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Peer-Review Publications


2019

M. Vockert, G. Weber, H. Bräuning, A. Surzhykov, C. Brandau, S. Fritzsche, S. Geyer, S. Hagmann, S. Hess, C. Kozhuharov, R. Märtin, N. Petridis, R. Hess, S. Trotsenko, Yu. A. Litvinov, J. Glorius, A. Gumberidze, M. Steck, S. Litvinov, T. Gassner, P.-M. Hillenbrand, M. Lestinsky, F. Nolden, M. S. Sanjari, U. Popp, C. Trageser, D. F. A. Winters, U. Spillmann, T. Krings, and Th. Stöhlker
Radiative electron capture as a tunable source of highly linearly polarized x rays
Phyical Review A 99, 052702 (2019)

Abstract: The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U92+. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.

G. Torgrimsson
Perturbative methods for assisted nonperturbative pair production
Physical Review D 99, 096002 (2019)

Abstract: In the dynamically assisted Schwinger mechanism, the pair production probability is significantly enhanced by including a weak, rapidly varying field in addition to a strong, slowly varying field. In a previous paper we showed that several features of dynamical assistance can be understood by a perturbative treatment of the weak field. Here we show how to calculate the prefactors of the higher-orders terms, which is important because the dominant contribution can come from higher orders. We give a new and independent derivation of the momentum spectrum using the worldline formalism, and extend our WKB approach to calculate the amplitude to higher orders. We show that these methods are also applicable to doubly assisted pair production.

C. Kohlfürst
Spin states in multiphoton pair production for circularly polarized light
Physical Review D 99, 096017 (2019)

Abstract: Scalar and fermionic particle pair production in rotating electric fields is investigated in the nonperturbative multiphoton regime. Angular momentum distribution functions in above-threshold pair production processes are calculated numerically within quantum kinetic theory and discussed on the basis of a photon absorption model. The particle spectra can be understood if the spin states of the particle-antiparticle pair are taken into account.

G. Torgrimsson
Thermally versus dynamically assisted Schwinger pair production
Physical Review D 99, 096007 (2019)

Abstract: We study electron-positron pair production by the combination of a strong, constant electric field and a thermal background. We show that this process is similar to dynamically assisted Schwinger pair production, where the strong field is instead assisted by another coherent field, which is weaker but faster. We treat the interaction with the photons from the thermal background perturbatively, while the interaction with the electric field is nonperturbative (i.e., a Furry picture expansion in α). At O(α2) we have ordinary perturbative Breit-Wheeler pair production assisted nonperturbatively by the electric field. Already at this order we recover the same exponential part of the probability as previous studies, which did not expand in α. This means that we do not have to consider higher orders, so our approach allows us to calculate the preexponential part of the probability, which has not been obtained before in this regime. Although the prefactor is in general subdominant compared to the exponential part, in this case it can be important because it scales as α2≪1 and is therefore much smaller than the prefactor at O(α0) (pure Schwinger pair production). We show that, because of the exponential enhancement, O(α2) still gives the dominant contribution for temperatures above a certain threshold, but, because of the small prefactor, the threshold is higher than what the exponential alone would suggest.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
Photon-Photon Scattering at the High-Intensity Frontier: Paraxial Beams
Journal of Physics: Conference Series 1206, 012016 (2019)

Abstract: Our goal is to study optical signatures of quantum vacuum nonlinearities in strong macroscopic electromagnetic fields provided by high-intensity laser beams. The vacuum emission scheme is perfectly suited for this task as it naturally distinguishes between incident laser beams, described as classical electromagnetic fields driving the effect, and emitted signal photons encoding the signature of quantum vacuum nonlinearity. Using the Heisenberg-Euler effective action, our approach allows for a reliable study of photonic signatures of QED vacuum nonlinearity in the parameter regimes accessible by all-optical high-intensity laser experiments. To this end, we employ an efficient, flexible numerical algorithm, which allows for a detailed study of the signal photons emerging in the collision of focused paraxial high-intensity laser pulses. Due to the high accuracy of our numerical solutions we predict the total number of signal photons, but also have full access to the signal photons’ characteristics, including their spectrum, propagation directions and polarizations. We discuss setups offering an excellent background-to-noise ratio, thus providing an important step towards the experimental verification of quantum vacuum nonlinearities.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
The Vacuum Emission Picture Beyond Paraxial Approximation
Journal of Physics: Conference Series 1206, 012017 (2019)

