# Referierte Publikationen

## 2020

C. Jauregui, C. Stihler, and J. Limpert
Transverse mode instability
Advances in Optics and Photonics 12, 429 (2020)

Abstract: This work presents a review on the effect of transverse mode instability in highpower fiber laser systems and the corresponding investigations led worldwide over the past decade. This paper includes a description of the experimental observations and the physical origin of this effect, as well as some of the proposed mitigation strategies.

J. Buldt, M. Mueller, H. Stark, C. Jauregui, and J. Limpert
Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power
Applied Physics B 126, 2 (2020)

Abstract: We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

J. Deprince, M. Bautista, S. Fritzsche, J. García, T. Kallman, C. Mendoza, P. Palmeri, and P. Quinet
Plasma-environment effects on K lines of astrophysical interest: III. IPs, K thresholds, radiative rates, and Auger widths in Fe IX - Fe XVI
Astronomy and Astrophysics 635, A70 (2020)

Abstract: Aims. In the context of black-hole accretion disks, we aim to compute the plasma-environment effects on the atomic parameters used to model the decay of K-vacancy states in moderately charged iron ions, namely Fe IX - Fe XVI. Methods. We used the fully relativistic multiconfiguration Dirac-Fock method approximating the plasma electron-nucleus and electron-electron screenings with a time-averaged Debye-Hückel potential. Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 105-107 K and 1018-1022 cm-3. Conclusions. This study confirms that the high-resolution X-ray spectrometers onboard the future XRISM and Athena space missions will be capable of detecting the lowering of the K edges of these ions due to the extreme plasma conditions occurring in accretion disks around compact objects.

D. Samoilenko, A. Volotka, and S. Fritzsche
Elastic photon scattering on hydrogenic atoms near resonances
Atoms 8, 12 (2020)

Abstract: Scattering of light on relativistic heavy ion beams is widely used for characterizing and tuning the properties of both the light and the ion beam. Its elastic component-Rayleigh scattering-is investigated in this work for photon energies close to certain electronic transitions because of its potential usage in the Gamma Factory initiative at CERN. The angle-differential cross-section, as well as the degree of polarization of the scattered light are investigated for the cases of 1s - 2p1/2 and 1s - 2p3/2 resonance transitions in H-like lead ions. In order to gauge the validity and uncertainty of frequently used approximations, we compare different methods. In particular, rigorous quantum electrodynamics calculations are compared with the resonant electric-dipole approximation evaluated within the relativistic and nonrelativistic formalisms. For better understanding of the origin of the approximation, the commonly used theoretical approach is explained here in detail. We find that in most cases, the nonrelativistic resonant electric-dipole approximation fails to describe the properties of the scattered light. At the same time, its relativistic variant agrees with the rigorous treatment within a level of 10% to 20%. These findings are essential for the design of an experimental setup exploiting the scattering process, as well as for the determination of the scattered light properties.

H. Gies, and J. Ziebell
Asymptotically safe QED
European Physical Journal C 80, 607 (2020)

Abstract: High-energy completeness of quantum electrodynamics (QED) can be induced by an interacting ultraviolet fixed point of the renormalization flow. We provide evidence for the existence of two of such fixed points in the subspace spanned by the gauge coupling, the electron mass and the Pauli spin-field coupling. Renormalization group trajectories emanating from these fixed points correspond to asymptotically safe theories that are free from the Landau pole problem. We analyze the resulting universality classes defined by the fixed points, determine the corresponding critical exponents, study the resulting phase diagram, and quantify the stability of our results with respect to a systematic expansion scheme. We also compute high-energy complete flows towards the long-range physics. We observe the existence of a renormalization group trajectory that interconnects one of the interacting fixed points with the physical low-energy behavior of QED as measured in experiment. Within pure QED, we estimate the crossover from perturbative QED to the asymptotically safe fixed point regime to occur somewhat above the Planck scale but far below the scale of the Landau pole.

