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Peer-Review Publications


S. Skruszewicz, S. Fuchs, J. J. Abel, J. Nathanael, J. Reinhard, C. Rödel, F. Wiesner, M. Wuensche, P. Wachulak, A. Bartnik, K. Janulewicz, H. Fiedorowicz, and G. G. Paulus
Coherence tomography with broad bandwidth extreme ultraviolet and soft X-ray radiation
Applied Physics B 127, 55 (2021)

Abstract: We present an overview of recent results on optical coherence tomography with the use of extreme ultraviolet and soft X-ray radiation (XCT). XCT is a cross-sectional imaging method that has emerged as a derivative of optical coherence tomography (OCT). In contrast to OCT, which typically uses near-infrared light, XCT utilizes broad bandwidth extreme ultraviolet (XUV) and soft X-ray (SXR) radiation (Fuchs et al in Sci Rep 6:20658, 2016). As in OCT, XCT\textquotesingle s axial resolution only scales with the coherence length of the light source. Thus, an axial resolution down to the nanometer range can be achieved. This is an improvement of up to three orders of magnitude in comparison to OCT. XCT measures the reflected spectrum in a common-path interferometric setup to retrieve the axial structure of nanometer-sized samples. The technique has been demonstrated with broad bandwidth XUV/SXR radiation from synchrotron facilities and recently with compact laboratory-based laser-driven sources. Axial resolutions down to 2.2 nm have been achieved experimentally. XCT has potential applications in three-dimensional imaging of silicon-based semiconductors, lithography masks, and layered structures like XUV mirrors and solar cells.

S. Sadashivaiah, J. Wolny, L. Scherthan, K. Jenni, A. Omlor, C. Mueller, I. Sergueev, M. Herlitschke, O. Leupold, H.-C. Wille, R. Röhlsberger, and V. Schuenemann
High-Repetition Rate Optical Pump-Nuclear Resonance Probe Experiments Identify Transient Molecular Vibrations after Photoexcitation of a Spin Crossover Material
Journal of Physical Chemistry Letters 12, 3240 (2021)

Abstract: Phonon modes play a vital role in the cooperative phenomenon of light-induced spin transitions in spin crossover (SCO) molecular complexes. Although the cooperative vibrations, which occur over several hundreds of picoseconds to nanoseconds after photoexcitation, are understood to play a crucial role in this phase transition, they have not been precisely identified. Therefore, we have performed a novel optical laser pump-nuclear resonance probe experiment to identify the Fe-projected vibrational density of states (pDOS) during the first few nanoseconds after laser excitation of the mononuclear Fe(II) SCO complex [Fe(PM-BiA)(2)(NCS)(2)]. Evaluation of the so obtained nanosecond-resolved pDOS yields an excitation of similar to 8% of the total volume of the complex from the low-spin to high-spin state. Density functional theory calculations allow simulation of the observed changes in the pDOS and thus identification of the transient inter- and intramolecular vibrational modes at nanosecond time scales.

H. Gies, F. Karbstein, and L. Klar
Quantum vacuum signatures in multicolor laser pulse collisions
Physical Review D 103, 076009 (2021)

Abstract: Quantum vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. A prominent signature of quantum vacuum nonlinearities driven by macroscopic fields are signal photons differing in characteristic properties such as frequency, propagation direction and polarization from the driving fields. We devise a strategy for the efficient tracing of the various vacuum-fluctuation-mediated interaction processes in order to identify the most prospective signal photon channels. As an example, we study the collision of up to four optical laser pulses and pay attention to sum and difference frequency generation. We demonstrate how this information can be used to enhance the signal photon yield in laser pulse collisions for a given total laser energy.

H. Kang, S. Chen, J. Chen, and G. Paulus
Frustrated double ionization of atoms in circularly polarized laser fields
New Journal of Physics 23, 033041 (2021)

Abstract: We theoretically study frustrated double ionization (FDI) of atoms subjected to intense circularly polarized laser pulses using a three-dimensional classical model. We find a \textasciigrave knee\textquotesingle structure of FDI probability as a function of intensity, which is similar to the intensity dependence of nonsequential double ionization probability. The observation of FDI is more favourable when using targets with low ionization potentials and short driving laser wavelengths. This is attributed to the crucial role of recollision therein, which can be experimentally inferred from the photoelectron momentum distribution generated by FDI. This work provides novel physical insights into FDI dynamics with circular polarization.

