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Peer-Review Publications


P. Balla, H. Tuennermann, S. Salman, M. Fan, S. Alisauskas, I. Hartl, and C. Heyl
Ultrafast serrodyne optical frequency translator
Nature Photonics 17, 187 (2023)

Abstract: The serrodyne principle enables an electromagnetic signal to be frequency shifted by applying a linear phase ramp in the time domain. This phenomenon has been exploited to frequency shift signals in the radiofrequency, microwave and optical regions of the electromagnetic spectrum over ranges of up to a few gigahertz, for example, to analyse the Doppler shift of radiofrequency signals for noise suppression and frequency stabilization. Here we employ this principle to shift the centre frequency of high-power femtosecond laser pulses over a range of several terahertz with the help of a nonlinear multi-pass cell. We demonstrate our method experimentally by shifting the central wavelength of a state-of-the-art 75 W frequency comb laser from 1,030 nm to 1,060 nm and to 1,000 nm. Furthermore, we experimentally show that this wavelength-shifting technique supports coherence characteristics at the few hertz-level while improving the temporal pulse quality. The technique is generally applicable to wide parameter ranges and different laser systems, enabling efficient wavelength conversion of high-power lasers to spectral regions beyond the gain bandwidth of available laser platforms.

A. Fedotov, A. Ilderton, F. Karbstein, B. King, D. Seipt, H. Taya, and G. Torgrimsson
Advances in QED with intense background fields
Physics Reports 1010, 1 (2023)

Abstract: Upcoming and planned experiments combining increasingly intense lasers and energetic particle beams will access new regimes of nonlinear, relativistic, quantum effects. This improved experimental capability has driven substantial progress in QED in intense background fields. We review here the advances made during the last decade, with a focus on theory and phenomenology. As ever higher intensities are reached, it becomes necessary to consider processes at higher orders in both the number of scattered particles and the number of loops, and to account for non-perturbative physics (e.g. the Schwinger effect), with extreme intensities requiring resummation of the loop expansion. In addition to increased intensity, experiments will reach higher accuracy, and these improvements are being matched by developments in theory such as in approximation frameworks, the description of finite-size effects, and the range of physical phenomena analysed. Topics on which there has been substantial progress include: radiation reaction, spin and polarisation, nonlinear quantum vacuum effects and connections to other fields including physics beyond the Standard Model.

A. Kirsche, R. Klas, M. Gebhardt, L. Eisenbach, W. Eschen, J. Buldt, H. Stark, J. Rothhardt, and J. Limpert
Continuously tunable high photon flux high harmonic source at 50 – 70 eV
Optics Express 31, 2744 (2023)

Abstract: A fully tunable table-top extreme ultraviolet source providing state-of-the-art photon flux at energies of 50-70 eV is presented. It is based on a nonlinear blueshift and subsequent high harmonic generation in a gas-filled capillary.

B. Minneker, R. Klas, J. Rothhardt, and S. Fritzsche
Critical Laser Intensity of Phase-Matched High-Order Harmonic Generation in Noble Gases
Photonics 10, 1 (2023)

Abstract: The efficient generation of high-order harmonic radiation has been a challenging task since the early days of strong-field physics. An essential requirement to achieve efficient high-order harmonic generation inside a gas medium is the phase matching of the high-order harmonic radiation and the incident laser pulse. The dominant contribution to the wave–vector mismatch Δk is associated with the ionization probability of the medium. In this work, we derive two analytical formulas to calculate the critical intensity of a general linearly polarized laser pulse that obey the phase-matching condition Δk=0. The analytic formulas are valid in the tunneling regime (ADK model) and the regime of the tunnel and multi-photon ionization (PPT model), respectively. We compare our results to numerical computations and discuss the scaling of the critical intensity depending on the pulse duration and the wavelength of a realistic incident laser pulse. The analytical approach demonstrated in this work is highly accurate and can compete with the existing numerical computational methods by an error of less than 1% and a decrease in the computation time of approximately 4 to 6 orders of magnitude. This enables complex theoretical studies of the efficiency scaling in HHG or to consider the effects of ground state depletion efficiently.

C. Liu, W. Eschen, L. Loetgering, D. Molina, R. Klas, A. Iliou, M. Steinert, S. Herkersdorf, A. Kirsche, T. Pertsch, F. Hillmann, J. Limpert, and J. Rothhardt
Visualizing the ultra-structure of microorganisms using table-top extreme ultraviolet imaging
PhotoniX 4, 1 (2023)

Abstract: Table-top extreme ultraviolet (EUV) microscopy offers unique opportunities for label-free investigation of biological samples. Here, we demonstrate ptychographic EUV imaging of two dried, unstained model specimens: germlings of a fungus (Aspergillus nidulans), and bacteria (Escherichia coli) cells at 13.5 nm wavelength. We find that the EUV spectral region, which to date has not received much attention for biological imaging, offers sufficient penetration depths for the identification of intracellular features. By implementing a position-correlated ptychography approach, we demonstrate a millimeter-squared field of view enabled by infrared illumination combined with sub-60 nm spatial resolution achieved with EUV illumination on selected regions of interest. The strong element contrast at 13.5 nm wavelength enables the identification of the nanoscale material composition inside the specimens. Our work will advance and facilitate EUV imaging applications and enable further possibilities in life science.


M. Seidel, F. Pressacco, O. Akcaalan, T. Binhammer, J. Darvill, N. Ekanayake, M. Frede, U. Grosse-Wortmann, M. Heber, C. Heyl, D. Kutnyakhov, C. Li, C. Mohr, J. Mueller, O. Puncken, H. Redlin, N. Schirmel, S. Schulz, A. Swiderski, H. Tavakol, H. Tuennermann, C. Vidoli, L. Wenthaus, N. Wind, L. Winkelmann, B. Manschwetus, and I. Hartl
Ultrafast MHz-Rate Burst-Mode Pump-Probe Laser for the FLASH FEL Facility Based on Nonlinear Compression of ps-Level Pulses from an Yb-Amplifier Chain
Laser & Photonics Reviews 1, 2100268 (2022)

Abstract: The Free-Electron Laser (FEL) FLASH offers the worldwide still unique capability to study ultrafast processes with high-flux, high-repetition rate extreme ultraviolet, and soft X-ray pulses. The vast majority of experiments at FLASH are of pump-probe type. Many of them rely on optical ultrafast lasers. Here, a novel FEL facility laser is reported which combines high average power output from Yb:YAG amplifiers with spectral broadening in a Herriott-type multipass cell and subsequent pulse compression to sub-100-fs durations. Compared to other facility lasers employing optical parametric amplification, the new system comes with significantly improved noise figures, compactness, simplicity, and power efficiency. Like FLASH, the optical laser operates with 10-Hz burst repetition rate. The bursts consist of 800-mu s long trains of up to 800 ultrashort pulses being synchronized to the FEL with femtosecond precision. In the experimental chamber, pulses with up to 50-mu J energy, 60-fs full-width half-maximum duration and 1-MHz rate at 1.03-mu m wavelength are available and can be adjusted by computer-control. Moreover, nonlinear polarization rotation is implemented to improve laser pulse contrast. First cross-correlation measurements with the FEL at the plane-grating monochromator photon beamline are demonstrated, exhibiting the suitability of the laser for user experiments at FLASH.

