, R.; Limpert, J.; Nolte, S.; T{\"{u}}nnermann, A. (Applied Optics, 2018)
Ultrashort laser pulses allow for in-volume processing of glass through non-linear absorption. This results in permanent material changes, largely independent of the processed glass, and it is of particular relevance for cleaving applications. In this paper, a laser with a wavelength of 1030 nm, pulse duration of 19 ps, repetition rate of 10 kHz, and burst regime consisting of either four or eight pulses, with an intra-burst pulse separation of 12.5 ns, is used. Subsequently, a Gaussian–Bessel focal line is generated in a fused silica substrate with the aid of an axicon configuration. We show how the structure of the modifications, including the length of material disruptions and affected zones, can be directly influenced by a reasonable choice of focus geometry, pulse energy, and burst regime. We achieve single-shot modifications with 2 μm in diameter and 7.6 mm in length, exceeding an aspect ratio of 1:3800. Furthermore, a maximum length of 10.8 mm could be achieved with a single shot.
hij:1, public:1,

Tailored orbital angular momentum in high-order harmonic generation with bicircular Laguerre-Gaussian beams - Paufler, W.; B{\"{o}}ning, B.; Fritzsche, S. (Physical Review A, 2018)
We report on a method to generate extreme ultraviolet vortices from high-order harmonic generation with two-color counter-rotating Laguerre-Gaussian (LG) beams that carry a well-defined orbital angular momentum (OAM). Our calculations show that the OAM of each harmonic can be directly controlled by the OAM of the incident LG modes. Furthermore, we show how the incoming LG modes have to be tailored, in order to generate every possible value of OAM in the emitted harmonics. In addition, we analyze the emitted harmonics with respect to their divergence and find that it decreases with the harmonic order and increases with the OAM of the emitted harmonic.
hij:1, public:1,

The plateau in above-threshold ionization: the keystone of rescattering physics - Becker, W.; Goreslavski, S. P.; Milo{{s}}evi{}, D. B.; Paulus, G.G. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2018)
A review is presented of the rescattering plateau in laser-induced above-threshold ionization and its various features as they were discovered over time. Several theoretical explanations are discussed, from simple momentum conservation to the quantum-mechanical improved strong-field approximation and the inherent quantum orbits or, alternatively, entirely classical methods. Applications of the plateau to the extraction of atomic or molecular potentials and to the characterization of the driving laser pulse are also surveyed.
hij:1, public:1,

THz Induced Nonlinear Effects in Materials at Intensities above 26 GW/cm2 - Woldegeorgis, A.; Kurihara, T.; Beleites, B.; Bossert, J.; Grosse, R.; Paulus, G.G.; Ronneberger, F.; Gopal, A. (Journal of Infrared, Millimeter, and Terahertz Waves, 2018)
Nonlinear refractive index and absorption coefficient are measured for common semiconductor material such as silicon and organic molecule such as lactose in the terahertz (THz) spectral regime extending from 0.1 to 3 THz. Terahertz pulses with field strengths in excess of 4.4 MV/cm have been employed. Transmittance and the transmitted spectrum were measured with Z-scan and single shot noncollinear electro-optic pump-probe techniques. The THz-induced change in the refractive index (Deltan) shows frequency-dependence and a maximum change of -0.128 at 1.37 THz in lactose and up to +0.169 at 0.15 THz in silicon was measured for a peak incident THz intensity of 26 GW/cm2. Furthermore, the refractive index variation shows a quadratic dependence on the incident THz field, implying the dominance of third-order nonlinearity.
hij:1, public:1,

Wavelength-dispersive spectroscopy in the hard x-ray regime of a heavy highly-charged ion: the 1s Lamb shift in hydrogen-like gold - Gassner, T.; Trassinelli, M.; Hess, R.; Spillmann, U.; Bana{ś}, D.; Blumenhagen, K.-H.; Bosch, F.; Brandau, C.; Chen, W.; Dimopoulou, C.; F{\"{o}}rster, E.; Grisenti, R.; et al. (New Journal of Physics, 2018)
Accurate spectroscopy of highly-charged high-Z ions in a storage ring is demonstrated to be feasible by the use of specially adapted crystal optics. The method has been applied for the measurement of the is Lamb shift in hydrogen-like gold (Au78+) in a storage ring through spectroscopy of the Lyman x-rays. This measurement represents the first result obtained for a high-Z element using high-resolution wavelength-dispersive spectroscopy in the hard x-ray regime, paving the way for sensitivity to higher-order QED effects.
hij:1, public:1,

An investigation on THz yield from laser-produced solid density plasmas at relativistic laser intensities - Herzer, S.; Woldegeorgis, A.; Polz, J.; Reinhard, A.; Almassarani, M.; Beleites, B.; Ronneberger, F.; Grosse, R.; Paulus, G.G.; Huebner, U.; May, T.; Gopal, A. (New Journal of Physics, 2018)
We experimentally characterize the generation of high-power terahertz radiation (THz) at the rear surface of a target irradiated by multiple laser pulses. A detailed dependence of the THz yield as a function of laser pulse duration, energy, target material and thickness is presented. We studied the THz radiation emitted mainly in two directions from the target rear surface, namely target normal (acceptance angle 0.87 sr) and non-collinear direction (perpendicular to the target normal direction{–}acceptance angle 4.12 sr). Independent measurements based on electro-optic diagnostics and pyroelectric detector were employed to estimate the THz yield. Most of the energy is emitted at large angles relative to the target normal direction. THz yield increases with incident laser intensity and thinner targets are better emitters of THz radiation compared to thicker ones.
hij:1, public:1,

Black holes in massive gravity as heat engines - Hendi, S.; Panah, B. E.; Panahiyan, S.; Liu, H.; Meng, X.-H. (Physics Letters B, 2018)
The paper at hand studies the heat engine provided by black holes in the presence of massive gravity. The main motivation is to investigate the effects of massive gravity on different properties of the heat engine. It will be shown that massive gravity parameters modify the efficiency of engine on a significant level. Furthermore, it will be pointed out that it is possible to have a heat engine for non-spherical black holes in massive gravity, and therefore, we will study the effects of horizon topology on the properties of heat engine. Surprisingly, it will be shown that the highest efficiency for the heat engine belongs to black holes with the hyperbolic horizon, while the lowest one belongs to the spherical black holes.
hij:1, public:1,

Chirp and carrier-envelope-phase effects in the multiphoton regime: measurements and analytical modeling of strong-field ionization of sodium - Zille, D.; Adolph, D.; M{\"{o}}ller, M.; Sayler, A. M.; Paulus, G.G. (New Journal of Physics, 2018)
We investigate the influence of chirp on carrier-envelope-phase (CEP)-dependent strong-field few-cycle laser-induced photoelectron spectra of sodium, well within the multiphoton regime. Our measurements and analytical model of this process reveal a simple chirp-dependence, which has the potential to be utilized as an online monitor of laser chirp. Moreover, this effect could extend single-shot measurements of the CEP using above-threshold ionization to longer, chirped pulses, and significantly lower the required pulse energies. Specifically, at a wavelength of 775 nm and an intensity of 6.5 x 10^12 W/cm^2 the CEP- and energy-dependent left-right asymmetries of emitted electrons are measured in a time-of-flight spectrometer. In these asymmetry maps, inclined stripe-like structures emerge, where the inclination is tunable with the chirp of the pulse. We report a simple analytical model, explaining the effect as the interference of electrons with even and odd angular momenta, located at energies in between adjacent above-threshold ionization peaks. As we demonstrate, the analytical model is in good agreement with the measurement, as well as with solutions of the three-dimensional time-dependent Schr{\"{o}}dinger equation. Further, the analytical model, which can easily be extended to other atoms, allows us to derive an equation, describing the relation between the slope of the inclined stripes and the chirp of the laser.
hij:1, public:1,

Cryogenic Current Comparators for Larger Beamlines - Seidel, P.; Tympel, V.; Neubert, R.; Golm, J.; Schmelz, M.; Stolz, R.; Zakosarenko, V.; Sieber, T.; Schwickert, M.; Kurian, F.; Schmidl, F.; St{\"{o}}hlker, T. (IEEE Transactions on Applied Superconductivity, 2018)
The non-destructive measurement of charged particle beams with intensities below 1 μA represents still a challenge in current R{ &}D efforts. Beam peak-intensities of modern high power accelerators are in the range of milli-amperes, but for a large number of experiments, the pulse lengths have to be increased by several orders of magnitude (slow extraction process) to avoid saturation in the detectors. At the same time, the intensities of exotic ion- or antiproton-beams – depending on the production yield – might be in the range of nano-amperes or even below. The solution of this measurement problem should moreover include the possibility to calibrate the electrical current with traceability to national standards.
hij:1, public:1,

Dispersive refraction of different light to heavy materials at MeV γ-ray energies - G{\"{u}}nther, M. M.; Volotka, A. V.; Jentschel, M.; Fritzsche, S.; St{\"{o}}hlker, T.; Thirolf, P. G.; Zepf, M. (Physical Review A, 2018)
The dispersive behavior of materials with atomic charge numbers varying from Z=4 (beryllium, Be) to Z=82 (lead, Pb) was investigated experimentally and theoretically at gamma-ray energies up to 2 MeV. The experiment was performed at the double-crystal gamma spectrometer GAMS6 of the Institut Laue-Langevin in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that, for the development of refractive optics at $γ$-ray energies beyond those currently in use, high-Z materials become increasingly attractive compared to the beryllium lens-stacks used at x-ray energies.
hij:1, public:1,

High resolution XUV Fourier transform holography on a table top - Tadesse, G.; Eschen, W.; Klas, R.; Hilbert, V.; Schelle, D.; Nathanael, A.; Zilk, M.; Steinert, M.; Schrempel, F.; Pertsch, T.; T{\"{u}}nnermann, A.; Limpert, J.; et al. (Scientific Reports, 2018)
Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.
hij:1, public:1,

One-center calculations of the electron-positron pair creation in low-energy collisions of heavy bare nuclei - Popov, R.; Bondarev, A.; Kozhedub, Y.; Maltsev, I.; Shabaev, V.; Tupitsyn, I.; Ma, X.; Plunien, G.; St{\"{o}}hlker, T. (The European Physical Journal D, 2018)
The probabilities of bound-free electron-positron pair creation are calculated for head-on collisions of bare uranium nuclei beyond the monopole approximation. The calculations are based on the numerical solving of the time-dependent Dirac equation in the target reference frame with multipole expansion of the projectile potential. In addition, the energy dependence of the pair-creation cross section is studied in the monopole approximation.
hij:1, public:1,

Probing Sizes and Shapes of Nobelium Isotopes by Laser Spectroscopy - Raeder, S.; Ackermann, D.; Backe, H.; Beerwerth, R.; Berengut, J. C.; Block, M.; Borschevsky, A.; Cheal, B.; Chhetri, P.; D{\"{u}}llmann, Ch. E.; Dzuba, V. A.; Eliav, E.; et al. (Physical Review Letters, 2018)
Until recently, ground-state nuclear moments of the heaviest nuclei could only be inferred from nuclear spectroscopy, where model assumptions are required. Laser spectroscopy in combination with modern atomic structure calculations is now able to probe these moments directly, in a comprehensive and nuclear-model-independent way. Here we report on unique access to the differential mean-square charge radii of 252,253,254No, and therefore to changes in nuclear size and shape. State-of-the-art nuclear density functional calculations describe well the changes in nuclear charge radii in the region of the heavy actinides, indicating an appreciable central depression in the deformed proton density distribution in 252,254No isotopes. Finally, the hyperfine splitting of 253No was evaluated, enabling a complementary measure of its (quadrupole) deformation, as well as an insight into the neutron single-particle wave function via the nuclear spin and magnetic moment.
hij:1, public:1,

Spectroscopic investigations of thulium doped YAG and YAP crystals between 77 K and 300 K for short-wavelength infrared lasers - K{\"{o}}rner, J.; L{\"{u}}hder, T.; Reiter, J.; Uschmann, I.; Marschner, H.; Jambunathan, V.; Lucianetti, A.; Mocek, T.; Hein, J.; Kaluza, M.C. (Journal of Luminescence, 2018)
We present detailed measurements of laser relevant cross sections of thulium doped yttrium-aluminum-garnet (Tm:YAG) and yttrium-aluminum-perovskite (Tm:YAP), including the absorption cross sections for the H63 to H43 transition near 800nm, and the absorption and emission cross sections for the transitions between the H63 and F43 manifolds in the short-wavelength infrared region. For Tm:YAP we present data for all polarization axes. The measurements were carried out at temperatures ranging from 80 K to 300 K. Furthermore, re-absorption free fluorescence lifetimes of the F43 to H63 transition at 77 K, 200 K and 29 5K were obtained using the pinhole method. We observed a significant enhancement of the fluorescence lifetime when cooling from room temperature to 77 K. The lifetime was increased from 9.42 ms to 15.22 ms in Tm:YAG and from 3.81 ms to 4.93 ms in Tm:YAP. This indicates that lifetime quenching is present at room temperature, which can be overcome, at least partially, by cryogenic cooling. These data are presented with the scope to qualify these materials for their use in a new generation of cryogenically cooled, short-wavelength infrared, high-energy class diode pumped solid state lasers utilizing the cross relaxation mechanism for pumping.
hij:1, public:1,

The Heidelberg compact electron beam ion traps - Micke, P.; Kuhn, S.; Buchauer, L.; Harries, J. R.; B{\"{u}}cking, T. M.; Blaum, K.; Cieluch, A.; Egl, A.; Hollain, D.; Kraemer, S.; Pfeifer, T.; Schmidt, P. O.; et al. (Review of Scientific Instruments, 2018)
Electron beam ion traps (EBITs) are ideal tools for both production and study of highly charged ions (HCIs). In order to reduce their construction, maintenance, and operation costs, we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA, and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar¹⁶⁺ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e.g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe²⁴⁺, achieving an electron-energy resolving power of E/ΔE > 1500 at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating the resonant photoexcitation of highly charged oxygen.
hij:1, public:1,

Thermodynamic control of soliton dynamics in liquid-core fibers - Chemnitz, M.; Scheibinger, R.; Gaida, C.; Gebhardt, M.; Stutzki, F.; Pumpe, S.; Kobelke, J.; T{\"{u}}nnermann, A.; Limpert, J.; Schmidt, M. A. (Optica, 2018)
Liquid-core fibers offer local external control over pulse dispersion due to their strong thermodynamic response, offering a new degree of freedom in accurate soliton steering for reconfigurable nonlinear light generation. Here, we show how to accurately control soliton dynamics and supercontinuum generation in carbon disulfide/silica fibers by temperature and pressure tuning, monitored via the spectral location and the onset energy of non-solitonic radiation. Simulations and phase-matching calculations based on an extended thermodynamic dispersion model of carbon disulfide confirm the experimental results, which allows us to demonstrate the potential of temperature detuning of liquid-core fibers for octave spanning recompressible supercontinuum generation in the near-infrared.
hij:1, public:1,