Abstract: Optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum can be conveniently described in terms of stimulated photon emission processes induced by strong classical, space-time dependent electromagnetic fields. Recent studies have adopted this approach to study collisions of Gaussian laser pulses in paraxial approximation. The present study extends these investigations beyond the paraxial approximation by using an efficient numerical solver for the classical input fields. This new numerical code allows for a consistent theoretical description of optical signatures of QED vacuum nonlinearities in generic electromagnetic fields governed by Maxwell’s equations in the vacuum, such as manifestly non-paraxial laser pulses. Our code is based on a locally constant field approximation of the Heisenberg-Euler effective Lagrangian. As this approximation is applicable for essentially all optical high-intensity laser experiments, our code is capable of calculating signal photon emission amplitudes in completely generic input field configurations, limited only by numerical cost.

J. Glorius, C. Langer, Z. Slavkovská, L. Bott, C. Brandau, B. Brückner, K. Blaum, X. Chen, S. Dababneh, T. Davinson, P. Erbacher, S. Fiebiger, T. Gaßner, K. Göbel, M. Groothuis, A. Gumberidze, G. Gyürky, M. Heil, R. Hess, R. Hensch, P. Hillmann, P.-M. Hillenbrand, O. Hinrichs, B. Jurado, T. Kausch, A. Khodaparast, T. Kisselbach, N. Klapper, C. Kozhuharov, D. Kurtulgil, G. Lane, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Yu. A. Litvinov, B. Löher, F. Nolden, N. Petridis, U. Popp, T. Rauscher, M. Reed, R. Reifarth, M. S. Sanjari, D. Savran, H. Simon, U. Spillmann, M. Steck, T. Stöhlker, J. Stumm, A. Surzhykov, T. Szücs, T. T. Nguyen, A. Taremi Zadeh, B. Thomas, S. Yu. Torilov, H. Törnqvist, M. Träger, C. Trageser, S. Trotsenko, L. Varga, M. Volknandt, H. Weick, M. Weigand, C. Wolf, P. J. Woods, and Y. M. Xing
Approaching the Gamow Window with Stored Ions: Direct Measurement of ¹²⁴Xe(p,γ) in the ESR Storage Ring
Physical Review Letters 122, 092701 (2019)

Abstract: We report the first measurement of low-energy proton-capture cross sections of 124Xe in a heavy-ion storage ring. 124Xe^54+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The 125Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.

Y. S. You, J. Lu, E. F. Cunningham, C. Rödel, and S. Ghimire
Crystal orientation-dependent polarization state of high-order harmonics
Optics Letters 44, 530 (2019)

Abstract: We analyze the crystal orientation-dependent polarization state of extreme ultraviolet high-order harmonics from bulk magnesium oxide crystals subjected to intense linearly polarized laser fields. We find that only along high-symmetry directions do high-order harmonics follow the polarization direction of the laser field. In general, there are strong deviations that depend on harmonic order, strength of the laser field, and crystal orientation. We use a real-space electron trajectory picture to understand the origin of polarization deviations. These results have implications in all-optical probing of electronic band structure in momentum space and valence charge distributions in real space, and in producing attosecond pulses with time-dependent polarization in compact setups.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
All-optical signatures of quantum vacuum nonlinearities in generic laser fields
Physical Review D 99, 016006 (2019)

Abstract: All-optical experiments at the high-intensity frontier offer a promising route to unprecedented precision tests of quantum electrodynamics in strong macroscopic electromagnetic fields. So far, most theoretical studies of all-optical signatures of quantum vacuum nonlinearity are based on simplifying approximations of the beam profiles and pulse shapes of the driving laser fields. Since precision tests require accurate quantitative theoretical predictions, we introduce an efficient numerical tool facilitating the quantitative theoretical study of all-optical signatures of quantum vacuum nonlinearity in generic laser fields. Our approach is based on the vacuum emission picture, and makes use of the fact that the dynamics of the driving laser fields are to an excellent approximation governed by classical Maxwell theory in vacuum. In combination with a Maxwell solver, which self-consistently propagates any given laser field configuration, this allows for accurate theoretical predictions of photonic signatures of vacuum nonlinearity in high-intensity laser experiments from first principles. We employ our method to simulate photonic signatures of quantum vacuum nonlinearity in laser pulse collisions involving a few-cycle pulse, and show that the angular and spectral distributions of the emitted signal photons deviate from those of the driving laser beams.