R. Steinbrügge, S. Kuehn, F. Nicastro, M. Gu, M. Togawa, M. Hoesch, J. Seltmann, I. Sergeev, F. Trinter, S. Bernitt, C. Shah, M. Leutenegger, and J. Lopez-Urrutia
X-Ray Photoabsorption of Density-sensitive Metastable States in Ne vii, Fe xxii, and Fe xxiii
Astrophysical Journal 941, 941 (2022)

Abstract: Metastable states of ions can be sufficiently populated in absorbing and emitting astrophysical media, enabling spectroscopic plasma-density diagnostics. Long-lived states appear in many isoelectronic sequences with an even number of electrons, and can be fed at large rates by various photonic and electronic mechanisms. Here, we experimentally investigate beryllium-like and carbon-like ions of neon and iron that have been predicted to exhibit detectable features in astrophysical soft X-ray absorption spectra. An ion population generated and excited by electron impact is subjected to highly monochromatic X-rays from a synchrotron beamline, allowing us to identify K alpha transitions from metastable states. We compare their energies and natural line widths with state-of-the-art theory and benchmark level population calculations at electron densities of 10(10.5) cm(-3).

S. Fritzsche
Application of Symmetry-Adapted Atomic Amplitudes
Atoms 10, 127 (2022)

Abstract: Following the work of Giulio Racah and others from the 1940s onward, the rotational symmetry of atoms and ions, e.g., the conservation of angular momentum, has been utilized in order to efficiently predict atomic behavior, from their level structure to the interaction with external fields, and up to the angular distribution and polarization of either emitted or scattered photons and electrons, while this rotational symmetry becomes apparent first of all in the block-diagonal structure of the Hamiltonian matrix, it also suggests a straight and consequent use of symmetry-adapted interaction amplitudes in expressing the observables of most atomic properties and processes. We here emphasize and discuss how atomic structure theory benefits from exploiting this symmetry, especially if open-shell atoms and ions in different charge states need to be combined with electrons in the continuum. By making use of symmetry-adapted amplitudes, a large number of excitation, ionization, recombination or even cascade processes can be formulated rather independently of the atomic shell structure and in a language close to the formal theory. The consequent use of these amplitudes in existing codes such as Grasp will therefore qualify them to deal with the recently emerging demands for developing general-purpose tools for atomic computations.

S. Strnat, J. Sommerfeldt, V. Yerokhin, W. Middents, T. Stöhlker, and A. Surzhykov
Circular Polarimetry of Hard X-rays with Rayleigh Scattering
Atoms 10, 140 (2022)

Abstract: We present a theoretical investigation of the elastic Rayleigh scattering of X-rays by atomic targets. Special attention is paid to the question of how the polarization of the scattered photons is affected if the incident light is itself polarized. In particular, we found that the circular polarization of the incoming X-rays may lead to a remarkable modification of the linear polarization of the scattered photons. Based on this \textasciigrave circular-to-linear-polarization-transfer\textquotesingle and on the fact that the linear polarization of X-rays can be conveniently observed by solid-state Compton detectors, we argue that Rayleigh scattering may be used as a tool for circular polarimetry of hard X-rays. To illustrate our proposal, we performed detailed calculations of 145 and 500 keV circularly polarized photons scattered by lead atoms. Based on these calculations, we found that the photon scattering under large angles with respect to the incident beam direction is most favorable for the circular polarimetry of hard X-rays. In particular, for 500 keV photon energy and scattering angles around 70 deg we found a remarkable modification of the linear polarization of scattered light for the case when the incident radiation is circularly polarized.

M. Lestinsky, E. Menz, H. Danared, C. Krantz, E. Lindroth, Z. Andelkovic, C. Brandau, A. Braeuning-Demian, S. Fedotova, W. Geithner, F. Herfurth, A. Kalinin, I. Kraus, U. Spillmann, G. Vorobyev, and T. Stöhlker
First Experiments with CRYRING@ESR
Atoms 10, 141 (2022)

Abstract: The low-energy heavy ion storage ring CRYRING was transported from Stockholm to Darmstadt, modernized and reconfigured, and recommissioned as CRYRING@ESR. The machine is now in operation with all installations in service and is available as a user facility for experiments proposed through the SPARC collaboration. During the 2020-2022 period, we brought a number of experimental installations into service and used them to measure first data: the ultra-cold electron cooler for merged-beam electron-ion collisions, the gas jet target for atomic collisions, a next-generation microcalorimeter-based X-ray spectroscopy setup, and others. Ions can be injected either in low charge states from a local ion source through a 300 keV/u RFQ linac, or in high charge states from the GSI accelerator chain through ESR. This allows for very broad access to ions across the entire periodic table. CRYRING@ESR is able to de- or accelerate ions and cool and store beams of isotopically pure species in a desired charge state. While the analysis is still largely ongoing, the first experimental data already show that the machine reached its expected performance level, and our high expectations regarding achievable resolution in spectroscopy experiments have been fulfilled. With access to new classes of ions available through ESR injection and a new generation of experimental instrumentation, CRYRING@ESR is a unique facility for experiments with heavy, highly charged ions. Here, we will review our present setup and machine performance, discuss the data from our first commissioning experiments and briefly preview the upcoming new installations for the coming years.

M. Jolly, S. Voikopoulos, E. Lamour, A. Mery, A. Braeuning-Demian, J.-Y. Chesnel, A. Gumberidze, M. Lestinsky, S. Mace, C. Prigent, J.-M. Ramillon, J. Rangama, P. Rousseau, D. Schury, U. Spillmann, S. Steydli, T. Stöhlker, M. Trassinelli, and D. Vernhet
Performance of a keV/u Ion Spectrometer for the FISIC Platform
Atoms 10, 146 (2022)

Abstract: The design and performances of a newly built electrostatic charge state analyzer constructed to act as a spectrometer for keV/u ions are reported. It consists of two 90 & LCIRC; curved electrodes enclosed by Matsuda electrodes. This setup was recently tested using Ar9+ and Ar12+ ion beams at an energy of 10 keV per charge unit. This spectrometer achieves a good separation of different charge states formed by electron capture processes during collisions between primary ions and the residual gas. Thanks to these first tests, we have identified up to three different background contributions on the detector that need to be reduced or suppressed.