S. Hendi, A. Tavakkoli, S. Panahiyan, B. Panah, and E. Hackmann
Simulation of geodesic trajectory of charged BTZ black holes in massive gravity
European Physical Journal C 80, 524 (2020)

Abstract: In order to classify and understand structure of the spacetime, investigation of the geodesic motions of massive and massless particles is a key tool. So the geodesic equation is a central equation of gravitating systems and the subject of geodesics in the black hole dictionary attracted much attention. In this paper, we give a full description of geodesic motions in three-dimensional spacetime. We investigate the geodesics near charged BTZ black holes and then generalize our prescriptions to the case of massive gravity. We show that electric charge is a critical parameter for categorizing the geodesic motions of both lightlike and timelike particles. In addition, we classify the type of geodesics based on the particle properties and geometry of spacetime.

J. Hornung, Y. Zobus, P. Boller, C. Brabetz, U. Eisenbarth, T. Kühl, Zs. Major, J. Ohland, M. Zepf, B. Zielbauer, and V. Bagnoud
Enhancement of the laser-driven proton source at PHELIX
High Power Laser Science and Engineering 8, e24 (2020)

Abstract: We present a study of laser-driven ion acceleration with micrometre and sub-micrometre thick targets, which focuses on the enhancement of the maximum proton energy and the total number of accelerated particles at the PHELIX facility. Using laser pulses with a nanosecond temporal contrast of up to and an intensity of the order of, proton energies up to 93 MeV are achieved. Additionally, the conversion efficiency at incidence angle was increased when changing the laser polarization to p, enabling similar proton energies and particle numbers as in the case of normal incidence and s-polarization, but reducing the debris on the last focusing optic.

V. Zaytsev, A. Surzhykov, V. Serbo, V. Kosheleva, M. Groshev, V. Yerokhin, V. Shabaev, and T. Stöhlker
Atomic processes with twisted electrons
Journal of Physics: Conference Series 1412, 052013 (2020)

Abstract: The present status of the fully-relativistic nonperturbative calculations of the fundamental atomic processes with twisted electrons is presented. In particular, the elastic (Mott) scattering, the radiative recombination, and for the very first time, the Bremsstrahlung processes are considered. The electron-ion interaction is accounted for in a nonperturbative manner, that allows obtaining reliable results for heavy systems. We investigate the influence of the "twistedness" of the incoming electron on the angular and polarization properties of the emitted electrons and photons for the elastic and inelastic scattering, respectively. It is found that these properties exhibit a strong dependence on the opening angle of the vortex electron beam in all processes considered.

M. Kiffer, S. Ringleb, N. Stallkamp, S. Kumar, B. Arndt, M. Vogel, W. Quint, and T. Stöhlker
Characterisation of ion bunches by a single-pass non-destructive charge counter
Journal of Physics: Conference Series 1412, 242004 (2020)

Abstract: Synopsis We present non-destructive single-pass ion bunch detection and characterisation by measuring the induced image charge in a detection electrode. The presented technique allows direct determination of ion kinetic energy, absolute ion number and spatial ion bunch length. We will show the results of corresponding measurements with bunches of low-energy highly charged ions and discuss the minimum detectable number of charges.

S. Hagmann, P. Hillenbrand, Y. Litvinov, U. Spillmann, and T. Stöhlker
Coincident mapping of e+and e-from free-free pair production in a magnetic toroidal lepton spectrometer
Journal of Physics: Conference Series 1412, 232004 (2020)

Abstract: We discuss the electron-optical properties of a toroidal magnetic sector spectrometer and its suitablilty for electron-positron pair spectroscopy in relativistic ion-atom collisions in the future HESR storage ring at FAIR. With the simultaneous mapping of electrons and positrons and geometric invariants in the lepton trajectorties this instrument offers a very high efficiency for studies of vector momentum correlation in free-free pair production.

P. Jagodziłski, D. Banaś, M. Pajek, A. Warczak, H. Beyer, A. Gumberidze, G. Weber, Th. Stöhlker, and M. Trassinelli
Concept and simulations of a high-resolution asymmetric von Hamos X-ray spectrometer for CRYRING@ESR electron cooler
Journal of Physics: Conference Series 1412, 132031 (2020)

Abstract: A concept of a high resolution asymmetric von Hamos X-ray spectrometer for the CRYRING@ESR electron cooler is described and its characteristics obtained by ray-tracing Monte-Carlo simulations are presented. The spectrometer will be used to study the QED e-ects in H-like medium-Z ions by measuring the energies of X-rays from radiative recombination of highly charged ions with cooling electrons, with a ppm precision of energy determination.