Charge radii and electromagnetic moments of ¹⁹⁵⁻²¹¹At - Cubiss, J. G.; Barzakh, A. E.; Seliverstov, M. D.; Andreyev, A. N.; Andel, B.; Antalic, S.; Ascher, P.; Atanasov, D.; Beck, D.; Bieron, J.; Blaum, K.; Borgmann, Ch.; et al. (Physical Review C, 2018)
Hyperfine-structure parameters and isotope shifts of 195–211At have been measured for the first time at CERN-ISOLDE, using the in-source resonance-ionization spectroscopy method. The hyperfine structures of isotopes were recorded using a triad of experimental techniques for monitoring the photo-ion current. The Multi-Reflection Time-of-Flight Mass Spectrometer, in connection with a high-resolution electron multiplier, was used as an ion-counting setup for isotopes that either were affected by strong isobaric contamination or possessed a long half-life; the ISOLDE Faraday cups were used for cases with high-intensity beams; and the Windmill decay station was used for short-lived, predominantly α-decaying nuclei. The electromagnetic moments and changes in the mean-square charge radii of the astatine nuclei have been extracted from the measured hyperfine-structure constants and isotope shifts. This was only made possible by dedicated state-of-the-art large-scale atomic computations of the electronic factors and the specific mass shift of atomic transitions in astatine that are needed for these extractions. By comparison with systematics, it was possible to assess the reliability of the results of these calculations and their ascribed uncertainties. A strong deviation in the ground-state mean-square charge radii of the lightest astatine isotopes, from the trend of the (spherical) lead isotopes, is interpreted as the result of an onset of deformation. This behavior bears a resemblance to the deviation observed in the isotonic polonium isotopes. Cases for shape coexistence have been identified in 197,199At, for which a significant difference in the charge radii for ground (9/2−) and isomeric (1/2+) states has been observed.
hij:1, public:1,

Electroweak Decays of Highly Charged Ions - Singh Sidhu, R.; Glorius, J.; Litvinov, Yu. A.; Pavi{}evi{}, M. K.; Sanjari, M. S.; St{\"{o}}hlker, T. (EPJ Web of Conferences, 2018)
In this contribution we review the present status of experimental studies of electroweak decays of highly charged ions. A particular focus will be given on the bound state beta decay measurement of 205Tl.
hij:1, public:1,

Enhanced energy absorption of high intensity laser pulses by targets of modulated surface - Cerchez, M.; Swantusch, M.; Toncian, M.; Zhu, X. M.; Prasad, R.; Toncian, T.; R{\"{o}}del, Ch.; J{\"{a}ckel, O. and Paulus, G.G. and Andreev, A. A. and Willi, O. (Applied Physics Letters, 2018)
Investigations of energy transfer of high intensity (I=5x10^19 W/cm2), ultrashort (<30 fs) Ti:Sa laser pulses to solid targets with a randomly rough surface have been performed. We investigated the influence of the target surface morphology on the efficiency of energy transfer of p-polarized laser pulses characterized by a very high contrast. Targets with a roughness r larger than 20% of the laser wavelength proved to absorb a remarkably large fraction of energy reaching up to 70%, almost independent of the incidence angle. Numerical simulations of various interaction conditionsare in agreement with the experimental data and confirm the effect of the target morphology and its surface parameters on the enhanced energy absorbed fraction.
hij:0, public:1,

Influence of a stray magnetic field on the measurement of long-range spin-spin interaction - Goswami, B.; Volotka, A. V.; Fritzsche, S. (Journal of Physics Communications, 2018)
We study the influence of an additional uncontrolled (stray) magnetic field upon the measurement of long-range spin-spin interaction strength of two spin-1/2 valence electrons bound in two separate ions at well-defined distances from each other. This stray field, which is neither perpendicular nor parallel to the line connecting two ions, could appear due to the Earth magnetic field, or, due to the slight angular misalignment between the applied magnetic field and the line connecting two ions. It is found that the presence of the stray magnetic field plays an important role in the dynamics of the spin-states of two electrons. If neglected in the analysis, moreover, such a stray field may affect the measurement of the spin-spin interaction strength, especially at smaller inter-spin distances.
hij:1, public:1,

Intense, directed neutron beams from a laser-driven neutron source at PHELIX - Kleinschmidt, A.; Bagnoud, V.; Deppert, O.; Favalli, A.; Frydrych, S.; Hornung, J.; Jahn, D.; Schaumann, G.; Tebartz, A.; Wagner, F.; Wurden, G.; Zielbauer, B.; et al. (Physics of Plasmas, 2018)
Laser-driven neutrons are generated by the conversion of laser-accelerated ions via nuclear reactions inside a converter material. We present results from an experimental campaign at the PHELIX laser at GSI in Darmstadt where protons and deuterons were accelerated from thin deuterated plastic foils with thicknesses in the μm and sub-μm range. The neutrons were generated inside a sandwich-type beryllium converter, leading to reproducible neutron numbers around 10^11 neutrons per shot. The angular distribution was measured with a high level of detail using up to 30 bubble detectors simultaneously. It shows a laser forward directed component of up to 1.42 × 10^10 neutrons per steradian, corresponding to a dose of 43 mrem scaled to a distance of 1 m from the converter.
hij:1, public:1,

Line strengths of QED-sensitive forbidden transitions in B-, Al-, F- and Cl-like ions - Bilal, M.; Volotka, A. V.; Beerwerth, R.; Fritzsche, S. (Physical Review A, 2018)
The magnetic dipole (M1) line strength between the fine-structure levels of the ground configurations in B-, F-, Al-, and Cl-like ions are calculated for the four elements argon, iron, molybdenum, and tungsten. Systematically enlarged multiconfiguration Dirac-Hartree-Fock (MCDHF) wave functions are employed to account for the interelectronic interaction with the Breit interaction included in first-order perturbation theory. The QED corrections are evaluated to all orders in αZ utilizing an effective potential approach. The calculated line strengths are compared with the results of other theories. The M1 transition rates are reported using accurate energies from the literature. Moreover, the lifetimes in the range of millisecond to picosecond are predicted including the contributions from the transition rate due to the E2 transition channel. The discrepancies of the predicted rates from those available from the literature are discussed and a benchmark data set of theoretical lifetimes is provided to support future experiments.
hij:1, public:1,

Magnetic solutions in Einstein-massive gravity with linear and nonlinear fields - Hendi, S.; Panah, B.; Panahiyan, S.; Momennia, M. (The European Physical Journal C, 2018)
The solutions of U(1) gauge-gravity coupling is one of the interesting models for analyzing the semi-classical nature of spacetime. In this regard, different well-known singular and nonsingular solutions have been taken into account. The paper at hand investigates the geometrical properties of the magnetic solutions by considering Maxwell and power Maxwell invariant (PMI) nonlinear electromagnetic fields in the context of massive gravity. These solutions are free of curvature singularity, but have a conic one which leads to presence of deficit/surplus angle. The emphasize is on modifications that these generalizations impose on deficit angle which determine the total geometrical structure of the solutions, hence, physical/gravitational properties. It will be shown that depending on the background spacetime [being anti de Sitter (AdS) or de Sitter (dS)], these generalizations present different effects and modify the total structure of the solutions differently.
hij:1, public:1,

Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers - Stihler, C.; Jauregui, C.; T{\"{u}}nnermann, A.; Limpert, J. (Light: Science { &} Applications, 2018)
Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.
hij:1, public:1,

Phase-space analysis of the Schwinger effect in inhomogeneous electromagnetic fields - Kohlf{\"{u}}rst, C. (The European Physical Journal Plus, 2018)
Schwinger pair production in spatially and temporally inhomogeneous electric and magnetic fields is studied. The focus is on the particle phase-space distribution within a high-intensity few-cycle pulse. Accurate numerical solutions of a quantum kinetic theory (DHW formalism) are presented in momentum space and, with the aid of coarse-graining techniques, in a mixed spatial-momentum representation. Additionally, signatures of the carrier-envelope phase as well as spin-field interactions are discussed on the basis of a trajectory-based model taking into account instantaneous pair production and relativistic single-particle dynamics. Although our simple semi-classical single-particle model cannot describe every aspect of the particle production process (quantum interferences), essential features such as spin-field interactions are captured.
hij:1, public:1,

Polarization reconstruction algorithm for a Compton polarimeter - Vockert, M.; Weber, G.; Spillmann, U.; Krings, T.; St{\"{o}}hlker, T. (Journal of Physics: Conference Series, 2018)
We present the technique of Compton polarimetry using X-ray detectors based on double-sided segmented semiconductor crystals that were developed within the SPARC collaboration. In addition, we discuss the polarization reconstruction algorithm with particular emphasis on systematic deviations between the observed detector response and our model function for the Compton scattering distribution inside the detector.
hij:1, public:1,

Sauter-Schwinger pair creation dynamically assisted by a plane wave - Torgrimsson, G.; Schneider, C.; Sch{\"{u}}tzhold, R. (Physical Review D, 2018)
We study electron-positron pair creation by a strong and constant electric field superimposed with a weaker transversal plane wave which is incident perpendicularly (or under some angle). Comparing the fully nonperturbative approach based on the world-line instanton method with a perturbative expansion into powers of the strength of the weaker plane wave, we find good agreement{–}provided that the latter is carried out to sufficiently high orders. As usual for the dynamically assisted Sauter-Schwinger effect, the additional plane wave induces an exponential enhancement of the pair-creation probability if the combined Keldysh parameter exceeds a certain threshold.
hij:1, public:1,

Velocity- and pointing-error measurements of a 300 000-r/min self-bearing permanent-magnet motor for optical applications - Breitkopf, S.; Lilienfein, N.; Achtnich, T.; Zwyssig, C.; T{\"{u}}nnermann, A.; Pupeza, I.; Limpert, J. (Review of Scientific Instruments, 2018)
Compact, ultra-high-speed self-bearing permanent-magnet motors enable a wide scope of applications including an increasing number of optical ones. For implementation in an optical setup, the rotors have to satisfy high demands regarding their velocity and pointing errors. Only a restricted number of measurements of these parameters exist and only at relatively low velocities. This manuscript presents the measurement of the velocity and pointing errors at rotation frequencies up to 5 kHz. The acquired data allow us to identify the rotor drive as the main source of velocity variations with fast fluctuations of up to 3.4 ns (RMS) and slow drifts of 23 ns (RMS) over ∼120 revolutions at 5 kHz in vacuum. At the same rotation frequency, the pointing fluctuated by 12 μrad (RMS) and 33 μrad (peak-to-peak) over ∼10 000 round trips. To our best knowledge, this states the first measurement of velocity and pointing errors at multi-kHz rotation frequencies and will allow potential adopters to evaluate the feasibility of such rotor drives for their application.
hij:1, public:1,

87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb: yttrium lithium fluoride amplifier - Hua, Y.; Liu, W.; Hemmer, M.; Zapata, L. E.; Zhou, G.; Schimpf, D. N.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A.; K{\"{a}rtner, F. X. and Chang, G. (Optics Letters, 2018)
We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-µJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers.
hij:1, public:1,

Analysis of angular momentum properties of photons emitted in fundamental atomic processes - Zaytsev, V. A.; Surzhykov, A. S.; Shabaev, V. M.; St{\"{o}}hlker, T. (Physical Review A, 2018)
Many atomic processes result in the emission of photons. Analysis of the properties of emitted photons, such as energy and angular distribution as well as polarization, is regarded as a powerful tool for gaining more insight into the physics of corresponding processes. Another characteristic of light is the projection of its angular momentum upon propagation direction. This property has attracted a special attention over the past decades due to studies of twisted (or vortex) light beams. Measurements being sensitive to this projection may provide valuable information about the role of angular momentum in the fundamental atomic processes. Here we describe a simple theoretical method for determination of the angular momentum properties of the photons emitted in various atomic processes. This method is based on the evaluation of expectation value of the total angular momentum projection operator. To illustrate the method, we apply it to the textbook examples of plane-wave, spherical-wave, and Bessel light. Moreover, we investigate the projection of angular momentum for the photons emitted in the process of the radiative recombination with ionic targets. It is found that the recombination photons do carry a nonzero projection of the orbital angular momentum.
hij:1, public:1,

Coherently combined 16-channel multicore fiber laser system - Klenke, A.; M{\"{u}}ller, M.; Stark, H.; Stutzki, F.; Hupel, C.; Schreiber, T.; T{\"{u}}nnermann, A.; Limpert, J. (Optics Letters, 2018)
We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled-aperture beam splitting and combination, together with temporal phasing, is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses is achieved with combination efficiencies around 80%.
hij:1, public:1,

Curvature bound from gravitational catalysis - Gies, H.; Martini, R. (Physical Review D, 2018)
We determine bounds on the curvature of local patches of spacetime from the requirement of intact long-range chiral symmetry. The bounds arise from a scale-dependent analysis of gravitational catalysis and its influence on the effective potential for the chiral order parameter, as induced by fermionic fluctuations on a curved spacetime with local hyperbolic properties. The bound is expressed in terms of the local curvature scalar measured in units of a gauge-invariant coarse-graining scale. We argue that any effective field theory of quantum gravity obeying this curvature bound is safe from chiral symmetry breaking through gravitational catalysis and thus compatible with the simultaneous existence of chiral fermions in the low-energy spectrum. With increasing number of dimensions, the curvature bound in terms of the hyperbolic scale parameter becomes stronger. Applying the curvature bound to the asymptotic safety scenario for quantum gravity in four spacetime dimensions translates into bounds on the matter content of particle physics models.
hij:1, public:1,

Interelectronic-interaction contribution to the nonlinear Zeeman effect in boronlike ions - Varentsova, A. S.; Agababaev, V. A.; Glazov, D. A.; Volchkova, A. M.; Volotka, A. V.; Shabaev, V. M.; Plunien, G. (Physical Review A, 2018)
Relativistic calculations of the second- and third-order contributions in magnetic field to the Zeeman splitting in boronlike ions are presented for the wide range of nuclear charge numbers Z=6−92. The interelectronic-interaction correction of the first order in 1/Z is evaluated to all orders in αZ. The higher order corrections in 1/Z are taken into account approximately by means of effective screening potentials. The obtained results are important for interpretation of experimental data on the Zeeman splitting in boronlike ions, in particular, for the ARTEMIS experiment presently implemented at GSI.
hij:1, public:1,