N. Marsic, W. Mueller, V. Tympel, T. Stöhlker, M. Stapelfeld, F. Schmidl, M. Schmelz, V. Zakosarenko, R. Stolz, D. Haider, T. Sieber, M. Schwickert, and H. De Gersem
Influence of Mechanical Deformations on the Performance of a Coaxial Shield for a Cryogenic Current Comparator
IEEE Transactions on Applied Superconductivity 32, 2500409 (2022)

Abstract: This article studies the impact of mechanical deformations on the performance of a coaxial-type cryogenic current comparator (CCC). Such deformations may become a concern as the size of the CCC increases (e.g., when used as a diagnostic device in a particle accelerator facility involving beamlines with a large diameter). In addition to static deformations, this article also discusses the effect of mechanical vibrations on the CCC performance.

K. A. Janulewicz, Ł. Węgrzyński, T. Fok, A. Bartnik, H. Fiedorowicz, S. Skruszewicz, M. Wünsche, E. Eckner, S. Fuchs, J. Reinhard, J. J. Abel, F. Wiesner, G. G. Paulus, C. Rödel, C. M. Kim, and P. W. Wachulak
Broadband soft X-ray source from a clustered gas target dedicated to high-resolution XCT and X-ray absorption spectroscopy
Optics Express 30, 47867 (2022)
No abstract availableLinkBibTeX
B. Baghdasaryan, C. Sevilla-Gutierrez, F. Steinlechner, and S. Fritzsche
Generalized description of the spatio-temporal biphoton state in spontaneous parametric down-conversion
Physical Review A 106, 063711 (2022)

Abstract: Spontaneous parametric down-conversion (SPDC) is a widely used source for photonic entanglement. Years of focused research have led to a solid understanding of the process, but a cohesive analytical description of the paraxial biphoton state has yet to be achieved. We derive a general expression for the spatio-temporal biphoton state that applies universally across common experimental settings and correctly describes the nonseparability of spatial and spectral modes. We formulate a criterion on how to decrease the coupling of the spatial from the spectral degree of freedom by taking into account the Gouy phase of interacting beams. This work provides new insights into the role of the Gouy phase in SPDC, and also into the preparation of engineered entangled states for multidimensional quantum information processing.

B. Baghdasaryan, F. Steinlechner, and S. Fritzsche
Maximizing the validity of the Gaussian approximation for the biphoton state from parametric down-conversion
Physical Review A 106, 063714 (2022)

Abstract: Spontaneous parametric down-conversion (SPDC) is widely used in quantum applications based on photonic entanglement. The efficiency of photon pair generation is often characterized by means of a sinc(L delta k/2) function, where L is the length of the nonlinear medium and delta k is the phase mismatch between the pump and down-converted fields. In theoretical investigations, the sinc behavior of the phase mismatch has often been approximated by a Gaussian function exp (-alpha x(2)) in order to derive analytical expressions for the SPDC process. Different values have been chosen in the literature for the optimization factor alpha, for instance, by comparing the widths of sinc and Gaussian functions or the momentum of down-converted photons. As a consequence, different values of alpha provide different theoretical predictions for the same setup. Therefore an informed and unique choice of this parameter is necessary. In this paper, we present a choice of alpha which maximizes the validity of the Gaussian approximation. Moreover, we also discuss the so-called super-Gaussian and cosine-Gaussian approximations as practical alternatives with improved predictive power for experiments.

H. Gies, F. Karbstein, and L. Klar
All-optical quantum vacuum signals in two-beam collisions
Physical Review D 106, 116005 (2022)

Abstract: We study the collision of two optical laser pulses in a pump-probe setup using beams with circular and elliptic cross section and estimate the number of discernible signal photons induced by quantum vacuum nonlinearities. In this analysis we study strategies to optimize the quantum vacuum signal discernible from the background of the driving lasers. One of the main results is that the collision of two maximally focused lasers does not lead to the best discernible signal. Instead, widening the focus typically improves the signal to background separation in the far field. For petawatt class lasers, an optimal choice of the focus waist yields several discernible photons per shot in contrast to no discernible signal for tight focusing. Further enhancement is possible by using an elliptical waist.

H. Gies, D. Gkiatas, and L. Zambelli
Background effective action with nonlinear massive gauge fixing
Physical Review D 106, 116013 (2022)

Abstract: We combine a recent construction of a Becchi-Rouet-Stora-Tyutin (BRST)-invariant, nonlinear massive gauge fixing with the background field formalism. The resulting generating functional preserves background -field invariance as well as BRST invariance of the quantum field manifestly. The construction features BRST-invariant mass parameters for the quantum gauge and ghost fields. The formalism employs a background Nakanishi-Lautrup field which is part of the nonlinear gauge-fixing sector and thus should not affect observables. We verify this expectation by computing the one-loop effective action and the beta function of the gauge coupling as an example. The corresponding Schwinger functional generating connected correlation functions acquires additional one-particle reducible terms that vanish on shell. We also study off-shell one -loop contributions in order to explore the consequences of a nonlinear gauge fixing scheme involving a background Nakanishi-Lautrup field. As an application, we show that our formalism straightforwardly accommodates nonperturbative information about propagators in the Landau gauge in the form of the so-called decoupling solution. Using this nonperturbative input, we find evidence for the formation of a gluon condensate for sufficiently large coupling, whose scale is set by the BRST-invariant gluon mass parameter.

B. Liu, M. Shi, M. Zepf, B. Lei, and D. Seipt
Accelerating Ions by Crossing Two Ultraintense Lasers in a Near-Critical Relativistically Transparent Plasma
Physical Review Letters 129, 274801 (2022)

Abstract: A new scheme of ion acceleration by crossing two ultraintense laser pulses in a near-critical relativistically transparent plasma is proposed. One laser, acting as a trigger, preaccelerates background ions in its radial direction via the laser-driven shock. Another crossed laser drives a comoving snowplow field which traps some of the preaccelerated ions and then efficiently accelerates them to high energies up to a few giga-electron-volts. The final output ion beam is collimated and quasimonoenergetic due to a momentum-selection mechanism. Particle-in-cell simulations and theoretical analysis show that the scheme is feasible and robust.