M. Gebhardt, E. Amuah, R. Klas, H. Stark, J. Buldt, A. Steinkopff, and J. Limpert
Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries
Optics Express 29, 6957 (2021)

Abstract: Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

L. Stoyanov, Y. Zhang, A. Dreischuh, and G. Paulus
Long-range quasi-non-diffracting Gauss-Bessel beams in a few-cycle laser field
Optics Express 29, 10997 (2021)

Abstract: Many applications ranging from nonlinear optics to material processing would benefit from pulsed ultrashort (quasi-)non-diffracting Gauss-Bessel beams (GBBs). Here we demonstrate a straightforward yet efficient method for generating such zeroth- and first-order GBBs using a single reflective spatial light modulator. Even in the sub-8-fs range there are no noticeable consequences for the measured pulse duration. The only effect is a weak coloring of the outer-lying satellite rings of the beams due to the spectrum spanning over more than 300 nm. The obtained beams have diffraction half-angles below 40 mu rad and reach propagation distances in excess of 1.5 m.

J. White, S. Wang, W. Eschen, and J. Rothhardt
Real-time phase-retrieval and wavefront sensing enabled by an artificial neural network
Optics Express 29, 1 (2021)

Abstract: In this manuscript we demonstrate a method to reconstruct the wavefront of focused beams from a measured diffraction pattern behind a diffracting mask in real-time. The phase problem is solved by means of a neural network, which is trained with simulated data and verified with experimental data. The neural network allows live reconstructions within a few milliseconds, which previously with iterative phase retrieval took several seconds, thus allowing the adjustment of complex systems and correction by adaptive optics in real time. The neural network additionally outperforms iterative phase retrieval with high noise diffraction patterns.

H. Stark, J. Buldt, M. Mueller, A. Klenke, and J. Limpert
1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system
Optics Letters 46, 5529 (2021)

Abstract: An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

S. Fritzsche, P. Palmeri, and S. Schippers
Atomic Cascade Computations
Symmetry-Basel 13, 520 (2021)

Abstract: Atomic cascades are ubiquitous in nature and they have been explored within very different scenarios, from precision measurements to the modeling of astrophysical spectra, and up to the radiation damage in biological matter. However, up to the present, a quantitative analysis of these cascades often failed because of their inherent complexity. Apart from utilizing the rotational symmetry of atoms and a proper distinction of different physical schemes, a hierarchy of useful approaches is therefore needed in order to keep cascade computations feasible. We here suggest a classification of atomic cascades and demonstrate how they can be modeled within the framework of the Jena Atomic Calculator. As an example, we shall compute within a configuration-average approach the stepwise decay cascade of atomic magnesium, following a 1s inner-shell ionization, and simulate the corresponding (final) ion distribution. Our classification of physical scenarios (schemes) and the hierarchy of computational approaches are both flexible to further refinements as well as to complex shell structures of the atoms and ions, for which the excitation and decay dynamics need to be modeled in good detail.

A.-L. Calendron, J. Meier, E. Kueny, S. Velten, L. Bocklage, R. Röhlsberger, and F. Kaertner
Bulk, cascaded pulse compression scheme and it s application to spin emitter characterization
Applied Optics 60, 912 (2021)

Abstract: The 35-fs-long pulses of a commercial Ti:sapphire amplifier are compressed to similar to 20 fs via self-phase modulation in bulk glass substrates. The cascading of both nonlinear broadening and dispersion compensation stages makes use of the increasing peak power in the successive nonlinear stages. As an application example, the compressed pulses are used for electro-optical sampling of terahertz waves created by optically pumped thin-film spin emitters. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

M. Gebhardt, T. Heuermann, R. Klas, C. Liu, A. Kirsche, M. Lenski, Z. Wang, C. Gaida, J. E. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre
Light: Science & Applications 10, 2021 (2021)

Abstract: Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

K. Heeg, A. Kaldun, C. Strohm, C. Ott, R. Subramanian, D. Lentrodt, J. Haber, H.-C. Wille, S. Goerttler, R. Rüffer, C. Keitel, R. Röhlsberger, T. Pfeifer, and J. Evers
Coherent X-ray-optical control of nuclear excitons
Nature 590, 401 (2021)