Isotope shifts from collinear laser spectroscopy of doubly charged yttrium isotopes - Vormawah, L. J.; Vil{\'{e}}n, M.; Beerwerth, R.; Campbell, P.; Cheal, B.; Dicker, A.; Eronen, T.; Fritzsche, S.; Geldhof, S.; Jokinen, A.; Kelly, S.; Moore, I. D.; et al. (Physical Review A, 2018)
Collinear laser spectroscopy has been performed on doubly charged ions of radioactive yttrium in order to study the isotope shifts of the 294.6-nm 5s2S1/2→5p2P1/2 line. The potential of such an alkali-metal-like transition to improve the reliability of atomic-field-shift and mass-shift factor calculations, and hence the extraction of nuclear mean-square radii, is discussed. Production of yttrium ion beams for such studies is available at the IGISOL IV Accelerator Laboratory, Jyv{\"{a}skyl{\"{a}, Finland. This newly recommissioned facility is described here in relation to the on-line study of accelerator-produced short-lived isotopes using collinear laser spectroscopy and application of the technique to doubly charged ions.
hij:1, public:1,

Lifetimes of relativistic heavy-ion beams in the High Energy Storage Ring of FAIR - Shevelko, V.; Litvinov, Yu. A.; St{\"{o}}hlker, T.; Tolstikhina, I. Yu. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2018)
The High Energy Storage Ring, HESR, will be constructed at the Facility for Antiproton and Ion Research, FAIR, Darmstadt. For the first time, it will be possible to perform experiments with cooled high-intensity stable and radioactive heavy ions at highly relativistic energies. To design experiments at the HESR, realistic estimations of beam lifetimes are indispensable. Here we report calculated cross sections and lifetimes for typical U88+, U90+, U92+, Sn49+ and Sn50+ ions in the energy range E = 400 MeV/u–5 GeV/u, relevant for the HESR. Interactions with the residual gas and with internal gas-jet targets are also considered.
hij:1, public:1,

Photon-photon scattering at the high-intensity frontier - Gies, H.; Karbstein, F.; Kohlf{\"{u}}rst, C.; Seegert, N. (Physical Review D, 2018)
The tremendous progress in high-intensity laser technology and the establishment of dedicated high-field laboratories in recent years have paved the way towards a first observation of quantum vacuum nonlinearities at the high-intensity frontier. We advocate a particularly prospective scenario, where three synchronized high-intensity laser pulses are brought into collision, giving rise to signal photons, whose frequency and propagation direction differ from the driving laser pulses, thus providing various means to achieve an excellent signal to background separation. Based on the theoretical concept of vacuum emission, we employ an efficient numerical algorithm which allows us to model the collision of focused high-intensity laser pulses in unprecedented detail. We provide accurate predictions for the numbers of signal photons accessible in experiment. Our study is the first to predict the precise angular spread of the signal photons, and paves the way for a first verification of quantum vacuum nonlinearity in a well-controlled laboratory experiment at one of the many high-intensity laser facilities currently coming online.
hij:1, public:1,

Probing baryogenesis through the Higgs boson self-coupling - Reichert, M.; Eichhorn, A.; Gies, H.; Pawlowski, J. M.; Plehn, T.; Scherer, M. M. (Physical Review D, 2018)
The link between a modified Higgs self-coupling and the strong first-order phase transition necessary for baryogenesis is well explored for polynomial extensions of the Higgs potential. We broaden this argument beyond leading polynomial expansions of the Higgs potential to higher polynomial terms and to nonpolynomial Higgs potentials. For our quantitative analysis we resort to the functional renormalization group, which allows us to evolve the full Higgs potential to higher scales and finite temperature. In all cases we find that a strong first-order phase transition manifests itself in an enhancement of the Higgs self-coupling by at least 50%, implying that such modified Higgs potentials should be accessible at the LHC.
hij:1, public:1,

Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold - Jauregui, C.; Stihler, C.; T{\"{u}}nnermann, A.; Limpert, J. (Optics Express, 2018)
A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of \ 600W, which is a factor of 2 above the mode instability threshold.
hij:1, public:1,

Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime - Becker, G. A.; Tietze, S.; Keppler, S.; Reisl{\"{o}}hner, J.; Bin, J. H.; Bock, L.; Brack, F.-E.; Hein, J.; Hellwing, M.; Hilz, P.; Hornung, M.; Kessler, A.; et al. (Plasma Physics and Controlled Fusion, 2018)
The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here{–}in addition to the longitudinal electric fields responsible for the forward acceleration of the protons{–}a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.
hij:1, public:1,

Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays - Klenke, A.; M{\"{u}}ller, M.; Stark, H.; T{\"{u}}nnermann, A.; Limpert, J. (Optics Express, 2018)
We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.
hij:1, public:1,

Strong-field ionization with twisted laser pulses - Paufler, W.; B{\"{o}}ning, B.; Fritzsche, S. (Physical Review A, 2018)
We apply quantum trajectory Monte Carlo computations in order to model strong-field ionization of atoms by twisted Bessel pulses and calculate photoelectron momentum distributions (PEMD). Since Bessel beams can be considered as an infinite superposition of circularly polarized plane waves with the same helicity, whose wave vectors lie on a cone, we compared the PEMD of such Bessel pulses to those of a circularly polarized pulse. We focus on the momentum distributions in propagation direction of the pulse and show how these momentum distributions are affected by experimental accessible parameters, such as the opening angle of the beam or the impact parameter of the atom with regard to the beam axis. In particular, we show that we can find higher momenta of the photoelectrons, if the opening angle is increased.
hij:1, public:1,

Unexpected impact of radiation friction: enhancing production of longitudinal plasma waves - Gelfer, E.; Elkina, N.; Fedotov, A. (Scientific Reports, 2018)
We study the penetration of ultra-intense (intensity I ≃ 10^(23–24) W/cm2) circularly polarized laser pulses into a thick subcritical plasma layer with accounting for radiation friction. We show that radiation pressure is enhanced due to radiation friction in the direction transverse to the laser pulse propagation, and that for stronger and longer laser pulses this mechanism dominates over the ordinary ponderomotive pressure, thus resulting in a substantionaly stronger charge separation than anticipated previously. We give estimates of the effect and compare them with the results of one and two dimensional particle-in-cell simulations. This effect can be important for laser-based acceleration schemes.
hij:1, public:1,

A study of electric field distribution in Benjamin type proportional counter using finite element method - Lin, Y.; Yuan, Y.; Fang, F.; Tan, Z. (Applied Radiation and Isotopes, 2018)
Tissue equivalent proportional counters (TEPCs) are commonly based on the Benjamin type of concept. Initially the electric field is optimized by pulse height measurement methods and only one optimum solution was established at that time. In this paper, the electric field distribution is analyzed and optimized using a three-dimensional finite element method. The calculations show that the characteristics of the radial electric field distribution of this type of counters can be equated to cylindrical counters using a pair of appropriate field shaping electrode. Furthermore, the paper analyzes the axial electric field distribution and the possibility of achieving a uniform electric field along its anode while reducing the size of Benjamin type proportional counter design down to 1/10 of currently feasible values with respect to the thinnest available anode wire diameters.
hij:1, public:1,

A systematic approach to numerical dispersion in Maxwell solvers - Blinne, A.; Schinkel, D.; Kuschel, S.; Elkina, N.; Rykovanov, S. G.; Zepf, M. (Computer Physics Communications, 2018)
The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.
hij:1, public:1,

Black Hole Solutions in Gauss-Bonnet-Massive Gravity in the Presence of Power-Maxwell Field - Hendi, S. H.; Eslam Panah, B.; Panahiyan, S. (Fortschritte der Physik, 2018)
Motivated by recent progresses in the field of massive gravity, the paper at hand investigates the thermodynamical structure of black holes with three specific generalizations: i) Gauss-Bonnet gravity which is motivated from string theory ii) PMI nonlinear electromagnetic field which is motivated from perspective of the QED correction iii) massive gravity which is motivated by obtaining the modification of standard general relativity. The exact solutions of this setup are extracted which are interpreted as black holes. In addition, thermodynamical quantities of the solutions are calculated and their critical behavior are studied. It will be shown that although massive and Gauss-Bonnet gravities are both generalizations in gravitational sector, they show opposing effects regarding the critical behavior of the black holes. Furthermore, a periodic effect on number of the phase transition is reported for variation of the nonlinearity parameter and it will be shown that for super charged black holes, system is restricted in a manner that prevents it to reach the critical point and to acquire phase transition. In addition, the effects of geometrical structure on thermodynamical phase transition will be highlighted.
hij:1, public:1,

K-shell ionization of heavy hydrogenlike ions - Novak, O.; Kholodov, R.; Surzhykov, A.; Artemyev, A. N.; St{\"{o}}hlker, T. (Physical Review A, 2018)
A theoretical study of the K-shell ionization of hydrogenlike ions, colliding with bare nuclei, is performed within the framework of the time-dependent Dirac equation. Special emphasis is placed on the ionization probability that is investigated as a function of impact parameter, collision energy, and nuclear charge. To evaluate this probability in a wide range of collisional parameters we propose a simple analytical expression for the transition amplitude. This expression contains three fitting parameters that are determined from the numerical calculations, based on the adiabatic approximation. In contrast to previous studies, our analytical expression for the transition amplitude and ionization probability accounts for the full multipole expansion of the two-center potential and allows accurate description of nonsymmetric collisions of nuclei with different atomic numbers Z1≠Z2. The calculations performed for both symmetric and asymmetric collisions indicate that the ionization probability is reduced when the difference between the atomic numbers of ions increases.
hij:1, public:1,

Nonunique and nonuniform mapping in few-body Coulomb-explosion imaging - Sayler, A. M.; Eckner, E.; McKenna, J.; Esry, B. D.; Carnes, K. D.; Ben-Itzhak, I.; Paulus, G.G. (Physical Review A, 2018)
Much of our knowledge of molecular geometry and interaction dynamics comes from indirect measurements of the molecular fragments following breakup. This technique{–}Coulomb-explosion imaging (CEI), i.e., determining the initial molecular configuration of a system from the momenta of the resulting fragments using knowledge of the particle interactions{–}is one of the fundamental tools of molecular physics. Moreover, CEI has been a staple of molecular studies for decades. Here we show that one often cannot assign a unique initial configuration to the few-body breakup of a polyatomic molecule given the measurement of the resulting fragments' momenta. Specifically, multiple initial configurations can result in identical momenta for a molecule breaking into three or more parts. Further, the nonunique and nonuniform mapping from the initial configuration to the measured momenta also significantly complicates the determination of molecular alignment at the time of breakup.
hij:1, public:1,

Observation of non-symmetric side-scattering during high-intensity laser-plasma interactions - Arunachalam, A. K.; Schwab, M. B.; S{\"{a}vert, A. and Kaluza, M.C. (New Journal of Physics, 2018)
Non-symmetric side-scattering has been observed during the interaction between a high-intensity laser pulse and under-dense argon plasma. The angle between the laser's forward direction and the scattered radiation is found to decrease for increasing electron densities ranging from 0.01 to 0.25n c , where n c is the critical density for the laser wavelength. We show that the observed features of the scattering cannot be described by Raman side-scattering but can be explained to be a consequence of the non-uniform density distribution of the plasma with the scattering angle being oriented along the direction of the resulting electron density gradient.
hij:1, public:1,

On-shot characterization of single plasma mirror temporal contrast improvement - Obst, L.; Metzkes-Ng, J.; Bock, S.; Cochran, G. E.; Cowan, T. E.; Oksenhendler, T.; Poole, P. L.; Prencipe, I.; Rehwald, M.; R{\"{o}}del, C.; Schlenvoigt, H.-P.; Schramm, U.; et al. (Plasma Physics and Controlled Fusion, 2018)
We report on the setup and commissioning of a compact recollimating single plasma mirror (PM) for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without PM is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the PM trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of 1 μm down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.
hij:1, public:1,

Photon polarization tensor in circularly polarized Hermite- and Laguerre-Gaussian beams - Karbstein, F.; Mosman, E. A. (Modern Physics Letters A, 2018)
We derive analytical expressions for the photon polarization tensor in circularly polarized Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) beams, complementing the corresponding results for linearly polarized beams obtained recently. As they are based upon a locally constant field approximation of the one-loop Heisenberg–Euler effective Lagrangian for quantum electrodynamics (QED) in constant fields, our results are generically limited to slowly varying electromagnetic fields, varying on spatial (temporal) scales much larger than the Compton wavelength (time) of the electron.
hij:1, public:1,

Probing ultrafast proton induced dynamics in transparent dielectrics - Taylor, M.; Coughlan, M.; Nersisyan, G.; Senje, L.; Jung, D.; Currell, F.; Riley, D.; Lewis, C. L. S.; Zepf, M.; Dromey, B. (Plasma Physics and Controlled Fusion, 2018)
A scheme has been developed permitting the spatial and temporal characterisation of ultrafast dynamics induced by laser driven proton bursts in transparent dielectrics. Advantage is taken of the high degree of synchronicity between the proton bursts generated during laser-foil target interactions and the probing laser to provide the basis for streaking of the dynamics. Relaxation times of electrons (<10⁻¹² s) are measured following swift excitation across the optical band gap for various glass samples. A temporal resolution of <500 fs is achieved demonstrating that these ultrafast dynamics can be characterized on a single-shot basis.
hij:1, public:1,

Real-Time Tomography of Gas-Jets with a Wollaston Interferometer - Adelmann, A.; Hermann, B.; Ischebeck, R.; Kaluza, M.; Locans, U.; Sauerwein, N.; Tarkeshian, R. (Applied Sciences, 2018)
A tomographic gas-density diagnostic using a Single-Beam Wollaston Interferometer able to characterize non-symmetric density distributions in gas jets is presented. A real-time tomographic algorithm is able to reconstruct three-dimensional density distributions. A Maximum Likelihood-Expectation Maximization algorithm, an iterative method with good convergence properties compared to simple back projection, is used. With the use of graphical processing units, real-time computation and high resolution are achieved. Two different gas jets are characterized: a kHz, piezo-driven jet for lower densities and a solenoid valve-based jet producing higher densities. While the first jet is used for free electron laser photon beam characterization, the second jet is used in laser wake field acceleration experiments. In this latter application, well-tailored and non-symmetric density distributions produced by a supersonic shock front generated by a razor blade inserted laterally to the gas flow, which breaks cylindrical symmetry, need to be characterized.
hij:1, public:1,

Thermal analysis of Yb-doped high-power fiber amplifiers with Al:P co-doped cores - Jauregui, C.; Stutzki, F.; T{\"{u}}nnermann, A.; Limpert, J. (Optics Express, 2018)
It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.
hij:1, public:1,