A. Borovik, G. Weber, V. Hilbert, H. Lin, P. Pfäfflein, B. Zhu, C. Hahn, M. Lestinsky, S. Schippers, Th. Stöhlker, and J. Rothhardt
Development of a detector to register low-energy, charge-changed ions from ionization experiments at CRYRING@ESR
Journal of Physics: Conference Series 1412, 242003 (2020)

Abstract: A detector setup for registering ion species between the poles of a dipole magnet at CRYRING@ESR has been developed. It is based on a scintillator delivering light via a quartz light guide onto a semiconductor photomultiplier. The detector is capable of operating in a strong magnetic field. It can be swiftly retracted from the exposition area during the beam injection into the ring and repositioned back for the measurement cycle to avoid unnecessary exposition and, thus, to increase the scintillator life time.

N. Stallkamp, S. Ringleb, B. Arndt, M. Kiffer, S. Kumar, G. Paulus, W. Quint, T. Stöhlker, and M. Vogel
HILITE-A well-defined ion target for laser experiments
Journal of Physics: Conference Series 1412, 092009 (2020)

Abstract: We present a Penning-trap-based setup for the study of light-matter interactions in the high-power and/or high-intensity laser regime, such as multi-photon ionization and field ionization. The setup applies ioncloud formation techniques to highly charged ions to the end of specific target preparation, as well as nondestructive detection techniques to identify and quantify the interaction educts and products.

A. Gumberidze, C. Kozhuharov, R. Zhang, S. Trotsenko, Y. Kozhedub, R. Du, H. Bois, F. Beyer, K.-H. Blumenhagen, C. Brandau, A. Bräuning-Demian, W. Chen, O. Forstner, B. Gao, T. Gassner, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, A. Kumar, M. Lestinsky, Y. Litvinov, N. Petridis, D. Schury, U. Spillmann, C. Trageser, M. Trassinelli, X. Tu, and T. Stöhlker
Impact parameter sensitive study of inner-shell atomic processes in Xe54+, Xe52+ → Xe collisions
Journal of Physics: Conference Series 1412, 142015 (2020)

Abstract: In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms. The projectile and target x-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35 - 70 fm.

J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Nonlinear Cooper minimum as a precise tool for understanding multiphoton photoionization
Journal of Physics: Conference Series 1412, 152017 (2020)

Abstract: A new approach to accurately assess multiphoton ionization is suggested. Vanishing of the dominant ionization channel in nonresonant (direct) multiphoton ionization is predicted for a specific incident photon energy. The exact energy position of such nonlinear Cooper minimum can be accurately measured and requires calculations of the complete electronic spectrum. Measurements of various observables at these photon energies are desirable for further evaluation of theoretical calculations at hitherto unreachable accuracy.

Y. Litvinov, T. Stöhlker, X. Ma, Y. Zhang, and T. Yamaguchi
Nuclear physics research at heavy ion accelerators: Precision studies with stored and cooled exotic nuclei
Journal of Physics: Conference Series 1401, 012001 (2020)

Abstract: This contribution is based on the plenary presentation at the 14th International Conference on Heavy Ion Accelerator Technology (HIAT-2018) in Lanzhou, China. Heavy-ion storage rings offer unparalleled opportunities for precision experiments in the realm of nuclear structure, atomic physics and astrophysics. A brief somewhat biased review of the presently ongoing research programs is given as well as the future projects are outlined. The limited space does not allow for detailed description of individual experiments, which shall - to some extent - be compensated by extended bibliography.

E. Menz, C. Hahn, P. Pfäfflein, G. Weber, and T. Stöhlker
Performance of a scintillator-based ion detector for CRYRING@ESR
Journal of Physics: Conference Series 1412, 232006 (2020)

Abstract: A detector based on the scintillator material YAP:Ce and capable of counting single ions is presented. The detector consists of a YAP:Ce crystal and a light guide operating in ultra high vacuum and a conventional photomultiplier outside the vacuum. The crystal demonstrated the necessary radiation hardness against heavy ion irradiation. The detector has been commissioned at CRYRING@ESR and its detection capabilities have been confirmed with beam from the local source.