T. Helk, M. Zürch, and C. Spielmann
Perspective: Towards single shot time-resolved microscopy using short wavelength table-top light sources
Structural Dynamics 6, 010902 (2019)

Abstract: Time-resolved imaging allows revealing the interaction mechanisms in the microcosm of both inorganic and biological objects. While X-ray microscopy has proven its advantages for resolving objects beyond what can be achieved using optical microscopes, dynamic studies using full-field imaging at the nanometer scale are still in their infancy. In this perspective, we present the current state of the art techniques for full-field imaging in the extreme-ultraviolet- and soft X-ray-regime which are suitable for single exposure applications as they are paramount for studying dynamics in nanoscale systems. We evaluate the performance of currently available table-top sources, with special emphasis on applications, photon flux, and coherence. Examples for applications of single shot imaging in physics, biology, and industrial applications are discussed.

2018

K. S. Schulze
Fundamental limitations of the polarization purity of x rays
APL Photonics 3, 126106 (2018)

Abstract: 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.

D. Wu, W. Yu, Y. Zhao, S. Fritzsche, and X. He
Characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids: The role of bremsstrahlung and radiation reactions
Matter and Radiation at Extremes 3, 293 (2018)

Abstract: In this work, characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids are investigated by means of a newly developed particle-in-cell (PIC) simulation code. The PIC code takes advantage of the recently developed ionization and collision dynamics models, which make it possible to model different types of materials based on their intrinsic atomic properties. Within the simulations, both bremsstrahlung and nonlinear Compton scatterings have been included. Different target materials and laser intensities are considered for studying the parameter-dependent features of X/γ-ray radiations. The relative strength and angular distributions of X/γ ray productions from bremsstrahlung and nonlinear Compton scatterings are compared to each other. The threshold under which the nonlinear Compton scatterings become dominant over bremsstrahlung is also outlined.

L. Obst-Huebl, T. Ziegler, F.-E. Brack, J. Branco, M. Bussmann, T. E. Cowan, C. B. Curry, F. Fiuza, M. Garten, M. Gauthier, S. Göde, S. H. Glenzer, A. Huebl, A. Irman, J. B. Kim, T. Kluge, S. D. Kraft, F. Kroll, J. Metzkes-Ng, R. Pausch, I. Prencipe, M. Rehwald, C. Rödel, H.-P. Schlenvoigt, U. Schramm, and K. Zeil
All-optical structuring of laser-driven proton beam profiles
Nature Communications 9, 5292 (2018)

Abstract: Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

M. Müller, A. Klenke, A. Steinkopff, H. Stark, A. Tünnermann, and J. Limpert
3.5 kW coherently combined ultrafast fiber laser
Optics Letters 43, 6037 (2018)

Abstract: An ultrafast laser based on the coherent beam combination of four ytterbium-doped step-index fiber amplifiers is presented. The system delivers an average power of 3.5 kW and a pulse duration of 430 fs at an 80 MHz repetition rate. The beam quality is excellent (M2 < 1.24·1.10), and the relative intensity noise is as low as 1% in the frequency span from 1 Hz to 1 MHz. The system is turn-key operable, as it features an automated spatial and temporal alignment of the interferometric amplification channels.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, and J. Limpert
Ultrafast thulium fiber laser system emitting more than 1  kW of average power
Optics Letters 43, 5853 (2018)

Abstract: 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.