γ-Ray Generation from Plasma Wakefield Resonant Wiggler - Lei, B.; Wang, J.; Kharin, V.; Zepf, M.; Rykovanov, S. (Physical Review Letters, 2018)
A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.
hij:1, public:1,

A sensitive EUV Schwarzschild microscope for plasma studies with sub-micrometer resolution - Zastrau, U.; R{\"{o}}del, C.; Nakatsutsumi, M.; Feigl, T.; Appel, K.; Chen, B.; D{\"{o}}ppner, T.; Fennel, T.; Fiedler, T.; Fletcher, L. B.; F{\"{o}}rster, E.; Gamboa, E.; et al. (Review of Scientific Instruments, 2018)
We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.
hij:1, public:1,

All-optical signatures of strong-field QED in the vacuum emission picture - Gies, H.; Karbstein, F.; Kohlf{\"{u}}rst, C. (Physical Review D, 2018)
We study all-optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum, using the collision of two focused high-intensity laser pulses as an example. The experimental signatures of quantum vacuum nonlinearities are encoded in signal photons, whose kinematic and polarization properties differ from the photons constituting the macroscopic laser fields. We implement an efficient numerical algorithm allowing for the theoretical investigation of such signatures in realistic field configurations accessible in experiment. This algorithm is based on a vacuum emission scheme and can readily be adapted to the collision of more laser beams or further involved field configurations. We solve the case of two colliding pulses in full 3+1-dimensional spacetime and identify experimental geometries and parameter regimes with improved signal-to-noise ratios.
hij:1, public:1,

Carbon chloride-core fibers for soliton mediated supercontinuum generation - Chemnitz, M.; Gaida, C.; Gebhardt, M.; Stutzki, F.; Kobelke, J.; T{\"{u}}nnermann, A.; Limpert, J.; Schmidt, M. A. (Optics Express, 2018)
We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4). By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 1.92 µm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4). The measured results match simulations of both the generalized and a hybrid nonlinear Schr{\"{o}}dinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity. We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics.
hij:1, public:1,

Enhanced Laser-Driven Ion Acceleration by Superponderomotive Electrons Generated from Near-Critical-Density Plasma - Bin, J. H.; Yeung, M.; Gong, Z.; Wang, H. Y.; Kreuzer, C.; Zhou, M. L.; Streeter, M. J. V.; Foster, P. S.; Cousens, S.; Dromey, B.; Meyer-ter-Vehn, J.; Zepf, M.; et al. (Physical Review Letters, 2018)
We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30 MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.
hij:1, public:1,

Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam - Cole, J. M.; Behm, K. T.; Gerstmayr, E.; Blackburn, T. G.; Wood, J. C.; Baird, C. D.; Duff, M. J.; Harvey, C.; Ilderton, A.; Joglekar, A. S.; Krushelnick, K.; Kuschel, S.; et al. (Physical Review X, 2018)
The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500 MeV) with an intense laser pulse (a₀>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ϵ꜀ᵣᵢₜ>30 MeV.
hij:1, public:1,

Far-field pattern formation by manipulating the topological charges of square-shaped optical vortex lattices - Stoyanov, L.; Maleshkov, G.; Zhekova, M.; Stefanov, I.; Neshev, D. N.; Paulus, G.G.; Dreischuh, A. (Journal of the Optical Society of America B, 2018)
In this work, we demonstrate experimentally the formation of 10 different structures consisting of bright beams with flat phase fronts in the focus of a lens (i.e., in the artificial far field). The basic structure used is a large, stable, square-shaped optical vortex (OV) array composed of vortices with alternating topological charges (TCs). The TCs of one individual OV, of a subarray of OVs, or of the complete OV lattice were erased/doubled in the cases of perfect superposition (on-site alignment) or are manipulated in phase in the cases of an offset between the vortices (off-site alignment). A dramatic reshaping of the beam is observed in the far field and shown to be in excellent agreement with numerical simulations.
hij:1, public:1,

Neon in ultrashort and intense x-rays from free electron lasers - Buth, C.; Beerwerth, R.; Obaid, R.; Berrah, N.; Cederbaum, L. S.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2018)
We theoretically examine neon atoms in ultrashort and intense x-rays from free electron lasers and compare our results with data from experiments conducted at the Linac Coherent Light Source. For this purpose, we treat in detail the electronic structure in all possible nonrelativistic cationic configurations using a relativistic multiconfiguration approach. The interaction with the x-rays is described in rate-equation approximation. To understand the mechanisms of the interaction, a path analysis is devised which allows us to investigate what sequences of photoionization and decay processes lead to a specific configuration and with what probability. Thereby, we uncover a connection to the mathematics of graph theory and formal languages. In detail, we study the ion yields and find that plain rate equations do not provide a satisfactory description. We need to extend the rate equations for neon to incorporate double Auger decay of a K-shell vacancy and photoionization shake off for neutral neon. Shake off is included for valence and core ionization; the former has hitherto been overlooked but has important consequences for the ion yields from an x-ray energy below the core ionization threshold. Furthermore, we predict the photon yields from xuv and x-ray fluorescence; these allow one insights into the configurations populated by the interaction with the x-rays. Finally, we discover that inaccuracies in those Auger decay widths employed in previous studies have only a minor influence on ion and photon yields.
hij:1, public:1,

New Nuclear Magnetic Moment of ²⁰⁹Bi: Resolving the Bismuth Hyperfine Puzzle - Skripnikov, L.; Schmidt, S.; Ullmann, J.; Geppert, C.; Kraus, F.; Kresse, B.; N{\"{o}}rtersh{\"{a}user, W. and Privalov, A. and Scheibe, B. and Shabaev, V. and Vogel, M. and Volotka, A. (Physical Review Letters, 2018)
A recent measurement of the hyperfine splitting in the ground state of Li-like 208Bi80+ has established a puzzle- the experimental result exhibits a 7σ deviation from the theoretical prediction. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment (μI) of 209Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of μI(209Bi) and combine it with nuclear magnetic resonance measurements of Bi(NO3)3 in nitric acid solutions and of the hexafluoridobismuthate(V) BiF−6 ion in acetonitrile. The result clearly reveals that μI(209Bi) is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.
hij:1, public:1,

Phase control of attosecond pulses in a train - Guo, C.; Harth, A.; Carlstrom, S.; Cheng, Y.-C.; Mikaelsson, S.; Marsell, E.; Heyl, C.; Miranda, M.; Gisselbrecht, M.; Gaarde, M.; Schafer, K.; Mikkelsen, A.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2018)
Ultrafast processes in matter can be captured and even controlled by using sequences of few-cycle optical pulses, which need to be well characterized, both in amplitude and phase. The same degree of control has not yet been achieved for few-cycle extreme ultraviolet pulses generated by high-order harmonic generation (HHG) in gases, with duration in the attosecond range. Here, we show that by varying the spectral phase and carrier-envelope phase (CEP) of a high-repetition rate laser, using dispersion in glass, we achieve a high degree of control of the relative phase and CEP between consecutive attosecond pulses. The experimental results are supported by a detailed theoretical analysis based upon the semi-classical three-step model for HHG.
hij:1, public:1,

Ponderomotive effects in multiphoton pair production - Kohlf{\"{u}}rst, C.; Alkofer, R. (Physical Review D, 2018)
The Dirac-Heisenberg-Wigner formalism is employed to investigate electron-positron pair production in cylindrically symmetric but otherwise spatially inhomogeneous, oscillating electric fields. The oscillation frequencies are hereby tuned to obtain multiphoton pair production in the nonperturbative threshold regime. An effective mass, as well as a trajectory-based semiclassical analysis, is introduced in order to interpret the numerical results for the distribution functions as well as for the particle yields and spectra. The results, including the asymptotic particle spectra, display clear signatures of ponderomotive forces.
hij:1, public:1,

Quantum interference in laser spectroscopy of highly charged lithiumlike ions - Amaro, P.; Loureiro, U.; Safari, L.; Fratini, F.; Indelicato, P.; St{\"{o}}hlker, T.; Santos, J. (Physical Review A, 2018)
We investigate the quantum interference induced shifts between energetically close states in highly charged ions, with the energy structure being observed by laser spectroscopy. In this work, we focus on hyperfine states of lithiumlike heavy-Z isotopes and quantify how much quantum interference changes the observed transition frequencies. The process of photon excitation and subsequent photon decay for the transition 2s→2p→2s is implemented with fully relativistic and full-multipole frameworks, which are relevant for such relativistic atomic systems. We consider the isotopes 207Pb79+ and 209Bi80+ due to experimental interest, as well as other examples of isotopes with lower Z, namely 141Pr56+ and 165Ho64+. We conclude that quantum interference can induce shifts up to 11% of the linewidth in the measurable resonances of the considered isotopes, if interference between resonances is neglected. The inclusion of relativity decreases the cross section by 35%, mainly due to the complete retardation form of the electric dipole multipole. However, the contribution of the next higher multipoles (e.g., magnetic quadrupole) to the cross section is negligible. This makes the contribution of relativity and higher-order multipoles to the quantum interference induced shifts a minor effect, even for heavy-Z elements.
hij:1, public:1,

Rayleigh scattering of twisted light by hydrogenlike ions - Peshkov, A. A.; Volotka, A. V.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2018)
The elastic Rayleigh scattering of twisted light and, in particular, the polarization (transfer) of the scattered photons have been analyzed within the framework of second-order perturbation theory and Dirac's relativistic equation. Special attention was paid hereby to the scattering on three different atomic targets: single atoms, a mesoscopic (small) target, and a macroscopic (large) target, which are all centered with regard to the beam axis. Detailed calculations of the polarization Stokes parameters were performed for C5+ ions and for twisted Bessel beams. It is shown that the polarization of scattered photons is sensitive to the size of an atomic target and to the helicity, the opening angle, and the projection of the total angular momentum of the incident Bessel beam. These computations indicate more that the Stokes parameters of the (Rayleigh) scattered twisted light may significantly differ from their behavior for an incident plane-wave radiation.
hij:1, public:1,

Spatio-Temporal Characterization of Pump-Induced Wavefront Aberrations in Yb3 + -Doped Materials - Tamer, I.; Keppler, S.; Hornung, M.; K{\"{o}}rner, J.; Hein, J.; Kaluza, M.C. (Laser { &} Photonics Reviews, 2018)
Abstract A comprehensive spatio-temporal characterization is presented describing the pump-induced wavefront aberrations in Yb3 + -doped YAG, CaF2, and fluorophosphate glass. Time-resolved interferometric measurements were performed to reveal the profiles of the total optical path differences (OPDs), which are described by the spatio-temporal superposition of thermal as well as electronic contributions, across the free aperture of the considered diode-pumped active materials. These contributions were individually determined by a COMSOL-based thermal profile model along with a detailed characterization of the electronic changes by measuring the single-pass gain and the spatial fluorescence profile. Due to the low quantum defect, the amplitude of the electronic component becomes comparable for all three materials and, in the case of Yb:CaF2, almost completely compensates the thermal component resulting from a pump pulse during the time frame of laser pulse amplification. Finally, all relevant material constants – such as the photoelastic constant and the polarizability difference – could be determined during this investigation, allowing the accurate modeling of the total pump-induced wavefront aberrations and subsequent optimization for laser systems worldwide employing these Yb3 + -doped materials.
hij:1, public:1,

The nuclear magnetic moment of 208Bi and its relevance for a test of bound-state strong-field QED - Schmidt, S.; Billowes, J.; Bissell, M.; Blaum, K.; Ruiz, R. G.; Heylen, H.; Malbrunot-Ettenauer, S.; Neyens, G.; N{\"{o}}rtersh{\"{a}user, W. and Plunien, G. and Sailer, S. and Shabaev, V. and Skripnikov, L. and Tupitsyn, I. and Volotka, A. and Yang, X. (Physics Letters B, 2018)
The hyperfine structure splitting in the 6p3S3/24→6p27sP1/24 transition at 307 nm in atomic 208Bi was measured with collinear laser spectroscopy at ISOLDE, CERN. The hyperfine A and B factors of both states were determined with an order of magnitude improved accuracy. Based on these measurements, theoretical input for the hyperfine structure anomaly, and results from hyperfine measurements on hydrogen-like and lithium-like 209Bi80+,82+, the nuclear magnetic moment of 208Bi has been determined to μ(Bi208)=+4.570(10) μN. Using this value, the transition energy of the ground-state hyperfine splitting in hydrogen-like and lithium-like 208Bi80+,82+ and their specific difference of −67.491(5)(148) meV are predicted. This provides a means for an experimental confirmation of the cancellation of nuclear structure effects in the specific difference in order to exclude such contributions as the cause of the hyperfine puzzle, the recently reported 7-σ discrepancy between experiment and bound-state strong-field QED calculations of the specific difference in the hyperfine structure splitting of 209Bi80+,82+.
hij:1, public:1,

Trident pair production in plane waves: Coherence, exchange, and spacetime inhomogeneity - Dinu, V.; Torgrimsson, G. (Physical Review D, 2018)
We study the trident process in inhomogeneous plane-wave background fields. We obtain compact analytical expressions for all terms in the probability, including the exchange part, for an arbitrarily shaped plane wave. We evaluate the probability numerically using complex deformation of light-front time integrals and derive various analytical approximations. Our results provide insights into the importance of the one-step and exchange parts of the probability relative to the two-step process, and into the convergence to the locally constant field approximation.
hij:1, public:1,

Ultrafast quantum control of ionization dynamics in krypton - H{\"{u}}tten, K.; Mittermair, M.; Stock, S. O.; Beerwerth, R.; Shirvanyan, V.; Riemensberger, J.; Duensing, A.; Heider, R.; Wagner, M. S.; Guggenmos, A.; Fritzsche, S.; Kabachnik, N. M.; et al. (Nature Communications, 2018)
Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump–probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.
hij:1, public:1,

Coherent Diffraction Imaging with Tabletop XUV Sources - Z{\"{u}}rch, M.; Guggenmos, A.; Jung, R.; Rothhardt, J.; Sp{\"{a}th, C. and T{\"{u}}mmler, J. and Demmler, S. and Haedrich, S. and Limpert, J. and T{\"{u}}nnermann, A. and Kleineberg, U. and Stiel, H. and Spielmann, C. (X-Ray Lasers 2016, 2018)
Coherent diffraction imaging (CDI) at wavelengths in the extreme ultraviolet range has become an important tool for nanoscale investigations. Employing laser-driven high harmonic sources allows for lab-scale applications such as cancer cell classification and phase-resolved surface studies in reflection geometry. The excellent beam properties support a spatial resolution below the wavelength, i.e., close to the Abbe limit. Unfortunately, the usually low photon flux of HHG sources limits their applicability. Recent advances in ultrafast fiber laser development cumulated in sources delivering average powers approaching the milliwatt level in the extreme ultraviolet. In comparison, a tabletop soft X-ray laser driven by moderate pump energies was recently employed for CDI featuring excellent temporal coherence and extraordinary high flux allowing for single-shot imaging.
hij:1, public:1,