T. Buhr, S. O. Stock, A. Perry-Sassmannshausen, S. Reinwardt, M. Martins, S. Ricz, A. Mueller, S. Fritzsche, and S. Schippers
Photoionization of low-charged silicon ions
Journal of Physics: Conference Series 1412, 152024 (2020)

Abstract: Single and multiple photoionization of Si1+, Si2+, and Si3+ ions have been investigated near the silicon K-edge using the PIPE setup at beamline P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. Pronounced resonance structures are observed for all ions which are associated with excitation or ionization of a K-shell electron. The experimental cross sections are compared with results from theoretical calculations.

M. Lestinsky, Z. Andelkovic, M. Bai, A. Bräuning-Demian, V. Chetvertkova, S. Fedotova, O. Geithner, W. Geithner, O. Gorda, F. Herfurth, A. Kalinin, A. Källberg, S. Litvinov, E. Menz, K. Mohr, R. Sanchez, T. Stöhlker, G. Vorobjev, and U. Weinrich
Status of CRYRING@ESR and preparations for first experiments
Journal of Physics: Conference Series 1412, 23007 (2020)

Abstract: CRYRING was moved from Stockholm to Darmstadt, modernized and integrated into the GSI/FAIR beamline topology behind ESR. As CRYRING@ESR, it will receive and store heavy, highly charged ions from all species the present accelerator chain is capable of producing. An extensive research program on low-energy atomic collisions, spectroscopy and nuclear reactions was proposed. The facility is gradually completing commissioning, ion beams from the local injector branch have already been stored and prototype experiments performed. We present the machine status and highlight some planned experiments.

D. Schury, A. Méry, J. Ramillon, L. Adoui, J.-Y. Chesnel, A. Lévy, S. Macé, C. Prigent, J. Rangama, P. Rousseau, S. Steydli, M. Trassinelli, D. Vernhet, A. Gumberidze, T. Stöhlker, A. Bruning-Demian, C. Hahn, U. Spillmann, and E. Lamour
The low energy beamline of the FISIC experiment: Current status of construction and performance
Journal of Physics: Conference Series 1412, 162011 (2020)

Abstract: Ion-ion collisions between slow (kev/u) and fast (MeV/u) ions play an important role in for example astrophysical or inertial fusion plasmas as well as in ion-matter interaction. In this regime the energy transfer is maximum, as all primary electronic processes reach their maximum. At the same time up to today no reliable experimental data exists while being difficult to treat accurately by theory. We present the current status and performance of the low energy beam-line of the FISIC experiment which aims at filling in the blanks in this regime.

L. Varga, K. Blaum, T. Davinson, J. Glorius, B. Jurado, C. Langer, C. Lederer-Woods, Y. Litvinov, R. Reifarth, Z. Slavkovská, T. Stöhlker, P. Woods, and Y. Xing
Towards background-free studies of capture reactions in a heavy-ion storage ring
Journal of Physics: Conference Series 1412, 232011 (2020)

Abstract: Stored and cooled highly-charged ions offer unprecedented capabilities for precision studies in realm of atomic-, nuclear-structure and astrophysics. In context of the latter, after the successful investigation of the cross section of 96Ru(p,γ) in 2009, in 2016 the first measurement of the 124Xe(p,γ)125Cs reaction was performed at the Experimental Storage Ring (ESR) at GSI.

Y. Xing, J. Glorius, L. Varga, L. Bott, C. Brandau, B. Brückner, R. Chen, 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. Langer, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Y. Litvinov, B. Löher, N. Petridis, U. Popp, M. Reed, R. Reifarth, M. Sanjari, H. Simon, Z. Slavkovská, U. Spillmann, M. Steck, T. Stöhlker, J. Stumm, T. Szücs, T. Nguyen, A. Zadeh, B. Thomas, S. Torilov, H. Törnqvist, C. Trageser, S. Trotsenko, M. Volknandt, M. Wang, M. Weigand, C. Wolf, P. Woods, Y. Zhang, and X. Zhou
Determination of luminosity for in-ring reactions: A new approach for the low-energy domain
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 982, 164367 (2020)

Abstract: Luminosity is a measure of the colliding frequency between beam and target and it is a crucial parameter for the measurement of absolute values, such as reaction cross sections. In this paper, we make use of experimental data from the ESR storage ring to demonstrate that the luminosity can be precisely determined by modelling the measured Rutherford scattering distribution. The obtained results are in good agreement with an independent measurement based on the x-ray normalization method. Our new method provides an alternative way to precisely measure the luminosity in low-energy stored-beam configurations. This can be of great value in particular in dedicated low-energy storage rings where established methods are difficult or impossible to apply.