S. Kühn, C. Cheung, N. S. Oreshkina, R. Steinbrügge, M. Togawa, S. Bernitt, L. Berger, J. Buck, M. Hoesch, J. Seltmann, F. Trinter, C. H. Keitel, M. G. Kozlov, S. G. Porsev, M. F. Gu, F. S. Porter, T. Pfeifer, M. A. Leutenegger, Z. Harman, M. S. Safronova, J. R. C. López-Urrutia, and C. Shah
New Measurement Resolves Key Astrophysical Fe XVII Oscillator Strength Problem
Physical Review Letters 129, 245001 (2022)
No abstract availableLinkBibTeX
K. S. Schulze
Determination of the reflectivity of crystals by ptychography
AIP Advances 12, 125219 (2022)
No abstract availableLinkBibTeX
R. Weissenbilder, S. Carlstrom, L. Rego, C. Guo, C. M. Heyl, P. Smorenburg, E. Constant, C. L. Arnold, and A. L'Huillier
How to optimize high-order harmonic generation in gases
Nature Reviews Physics 4, 713 (2022)

Abstract: High-order harmonic generation (HHG) in gases leads to short-pulse extreme ultraviolet (XUV) radiation that is useful in a number of applications, such as attosecond science and nanoscale imaging. However, this process depends on many parameters, and there is still no consensus on how to choose the target geometry to optimize the source efficiency. We review the physics of HHG with emphasis on the macroscopic aspects of the nonlinear interaction, discussing the influence of length of medium, pressure, and intensity of the driving laser on the HHG conversion efficiency. Efficient HHG can be realized over a large range of pressures and medium lengths, if these follow a certain hyperbolic equation. This explains the large versatility in gas target designs for efficient HHG and provides design guidance for future high-flux XUV sources.

M. Lenski, T. Heuermann, M. Gebhardt, Z. Wang, C. Gaida, C. Jauregui, and J. Limpert
Inband-pumped, high-power thulium-doped fiber amplifiers for an ultrafast pulsed operation
Optics Express 30, 44270 (2022)

Abstract: We investigate the influence of the pump wavelength on the high-power amplification of large-mode area, thulium-doped fibers which are suitable for an ultrashort pulsed operation in the 2 mu m wavelength region. By pumping a standard, commercially available photonic crystal fiber in an amplifier configuration at 1692 nm, a slope efficiency of 80 % at an average output power of 60 W could be shown. With the help of simulations we investigate the effect of cross-relaxations on the efficiency and the thermal behavior. We extend our investigations to a rod-type, large-pitch fiber with very large mode area, which is exceptionally suited for high-energy ultrafast operation. Pumping at 1692 nm leads to a slope efficiency of 74% with a average output power of 67 W, instead of the 38 % slope efficiency obtained when pumping at 793 nm. These results pave the way to highly efficient 2 mu m fiber-based CPA systems.

P. Pfäfflein, S. Allgeier, S. Bernitt, A. Fleischmann, M. Friedrich, C. Hahn, D. Hengstler, M. O. Herdrich, A. Kalinin, F. M. Kröger, P. Kuntz, M. Lestinsky, B. Loeher, E. B. Menz, T. Over, U. Spillmann, G. Weber, B. Zhu, C. Enss, and T. Stöhlker
Integration of maXs-type microcalorimeter detectors for high-resolution x-ray spectroscopy into the experimental environment at the CRYRING@ESR electron cooler
Physica Scripta 97, 114005 (2022)

Abstract: We report on the first integration of novel magnetic microcalorimeter detectors (MMCs), developed within SPARC (Stored Particles Atomic Physics Research Collaboration), into the experimental environment of storage rings at GSI(6), Darmstadt, namely at the electron cooler of CRYRING@ESR. Two of these detector systems were positioned at the 0 degrees and 180 degrees view ports of the cooler section to obtain high-resolution x-ray spectra originating from a stored beam of hydrogen-like uranium interacting with the cooler electrons. While previous test measurements with microcalorimeters at the accelerator facility of GSI were conducted in the mode of well-established stand-alone operation, for the present experiment we implemented several notable modifications to exploit the full potential of this type of detector for precision x-ray spectroscopy of stored heavy ions. Among these are a new readout system compatible with the multi branch system data acquisition platform of GSI, the synchronization of a quasi-continuous energy calibration with the operation cycle of the accelerator facility, as well as the first exploitation of the maXs detectors time resolution to apply coincidence conditions for the detection of photons and charge-changed ions.

B. Minneker, B. Boening, and S. Fritzsche
Generalized nondipole strong-field approximation of high-order harmonic generation
Physical Review A 106, 053109 (2022)

Abstract: The strong-field approximation (SFA) is a widely used theoretical framework that describes the process of high-order harmonic generation of atoms and molecules. Here, we propose a generalization of the dipole SFA towards weakly relativistic contributions to the laser-electron interaction. These weakly relativistic contributions are closely related to the spatial structure of the light field and imply a correction of the relativistic order 1/c. Within this generalized nondipole SFA (GN-SFA), we demonstrate how to obtain explicit results and discuss their physical aspects. This approach enables one to investigate the nondipole effects of linear polarized plane waves as well as the influence of structured light fields, such as twisted light, that have not yet been captured by the currently available models. In order to utilize our generalized nondipole SFA, we consider a linearly polarized plane wave and demonstrate the decrease of the harmonic yield that is directly related to the nondipole effects of the laser field. Furthermore, we discuss the GN-SFA with regard to other nondipole SFA approaches by determining their physical and mathematical context. Finally, the GN-SFA is a powerful theoretical framework that extends the nonrelativistic SFA rigorously to the weakly relativistic regime and therefore will be a useful model for further theoretical investigations.

M. Metternich, H. Nazary, D. Schumacher, C. Brabetz, F. Kroll, F.-E. Brack, M. Ehret, A. Blazevic, U. Schramm, V. Bagnoud, and M. Roth
Analyzing the filamentation of MeV-range proton bunches in a laser-driven ion beamline and optimizing their peak intensity
Physical Review Accelerators and Beams 25, 111301 (2022)

Abstract: In this article, we report on the latest investigations and achievements in proton beam shaping with our laser-driven ion beamline at GSI Helmholtzzentrum fiir Schwerionenforschung GmbH. This beamline was realized within the framework of the Laser Ion Generation, Handling, and Transport (LIGHT) collaboration to study the combination of laser-driven ion beams with conventional accelerator components. At its current state, the ions are accelerated by the high-power laser PHELIX via target normal sheath acceleration, and two pulsed high-magnetic solenoids are used for energy selection, transport, and transverse focusing. In between the two solenoids, there is a rf cavity that gives the LIGHT beamline the capability to longitudinally manipulate and temporally compress ion bunches to sub-nanosecond durations. To get optimal results, the rf cavity has to be synchronized with the PHELIX laser and therefore a reliable measurement of the temporal ion beam profile is necessary. In the past, these measurements showed unexpected correlations between the temporal beam profile and the phase as well as the electric field strength of the cavity. In this article, we present a numerical simulation of the beam transport through the LIGHT beamline which explains this behavior by a beam filamentation. We also report on our latest experimental campaigns, in which we combined transverse and longitudinal focusing for the first time. This led to proton bunches with a peak intensity of (3.28 +/- 0.24) x 10(8) protons/ (ns mm(2)) at a central energy of (7.72 +/- 0.14) MeV. The intensity refers to a circle with a diameter of (1.38 +/- 0.02) mm that encloses 50% of the protons in the focal spot at the end of the beamline. The temporal bunch width at this position was (742 +/- 40) ps (FWHM).