Considerations towards the possibility of the observation of parity nonconservation in highly charged ions in storage rings - Bondarevskaya, A. A.; Chubukov, D. V.; Mistonova, E. A.; Lyashchenko, K. N.; Andreev, O. Y.; Surzhykov, A.; Labzowsky, L. N.; Plunien, G.; Liesen, D.; Bosch, F.; St{\"{o}}hlker, T. (Physica Scripta, 2018)
The feasibility of an experiment for the observation of parity nonconserving effects using He-like highly charged ions in storage rings is discussed theoretically. The basic idea is the observation of an asymmetry in the emission of the hyperfine quenched transition (1s2s)¹S₀→ (1s)²¹S₀+γ with respect to the direction of a beam of ions with polarized nuclei. It will be shown, that for such an experiment 151Eu^61+ ions with nuclear spin I=5/2 in the excited electronic state (1s2s)¹S₀ with zero total electron angular momentum and polarized nuclei are the best available candidates. The nuclei can be polarized if H-like Eu^62+ ions capture an electron from a polarized electron beam which overlays the ion beam on some part of the ring and travels with nearly the same velocity. It is suggested, to monitor steadily the degree of the nuclear polarization by the observation of the selective laser excitation and subsequent decay of the Zeeman sublevels of the excited hyperfine states of the ionic ground state. Estimates for the observation time aiming at an accuracy of PNC measurement of about 0.1 % are given.
hij:1, public:1,

Higher-Dimensional Caustics in Nonlinear Compton Scattering - Kharin, V.; Seipt, D.; Rykovanov, S. (Physical Review Letters, 2018)
A description of the spectral and angular distributions of Compton scattered light in collisions of intense laser pulses with high-energy electrons is unwieldy and usually requires numerical simulations. However, due to the large number of parameters affecting the spectra such numerical investigations can become computationally expensive. Using methods of catastrophe theory we predict higher-dimensional caustics in the spectra of the Compton scattered light, which are associated with bright narrow-band spectral lines, and in the simplest case can be controlled by the value of the linear chirp of the pulse. These findings require no full-scale calculations and have direct consequences for the photon yield enhancement of future nonlinear Compton scattering x-ray or gamma-ray sources.
hij:1, public:1,

Isolated proton bunch acceleration by a petawatt laser pulse - Hilz, P.; Ostermayr, T. M.; Huebl, A.; Bagnoud, V.; Borm, B.; Bussmann, M.; Gallei, M.; Gebhard, J.; Haffa, D.; Hartmann, J.; Kluge, T.; Lindner, F. H.; et al. (Nature Communications, 2018)
Often, the interpretation of experiments concerning the manipulation of the energy distribution of laser-accelerated ion bunches is complicated by the multitude of competing dynamic processes simultaneously contributing to recorded ion signals. Here we demonstrate experimentally the acceleration of a clean proton bunch. This was achieved with a microscopic and three-dimensionally confined near critical density plasma, which evolves from a 1 µm diameter plastic sphere, which is levitated and positioned with micrometer precision in the focus of a Petawatt laser pulse. The emitted proton bunch is reproducibly observed with central energies between 20 and 40 MeV and narrow energy spread (down to 25%) showing almost no low-energetic background. Together with three-dimensional particle-in-cell simulations we track the complete acceleration process, evidencing the transition from organized acceleration to Coulomb repulsion. This reveals limitations of current high power lasers and viable paths to optimize laser-driven ion sources.
hij:1, public:1,

LCLS in-photon out: fluorescence measurement of neon using soft x-rays - Obaid, R.; Buth, C.; Dakovski, G. L.; Beerwerth, R.; Holmes, M.; Aldrich, J.; Lin, M.-F.; Minitti, M.; Osipov, T.; Schlotter, W.; Cederbaum, L. S.; Fritzsche, S.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2018)
We measured the fluorescence photon yield of neon upon soft x-ray ionization (∼1200 eV) from the x-ray free-electron laser at Linac Coherent Light Source, and demonstrated the usage of a grazing incidence spectrometer with a variable line spacing grating to perform x-ray fluorescence spectroscopy on a gas phase system. Our measurements also allowed us to estimate the focal size of the beam from the theoretical description developed, in terms of the rate equation approximation accounting for photoionization shake off of neutral neon and double auger decay of single core holes.
hij:1, public:1,

Plasma Channel Undulator for Narrow-Bandwidth X-Ray Generation - Rykovanov, S. G.; Wang, J. W.; Kharin, V. Yu.; Lei, B.; Schroeder, C. B.; Geddes, C. G. R.; Esarey, E.; Leemans, W. P. (X-Ray Lasers 2016, 2018)
A new plasma channel undulator concept based on the wakefields generated by short intense laser pulse undergoing centroid oscillations inside parabolic plasma channel is presented. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be in the submillimeter range, while its strength can reach unity. Two-dimensional particle-in-cell simulations of the laser pulse propagation and wakefields are presented. Spontaneous radiation produced by the electron beam inside the plasma undulator is calculated.
hij:1, public:1,

Renormalization group flow of the Higgs potential - Gies, H.; Sondenheimer, R. (Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 2018)
We summarize results for local and global properties of the effective potential for the Higgs boson obtained from the functional renormalization group, which allows one to describe the effective potential as a function of both scalar field amplitude and renormalization group scale. This sheds light onto the limitations of standard estimates which rely on the identification of the two scales and helps in clarifying the origin of a possible property of meta-stability of the Higgs potential. We demonstrate that the inclusion of higher-dimensional operators induced by an underlying theory at a high scale (GUT or Planck scale) can relax the conventional lower bound on the Higgs mass derived from the criterion of absolute stability. This article is part of the Theo Murphy meeting issue cosmology.
hij:1, public:1,

Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling - Goy, C.; Potenza, M.; Dedera, S.; Tomut, M.; Guillerm, E.; Kalinin, A.; Voss, K.-O.; Schottelius, A.; Petridis, N.; Prosvetov, A.; Tejeda, G.; Fernandez, J.; et al. (Physical Review Letters, 2018)
The fast evaporative cooling of micrometer-sized water droplets in a vacuum offers the appealing possibility to investigate supercooled water{–}below the melting point but still a liquid{–}at temperatures far beyond the state of the art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a train of perfectly uniform water droplets allows us to measure the variation in droplet size resulting from evaporative mass losses with an absolute precision of better than 0.2%. This finding proves crucial to an unambiguous determination of the droplet temperature. In particular, we find that a fraction of water droplets with an initial diameter of 6379±12 nm remain liquid down to 230.6±0.6 K. Our results question temperature estimates reported recently for larger supercooled water droplets and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.
hij:0, public:1,

Toward multi-kW femtosecond fiber lasers based on mutlicore fibers - Limpert, J. (2018 IEEE Photonics Society Summer Topical Meeting Series (SUM), 2018)
The achievements and the potential of coherent pulse addition as a performance scaling approach of ultrafast laser systems will be reviewed. Active multicore arrangements pursue that concept in a most compact way. First very promising results will be presented.
hij:1, public:1,

A functional perspective on emergent supersymmetry - Gies, H.; Hellwig, T.; Wipf, A.; Zanusso, O. (Journal of High Energy Physics, 2017)
We investigate the emergence of N = 1 supersymmetry in the long-range behavior of three-dimensional parity-symmetric Yukawa systems. We discuss a renormalization approach that manifestly preserves supersymmetry whenever such symmetry is realized, and use it to prove that supersymmetry-breaking operators are irrelevant, thus proving that such operators are suppressed in the infrared. All our findings are illustrated with the aid of the ϵ-expansion and a functional variant of perturbation theory, but we provide numerical estimates of critical exponents that are based on the non-perturbative functional renormalization group.
hij:0, public:1,

Characterization of two ultrashort laser pulses using interferometric imaging of self-diffraction - Leithold, C.; Reisl{\"{o}}hner, J.; Gies, H.; Pfeiffer, A. N. (Optics Letters, 2017)
Noncollinear pulse characterization methods can be applied to over-octave spanning waveforms, but geometrical effects in the nonlinear medium such as beam smearing and critical sensitivity to beam alignment hinder their accurate application. Here, a method is introduced for the temporal and spatial characterization of two pulses by interferometric, spectrally resolved imaging of self-diffraction. Geometrical effects are resolved by the method and, therefore, do not limit the accuracy. Two methods for quantitative pulse retrieval are presented. One method is analytical and very fast; the other method is iterative and more robust if applied to noisy data.
hij:1, public:1,

Ground-state hyperfine splitting for Rb, Cs, Fr, Ba⁺, and Ra⁺ - Ginges, J. S. M.; Volotka, A. V.; Fritzsche, S. (Physical Review A, 2017)
We have systematically investigated the ground-state hyperfine structure for alkali-metal atoms 87Rb, 33Cs, and 211Fr and alkali-metal-like ions 135Ba^+ and 225Ra^+, which are of particular interest for parity violation studies. The quantum electrodynamic one-loop radiative corrections have been rigorously evaluated within an extended Furry picture employing core-Hartree and Kohn-Sham atomic potentials. Moreover, the effect of the nuclear magnetization distribution on the hyperfine structure intervals has been studied in detail and its uncertainty has been estimated. Finally, the theoretical description of the hyperfine structure has been completed with full many-body calculations performed in the all-orders correlation potential method.
hij:1, public:1,

Optically transparent solid electrodes for precision Penning traps - Wiesel, M.; Birkl, G.; Ebrahimi, M. S.; Martin, A.; Quint, W.; Stallkamp, N.; Vogel, M. (Review of Scientific Instruments, 2017)
We have conceived, built, and operated a cryogenic Penning trap with an electrically conducting yet optically transparent solid electrode. The trap, dedicated to spectroscopy and imaging of confined particles under large solid angles, is of “half-open” design with one open endcap and one closed endcap that mainly consists of a glass window coated with a highly transparent conductive layer. This arrangement allows for the trapping of externally or internally produced particles and yields flexible access for optical excitation and efficient light collection from the trapping region. At the same time, it is electrically closed and ensures long-term ion confinement under well-defined conditions. With its superior surface quality and its high as well as homogeneous optical transmission, the window electrode is an excellent replacement for partially transmissive electrodes that use holes, slits, metallic meshes, and the like.
hij:1, public:1,

Photon polarization tensor in pulsed Hermite- and Laguerre-Gaussian beams - Karbstein, F.; Mosman, E. A. (Physical Review D, 2017)
In this article, we provide analytical expressions for the photon polarization tensor in pulsed Hermite- and Laguerre-Gaussian laser beams. Our results are based on a locally constant field approximation of the one-loop Heisenberg-Euler effective Lagrangian for quantum electrodynamics. Hence, by construction they are limited to slowly varying electromagnetic fields, varying on spatial and temporal scales significantly larger than the Compton wavelength/time of the electron. The latter criterion is fulfilled by all laser beams currently available in the laboratory. Our findings will, e.g., be relevant for the study of vacuum birefringence experienced by probe photons brought into collision with a high-intensity laser pulse which can be represented as a superposition of either Hermite- or Laguerre-Gaussian modes.
hij:1, public:1,

Plasma channel undulator excited by high-order laser modes - Wang, J. W.; Schroeder, C. B.; Li, R.; Zepf, M.; Rykovanov, S. G. (Scientific Reports, 2017)
The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3{\^{a}}EUR%0kA level, making such an undulator a potential candidate towards a table-top FEL.
hij:1, public:1,

Resonance-enhanced multi-octave supercontinuum generation in antiresonant hollow-core fibers - Sollapur, R.; Kartashov, D.; Z{\"{u}}rch, M.; Hoffmann, A.; Grigorova, T.; Sauer, G.; Hartung, A.; Schwuchow, A.; Bierlich, J.; Kobelke, J.; Chemnitz, M.; Schmidt, M. A.; et al. (Light: Science { &} Applications, 2017)
Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 μm at an output energy of ∼ 23 μJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid‐infrared spectral range.
hij:1, public:1,

Single-shot, real-time carrier-envelope phase measurement and tagging based on stereographic above-threshold ionization at short-wave infrared wavelengths - Zhang, Y.; Kellner, P.; Adolph, D.; Zille, D.; Wustelt, P.; W{\"{u}}rzler, D.; Skruszewicz, S.; M{\"{o}}ller, M.; Sayler, A. M.; Paulus, G.G. (Optics Letters, 2017)
A high-precision, single-shot, and real-time carrier-envelope phase (CEP) measurement at 1.8 μm laser wavelength based on stereographic photoelectron spectroscopy is presented. A precision of the CEP measurement of 120 mrad for each and every individual laser shot for a 1 kHz pulse train with randomly varying CEP is demonstrated. Simultaneous to the CEP measurement, the pulse lengths are characterized by evaluating the spatial asymmetry of the measured above-threshold ionization (ATI) spectra of xenon and referenced to a standard pulse-duration measurement based on frequency-resolved optical gating. The validity of the CEP measurement is confirmed by implementing phase tagging for a CEP-dependent measurement of ATI in xenon with high energy resolution.
hij:1, public:1,

Enhanced absorption and cavity effects of three-photon pumped ZnO nanowires - Hollinger, R.; Samsonova, Z.; Gupta, D.; Spielmann, C.; R{\"{o}}der, R.; Trefflich, L.; Ronning, C.; Kartashov, D. (Applied Physics Letters, 2017)
Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.
hij:1, public:1,

First determination of ground state electromagnetic moments of ⁵³Fe - Miller, A. J.; Minamisono, K.; Rossi, D. M.; Beerwerth, R.; Brown, B. A.; Fritzsche, S.; Garand, D.; Klose, A.; Liu, Y.; Maass, B.; Mantica, P. F.; M{\"{u}}ller, P.; et al. (Physical Review C, 2017)
The hyperfine coupling constants of neutron deficient 53Fe were deduced from the atomic hyperfine spectrum of the 3d^6 4s^2 ^5D_4 <-> 3d^6 4s 4p ^5F_5 transition, measured using the bunched-beam collinear laser spectroscopy technique. The low-energy 53Fe beam was produced by projectile-fragmentation reactions followed by gas stopping, and used for the first time for laser spectroscopy. Ground state magnetic-dipole and electric-quadrupole moments were determined as μ = -0.65(1)μ_N and Q = +35(15)e^2 fm^2, respectively. The multiconfiguration Dirac-Fock method was used to calculate the electric field gradient to deduce Q from the quadrupole hyperfine coupling constant, since the quadrupole coupling constant has not been determined for any Fe isotopes. Both experimental values agree well with nuclear shell model calculations using the GXPF1A effective interaction performed in a full fp shell model space, which support the soft nature of the 56Ni nucleus.
hij:1, public:1,