X. Xu, Y. Zhang, H. Zhang, H. Lu, W. Zhou, C. Zhou, B. Dromey, S. Zhu, M. Zepf, X. He, and B. Qiao
Production of 100-TW single attosecond x-ray pulse
Optica 7, 355 (2020)

Abstract: Attosecond light sources have provided insight into the fastest atomic-scale electronic dynamics. True attosecond-pump–attosecond-probe experiments require a single attosecond pulse at high intensity and large photon energy, a challenge that has yet to be conquered. Here we show 100-TW single attosecond x-ray pulses with unprecedented intensity of 1021 W/cm2 and duration 8.0 as can be produced by intense laser irradiation of a capacitor-nanofoil target composed of two separate nanofoils. In the interaction, a strong electrostatic potential develops between the two foils, which drags electrons out of the second foil and piles them up in vacuum, forming an ultradense relativistic electron nanobunch. This nanobunch reaches both high density and high energy in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single, intense attosecond pulse. Such a pulse enables the capture and control of electron motion at the picometer–attosecond scale.

J. Körner, M. Krüger, J. Reiter, A. Münzer, J. Hein, and M. Kaluza
Temperature dependent spectroscopic study of Yb3+-doped KG(WO4)2, KY(WO4)2, YAlO3 and YLiF4 for laser applications
Optical Materials Express 10, 2425 (2020)

Abstract: We present a study on temperature dependent spectroscopic data for Yb:KGW, Yb:KYW and Yb:YLF between 80K and 280K and Yb:YAP between 100K and 300 K. Absorption and emission cross sections are determined. The latter ones are obtained by using a combination of the McCumber relation and the Füchtbauer-Ladenburg equation. Fluorescence lifetimes are measured within a setup optimized for the suppression of re-absorption and compared to the radiative lifetimes calculated from the previously determined cross sections to cross check the validity of the measurements. The cross sections are evaluated with regard to the materials' potential for supporting the generation of ultra-short laser pulses, low quantum defect lasing and requirements for suitable diode laser pump sources.

L. Lamaignère, G. Toci, B. Patrizi, M. Vannini, A. Pirri, S. Fanetti, R. Bini, G. Mennerat, A. Melninkaitis, L. Lukas, and J. Hein
Determination of non-linear refractive index of laser crystals and ceramics via different optical techniques
Optical Materials: X 8, 100065 (2020)

Abstract: The exact knowledge of optical material parameters is crucial for laser systems design. Therefore, the work reported herein was dedicated to the determination of an important parameter that is typically not known or only known with insufficient precision: the Kerr coefficient determined by the third order non-linearity, also called the n2-parameter. The optical Kerr effect is responsible for the accumulated nonlinear phase (the B-integral) in high energy laser amplifiers, which often represents a serious limitation. Therefore, the knowledge of n2 is especially required for new types of laser materials. In this paper we report measurements carried out on the widely used optical material Ytterbium-doped Yttrium Aluminium Garnet (Yb:YAG) ceramics. Furthermore, the new Neodymium-doped Calcium Fluoride (Nd:CaF2) crystal was investigated. Specifically, three different approaches have been employed to determine experimentally the nonlinear refractive index of these materials. Thus classical Z-scan technique (at two different wavelengths), the degenerated four waves mixing and the time-resolved digital holography techniques, were compared. These different approaches have permitted the precise measurements of these parameters as well as their dispersion estimations.

I. Tamer, M. Hellwing, Y. Azamoum, M. Hornung, S. Keppler, F. Schorcht, J. Hein, and M. Kaluza
Few-cycle fs-pumped NOPA with passive ultrabroadband spectral shaping
Optics Express 28, 19034 (2020)

Abstract: A compact, femtosecond-pumped noncollinear optical parametric amplifier (NOPA) is presented with a passive spectral shaping technique, employed to produce a flat-top-like ultrabroadband output spectrum. The NOPA is pumped by a dedicated 2 mJ, 120 fs Yb3+- based CPA system, which generates both the second harmonic pump pulse and white light supercontinuum as the signal pulse. A chirped mirror pair pre-compensates the material GVD within the optical path of the signal pulse to produce a near-FTL pulse duration at the OPA crystal output. By optimizing both the pump/signal cross angle and the pump/signal delay, the 40 cm × 40 cm footprint, single-pass, fs-pumped, direct NOPA (non-NOPCPA) system generates a record 20 μJ, 11 fs pulses at 820 nm central wavelength with a bandwidth of 230 nm FWHM, to be used as an ultrashort optical probe pulse for relativistic laser-plasma interactions at the petawatt-class POLARIS laser system.