I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, and Th. Stöhlker
Electron-positron pair production in slow collisions of heavy nuclei beyond the monopole approximation
Phyical Review A 98, 062709 (2018)

Abstract: Electron-positron pair production in low-energy collisions of heavy nuclei is considered beyond the monopole approximation. The calculation method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. Bound-free and free-free pair-production probabilities as well as the energy spectra of the emitted positrons are calculated for the collisions of bare uranium nuclei. The calculations are performed for collision energy near the Coulomb barrier for different values of the impact parameter. The obtained results are compared with the corresponding values calculated in the monopole approximation.

F. Karbstein, M. Wagner, and M. Weber
Determination of ΛMSbar (nf=2) and analytic parametrization of the static quark-antiquark potential
Physical Review D 98, 114506 (2018)

Abstract: While lattice QCD allows for reliable results at small momentum transfers (large quark separations), perturbative QCD is restricted to large momentum transfers (small quark separations). The latter is determined up to a reference momentum scale Λ, which is to be provided from outside, e.g., from experiment or lattice QCD simulations. In this article, we extract ΛMSbar for QCD with nf=2 dynamical quark flavors by matching the perturbative static quark-antiquark potential in momentum space to lattice results in the intermediate momentum regime, where both approaches are expected to be applicable. In a second step, we combine the lattice and the perturbative results to provide a complete analytic parametrization of the static quark-antiquark potential in position space up to the string breaking scale. As an exemplary phenomenological application of our all-distances potential, we compute the bottomonium spectrum in the static limit.

A. H. Woldegeorgis, B. Beleites, F. Ronneberger, R. Grosse, and A. Gopal
Investigating the influence of incident laser wavelength and polarization on particle acceleration and terahertz generation
Physical Review E 98, 061201 (2018)

Abstract: 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.

V. Zakosarenko, M. Schmelz, T. Schönau, S. Anders, J. Kunert, V. Tympel, R. Neubert, F. Schmidl, P. Seidel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, and R. Stolz
Coreless SQUID-based cryogenic current comparator for non-destructive intensity diagnostics of charged particle beams
Superconductor Science and Technology 32, 014002 (2018)

Abstract: We report on a novel concept and prototype development of a coreless SQUID-based charged-particle beam monitor as a non-destructive diagnostic tool for accelerator facilities. Omitting the typically used pickup coil with a high magnetic permeability core leads to a significant improvement in low-frequency noise performance. Moreover, a revised shielding geometry allows for very compact and rather lightweight device designs. Based on highly sensitive SQUIDs featuring sub-micron cross-type Josephson tunnel junctions, our prototype device exhibits a current sensitivity of about 6 pA Hz^(−1/2) in the white noise region. Together with a measured shielding factor of about 135 dB this opens up the way for its widespread use in modern accelerator facilities.

S. Kraft-Bermuth, D. Hengstler, P. Egelhof, C. Enss, A. Fleischmann, M. Keller, and T. Stöhlker
Microcalorimeters for X-Ray Spectroscopy of Highly Charged Ions at Storage Rings
Atoms 6, 59 (2018)

Abstract: 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.

V. A. Agababaev, D. A. Glazov, A. V. Volotka, D. V. Zinenko, V. M. Shabaev, and G. Plunien
Ground-state g factor of middle-Z boronlike ions
Journal of Physics: Conference Series 1138, 012003 (2018)

Abstract: 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.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, J. Antonio-Lopez, A. Schülzgen, R. Amezcua-Correa, A. Tünnermann, I. Pupeza, and J. Limpert
Watt-scale super-octave mid-infrared intrapulse difference frequency generation
Light: Science & Applications 7, 94 (2018)

Abstract: 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.

D. Jahn, D. Schumacher, C. Brabetz, J. Ding, S. Weih, F. Kroll, F. Brack, U. Schramm, A. Blažević, and M. Roth
First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 173 (2018)

Abstract: 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 für 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.

J. Ding, D. Schumacher, D. Jahn, A. Blažević, and M. Roth
Simulation studies on generation, handling and transport of laser-accelerated carbon ions
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 168 (2018)

Abstract: 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.

A. Bernhard, V. A. Rodríguez, S. Kuschel, M. Leier, P. Peiffer, A. Sävert, M. Schwab, W. Werner, C. Widmann, A. Will, A.-S. Müller, and M. Kaluza
Progress on experiments towards LWFA-driven transverse gradient undulator-based FELs
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 391 (2018)

Abstract: 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.