Impact of generalized Yukawa interactions on the lower Higgs-mass bound - Gies, H.; Sondenheimer, R.; Warschinke, M. (The European Physical Journal C, 2017)
We investigate the impact of operators of higher canonical dimension on the lower Higgs-mass consistency bound by means of generalized Higgs{–}Yukawa interactions. Analogously to higher-order operators in the bare Higgs potential in an effective field theory approach, the inclusion of higher-order Yukawa interactions, e.g., ϕ^3ψ^¯ψ, leads to a diminishing of the lower Higgs-mass bound and thus to a shift of the scale of new physics towards larger scales by a few orders of magnitude without introducing a metastability in the effective Higgs potential. We observe that similar renormalization group mechanisms near the weak-coupling fixed point are at work in both generalizations of the microscopic action. Thus, a combination of higher-dimensional operators with generalized Higgs as well as Yukawa interactions does not lead to an additive shift of the lower mass bound, but it relaxes the consistency bounds found recently only slightly. On the method side, we clarify the convergence properties of different projection and expansion schemes for the Yukawa potential used in the functional renormalization group literature so far.
hij:1, public:1,

In-gas laser ionization and spectroscopy of actinium isotopes near the N=126 closed shell - Granados, C.; Creemers, P.; Ferrer, R.; Gaffney, L. P.; Gins, W.; de Groote, R.; Huyse, M.; Kudryavtsev, Yu.; Mart{\'{i}}nez, Y.; Raeder, S.; Sels, S.; Van Beveren, C.; et al. (Physical Review C, 2017)
The in-gas laser ionization and spectroscopy (IGLIS) technique was applied on the 212-215Ac isotopes, produced at the Leuven Isotope Separator On-Line (LISOL) facility by using the in-gas-cell and the in-gas-jet methods. The first application under on-line conditions of the in-gas-jet laser spectroscopy method showed a superior performance in terms of selectivity, spectral resolution, and efficiency in comparison with the in-gas-cell method. Following the analysis of both experiments, the magnetic-dipole moments for the 212-215Ac isotopes, electric-quadrupole moments and nuclear spins for the 214,215Ac isotopes are presented and discussed. A good agreement is obtained with large-scale nuclear shell-model calculations by using a 208Pb core.
hij:1, public:1,

Intense attosecond pulses from laser-irradiated near-critical-density plasmas - Zhang, Y.; Qiao, B.; Xu, X.; Chang, H.; Lu, H.; Zhou, C.; Zhang, H.; Zhu, S.; Zepf, M.; He, X. (Optics Express, 2017)
A novel practical and efficient way of obtaining intense attosecond pulses is proposed, where the near-critical-density (NCD) plasma target satisfying n₀/a₀n꜀ ≈ 1 is used. The unique interaction dynamics in NCD plasmas have been identified theoretically and by particle-in-cell simulations, which show that three distinct dense electron nanobunches are formed each half a laser cycle and two of them can induce intense attosecond pulses in respectively the reflected and the transmitted directions by the so-called “coherent synchrotron emission” (CSE) mechanism [experimentally confirmed in Nat. Phys. 8, 804 (2012)]. Comparing with CSE in solids, here not only the required stringent conditions on laser and target are relaxed, but also the radiation intensities are enhanced by two orders of magnitude. It is shown that relativistically intense attosecond X-ray pulses with intensity 10¹⁹ W/cm² and duration 50 as can be robustly obtained in both directions by currently available driving lasers at intensities of 10²⁰ W/cm².
hij:1, public:1,

Three dimensional magnetic solutions in massive gravity with (non)linear field - Hendi, S.; Panah, B. E.; Panahiyan, S.; Momennia, M. (Physics Letters B, 2017)
The Noble Prize in physics 2016 motivates one to study different aspects of topological properties and topological defects as their related objects. Considering the significant role of the topological defects (especially magnetic strings) in cosmology, here, we will investigate three dimensional horizonless magnetic solutions in the presence of two generalizations: massive gravity and nonlinear electromagnetic field. The effects of these two generalizations on properties of the solutions and their geometrical structure are investigated. The differences between de Sitter and anti de Sitter solutions are highlighted and conditions regarding the existence of phase transition in geometrical structure of the solutions are studied.
hij:1, public:1,

Using the focal phase to control attosecond processes - Hoff, D.; Kr{\"{u}}ger, M.; Maisenbacher, L.; Paulus, G.; Hommelhoff, P.; Sayler, A. (Journal of Optics, 2017)
The spatial evolution of the electric field of focused broadband light is crucial for many emerging attosecond technologies. Here the effects of the input beam parameters on the evolution of few-cycle laser pulses in the focus are discussed. Specifically, we detail how the frequency-dependent input beam geometry, chirp and chromatic aberration can affect the spatial dependence of the carrier-envelope phase (CEP), central frequency and pulse duration in the focus. These effects are confirmed by a direct, three-dimensional measurement of the CEP-evolution in the focus of a typical few-cycle pulse laser using electron rescattering at metal nanotips in combination with a CEP-metre. Moreover, we demonstrate a simple measurement technique to estimate the focal CEP evolution by input-beam parameters. These parameters can be used in novel ways in order to control attosecond dynamics and tailor highly nonlinear light–matter interactions.
hij:1, public:1,

Velocity map imaging of scattering dynamics in orthogonal two-color fields - W{\"{u}}rzler, D.; Eicke, N.; M{\"{o}}ller, M.; Seipt, D.; Sayler, A. M.; Fritzsche, S.; Lein, M.; Paulus, G.G. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2017)
In strong-field ionization processes, two-color laser fields are frequently used for controlling sub-cycle electron dynamics via the relative phase of the laser fields. Here we apply this technique to velocity map imaging spectroscopy using an unconventional orientation with the polarization of the ionizing laser field perpendicular to the detector surface and the steering field parallel to it. This geometry allows not only to image the phase-dependent photoelectron momentum distribution (PMD) of low-energy electrons that interact only weakly with the ion (direct electrons), but also to investigate the low yield of higher-energy rescattered electrons. Phase-dependent measurements of the PMD of neon and xenon demonstrate control over direct and rescattered electrons. The results are compared with semi-classical calculations in three dimensions including elastic scattering at different orders of return and with solutions of the three-dimensional time-dependent Schr{\"{o}}dinger equation.
hij:1, public:1,

Attosecond streaking with twisted X waves and intense infrared pulses - B{\"{o}}ning, B.; Paufler, W.; Fritzsche, S. (Physical Review A, 2017)
We investigate the photoionization of atoms by attosecond X waves carrying orbital angular momentum in the presence of a strong, linearly polarized, near infrared (NIR) laser pulse. In the plane-wave case, the streaking of photoelectrons by the NIR pulse has been used to characterize the ionizing pulse. In contrast to plane-wave pulses, X waves have a spatially dependent temporal profile, which modifies the ionization process. Here we explore theoretically the influence of this complex pulse structure on the streaking of photoelectrons for both localized and macroscopically extended targets. On the basis of the strong-field approximation, we find that the streaking spectra of localized targets sensitively depend on the opening angle of the X wave and the position of the atomic target relative to the beam axis. For macroscopically extended targets, we find that the streaking spectra do not depend on the parameters characterizing the twist of the X wave.
hij:1, public:1,

Binding energies of the 1s2 2s2 2pj states in boronlike argon - Malyshev, A.; Glazov, D.; Volotka, A.; Tupitsyn, I.; Shabaev, V.; Plunien, G. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
The binding energies of the ground 1s2 2s2 2p1/2 and first excited 1s2 2s2 2p3/2 states in boronlike argon are rigorously evaluated. The calculations are performed by the QED perturbation theory in the framework of the extended Furry picture taking into account all the relevant first- and second-order radiative and correlation corrections. The third- and higher-order interelectronic-interaction effects are considered within the Breit approximation. The relativistic nuclear recoil effect is taken into account. In comparison with the previous calculations of the binding energies in boronlike argon the accuracy of the theoretical predictions has been significantly improved.
hij:1, public:1,

Dielectronic recombination of highly charged ions with spin-polarized electrons - Wu, Z.; Volotka, A.; Dong, C.; Fritzsche, S. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Angular distribution and linear polarization of photon emission following dielectronic recombination of initially lithium-like ions with spin-polarized electrons are studied. In particular, a general expression is derived for the alignment parameter of the doubly excited states produced via the resonant capture of spin-polarized electrons. By means of the alignment parameter, moreover, the angular distribution and linear polarization of the subsequently emitted photons are further obtained. Detailed computations are performed for the 1s2 2s J0=1/2+εe-→1s2s2 2p1/2 J=1→1s2 2s2 Jf=0+γ resonant electron capture and subsequent radiative decay of iodine ions. It is found that the spin polarization of the incident electrons changes only the q=±1 components of the alignment parameter A2q. As a consequence, the electron spin polarization contributes weakly to the γ photon angular distribution and linear polarization that are dominantly determined by the A20 parameter.
hij:1, public:1,

Higher-order perturbative relativistic calculations for few-electron atoms and ions - Glazov, D.; Malyshev, A.; Volotka, A.; Shabaev, V.; Tupitsyn, I.; Plunien, G. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
An effective computational method is developed for electronic-structure calculations in few-electron atoms and ions on the basis of the Dirac-Coulomb-Breit Hamiltonian. The recursive formulation of the perturbation theory provides an efficient access to the higher-order contributions of the interelectronic interaction. Application of the presented approach to the binding energies of lithiumlike and boronlike systems is demonstrated. The results obtained are in agreement with the large-scale configuration interaction Dirac-Fock-Sturm method and other all-order calculations.
hij:1, public:1,

Hyperfine induced effects on the angular distribution of the dielectronic hypersatellite line - Zaytsev, V.; Fritzsche, S.; Surzhykov, A.; Shabaev, V. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Abstract We investigate the dielectronic recombination (DR) of an electron and a highly-charged ion with non-zero nuclear spin. We assume that the incident electron is captured into doubly-excited 1s2κκ′J=0,1,2 levels of Be-like ions just above of its autoionization threshold. The angular distribution of the subsequent radiative emission is investigated especially for its dependence upon the nuclear spin and the nuclear magnetic moment. While the hyperfine and even the fine-structure of the ions cannot be resolved in typical DR experiments, we found the photon angular distribution, following the decay of the 1s2 2p3/2nsJ=1,2 DR resonance very sensitive to the nuclear parameters.
hij:1, public:1,

Impact parameter sensitive study of inner-shell atomic processes in the experimental storage ring - Gumberidze, A.; Kozhuharov, C.; Zhang, R.; Trotsenko, S.; Kozhedub, Y.; DuBois, R.; Beyer, H.; Blumenhagen, K.-H.; Brandau, C.; Br{\"{a}uning-Demian, A. and Chen, W. and Forstner, O. and Gao, B. and Gassner, T. and Grisenti, R. and Hagmann, S. and Hillenbrand, P.-M. and Indelicato, P. and Kumar, A. and Lestinsky, M. and Litvinov, Yu. A. and Petridis, N. and Schury, D. and Spillmann, U. and Trageser, C. and Trassinelli, M. and Tu, X. and St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70 fm.
hij:1, public:1,

Intensities of K-X-ray satellite and hypersatellite target radiation in Bi83+-Xe @70MeV/u collisions - Kozhedub, Y.; Bondarev, A.; Cai, X.; Gumberidze, A.; Hagmann, S.; Kozhuharov, C.; Maltsev, I.; Plunien, G.; Shabaev, V.; Shao, C.; St{\"{o}}hlker, T.; Tupitsyn, I.; et al. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Non-perturbative calculations of the relativistic quantum dynamics of electrons in the Bi83+-Xe collisions at 70 AMeV are performed. A method of calculation employs an independent particle model with effective single-electron Dirac-Kohn-Sham operator. Solving of the single-electron equations is based on the coupled-channel approach with atomic-like Dirac-Sturm-Fock orbitals, localized at the ions (atoms). Special attention is paid to the inner-shell processes. Intensities of the K satellite and hypersatellite target radiation are evaluated. The role of the relativistic effects is studied.
hij:1, public:1,

MCDF calculations of Auger cascade processes - Beerwerth, R.; Fritzsche, S. (The European Physical Journal D, 2017)
We model the multiple ionization of near-neutral core-excited atoms where a cascade of Auger processes leads to the emission of several electrons. We utilize the multiconfiguration Dirac-Fock (MCDF) method to generate approximate wave functions for all fine-structure levels and to account for all decays between them. This approach allows to compute electron spectra, the population of final-states and ion yields, that are accessible in many experiments. Furthermore, our approach is based on the configuration interaction method. A careful treatment of correlation between electronic configurations enables one to model three-electron processes such as an Auger decay that is accompanied by an additional shake-up transition. Here, this model is applied to the triple ionization of atomic cadmium, where we show that the decay of inner-shell 4p holes to triply-charged final states is purely due to the shake-up transition of valence 5s electrons.
hij:1, public:1,

Near L-edge Single and Multiple Photoionization of Singly Charged Iron Ions - Schippers, S.; Martins, M.; Beerwerth, R.; Bari, S.; Holste, K.; Schubert, K.; Viefhaus, J.; Savin, D. W.; Fritzsche, S.; M{\"{u}}ller, A. (The Astrophysical Journal, 2017)
Absolute cross-sections for m -fold photoionization (m=1, ... , 6 ) of Fe+ by a single photon were measured employing the photon–ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680–920 eV, which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of ±0.2 eV. The cross-section for Fe+ photoabsorption is derived as the sum of the individually measured cross-sections for m -fold ionization. Calculations of the Fe+ absorption cross-sections were carried out using two different theoretical approaches, Hartree–Fock including relativistic extensions and fully relativistic multiconfiguration Dirac–Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross-sections are in good agreement with each other and with the experimental results. In addition, the complex de-excitation cascades after the creation of inner-shell holes in the Fe+ ion were tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.
hij:1, public:1,

Nonlinear pulse compression to 43 W GW-class few-cycle pulses at 2 μm wavelength - Gebhardt, M.; Gaida, C.; Heuermann, T.; Stutzki, F.; Jauregui, C.; Antonio-Lopez, J.; Schulzgen, A.; Amezcua-Correa, R.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2017)
High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.
hij:1, public:1,