A. Mayer, W. Grosinger, J. Fellinger, G. Winkler, L. Perner, S. Droste, S. Salman, C. Li, C. Heyl, I. Hartl, and O. Heckl
Flexible all-PM NALM Yb:fiber laser design for frequency comb applications: Operation regimes and their noise properties
Optics Express 28, 18946 (2020)

Abstract: We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.035 ps2) to normal (+0.015 ps2). We put a particular focus on the characterization of the intensity noise as well as the free-running linewidth of the carrier-envelope-offset (CEO) frequency as a function of the different operation regimes. We observe that operation points far from the spontaneous emission peak of Yb (~1030 nm) and close to zero intracavity dispersion can be found, where the influence of pump noise is strongly suppressed. For such an operation point, we show that a CEO linewidth of less than 10-kHz at 1 s integration can be obtained without any active stabilization.

H. Kang, S. Chen, W. Chu, J. Yao, J. Chen, X. Jun Liu, Y. Cheng, and Z. Xu
Nonsequential double ionization of alkaline-earth metal atoms by intense mid-infrared femtosecond pulses
Optics Express 28, 19325 (2020)

Abstract: A systematic study of nonsequential double ionization (NSDI) of alkaline-earth metal atoms with mid-infrared femtosecond pulses is reported. We find that the measured NSDI yield shows a strong target dependence and it is more suppressed for alkaline-earth metal with higher ionization potential. The observation is attributed to the differences in the recollision induced excitation and ionization cross sections of alkaline-earth metals. This work indicates that NSDI of alkaline-earth metals can be generally understood within recollision picture and sheds light on ultrafast control of electron correlation and dynamics of ionic excited states during NSDI of atoms with complex structures.

C. Aleshire, A. Steinkopff, C. Jauregui, A. Klenke, A. Tünnermann, and J. Limpert
Simplified design of optical elements for filled-aperture coherent beam combination
Optics Express 28, 21035 (2020)

Abstract: A simplification strategy for segmented mirror splitters (SMS) used as beam combiners is presented. These devices are useful for compact beam division and the combination of linear and 2-D arrays. However, the standard design requires unique thin-film coating sections for each input beam; thus, potential for scaling to high beam-counts is limited due to manufacturing complexity. Taking advantage of the relative insensitivity of the beam combination process to amplitude variations, numerical techniques are used to optimize highly simplified designs with only one, two or three unique coatings. It is demonstrated that with correctly chosen coating reflectivities, the simplified optics are capable of high combination efficiency for several tens of beams. The performance of these optics as beam splitters in multicore fiber amplifier systems is analyzed, and inhomogeneous power distribution of the simplified designs is noted as a potential source of combining loss in such systems. These simplified designs may facilitate further scaling of filled-aperture coherently combined systems in linear array or 2-D array formats.

M. Müller, C. Aleshire, A. Klenke, E. Haddad, F. Légaré, A. Tünnermann, and J. Limpert
10.4 kW coherently combined ultrafast fiber laser
Optics Letters 45, 3083 (2020)

Abstract: An ultrafast laser delivering 10.4 kW average output power based on a coherent combination of 12 step-index fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at an 80 MHz repetition rate, and has a high beam quality (M2 ≤ 1.2) and a low relative intensity noise of 0.56% in the frequency range of 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

N. Geib, R. Hollinger, E. Haddad, P. Herrmann, F. Légaré, T. Pertsch, C. Spielmann, M. Zürch, and F. Eilenberger
Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared
Optics Letters 45, 5295 (2020)

Abstract: In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

L. Li, J. Koliyadu, H. Donnelly, D. Alj, O. Delmas, M. Ruiz-Lopez, O. de la Rochefoucauld, G. Dovillaire, M. Fajardo, C. Zhou, S. Ruan, B. Dromey, M. Zepf, and P. Zeitoun
High numerical aperture Hartmann wave front sensor for extreme ultraviolet spectral range
Optics Letters 45, 4248 (2020)