Ş. Zähter, O. N. Rosmej, P. Beloiu, A. Bogdanov, A. Golubev, M. Gyrdymov, J. Jacoby, A. Kantsyrev, R. Loetzsch, M. Nicolai, V. Panyushkin, A. Skobliakov, P. M. Tavana, I. Uschmann, N. Zahn, and C. Spielmann
Monitoring of the heavy-ion beam distribution using poly- and monochromatic x-ray fluorescence imaging
Review of Scientific Instruments 93, 113301 (2022)

Abstract: In this work, the first proof of the principal of an in situ diagnostics of the heavy-ion beam intensity distribution in irradiation of solid targets is proposed. In this scheme, x-ray fluorescence that occurs in the interaction of heavy-ions with target atoms is used for imaging purposes. The x-ray conversion to optical radiation and a transport-system was developed, and its first test was performed in experiments at the Universal Linear Accelerator in Darmstadt, Germany. The Au-beam intensity distribution on thin foils and Cu-mesh targets was imaged using multiple x-ray pinholes (polychromatic imaging) and 2D monochromatic imaging of Cu Kα radiation by using a toroidally bent silicon crystal. The presented results are of importance for application in experiments on the investigation of the equation of states of high energy density matter
using high intensity GeV/u heavy-ion beams of ≥10^10 particles/100 ns.

D. Schwickert, M. Ruberti, P. Kolorenc, A. Przystawik, S. Skruszewicz, M. Sumfleth, M. Braune, L. Bocklage, L. Carretero, M. Czwalinna, D. Diaman, S. Duesterer, M. Kuhlmann, S. Palutke, R. Röhlsberger, J. Roensch-Schulenburg, S. Toleikis, S. Usenko, J. Viefhaus, A. Vorobiov, M. Martins, D. Kip, V. Averbukh, J. Marangos, and T. Laarmann
Charge-induced chemical dynamics in glycine probed with time-resolved Auger electron spectroscopy
Structural Dynamics 9, 064301 (2022)

Abstract: In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of (-0.3 +/- 0.1 ) pi <= phi <= ( 0.1 +/- 0.2 ) pi and 18.2(-1.4)(+1.7) <= T <= 23.9(-1.1)(+1.2 )fs. Furthermore, for long delays, the data allow us to pinpoint the driving vibrational modes of chemical dynamics mediating charge-induced bond cleavage along different reaction coordinates. (C) 2022 Author(s).

S. Fritzsche, and J. Hofbrucker
Biharmonic (omega, 2 omega) ionization of atoms by elliptically-polarized light. Carving the photoelectron angular distributions
New Journal of Physics 24, 103031 (2022)

Abstract: The biharmonic (omega, 2 omega) photoionization of atomic inner-shell electrons opens up new perspectives for studying nonlinear light-atom interactions at intensities in the transition regime from weak to strong-field physics. In particular, the control of the frequency and polarization of biharmonic beams enables one to carve the photoelectron angular distribution and to enhance the resolution of ionization measurements by the (simultaneous) absorption of photons. Apart from its quite obvious polarization dependence, the photoelectron angular distributions are sensitive also to the (relative) intensity, the phase difference and the temporal structure of the incoming beam components, both at resonant and nonresonant frequencies. Here, we describe and analyze several characteristic features of biharmonic ionization in the framework of second-order perturbation theory and (so-called) ionization pathways, as they are readily derived from the interaction of inner-shell electrons with the electric-dipole field of the incident beam. We show how the photoelectron angular distribution and elliptical dichroism can be shaped in rather an unprecedented way by just tuning the properties of the biharmonic field. Since such fields are nowadays accessible from high-harmonic sources or free-electron lasers, these and further investigations might help extract photoionization amplitudes or the phase difference of incoming beams.

V. Shumakova, V. Pecile, J. Fellinger, M. Leskowschek, P. Aldia, A. Mayer, L. Perner, S. Salman, M. Fan, P. Balla, S. Schilt, C. Heyl, I. Hartl, G. Porat, and O. Heckl
Spectrally tunable high-power Yb:fiber chirped-pulse amplifier
Photonics Research 10, 2309 (2022)

Abstract: Tailoring the properties of the driving laser to the need of applications often requires compromises among laser stability, high peak and average power levels, pulse duration, and spectral bandwidth. For instance, spectroscopy with optical frequency combs in the extreme/visible ultraviolet spectral region requires a high peak power of the near-IR driving laser, and therefore high average power, pulse duration of a few tens of fs, and maximal available spectral bandwidth. Contrarily, the parametric conversion efficiency is higher for pulses with a duration in the 100-fs range due to temporal walk-off and coating limitations. Here we suggest an approach to adjust the spectral characteristics of high-power chirped-pulse amplification (CPA) to the requirements of different nonlinear frequency converters while preserving the low-phase-noise (PN) properties of the system. To achieve spectral tunability, we installed a mechanical spectral shaper in a free-space section of the stretcher of an in-house-developed ytterbium-fiber-based CPA system. The CPA system delivers 100 W of average power at a repetition rate of 132.4 MHz. While gaining control over the spectral properties, we preserve the relative-intensity-noise and PN properties of the system. The high-power CPA can easily be adjusted to deliver either a spectrum ideal for mid-IR light generation (full width at half maximum of similar to 11 nm, compressed pulse duration of 230 fs) or a spectrum ideal for highly nonlinear processes such as high-harmonic generation (-10 dB level of >50 nm, transform-limited pulse duration of similar to 65 fs).