Nuclear magnetic shielding in boronlike ions - Volchkova, A.; Varentsova, A.; Zubova, N.; Agababaev, V.; Glazov, D.; Volotka, A.; Shabaev, V.; Plunien, G. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
The relativistic treatment of the nuclear magnetic shielding effect in boronlike ions is presented. The leading-order contribution of the magnetic-dipole hyperfine interaction is calculated. Along with the standard second-order perturbation theory expression, the solutions of the Dirac equation in the presence of magnetic field are employed. All methods are found to be in agreement with each other and with the previous calculations for hydrogenlike and lithiumlike ions. The effective screening potential is used to account approximately for the interelectronic interaction.
hij:1, public:1,

Pair production in low-energy collisions of uranium nuclei beyond the monopole approximation - Maltsev, I.; Shabaev, V.; Tupitsyn, I.; Kozhedub, Y.; Plunien, G.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
A method for calculation of electron-positron pair production in low-energy heavy-ion collisions beyond the monopole approximation is presented. The method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. The one-electron wave functions are expanded in the finite basis set constructed on the two-dimensional spatial grid. Employing the developed approach the probabilities of bound-free pair production are calculated for collisions of bare uranium nuclei at the energy near the Coulomb barrier. The obtained results are compared with the corresponding values calculated in the monopole approximation.
hij:1, public:1,

Quadratic Zeeman effect in boronlike argon - Agababaev, V.; Volchkova, A.; Varentsova, A.; Glazov, D.; Volotka, A.; Shabaev, V.; Plunien, G. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Abstract A theoretical investigation of the second-order Zeeman effect in boronlike ions is presented. Rigorous calculations of the one-photon-exchange and one-loop QED corrections allow for predictions of the corresponding theoretical values for boronlike argon with an accuracy of about 2%. The obtained results are important in view of the forthcoming measurements of the Zeeman splitting in 40Ar13+ at GSI (ARTEMIS experiment).
hij:1, public:1,

Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms - Hofbrucker, J.; Volotka, A.; Fritzsche, S. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms are studied theoretically within the framework of second-order perturbation theory. The non-relativistic results are compared with the relativistic calculations in the dipole and no-pair approximations as well as with the complete relativistic approach. The calculations are performed in both velocity and length gauges. Our results show a significant decrease of the total cross section for heavy atoms as compared to the non-relativistic treatment, which is mainly due to the relativistic wavefunction contraction. The effects of higher multipoles and negative continuum energy states counteract the relativistic contraction contribution, but are generally much weaker. While the effects beyond the dipole approximation are equally important in both gauges, the inclusion of negative continuum energy states visibly contributes to the total cross section only in the velocity gauge.
hij:1, public:1,

Spectral and spatial characterisation of laser-driven positron beams - Sarri, G.; Warwick, J.; Schumaker, W.; Poder, K.; Cole, J.; Doria, D.; Dzelzainis, T.; Krushelnick, K.; Kuschel, S.; Mangles, S. P. D.; Najmudin, Z.; Romagnani, L.; et al. (Plasma Physics and Controlled Fusion, 2017)
The generation of high-quality relativistic positron beams is a central area of research in experimental physics, due to their potential relevance in a wide range of scientific and engineering areas, ranging from fundamental science to practical applications. There is now growing interest in developing hybrid machines that will combine plasma-based acceleration techniques with more conventional radio-frequency accelerators, in order to minimise the size and cost of these machines. Here we report on recent experiments on laser-driven generation of high-quality positron beams using a relatively low energy and potentially table-top laser system. The results obtained indicate that current technology allows to create, in a compact setup, positron beams suitable for injection in radio-frequency accelerators.
hij:1, public:1,

Streak Camera for Strong-Field Ionization - K{\"{u}}bel, M.; Dube, Z.; Naumov, A. Yu.; Spanner, M.; Paulus, G.G.; Kling, M. F.; Villeneuve, D. M.; Corkum, P. B.; Staudte, A. (Physical Review Letters, 2017)
Ionization of an atom or molecule by a strong laser field produces suboptical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a midinfrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the midinfrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale.
hij:1, public:1,

Tadpole diagrams in constant electromagnetic fields - Karbstein, F. (Journal of High Energy Physics, 2017)
We show how all possible one-particle reducible tadpole diagrams in constant electromagnetic fields can be constructed from one-particle irreducible constant-field diagrams. The construction procedure is essentially algebraic and involves differentiations of the latter class of diagrams with respect to the field strength tensor and contractions with derivatives of the one-particle irreducible part of the Heisenberg-Euler effective Lagrangian in constant fields. Specific examples include the two-loop addendum to the Heisenberg-Euler effective action as well as a novel one-loop correction to the charged particle propagator in constant electromagnetic fields discovered recently. As an additional example, the approach devised in the present article is adopted to derive the tadpole contribution to the two-loop photon polarization tensor in constant fields for the first time.
hij:1, public:1,

Temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra - Buldt, J.; M{\"{u}}ller, M.; Klas, R.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2017)
We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra. A measured temporal contrast enhancement by at least seven orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.
hij:1, public:1,

Theoretical analysis of the electron bridge process in 229Th3+ - M{\"{u}}ller, R.; Volotka, A.; Fritzsche, S.; Surzhykov, A. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
We investigate the deexcitation of the 229Th nucleus via the excitation of an electron. Detailed calculations are performed for the enhancement of the nuclear decay width due to the so called electron bridge (EB) compared to the direct photoemission from the nucleus. The results are obtained for triply ionized thorium by using a B-spline pseudo basis approach to solve the Dirac equation for a local xα potential. This approach allows for an approximation of the full electron propagator including the positive and negative continuum. We show that the contribution of continua slightly increases the enhancement compared to a propagator calculated by a direct summation over bound states. Moreover we put special emphasis on the interference between the direct and exchange Feynman diagrams that can have a strong influence on the enhancement.
hij:1, public:1,

Third-order Zeeman effect in highly charged ions - Varentsova, A.; Agababaev, V.; Volchkova, A.; Glazov, D.; Volotka, A.; Shabaev, V.; Plunien, G. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
The contribution of the third order in magnetic field to the Zeeman splitting of the ground state of hydrogenlike, lithiumlike, and boronlike ions in the range Z=6-82 is investigated within the relativistic approach. Both perturbative and non-perturbative methods of calculation are employed and found to be in agreement. For lithiumlike and boronlike ions the interelectronic-interaction effects are taken into account within the approximation of the local screening potential. The contribution of the third-order effect in low- and medium-Z boronlike ions is found to be important for anticipated high-precision measurements.
hij:1, public:1,

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions - Siek, F.; Neb, S.; Bartz, P.; Hensen, M.; Str{\"{u}}ber, C.; Fiechter, S.; Torrent-Sucarrat, M.; Silkin, V.; Krasovskii, E.; Kabachnik, N.; Fritzsche, S.; Mui{\ {n}}o, R.; et al. (Science, 2017)
Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe2) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe2, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.
hij:1, public:1,

Dilatonic black holes in gravity's rainbow with a nonlinear source: the effects of thermal fluctuations - Hendi, S. H.; Panah, B.; Panahiyan, S.; Momennia, M. (The European Physical Journal C, 2017)
This paper is devoted to an investigation of nonlinearly charged dilatonic black holes in the context of gravity's rainbow with two cases: (1) by considering the usual entropy, (2) in the presence of first order logarithmic correction of the entropy. First, exact black hole solutions of dilatonic Born{–}Infeld gravity with an energy dependent Liouville-type potential are obtained. Then, thermodynamic properties of the mentioned cases are studied, separately. It will be shown that although mass, entropy and the heat capacity are modified due to the presence of a first order correction, the temperature remains independent of it. Furthermore, it will be shown that divergences of the heat capacity, hence phase transition points are also independent of a first order correction, whereas the stability conditions are highly sensitive to variation of the correction parameter. Except for the effects of a first order correction, we will also present a limit on the values of the dilatonic parameter and show that it is possible to recognize AdS and dS thermodynamical behaviors for two specific branches of the dilatonic parameter. In addition, the effects of nonlinear electromagnetic field and energy functions on the thermodynamical behavior of the solutions will be highlighted and dependency of critical behavior, on these generalizations will be investigated.
hij:1, public:1,

Enhancing laser-driven proton acceleration by using micro-pillar arrays at high drive energy - Khaghani, D.; Lobet, M.; Borm, B.; Burr, L.; G{\"{a}rtner, F. and Gremillet, L. and Movsesyan, L. and Rosmej, O. and Toimil-Molares, M. and Wagner, F. and Neumayer, P. (Scientific Reports, 2017)
The interaction of micro- and nano-structured target surfaces with high-power laser pulses is being widely investigated for its unprecedented absorption efficiency. We have developed vertically aligned metallic micro-pillar arrays for laser-driven proton acceleration experiments. We demonstrate that such targets help strengthen interaction mechanisms when irradiated with high-energy-class laser pulses of intensities 10^17–18 W/cm2. In comparison with standard planar targets, we witness strongly enhanced hot-electron production and proton acceleration both in terms of maximum energies and particle numbers. Supporting our experimental results, two-dimensional particle-in-cell simulations show an increase in laser energy conversion into hot electrons, leading to stronger acceleration fields. This opens a window of opportunity for further improvements of laser-driven ion acceleration systems.
hij:1, public:1,

Influence of plasma environment on K-line emission in highly ionized iron atoms evaluated using a Debye–H{\"{u}}ckel model - Deprince, J.; Fritzsche, S.; Kallman, T.; Palmeri, P.; Quinet, P. (Canadian Journal of Physics, 2017)
The influence of plasma environment on the atomic parameters associated with the K-vacancy states has been investigated theoretically for several iron ions. To do this, a time-averaged Debye–H{\"{u}}ckel potential for both the electron–nucleus and electron–electron interactions has been considered in the framework of relativistic multiconfiguration Dirac–Fock computations. More particularly, the plasma screening effects on ionization potentials, K-thresholds, transition energies, and radiative rates have been estimated in the astrophysical context of accretion disks around black holes. In the present paper, we describe the behaviour of those atomic parameters for Ne-, Na-, Ar-, and K-like iron ions.
hij:1, public:1,

Ab initio calculations of energy levels, transition rates and lifetimes in Ni xii - Bilal, M.; Beerwerth, R.; Volotka, A. V.; Fritzsche, S. (Monthly Notices of the Royal Astronomical Society, 2017)
We report large-scale multi-configuration Dirac–Hartree–Fock calculations and relativistic configuration interaction calculations for allowed E1 and forbidden transitions (M1, E2, M2) among the fine structure levels of the 3s^2 3p^5, 3s 3p^6 and 3s^2 3p^4 3d configurations for Ni xii. In our systematically enlarged wave functions, we incorporated the effects of relativity, all important electron correlations and rearrangement of the bound electron density within two different computational models. We compare our calculated energies for the fine structure levels with previous calculations and experiments. We validate all the tentative experimental lines recently identified by Del Zanna { &} Badnell with one exception. We discuss the consistency of our transition rates in comparison to semi-empirical predictions. We present ab initio lifetime values by taking into account all allowed E1 and forbidden transitions (M1, E2, M2) rates among lowest 31 levels. Our results for lifetime values are better than previously reported ab initio and semi-empirical values as compared to available experiments, thus, providing reliable predictions in the prospects of future experiments.
hij:1, public:1,

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface - Koz{\'{a}}k, M.; Beck, P.; Deng, H.; McNeur, J.; Sch{\"{o}}nenberger, N.; Gaida, C.; Stutzki, F.; Gebhardt, M.; Limpert, J.; Ruehl, A.; Hartl, I.; Solgaard, O.; et al. (Optics Express, 2017)
We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element’s surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.
hij:1, public:1,

Dynamics of electron injection in a laser-wakefield accelerator - Xu, J.; Buck, A.; Chou, S.-W.; Schmid, K.; Shen, B.; Tajima, T.; Kaluza, M.C.; Veisz, L. (Physics of Plasmas, 2017)
The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way - called shock-front injection - utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r.m.s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages.
hij:1, public:1,

Ground-state ionization energies of boronlike ions - Malyshev, A. V.; Glazov, D. A.; Volotka, A. V.; Tupitsyn, I. I.; Shabaev, V. M.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2017)
High-precision QED calculations of ground-state ionization energies are performed for all boronlike ions with nuclear charge numbers in the range 16≤Z≤96. Rigorous QED calculations are performed within the extended Furry picture and include all many-electron QED effects up to the second order of the perturbation theory. The contributions of third- and higher-order electron-correlation effects are accounted for within the Breit approximation. Nuclear recoil and nuclear polarization effects are taken into account as well. In comparison with previous evaluations of the ground-state ionization energies of boronlike ions the accuracy of the theoretical predictions is improved significantly.
hij:1, public:1,

Magetostatic amplifier with tunable maximum by twisted-light plasma interactions - Wu, D.; Wang, J. W. (Plasma Physics and Controlled Fusion, 2017)
Laser beams with Laguerre–Gaussian (LG) mode carry orbital angular momentum (OAM); however, when interacting with plasmas, the net angular momentum acquired by plasmas is basically zero after interaction. Here, we find when there exists a small magetostatic seed along the laser propagation direction, the barrier would be broken, giving rise to dramatic angular momentum transfer from LG-lasers to plasmas. Hence, the net OAM remaining in the plasmas system would continuously enhance the magetostatic field, until the corresponding Larmor frequency of electrons is comparable to the laser frequency in vacuum. Three-dimensional particle-in-cell simulations are performed to confirm our theory, producing spatial-uniform, temporal-stable and extremely-intense magetostatic fields.
hij:1, public:1,

Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source - Fuchs, S.; W{\"{u}}nsche, M.; Nathanael, J.; Abel, J. J.; R{\"{o}}del, C.; Biedermann, J.; Reinhard, J.; H{\"{u}}bner, U.; Paulus, G.G. (Optica, 2017)
Extreme ultraviolet microscopy is technologically demanding and thus largely confined to synchrotron radiation facilities. However, specific benefits like high resolution and exceptional material contrast provide strong motivation for the development of table-top alternatives. We report on the first demonstration of coherence tomography, i.e., noninvasive cross-sectional imaging, with high harmonics. A depth resolution of 24 nm and very good material contrast are achieved. Excessively demanding optics for extreme ultraviolet radiation are avoided and artifacts due to the elementary geometry are suppressed with a novel three-step one-dimensional phase-retrieval algorithm. The images are recorded in reflection geometry, facilitating the analysis of, e.g., operating semiconductor samples.
hij:1, public:1,