Abstract: We present a novel, to the best of our knowledge, Hartmann wave front sensor for extreme ultraviolet (EUV) spectral range with a numerical aperture (NA) of 0.15. The sensor has been calibrated using an EUV radiation source based on gas high harmonic generation. The calibration, together with simulation results, shows an accuracy beyond λ/39 root mean square (rms) at λ = 32 nm. The sensor is suitable for wave front measurement in the 10 nm to 45 nm spectral regime. This compact wave front sensor is high-vacuum compatible and designed for in situ operations, allowing wide applications for up-to-date EUV sources or high-NA EUV optics.

H. Kang, S. Chen, Z. Lin, W. Chu, J. Yao, W. Quan, J. Chen, X. Liu, Y. Cheng, and Z. Xu
Comparative study of strong-field ionization of alkaline-earth-metal atoms
Physical Review A 101, 053433 (2020)

Abstract: We report on a comparative study of strong-field ionization of alkaline-earth-metal atoms by intense femtosecond laser pulses from near-infrared to midinfrared wavelengths. By collecting the ionization signals only produced within the central portion of the laser focus, the focus volume effect is largely reduced and the saturation intensities for different alkaline-earth-metal atoms are reliably determined, which permits us to directly test the strong-field-ionization theories. We demonstrate that the Perelomov-Popov-Terent'ev model accurately predicts the experimental ionization yields and saturation intensities in general for arbitrary values of the Keldysh parameter, while the Ammosov-Delone-Krainov simulations agree with the experiments for the tunneling-ionization regime and also for the regime when the Keldysh parameter is around 1. Our work presents benchmark data for strong-field ionization of alkaline-earth metals over a broad range of laser parameters and confirms the validity of Keldysh's picture for such atoms.

B. Grabiger, B. Marx-Glowna, I. Uschmann, R. Loetzsch, G. Paulus, and K. Schulze
A highly sensitive imaging polarimeter in the x-ray regime
Applied Physics Letters 117, 201102 (2020)

Abstract: We report on the development of a highly sensitive imaging polarimeter that allows for the investigation of polarization changing properties of materials in the x-ray regime. By combining a microfocus rotating anode, collimating multilayer mirrors, and two germanium polarizer crystals, we achieved a polarization purity of the two orthogonal linear polarization states of 8 × 10−8. This enables the detection of an ellipticity on the same order or a rotation of the polarization plane of 6 arcsec. The high sensitivity combined with the imaging techniques allows us to study the microcrystalline structure of materials. As an example, we investigated beryllium sheets of different grades, which are commonly used for fabricating x-ray lenses, with a spatial resolution of 200 μm, and observed a strong degradation of the polarization purity due to the polycrystalline nature of beryllium. This makes x-ray lenses made of beryllium unsuitable for imaging polarimeter with higher spatial resolution. The results are important for the development of x-ray optical instruments that combine high spatial resolution and high sensitivity to polarization.

Y. X. Geng, D. Wu, W. Yu, Z. M. Sheng, S. Fritzsche, Q. Liao, M. J. Wu, X. H. Xu, D. Y. Li, W. J. Ma, H. Y. Lu, Y. Y. Zhao, X. T. He, J. E. Chen, C. Lin, and X. Q. Yan
Proton beams from intense laser-solid interaction: Effects of the target materials
Matter and Radiation at Extremes 5, 064402 (2020)
F. Roeder, M. Shalaby, B. Beleites, F. Ronneberger, and A. Gopal
THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA
Optics Express 28, 36274 (2020)

Abstract: Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.

B. Böning, and S. Fritzsche
Partial-wave representation of the strong-field approximation
Physical Review A 102, 053108 (2020)

Abstract: The strong-field approximation (SFA) has been widely applied to model ionization processes in short and intense laser pulses. Several approaches have been suggested in order to overcome certain limitations of the original SFA formulation with regard to the representation of the initial bound and final continuum states of the emitted electron as well as a suitable description of the driving laser pulse. We here present a reformulation of the SFA in terms of partial waves and spherical tensor operators that supports a quite simple implementation and the comparison of different treatments of the active (photo)electron and the laser pulses. In particular, this reformulation helps to adapt the SFA to experimental setups, and it paves the way to extend the strong-field theory toward the study of nondipole contributions in light-atom interactions as well as of many-particle correlations in strong-field ionization processes. A series of detailed computations have been carried out in order to confirm the validity of the reformulation and to show how the representation of the bound and continuum states affects the predicted above-threshold ionization spectra and related observables.