Abstract: Coherent control of quantum dynamics is key to a multitude of fundamental studies and applications1. In the visible or longer-wavelength domains, near-resonant light fields have become the primary tool with which to control electron dynamics2. Recently, coherent control in the extreme-ultraviolet range was demonstrated3, with a few-attosecond temporal resolution of the phase control. At hard-X-ray energies (above 5--10 kiloelectronvolts), Mössbauer nuclei feature narrow nuclear resonances due to their recoilless absorption and emission of light, and spectroscopy of these resonances is widely used to study the magnetic, structural and dynamical properties of matter4,5. It has been shown that the power and scope of Mössbauer spectroscopy can be greatly improved using various control techniques6--16. However, coherent control of atomic nuclei using suitably shaped near-resonant X-ray fields remains an open challenge. Here we demonstrate such control, and use the tunable phase between two X-ray pulses to switch the nuclear exciton dynamics between coherent enhanced excitation and coherent enhanced emission. We present a method of shaping single pulses delivered by state-of-the-art X-ray facilities into tunable double pulses, and demonstrate a temporal stability of the phase control on the few-zeptosecond timescale. Our results unlock coherent optical control for nuclei, and pave the way for nuclear Ramsey spectroscopy17 and spin-echo-like techniques, which should not only advance nuclear quantum optics18, but also help to realize X-ray clocks and frequency standards19. In the long term, we envision time-resolved studies of nuclear out-of-equilibrium dynamics, which is a long-standing challenge in Mössbauer science20.

M. Dragowski, M. Adamus, G. Weber, and M. Wlodarczyk
Polarized electron Mott scattering model for the Geant4 Monte Carlo toolkit
Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 488, 37 (2021)

Abstract: A new method for simulation of polarized electron interactions with matter, based on the Geant4 Monte Carlo toolkit, is presented. The extension consists of a Mott scattering model taking into account the polarization dependence of the cross section, as well as the change of electron polarization in the scattering. The results regarding azimuthal asymmetry in Mott scattering of polarized electron beams off gold and lead targets are compared to available experimental data for energies up to 14 MeV.

F. Wiesner, M. Wünsche, J. Reinhard, J. Abel, J. Nathanael, S. Skruszewicz, C. Rödel, S. Yulin, A. Gawlik, G. Schmidl, U. Huebner, J. Plentz, G. Paulus, and S. Fuchs
Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography
Optica 8, 230 (2021)

Abstract: Scientific and technological progress depend substantially on the ability to image on the nanoscale. In order to investigate complex, functional, nanoscopic structures like, e.g., semiconductor devices, multilayer optics, or stacks of 2D materials, the imaging techniques not only have to provide images but should also provide quantitative information. We report the material-specific characterization of nanoscopic buried structures with extreme ultraviolet coherence tomography. The method is demonstrated at a laser-driven broadband extreme ultraviolet radiation source, based on high-harmonic generation. We show that, besides nanoscopic axial resolution, the spectral reflectivity of all layers in a sample can be obtained using algorithmic phase reconstruction. This provides localized, spectroscopic, material-specific information of the sample. The method can be applied in, e.g., semiconductor production, lithographic mask inspection, or quality control of multilayer fabrication. Moreover, it paves the way for the investigation of ultrafast nanoscopic effects at functional buried interfaces. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.

L. Stoyanov, M. Zhekova, A. Stefanov, B. Ivanov, I. Stefanov, G. Paulus, and A. Dreischuh
Generation of long range low-divergent Gauss-Bessel beams by annihilating optical vortices
Optics Communications 480, 126510 (2021)

Abstract: Bessel beams are remarkable since they do not diverge. Accordingly, they have numerous applications ranging from precision laser micro-machining to laser particle acceleration. We demonstrate a novel approach for generating long-range Gauss-Bessel beams. A ring-shaped beam is produced by imprinting a vortex with high topological charge in a Gaussian beam. The phase singularities are thereafter removed and the ring-shaped beam focused/Fourier-transformed by a thin lens. This results in a remarkably good realization of a Gauss- Bessel beam. Divergence angles in the microradian range and Gauss-Bessel beam lengths up to 2.5 m behind the focal plane of the lens are demonstrated.