Photoexcitation of atoms by Laguerre-Gaussian beams - Peshkov, A. A.; Seipt, D.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2017)
In a recent experiment, Schmiegelow et al. [Nat. Commun. 7, 12998 (2016)] investigated the magnetic sublevel population of Ca^+ ions in a Laguerre-Gaussian light beam if the target atoms were just centered along the beam axis. They demonstrated in this experiment that the sublevel population of the excited atoms is uniquely defined by the projection of the orbital angular momentum of the incident light. However, little attention has been paid so far to the question of how the magnetic sublevels are populated when atoms are displaced from the beam axis by some impact parameter b. Here, we analyze this sublevel population for different atomic impact parameters in first-order perturbation theory and by making use of the density-matrix formalism. Detailed calculations are performed especially for the 4s ^2S_1/2 -> 3d ^2D_5/2 transition in Ca^+ ions and for the vector potential of a Laguerre-Gaussian beam in Coulomb gauge. It is shown that the magnetic sublevel population of the excited ^2D_5/2 level varies significantly with the impact parameter and is sensitive to the polarization, the radial index, as well as the orbital angular momentum of the incident light beam.
hij:1, public:1,

Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses - Scullion, C.; Doria, D.; Romagnani, L.; Sgattoni, A.; Naughton, K.; Symes, D. R.; McKenna, P.; Macchi, A.; Zepf, M.; Kar, S.; Borghesi, M. (Physical Review Letters, 2017)
The acceleration of ions from ultrathin (10–100 nm) carbon foils has been investigated using intense (∼6×10²⁰ W cm⁻²) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25−30 MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.
hij:1, public:1,

Angle-resolved x-ray spectroscopic scheme to determine overlapping hyperfine splittings in highly charged heliumlike ions - Wu, Z. W.; Volotka, A. V.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2017)
An angle-resolved x-ray spectroscopic scheme is presented for determining the hyperfine splitting of highly charged ions. For heliumlike ions, in particular, we propose to measure either the angular distribution or polarization of the 1s2p ^3P_1,F -> 1s^2 ^1S_0,F_f emission following the stimulated decay of the initial 1s 2s ^1S_0,F_i level. It is found that both the angular and polarization characteristics of the emitted x-ray photons strongly depend on the (relative) splitting of the partially overlapping hyperfine 1s 2p ^3P_1,F resonances and may thus help resolve their hyperfine structure. The proposed scheme is feasible with present-day photon detectors and allows a measurement of the hyperfine splitting of heliumlike ions with a relative accuracy of about 10^-4.
hij:1, public:1,

Auger decay of 4d inner-shell holes in atomic Hg leading to triple ionization - Andersson, J.; Beerwerth, R.; Roos, A.; Squibb, R. J.; Singh, R.; Zagorodskikh, S.; Talaee, O.; Koulentianos, D.; Eland, J. H. D.; Fritzsche, S.; Feifel, R. (Physical Review A, 2017)
Formation of triply ionized states upon the creation of 4d inner-shell holes in atomic Hg is investigated by using synchrotron radiation of 730 eV photon energy and a versatile multielectron coincidence detection technique in combination with multiconfiguration Dirac-Fock calculations. By carefully selecting Coster-Kronig electrons detected only in coincidence with a 4d photoelectron, the Coster-Kronig spectrum has been extracted and the corresponding branching ratios of the 4d hole have been determined. The results are found to differ from previously established experimental ratios based on electron impact ionization but to agree now better with theory. We also present an Auger cascade analysis of pathways leading to triply ionized states of atomic Hg upon removal of a 4d inner-shell electron.
hij:1, public:1,

High Average Power Near-Infrared Few-Cycle Lasers - Rothhardt, J.; H{\"{a}drich, S. and Delagnes, J. C. and Cormier, E. and Limpert, J. (Laser and Photonics Reviews, 2017)
Ultra-short laser pulses with only a few optical cycles duration have gained increasing importance during the recent decade and are currently employed in many laboratories worldwide. In addition, modern laser technology nowadays can provide few-cycle pulses at very high average power which advances established studies and opens exciting novel research opportunities. In this paper, the two complementary approaches for providing few-cycle pulses at high average power, namely optical parametric amplification and nonlinear pulse compression, are reviewed and compared. In addition, their limitations and future scaling potential are discussed. Furthermore, selected applications particularly taking advantage of the high average power and high repetition rate are presented.
hij:1, public:1,

High-average-power femtosecond laser at 258 nm - M{\"{u}}ller, M.; Klenke, A.; Gottschall, T.; Klas, R.; Rothhardt, C.; Demmler, S.; Rothhardt, J.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2017)
We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M^2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.
hij:1, public:1,

Non-Abelian Higgs models: Paving the way for asymptotic freedom - Gies, H.; Zambelli, L. (Physical Review D, 2017)
Asymptotically free renormalization group trajectories can be constructed in non-Abelian Higgs models with the aid of generalized boundary conditions imposed on the renormalized action. We detail this construction within the languages of simple low-order perturbation theory, effective field theory, as well as modern functional renormalization group equations. We construct a family of explicit scaling solutions using a controlled weak-coupling expansion in the ultraviolet, and obtain a standard Wilsonian renormalization group relevance classification of perturbations about scaling solutions. We obtain global information about the quasifixed function for the scalar potential by means of analytic asymptotic expansions and numerical shooting methods. Further analytical evidence for such asymptotically free theories is provided in the large-N limit. We estimate the long-range properties of these theories and identify initial/boundary conditions giving rise to a conventional Higgs phase.
hij:1, public:1,

Photoelectron distribution of nonresonant two-photon ionization of neutral atoms - Hofbrucker, J.; Volotka, A. V.; Fritzsche, S. (Physical Review A, 2017)
Photoelectron angular distributions following the nonresonant two-photon K-shell ionization of neutral atoms are studied theoretically. Using the independent particle approximation and relativistic second-order perturbation theory, the contributions of screening and relativistic effects to the photoelectron angular distribution are evaluated. A simple nonrelativistic expression is presented for the angle-differential cross section in dipole approximation for two-photon ionization by elliptically polarized photons, and its limitations are analyzed numerically. Moreover, we show that screening effects of the inactive electrons can significantly affect the photoelectron distributions and can also lead to a strong elliptical dichroism. Numerical results are presented for the case of two-photon K-shell ionization of neutral Ne, Ge, Xe, and U atoms.
hij:1, public:1,

Transverse Coherence Limited Coherent Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies - Z{\"{u}}rch, M.; Jung, R.; Sp{\"{a}th, C. and T{\"{u}}mmler, J. and Guggenmos, A. and Attwood, D. and Kleineberg, U. and Stiel, H. and Spielmann, C. (Scientific Reports, 2017)
Coherent diffraction imaging (CDI) in the extreme ultraviolet has become an important tool for nanoscale investigations. Laser-driven high harmonic generation (HHG) sources allow for lab scale applications such as cancer cell classification and phase-resolved surface studies. HHG sources exhibit excellent coherence but limited photon flux due poor conversion efficiency. In contrast, table-top soft X-ray lasers (SXRL) feature excellent temporal coherence and extraordinary high flux at limited transverse coherence. Here, the performance of a SXRL pumped at moderate pump energies is evaluated for CDI and compared to a HHG source. For CDI, a lower bound for the required mutual coherence factor of |μ12| ≥ 0.75 is found by comparing a reconstruction with fixed support to a conventional characterization using double slits. A comparison of the captured diffraction signals suggests that SXRLs have the potential for imaging micron scale objects with sub-20 nm resolution in orders of magnitude shorter integration time compared to a conventional HHG source. Here, the low transverse coherence diameter limits the resolution to approximately 180 nm. The extraordinary high photon flux per laser shot, scalability towards higher repetition rate and capability of seeding with a high harmonic source opens a route for higher performance nanoscale imaging systems based on SXRLs.
hij:1, public:1,

Electro-optically controlled divided-pulse amplification - Stark, H.; M{\"{u}}ller, M.; Kienel, M.; Klenke, A.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2017)
A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.
hij:1, public:1,

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter - Cayzac, W.; Frank, A.; Ortner, A.; Bagnoud, V.; Basko, M. M.; Bedacht, S.; Bl{\"{a}ser, C. and Blazevic, A. and Busold, S. and Deppert, O. and Ding, J. and Ehret, M. and Fiala, P. and Frydrych, S. and Gericke, D. O. and Hallo, L. and Helfrich, J. and Jahn, D. and Kjartansson, E. and Knetsch, A. and Kraus, D. and Malka, G. and Neumann, N. W. and P{\'{e}}pitone, K. and Pepler, D. and Sander, S. and Schaumann, G. and Schlegel, T. and Schroeter, N. and Schumacher, D. and Seibert, M. (Nature Communications, 2017)
The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.
hij:1, public:1,

High flux, beamed neutron sources employing deuteron-rich ion beams from D2O-ice layered targets - Alejo, A.; Krygier, A. G.; Ahmed, H.; Morrison, J. T.; Clarke, R. J.; Fuchs, J.; Green, A.; Green, J. S.; Jung, D.; Kleinschmidt, A.; Najmudin, Z.; Nakamura, H.; et al. (Plasma Physics and Controlled Fusion, 2017)
A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D₂O ice at the rear side, irradiated by sub-petawatt laser pulses ( 200 J, 750 fs) at peak intensity ∼2×10²⁰ W cm⁻². The neutrons were preferentially produced in a beam of 70° FWHM cone along the ion beam forward direction, with maximum energy up to 40 MeV and a peak flux along the axis ∼2×10⁹ n sr⁻¹ for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)³He reaction using the deuteron beam produced by the ice-layered target.
hij:1, public:1,

Hybrid soliton dynamics in liquid-core fibres - Chemnitz, M.; Gebhardt, M.; Gaida, C.; Stutzki, F.; Kobelke, J.; Limpert, J.; T{\"{u}}nnermann, A.; Schmidt, M. (Nature Communications, 2017)
The discovery of optical solitons being understood as temporally and spectrally stationary optical states has enabled numerous innovations among which, most notably, supercontinuum light sources have become widely used in both fundamental and applied sciences. Here, we report on experimental evidence for dynamics of hybrid solitons{–}a new type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres. Octave-spanning supercontinua in the mid-infrared region are observed when pumping the hybrid waveguide with a 460 fs laser (1.95 μm) in the anomalous dispersion regime at nanojoule-level pulse energies. A detailed numerical analysis well correlated with the experiment uncovers clear indicators of emerging hybrid solitons, revealing their impact on the bandwidth, onset energy and noise characteristics of the supercontinua. Our study highlights liquid-core fibres as a promising platform for fundamental optics and applications towards novel coherent and reconfigurable light sources.
hij:1, public:1,

Magnetic brane solutions in Gauss–Bonnet–Maxwell massive gravity - Hendi, S. H.; Panah, B. E.; Panahiyan, S.; Momennia, M. (Physics Letters B, 2017)
Magnetic branes of Gauss–Bonnet–Maxwell theory in the context of massive gravity is studied in detail. Exact solutions are obtained and their interesting geometrical properties are investigated. It is argued that although these horizonless solutions are free of curvature singularity, they enjoy a cone-like geometry with a conic singularity. In order to investigate the effects of various parameters on the geometry of conic singularity, its corresponding deficit angle is studied. It will be shown that despite the effects of Gauss–Bonnet gravity on the solutions, deficit angle is free of Gauss–Bonnet parameter. On the other hand, the effects of massive gravity, cosmological constant and electrical charge on the deficit angle will be explored. Also, a brief discussion related to possible geometrical phase transition of these topological objects is given.
hij:1, public:1,

Pulse Duration of Seeded Free-Electron Lasers - Finetti, P.; H{\"{o}}ppner, H.; Allaria, E.; Callegari, C.; Capotondi, F.; Cinquegrana, P.; Coreno, M.; Cucini, R.; Danailov, M.; Demidovich, A.; De Ninno, G.; Di Fraia, M.; et al. (Physical Review X, 2017)
The pulse duration, and, more generally, the temporal intensity profile of free-electron laser (FEL) pulses, is of utmost importance for exploring the new perspectives offered by FELs; it is a nontrivial experimental parameter that needs to be characterized. We measured the pulse shape of an extreme ultraviolet externally seeded FEL operating in high-gain harmonic generation mode. Two different methods based on the cross-correlation of the FEL pulses with an external optical laser were used. The two methods, one capable of single-shot performance, may both be implemented as online diagnostics in FEL facilities. The measurements were carried out at the seeded FEL facility FERMI. The FEL temporal pulse characteristics were measured and studied in a range of FEL wavelengths and machine settings, and they were compared to the predictions of a theoretical model. The measurements allowed a direct observation of the pulse lengthening and splitting at saturation, in agreement with the proposed theory.
hij:1, public:1,

Soft x-ray induced femtosecond solid-to-solid phase transition - Tavella, F.; H{\"{o}}ppner, H.; Tkachenko, V.; Medvedev, N.; Capotondi, F.; Golz, T.; Kai, Y.; Manfredda, M.; Pedersoli, E.; Prandolini, M. J.; Stojanovic, N.; Tanikawa, T.; et al. (High Energy Density Physics, 2017)
Abstract Soft x-rays were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. This scheme enabled for the first time the measurement of a phase transition on a timescale of ∼150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.
hij:1, public:1,

Spin-dependent quantum theory of high-order above-threshold ionization - Zille, D.; Seipt, D.; M{\"{o}}ller, M.; Fritzsche, S.; Paulus, G.G.; Milo{{s}}evi{}, D. B. (Physical Review A, 2017)
The strong-field-approximation theory of high-order above-threshold ionization of atoms is generalized to include the electron spin. The obtained rescattering amplitude consists of a direct and exchange part. On the examples of excited He atoms as well as Li^+ and Be^2+ ions, it is shown that the interference of these two amplitudes leads to an observable difference between the photoelectron momentum distributions corresponding to different initial spin states: Pronounced minima appear for singlet states, which are absent for triplet states.
hij:1, public:1,

Studying the Dynamics of Relativistic Laser-Plasma Interaction on Thin Foils by Means of Fourier-Transform Spectral Interferometry - Bagnoud, V.; Hornung, J.; Schlegel, T.; Zielbauer, B.; Brabetz, C.; Roth, M.; Hilz, P.; Haug, M.; Schreiber, J.; Wagner, F. (Physical Review Letters, 2017)
We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^19 W/cm2 range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.
hij:1, public:1,

Commissioning of a Si(Li) Compton polarimeter with improved energy resolution - Vockert, M.; Weber, G.; Spillmann, U.; Krings, T.; Herdrich, M.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Abstract On the basis of a double-side segmented Si(Li) crystal a new Compton polarimeter was developed within the SPARC collaboration. The new detector is equipped with a cryogenic first stage of the preamplifiers to improve the energy resolution compared to previous detectors with preamplifiers operating at room temperature. We present first results from a commissioning measurement of the new instrument at the ESR storage ring of GSI in Darmstadt, Germany and contrast it with the performance of an precursor polarimeter system.
hij:1, public:1,