A. Woldegeorgis, T. Kurihara, M. Almassarani, B. Beleites, R. Grosse, F. Ronneberger, and A. Gopal
Multi-MV/cm longitudinally polarized terahertz pulses from laser–thin foil interaction
Optica 5, 1474 (2018)

Abstract: 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.

C. Gaida, T. Heuermann, M. Gebhardt, E. Shestaev, T. P. Butler, D. Gerz, N. Lilienfein, P. Sulzer, M. Fischer, R. Holzwarth, A. Leitenstorfer, I. Pupeza, and J. Limpert
High-power frequency comb at 2  μm wavelength emitted by a Tm-doped fiber laser system
Optics Letters 43, 5178 (2018)

Abstract: 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.

A. Surzhykov, V. A. Yerokhin, S. Fritzsche, and A. V. Volotka
Diagnostics of polarization purity of x rays by means of Rayleigh scattering
Phyical Review A 98, 053403 (2018)

Abstract: 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.

J. Wang, W. Yu, M. Y. Yu, S. Rykovanov, J. Ju, S. Luan, K. Li, Y. Leng, R. Li, and Z.-M. Sheng
Very-long distance propagation of high-energy laser pulse in air
Physics of Plasmas 25, 113111 (2018)

Abstract: 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.

R. Battesti, J. Beard, S. Böser, N. Bruyant, D. Budker, S. A. Crooker, E. J. Daw, V. V. Flambaum, T. Inada, I. G. Irastorza, F. Karbstein, D. L. Kim, M. G. Kozlov, Z. Melhem, A. Phipps, P. Pugnat, G. Rikken, C. Rizzo, M. Schott, Y. K. Semertzidis, H. H. t. Kate, and G. Zavattini
High magnetic fields for fundamental physics
Physics Reports 765-766, 1 (2018)

Abstract: 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.

C. M. Heyl, S. B. Schoun, G. Porat, H. Green, and J. Ye
A nozzle for high-density supersonic gas jets at elevated temperatures
Review of Scientific Instruments 89, 113114 (2018)

Abstract: 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.

K. T. Behm, J. M. Cole, A. S. Joglekar, E. Gerstmayr, J. C. Wood, C. D. Baird, T. G. Blackburn, M. Duff, C. Harvey, A. Ilderton, S. Kuschel, S. P. D. Mangles, M. Marklund, P. McKenna, C. D. Murphy, Z. Najmudin, K. Poder, C. P. Ridgers, G. Sarri, G. M. Samarin, D. Symes, J. Warwick, M. Zepf, K. Krushelnick, and A. G. R. Thomas
A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV
Review of Scientific Instruments 89, 113303 (2018)

Abstract: 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.

N. Jayakumar, R. Sollapur, A. Hoffmann, T. Grigorova, A. Hartung, A. Schwuchow, J. Bierlich, J. Kobelke, M. A. Schmidt, and C. Spielmann
Polarization evolution in single-ring antiresonant hollow-core fibers
Applied Optics 57, 8529 (2018)

Abstract: 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.

J. Kirtzel, S. Madhavan, N. Wielsch, A. Blinne, Y. Hupfer, J. Linde, K. Krause, A. Svatoš, and E. Kothe
Enzymatic Bioweathering and Metal Mobilization From Black Slate by the Basidiomycete Schizophyllum commune
Frontiers in microbiology 9, 2545 (2018)

Abstract: 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.

J. Rothhardt, G. Tadesse, W. Eschen, and J. Limpert
Table-top nanoscale coherent imaging with XUV light
Journal of Optics 20, 113001 (2018)

Abstract: 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.

M. Ruijter, V. Y. Kharin, and S. G. Rykovanov
Analytical solutions for nonlinear Thomson scattering including radiation reaction
Journal of Physics B: Atomic, Molecular and Optical Physics 51, 225701 (2018)

Abstract: 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.