M. Ruijter, V. Petrillo, and M. Zepf
Decreasing the bandwidth of linear and nonlinear Thomson scattering radiation for electron bunches with a finite energy spread
Physical Review ST - Accelerators and Beams 24, 020702 (2021)

Abstract: Relative narrow bandwidth-high energy radiation can be produced through Thomson scattering, where highly relativistic electrons collide with a laser pulse. The bandwidth of such a source is determined, among others factors, by the bandwidth of the laser pulse and the energy spread of the electrons. Here we investigate how the bandwidth of such a source can be minimized, with a particular emphasis on electron bunches with a correlated energy spread of several percent, that are typical for plasma based accelerator schemes. We show that by introducing a chirp on the laser pulse it is possible to compensate the broadening effect due to the energy spread of the electrons, and obtain the same bandwidth as a quasi-monochromatic plane wave laser pulse colliding with a monoenergetic electron bunch. Ultimately, the bandwidth of a Thomson source is limited by the acceptance angle and the initial transverse momentum of electrons (emittance).

B. Böning, P. Abele, W. Paufler, and S. Fritzsche
A strong-field approach with realistic wave functions to the above-threshold ionization of Ba+
Journal of Physics B: Atomic, Molecular and Optical Physics 54, 025602 (2021)

Abstract: We study the above-threshold ionization of atoms in intense circularly polarized laser pulses. In order to compute photoelectron energy spectra, we apply the strong-field approximation with different models for the initial state wave function. Specifically, we compare the spectra for singly ionized Barium (Ba^+) using hydrogenic wave functions and realistic one-particle wave functions obtained by multiconfiguration Dirac–Hartree–Fock computations, respectively. As a particular example, we discuss the dependence of the photoelectron spectra on the magnetic quantum number m of the initial state and we reproduce the well known m-selectivity in strong-field ionization. Here, we show that the photoelectron spectra exhibit noticeable differences for the two models of the initial state and that the m-selectivity is enhanced when realistic wave functions are used. We conclude that the description of strong-field processes within the strong-field approximation will benefit from a realistic description of the initial atomic state.

A.-L. Viotti, S. Alisauskas, A. Bin Wahid, P. Balla, N. Schirmel, B. Manschwetus, I. Hartl, and C. Heyl
60 fs, 1030nm FEL pump--probe laser based on a multi-pass post-compressed Yb:YAG source
Journal of Synchrotron Radiation 28, 36 (2021)

Abstract: This paper reports on nonlinear spectral broadening of 1.1ps pulses in a gas-filled multi-pass cell to generate sub-100fs optical pulses at 1030nm and 515nm at pulse energies of 0.8mJ and 225\textdollar µ \textdollar J, respectively, for pump--probe experiments at the free-electron laser FLASH. Combining a 100kHz Yb:YAG laser with\textasciitilde 180W in-burst average power and a post-compression platform enables reaching simultaneously high average powers and short pulse durations for high-repetition-rate FEL pump--probe experiments.

B. Marx-Glowna, I. Uschmann, K. Schulze, H. Marschner, H.-C. Wille, K. Schlage, T. Stöhlker, R. Röhlsberger, and G. Paulus
Advanced X-ray polarimeter design for nuclear resonant scattering
Journal of Synchrotron Radiation 28, 120 (2021)

Abstract: This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of Fe-57. By using four asymmetric reflections at asymmetry angles of alpha(1) = -28 degrees, alpha(2) = 28 degrees, alpha(3) = -28 degrees and alpha(4) = 28 degrees, the degree of polarization purity could be improved to 2.2 x 10(-9). For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.

K. S. Schulze, R. Lötzsch, R. Rüffer, I. Uschmann, R. Röhlsberger, and G. G. Paulus
X-ray dichroism in polyimide caused by non-resonant scattering
Journal of Synchrotron Radiation 28, 176 (2021)

Abstract: Dichroism is one of the most important optical effects in both the visible and the X-ray range. Besides absorption, scattering can also contribute to dichroism. This paper demonstrates that, based on the example of polyimide, materials can show tiny dichroism even far from electronic resonances due to scattering. Although the effect is small, it can lead to a measurable polarization change and might have influence on highly sensitive polarimetric experiments.