F. Liu, S. Li, Z. Chen, B. Böning, and S. Fritzsche
Nonsequential double ionization of Ne with elliptically polarized laser pulses
Physical Review A 106, 043120 (2022)

Abstract: We show through simulation that the improved quantitative rescattering model (QRS) can successfully predict the nonsequential double ionization (NSDI) process by intense elliptically polarized laser pulses. Using the QRS model, we calculate the correlated two-electron and ion momentum distributions of NSDI in Ne exposed to intense elliptically polarized laser pulses with a wavelength of 788 nm at a peak intensity of 5.0 x 10(14) W/cm(2). We analyze the asymmetry in the doubly charged ion momentum spectra observed by Kang et al. [Phys. Rev. Lett. 120, 223204 (2018)] in going from linearly to elliptically polarized laser pulses. Our model reproduces the experimental data well. Furthermore, we find that the ellipticity-dependent asymmetry arises from the drift velocity along the minor axis of the elliptic polarization. We explain how the correlated electron momentum distributions along the minor axis provide access to the subcycle dynamics of recollision. From these findings, we expect that we can extend the QRS model for NSDI toward more complicated laser fields in the future.

B. Böning, and S. Fritzsche
Steering the longitudinal photoelectron momentum in the above-threshold ionization with two not-quite-collinear laser beams
Physical Review A 106, 043102 (2022)

Abstract: Strong-field atomic processes, driven by long-wavelength laser beams, are known to be affected by magnetic forces. In such beams, the Lorentz force pushes the photoelectrons along the beam direction and prevents their rescattering or recombination with the parent ions. In high-order harmonic generation (HHG), therefore, the yield of energetic photons is markedly suppressed, rendering x-ray radiation sources from high harmonics so far impractical. To compensate these magnetic forces and to reenable HHG at long wavelengths, a setup of two not quite collinear beams has been suggested recently but not much analyzed beyond classical arguments and with respect to accessible laser parameters. Using the nondipole strong-field approximation, we here investigate when the longitudinal momentum of the photoelectrons vanishes and how this noncollinear setup explicitly depends on the wavelength and intensity of the driving beams. We also demonstrate that an optimal crossing angle delta 0 between these beams always exists for which the fraction of the returning electrons is maximized. This rather simple steering of the longitudinal momentum will allow an efficient HHG with driving beams deep in the midinfrared.

S. Fritzsche, and B. Boening
Strong-Field Ionization Amplitudes for Atomic Many-Electron Targets
Atoms 10, 70 (2022)

Abstract: The strong-field approximation (SFA) has been widely applied in the literature to model the ionization of atoms and molecules by intense laser pulses. A recent re-formulation of the SFA in terms of partial waves and spherical tensor operators helped adopt this approach to account for realistic atomic potentials and pulses of different shape and time structure. This re-formulation also enables one to overcome certain limitations of the original SFA formulation with regard to the representation of the initial-bound and final-continuum wave functions of the emitted electrons. We here show within the framework of JAC, the Jena Atomic Calculator, how the direct SFA ionization amplitude can be readily generated and utilized in order to compute above-threshold ionization (ATI) distributions for many-electron targets and laser pulses of given frequency, intensity, polarization, pulse duration and carrier-envelope phase. Examples are shown for selected ATI energy, angular as well as momentum distributions in the strong-field ionization of atomic krypton. We also briefly discuss how this approach can be extended to incorporate rescattering and high-harmonic processes into the SFA amplitudes.

L. Hoffmann, S. Jamnuch, C. Schwartz, T. Helk, S. Raj, H. Mizuno, R. Mincigrucci, L. Foglia, E. Principi, R. Saykally, W. Drisdell, S. Fatehi, T. Pascal, and M. Zuerch
Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge
Journal of Physical Chemistry Letters 13, 8963 (2022)

Abstract: The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm(2)). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

J. Abel, F. Wiesner, J. Nathanael, J. Reinhard, M. Wünsche, G. Schmidl, A. Gawlik, U. Huebner, J. Plentz, C. Rödel, G. Paulus, and S. Fuchs
Absolute EUV reflectivity measurements using a broadband high-harmonic source and an in situ single exposure reference scheme
Optics Express 30, 35671 (2022)

Abstract: We present a tabletop setup for extreme ultraviolet (EUV) reflection spectroscopy in the spectral range from 40 to 100 eV by using high-harmonic radiation. The simultaneous measurements of reference and sample spectra with high energy resolution provide precise and robust absolute reflectivity measurements, even when operating with spectrally fluctuating EUV sources. The stability and sensitivity of EUV reflectivity measurements are crucial factors for many applications in attosecond science, EUV spectroscopy, and nano-scale tomography. We show that the accuracy and stability of our in situ referencing scheme are almost one order of magnitude better in comparison to subsequent reference measurements. We demonstrate the performance of the setup by reflective near-edge x-ray absorption fine structure measurements of the aluminum L2/3 absorption edge in alpha-Al2O3 and compare the results to synchrotron measurements.

J. Wang, M. Zepf, Y. Leng, R. Li, and S. Rykovanov
Self-torqued harmonics and attosecond pulses driven by time-delayed relativistic vortex lasers
Physical Review A 106, 033515 (2022)

Abstract: Light carrying time-varying orbital angular momentum (OAM) is a recently discovered type of structured electromagnetic field compared with a typical vortex field whose OAM is static. Such so-called self-torqued light is employed for manipulating the fast magnetic, topological, and quantum excitations and increasing its intensity and having access to shorter pulse durations would be of great benefit. Here we theoretically and numerically demonstrate the generation of intense self-torqued harmonics and attosecond pulses in the relativistic regime, driven by two time-delayed relativistic vortex lasers with different OAMs l1 and l2. The OAM of the nth harmonic spans nl1 to nl2, and the OAM of the attosecond pulses changes from l1 to l2. Such intense self-torqued harmonics and attosecond pulses may offer alternative possibilities in ultrafast spectroscopy.

L. Yue, R. Hollinger, C. Uzundal, B. Nebgen, Z. Gan, E. Najafidehaghani, A. George, C. Spielmann, D. Kartashov, A. Turchanin, D. Qiu, M. Gaarde, and M. Zuerch
Signatures of Multiband Effects in High-Harmonic Generation in Monolayer MoS2 br
Physical Review Letters 129, 147401 (2022)

Abstract: High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of highorder harmonics generated in monolayer MoS2, as a function of crystal orientation relative to the midinfrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multiband recombination dipoles.

S. Röder, Y. Zobus, C. Brabetz, and V. Bagnoud
How the laser beam size conditions the temporal contrast in pulse stretchers of chirped-pulse amplification lasers
High Power Laser Science and Engineering 10, 34 (2022)

Abstract: In this work, we propose and verify experimentally a model that describes the concomitant influence of the beam size and optical roughness on the temporal contrast of optical pulses passing through a pulse stretcher in chirped-pulse amplification laser systems. We develop an analytical model that is capable of predicting the rising edge caused by the reflection from an optical element in a pulse stretcher, based on the power spectral density of the surface and the spatial beam profile on the surface. In an experimental campaign, we characterize the temporal contrast of a laser pulse that passed through either a folded or an unfolded stretcher design and compare these results with the analytical model. By varying the beam size for both setups, we verify that optical elements in the near- and the far-field act opposed to each with respect to the temporal contrast and that the rising edge caused by a surface benefits from a larger spatial beam size on that surface.