T. Sato, M. Asai, A. Borschevsky, R. Beerwerth, Y. Kaneya, H. Makii, A. Mitsukai, Y. Nagame, A. Osa, A. Toyoshima, K. Tsukada, M. Sakama, S. Takeda, K. Ooe, D. Sato, Y. Shigekawa, S.-i. Ichikawa, C. Düllmann, J. Grund, D. Renisch, J. Kratz, M. Schädel, E. Eliav, U. Kaldor, S. Fritzsche, and T. Stora
First Ionization Potentials of Fm, Md, No, and Lr: Verification of Filling-Up of 5f Electrons and Confirmation of the Actinide Series
Journal of the American Chemical Society 140, 14609 (2018)

Abstract: 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.

A. V. Maiorova, S. Fritzsche, R. A. Müller, and A. Surzhykov
Elastic scattering of twisted electrons by diatomic molecules
Phyical Review A 98, 042701 (2018)

Abstract: 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.

J. Palaudoux, T. Kaneyasu, L. Andric, S. Carniato, G. Gamblin, F. Penent, Y. Hikosaka, E. Shigemasa, K. Ito, S. Fritzsche, E. Kukk, S. Sheinerman, R. F. Fink, P. Lablanquie, and R. Püttner
Selectivity of the Br 3d⁻1 Auger decays in HBr
Phyical Review A 98, 043406 (2018)

Abstract: 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.

L. Yue, P. Wustelt, A. Sayler, F. Oppermann, M. Lein, G. Paulus, and S. Gräfe
Strong-field polarizability-enhanced dissociative ionization
Phyical Review A 98, 043418 (2018)

Abstract: 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.

B. Panah, S. Hendi, S. Panahiyan, and M. Hassaine
BTZ dilatonic black holes coupled to Maxwell and Born-Infeld electrodynamics
Physical Review D 98, 084006 (2018)

Abstract: 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.

C. Schneider, G. Torgrimsson, and R. Schützhold
Discrete worldline instantons
Physical Review D 98, 085009 (2018)

Abstract: 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.

S. Kuschel, M. B. Schwab, M. Yeung, D. Hollatz, A. Seidel, W. Ziegler, A. Sävert, M. C. Kaluza, and M. Zepf
Controlling the Self-Injection Threshold in Laser Wakefield Accelerators
Physical Review Letters 121, 154801 (2018)

Abstract: 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.

Z. Samsonova, S. Höfer, R. Hollinger, T. Kämpfer, I. Uschmann, R. Röder, L. Trefflich, O. Rosmej, E. Förster, C. Ronning, D. Kartashov, and C. Spielmann
Hard X-ray generation from ZnO nanowire targets in a non-relativistic regime of laser-solid interactions
Applied Sciences 8, 1728 (2018)

Abstract: 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.

A. Klenke, M. Muller, H. Stark, M. Kienel, C. Jauregui, A. Tünnermann, and J. Limpert
Coherent Beam Combination of Ultrafast Fiber Lasers
IEEE Journal of Selected Topics in Quantum Electronics 24, 1 (2018)

Abstract: 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.

R. Lachheb, A. Herrmann, A. Assadi, J. Reiter, J. Körner, J. Hein, C. Rüssel, R. Maâlej, and K. Damak
Judd–Ofelt analysis and experimental spectroscopic study of erbium doped phosphate glasses
Journal of Luminescence 201, 245 (2018)

Abstract: 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.

K. Krushelnick, A. E. Dangor, M. Kaluza, S. P. D. Mangles, C. D. Murphy, Z. Najmudin, and A. G. R. Thomas
Observation of anomalous side-scattering in laser wakefield accelerators
Laser and Particle Beams 36, 391 (2018)

Abstract: 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.

T. Heuermann, C. Gaida, M. Gebhardt, and J. Limpert
Thulium-doped nonlinear fiber amplifier delivering 50  fs pulses at 20  W of average power
Optics Letters 43, 4441 (2018)

Abstract: 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.

J. S. M. Ginges, and A. V. Volotka
Testing atomic wave functions in the nuclear vicinity: The hyperfine structure with empirically deduced nuclear and quantum electrodynamic effects
Phyical Review A 98, 032504 (2018)

Abstract: 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.

F. Karbstein
Vacuum birefringence in the head-on collision of x-ray free-electron laser and optical high-intensity laser pulses
Physical Review D 98, 056010 (2018)

Abstract: 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.