D. Seipt, and B. King
Spin- and polarization-dependent locally-constant-field-approximation rates for nonlinear Compton and Breit-Wheeler processes
Physical Review A 102, 052805 (2020)

Abstract: In this paper we derive and discuss the completely spin- and photon-polarization-dependent probability rates for nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. The locally constant field approximation, which is essential for applications in plasma-QED simulation codes, is rigorously derived from the strong-field QED matrix elements in the Furry picture for a general plane-wave background field. We discuss important polarization correlation effects in the spectra of both processes. Asymptotic limits for both small and large values of $\chi$ are derived and their spin and polarization dependence is discussed.

S. Panahiyan, and S. Fritzsche
Controllable simulation of topological phases and edge states with quantum walk
Physics Letters A 384, 126828 (2020)
M. Coughlan, H. Donnelly, N. Breslin, C. Arthur, G. Nersisyan, M. Yeung, B. Villagomez-Bernabe, M. Afshari, F. Currell, M. Zepf, and B. Dromey
Ultrafast dynamics and evolution of ion-induced opacity in transparent dielectrics
New Journal of Physics 22, 103023 (2020)
Q. Lu, C. L. Yan, G. Q. Xu, N. Fu, Y. Yang, Y. Zou, A. V. Volotka, J. Xiao, N. Nakamura, and R. Hutton
Direct measurements for the fine-structure splitting of S VIII and Cl IX
Physical Review A 102, 042817 (2020)
F. M. Kröger, G. Weber, M. O. Herdrich, 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, Gy. Gyürky, S. Hagmann, C. Hahn, 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, M. Reed, R. Reifarth, M. S. Sanjari, H. Simon, U. Spillmann, M. Steck, J. Stumm, T. Szücs, T. T. Nguyen, A. T. Zadeh, B. Thomas, S. Yu. Torilov, H. Törnqvist, C. Trageser, S. Trotsenko, M. Volknandt, M. Weigand, C. Wolf, P. J. Woods, V. P. Shevelko, I. Yu. Tolstikhina, and Th. Stöhlker
Electron capture of Xe54+ in collisions with H2 molecules in the energy range between 5.5 and 30.9 MeV/u
Physical Review A 102, 042825 (2020)

Abstract: The electron-capture process was studied for Xe54+ colliding with H2 molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.

J. Hofbrucker, A. V. Volotka, J. Szlachetko, and S. Fritzsche
Enhanced polarization transfer to the characteristic L alpha x-ray lines near the nonlinear Cooper minimum of two-photon ionization
Physical Review A 102, 042807 (2020)
Z. W. Wu, Z. Q. Tian, J. Jiang, C. Z. Dong, and S. Fritzsche
Hyperfine-induced effects on angular emission of the magnetic-quadrupole line 1s2p(3/2) P-3(2) -> 1s(2) S-1(0) following electron-impact excitation of Tl79+ ions
Physical Review A 102, 042813 (2020)
A. V. Volotka, A. Surzhykov, and S. Fritzsche
Rayleigh scattering of linearly polarized light: Scenario of the complete experiment
Physical Review A 102, 042814 (2020)
J. Sommerfeldt, R. A. Mueller, A. V. Volotka, S. Fritzsche, and A. Surzhykov
Vacuum polarization and finite-nuclear-size effects in the two-photon decay of hydrogenlike ions
Physical Review A 102, 042811 (2020)
S. Panahiyan, W. Chen, and S. Fritzsche
Fidelity susceptibility near topological phase transitions in quantum walks
Physical Review B 102, 134111 (2020)
V. Popov, V. M. Shabaev, D. A. Telnov, I. I. Tupitsyn, I. A. Maltsev, Y. S. Kozhedub, I. Bondarev, V. Kozin, X. Ma, G. Plunien, T. Stöhlker, D. A. Tumakov, and V. A. Zaytsev
How to access QED at a supercritical Coulomb field
Physical Review D 102, 076005 (2020)