F. Wiesner, S. Skruszewicz, C. Rödel, J. Abel, J. Reinhard, M. Wünsche, J. Nathanael, M. Grünewald, U. Hübner, G. Paulus, and S. Fuchs
Characterization of encapsulated graphene layers using extreme ultraviolet coherence tomography
Optics Express 30, 32267 (2022)

Abstract: Many applications of two-dimensional materials such as graphene require the encapsulation in bulk material. While a variety of methods exist for the structural and functional characterization of uncovered 2D materials, there is a need for methods that image encapsulated 2D materials as well as the surrounding matter. In this work, we use extreme ultraviolet coherence tomography to image graphene flakes buried beneath 200 nm of silicon. We show that we can identify mono-, bi-, and trilayers of graphene and quantify the thickness of the silicon bulk on top by measuring the depth-resolved reflectivity. Furthermore, we estimate the quality of the graphene interface by incorporating a model that includes the interface roughness. These results are verified by atomic force microscopy and prove that extreme ultraviolet coherence tomography is a suitable tool for imaging 2D materials embedded in bulk materials.

F. Karbstein, D. Ullmann, E. Mosman, and M. Zepf
Direct Accessibility of the Fundamental Constants Governing Light-by-Light Scattering
Physical Review Letters 129, 061802 (2022)

Abstract: Quantum field theory predicts that the vacuum exhibits a nonlinear response to strong electromagnetic fields. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. We present proof of concept and detailed theoretical analysis of an experimental setup for precision measurements of the quantum vacuum signal generated by the collision of a brilliant x-ray probe with a high-intensity pump laser. The signal features components polarized parallel and perpendicularly to the incident x-ray probe. Our proof-of-concept measurements show that the background can be efficiently suppressed by many orders of magnitude which should not only facilitate a detection of the perpendicularly polarized component of the nonlinear vacuum response, but even make the parallel polarized component experimentally accessible for the first time. Remarkably, the angular separation of the signal from the intense x-ray probe enables precision measurements even in the presence of pump fluctuations and alignment jitter. This provides direct access to the low-energy constants governing light-by-light scattering.

H. Stark, C. Grebing, J. Buldt, A. Klenke, and J. Limpert
Divided-pulse nonlinear compression in a multipass cell
Journal of Physics-Photonics 4, 035001 (2022)

Abstract: The pulse-energy and peak-power limitations of a gas-filled multipass cell (MPC) for nonlinear pulse compression are surpassed by applying a burst of four temporally separated pulses instead of a single one. The burst is generated by two birefringent crystals and contains 1 mJ of energy per pulse replica. It is then spectrally broadened in an Argon-filled MPC and recombined into a single pulse by a second set of birefringent crystals. The combined pulse is compressed by chirped mirrors to a pulse duration of 32 fs and a pulse energy of 3.4 mJ. An excellent passive stability and a high system efficiency of >90% are reached. Using the 4-pulse burst, the overall output energy supported by the MPC is doubled in comparison to single-pulse operation.

E. Escoto, A.-L. Viotti, S. Alisauskas, H. Tunnermann, I. Hartl, and C. Heyl
Temporal quality of post-compressed pulses at large compression factors
Journal of the Optical Society of America B 39, 1694 (2022)

Abstract: Post-compression of ultra-short laser pulses via self-phase modulation is routinely employed for the generation of laser pulses with optical bandwidths reaching far beyond the laser gain limitations. Although high compression factors can be routinely achieved, the compressed pulses typically suffer from temporal quality degradation. We numerically and experimentally analyze the deterioration of different measures of temporal quality with increasing compression factor and show how appropriate dispersion management and cascading of the post-compression process can be employed to limit the impact of this effect. The demonstrated saturation of pulse quality degradation at large compression factors puts novel femtosecond laser architectures based on post-compressed picosecond or even nanosecond laser systems in sight.

P. Gierschke, C. Grebing, M. Abdelaal, M. Lenski, J. Buldt, Z. Wang, T. Heuermann, M. Mueller, M. Gebhardt, J. Rothhardt, and J. Limpert
Nonlinear pulse compression to 51-W average power GW-class 35-fs pulses at 2-mu m wavelength in a gas-filled multi-pass cell
Optics Letters 47, 3511 (2022)

Abstract: We report on the generation of GW-class peak power, 35-fs pulses at 2-mu m wavelength with an average power of 51 W at 300-kHz repetition rate. A compact, krypton-filled Herriott-type cavity employing metallic mirrors is used for spectral broadening. This multi-pass compression stage enables the efficient post compression of the pulses emitted by an ultrafast coherently combined thulium-doped fiber laser system. The presented results demonstrate an excellent preservation of the input beam quality in combination with a power transmission as high as 80%. These results show that multi-pass cell based post-compression is an attractive alternative to nonlinear spectral broadening in fibers, which is commonly employed for thulium-doped and other mid-infrared ultra-fast laser systems. Particularly, the average power scalability and the potential to achieve few-cycle pulse durations make this scheme highly attractive. (C) 2022 Optica Publishing Group

S. Ringleb, M. Kiffer, N. Stallkamp, S. Kumar, J. Hofbrucker, B. Reich, B. Arndt, G. Brenner, M. Ruiz-Lopéz, S. Düsterer, M. Vogel, K. Tiedtke, W. Quint, T. Stöhlker, and G. G. Paulus
High-intensity laser experiments with highly charged ions in a Penning trap
Physica Scripta 97, 084002 (2022)

Abstract: We have conceived and built the HILITE (High-Intensity Laser-Ion Trap Experiment) Penning-trap setup for the production, confinement and preparation of pure ensembles of highly charged ions in a defined quantum state as a target for various high-intensity lasers. This enables a broad suite of laser-ion interaction studies at high photon energies and/or intensities, such as non-linear photo-ionisation studies. The setup has now been used to perform experiments at one such laser facility, namely the FLASH Free-Electron Laser at DESY in Hamburg, Germany. We describe the experimental possibilities of the apparatus, the results of the first measurements and future experiments at other laser facilities.

Y. Hikosaka, and S. Fritzsche
Coster-Kronig and super Coster-Kronig transitions from the Xe 4s core-hole state
Physical Chemistry Chemical Physics 24, 17535 (2022)

Abstract: Coster-Kronig and super Coster-Kronig transitions from the Xe 4s core-hole state are investigated by coincidence detection of all the emitted electrons and product ions. The branching ratios of the transitions are determined by analyzing the coincidence data and comparing them to calculations. Subsequent decay pathways following these first-step Auger decays are also clarified.

S. Schippers, A. Hamann, A. Perry-Sassmannshausen, T. Buhr, A. Muller, M. Martins, S. Reinwardt, F. Trinter, and S. Fritzsche
Multiple photodetachment of oxygen anions via K-shell excitation and ionization: Direct double-detachment processes and subsequent deexcitation cascades
Physical Review A 106, 013114 (2022)

Abstract: Experimental cross sections for m-fold photodetachment (m = 2-5) of oxygen anions via K-shell excitation and ionization were measured in the photon-energy range of 525-1500 eV using the photon-ion merged-beams technique at a synchrotron light source. The measured cross sections exhibit clear signatures of direct double detachment, including double K-hole creation. The shapes of the double-detachment cross sections as a function of photon energy are in accord with Pattard s [J. Phys. B 35, L207 (2002)] empirical scaling law. We have also followed the complex de-excitation cascades that evolve subsequently to the initial double-detachment events by systematic large-scale cascade calculations. The resulting theoretical product charge-state distributions are in good agreement with the experimental findings.

J. Hofbrucker, S. Ramakrishna, A. V. Volotka, and S. Fritzsche
Polarization effects in the total rate of biharmonic omega+3 omega ionization of atoms
Physical Review A 106, 013118 (2022)

Abstract: The total ionization rate of biharmonic (omega + 3 omega) ionization is studied within the independent particle approximation and the third-order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counter-rotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between omega and 3 omega light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon L shell electrons.

R. N. Soguel, A. V. Volotka, and S. Fritzsche
QED approach to valence-hole excitation in closed-shell systems
Physical Review A 106, 012802 (2022)

Abstract: An ab initio QED approach to treat a valence-hole excitation in closed-shell systems is developed in the framework of the two-time Greens-function method. The derivation considers a redefinition of the vacuum state and its excitation as a valence-hole pair. The proper two-time Greens function, whose spectral representation confirms the poles at valence-hole excitation energies, is proposed. An contour integral formula which connects the energy corrections and the Greens function is also presented. First-order corrections to the valence-hole excitation energy involving self-energy, vacuum polarization, and one-photon-exchange terms are explicitly derived in the redefined vacuum picture. Reduction to the usual vacuum electron propagators is shown, which agrees in the Breit approximation with the many-body perturbation theory expressions for the valence-hole excitation energy.

S. Fritzsche, and B. Böning
Lorentz-force shifts in strong-field ionization with mid-infrared laser fields
Physical Review Research 4, 033031 (2022)

Abstract: In the past, the ionization of atoms and molecules by strong, mid-infrared (IR) laser fields has attracted recurrent interest. Measurements with different IR pulses have demonstrated the crucial role of the magnetic field on the electron dynamics, classically known as the Lorentz force F-L = q (epsilon + v x B), that acts upon all particles with charge q in motion. These measurements also require the advancement of theory beyond the presently applied methods. In particular, the strong-field approximation (SFA) is typically based on the dipole approximation alone and neglects both the magnetic field and the spatial dependence of the driving electric field. Here we show and discuss that several, if not most, observations from strong-field ionization experiments with mid-IR fields can be quantitatively explained within the framework of SFA, if the Lorentz force is taken into account by nondipole Volkov states in the formalism. The details of such a treatment are analyzed for the (peak) shifts of the polar-angle distribution of above-threshold ionization photoelectrons along the laser propagation, the steering of electron momenta by two not quite collinear laser beams, or the enhanced momentum transfer to photoelectrons in standing-light fields. Moreover, the same formalism promises to explain the generation of high harmonics and other strong-field rescattering phenomena when driven by mid-IR laser fields. All these results show how strong-field processes can be understood on equal footings within the SFA, if one goes beyond the commonly applied dipole approximation.

D. Erb, J. Perlich, V. Roth, R. Röhlsberger, and K. Schlage
Real-Time Observation of Temperature-Induced Surface Nanofaceting in M-Plane alpha-Al2O3
ACS Applied Materials & Interfaces 14, 31373 (2022)

Abstract: The spontaneous crystal surface reconstruction of M-plane alpha-Al2O3 is employed for nanopatterning and nanofabrication in various fields of research including, among others, magnetism, superconductivity, and optoelectronics. In this reconstruction process the crystalline surface transforms from a planar morphology to one with a nanoscale ripple patterning. However, the high sample temperature required to induce surface reconstruction made in situ studies of the process seem unfeasible. The kinetics of ripple pattern formation therefore remained uncertain, and thus production of templates for nanofabrication could not advance beyond a trial-and-error stage. We present an approach combining in situ real-time grazing incidence small-angle X-ray scattering experiments (GISAXS) with model-based analysis and with ex situ atomic force microscopy (AFM) to observe this morphological transition in great detail. Our approach provides time-resolved information about all relevant morphological parameters required to trace the surface topography on the nanometer scale during reconstruction, i.e., the time dependence of the pattern wavelength, the ripple length, width, and height, and thus their facet angles. It offers a comprehensive picture of this process exemplified by a M-plane alpha-Al2O3 surface annealed at 1325 degrees C for 930 min. Fitting the model parameters to the experimental GISAXS data revealed a Johnson-Mehl-Avrami-Kolmogorov type of behavior for the pattern wavelength and a predominantly linear time dependence of the other parameters. In this case the reconstruction resulted in a crystalline surface fully patterned with asymmetric ripple-shaped nanostructures of 75 nm periodicity, 15 nm in height, and 630 nm in length. By elucidating the time dependence of these morphological parameters, this study shows a powerful way to significantly advance the predictability of annealing outcome and thus to efficiently customize nanopatterned alpha-Al2O3 templates for improved nanofabrication routines.

S. Schippers, S. Stock, T. Buhr, A. Perry-Sassmannshausen, S. Reinwardt, M. Martins, A. Mueller, and S. Fritzsche
Near K-edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions
Astrophysical Journal 931, 100 (2022)

Abstract: Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si+ and Si2+ ions and for double photoionization of Si3+ ions by a single photon. The experiments employed the photon-ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835-1900 eV comprises resonances associated with the excitation of a 1s electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole lifetime of K-shell ionized neutral silicon is inferred. In addition, theoretical cross sections for photoabsorption and multiple photoionization were obtained from large-scale multiconfiguration Dirac-Hartree-Fock calculations. The present calculations agree with the experiment much better than previously published theoretical results. The importance of an accurate energy calibration of laboratory data is pointed out. The present benchmark results are particularly useful for discriminating between silicon absorption in the gaseous and in the solid component (dust grains) of the interstellar medium.