R. Hollinger, P. Herrmann, V. Korolev, M. Zapf, V. Shumakova, R. Roeder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO
Nanomaterials 11, 4 (2021)

Abstract: The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 mu m. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

A. T. Schmitt, Y. Joly, K. S. Schulze, B. Marx-Glowna, I. Uschmann, B. Grabiger, H. Bernhardt, R. Lötzsch, A. Juhin, J. Debray, H.-C. Wille, H. Yavaş, G. G. Paulus, and R. Röhlsberger
Disentangling x-ray dichroism and birefringence via high-purity polarimetry
Optica 8, 56 (2021)

Abstract: High-brilliance synchrotron radiation sources have opened new avenues for x-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear x-ray polarizers in a crossed setting, polarization extinction ratios down to 10⁻¹⁰ can be achieved. This renders the method sensitive to probe the tiniest optical anisotropies that would occur, for example, in strong-field quantum electrodynamics due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La₂CuO₄ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and x-ray lasers, where polarization extinction ratios of 10⁻¹² can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.

S. Strnat, V. A. Yerokhin, A. V. Volotka, G. Weber, S. Fritzsche, R. A. Müller, and A. Surzhykov
Polarization studies on Rayleigh scattering of hard x rays by closed-shell atoms
Physical Review A 103, 012801 (2021)

Abstract: We present a theoretical study on the elastic Rayleigh scattering of x-ray photons by closed-shell atoms. Special attention is paid to the transfer of linear polarization from the incident to the outgoing photons. To study this process, we apply the density-matrix formalism combined with the relativistic perturbation theory. This formalism enables us to find general relations between the Stokes parameters of the incident and scattered photons. By using these expressions, we revisit the recent proposal to use Rayleigh scattering for the analysis of the polarization purity of synchrotron radiation. We show that this analysis can be performed without any need for the theoretically calculated scattering amplitudes, if the linear polarization of the scattered light is measured simultaneously at the azimuthal angles 0 degrees and 45 degrees with respect to the plane of the synchrotron. To illustrate our approach, we present detailed calculations for scattering of 145 keV photons by lead atoms.

S. Panahiyan, and S. Fritzsche
Toward simulation of topological phenomena with one-, two-, and three-dimensional quantum walks
Physical Review A 103, 012201 (2021)

Abstract: We study the simulation of the topological phases in three subsequent dimensions with quantum walks. We focus mainly on the completion of a table for the protocols of the quantum walk that could simulate different families of the topological phases in one, two, and three dimensions. We also highlight the possible boundary states that can be observed for each protocol in different dimensions and extract the conditions for their emergences. To further enrich the simulation of the topological phenomena, we include step-dependent coins in the evolution operators of the quantum walks. This leads to step dependence of the simulated topological phenomena and their properties which introduces dynamicity as a feature of simulated topological phases and boundary states. This dynamicity provides the step number of the quantum walk as a means to control and engineer the numbers of topological phases and boundary states, their numbers, types, and even occurrences.

A. Seidel, J. Osterhoff, and M. Zepf
Characterizing ultralow emittance electron beams using structured light fields
Physical Review Accelerators and Beams 24, 012803 (2021)

Abstract: Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Current methods for the characterization of low emittance electron beams such as pepperpot measurements or beam focus scanning are limited in their capability to resolve emittances in the sub 0.1 mm mrad regime. Here we propose a novel, highly sensitive method for the single shot characterization of the beam waist and emittance using interfering laser beams. In this scheme, two laser pulses are focused under an angle creating a gratinglike interference pattern. When the electron beam interacts with the structured laser field, the phase space of the electron beam becomes modulated by the laser ponderomotive force and results in a modulated beam profile after further electron beam phase advance, which allows for the characterization of ultralow emittance beams. 2D PIC simulations show the effectiveness of the technique for normalized emittances in the range of epsilon(n) = 1/20.01; 1] mm mrad.

L. Wollenweber, T. R. Preston, A. Descamps, V. Cerantola, A. Comley, J. H. Eggert, L. B. Fletcher, G. Geloni, D. O. Gericke, S. H. Glenzer, S. Goede, J. Hastings, O. S. Humphries, A. Jenei, O. Karnbach, Z. Konopkova, R. Lötzsch, B. Marx-Glowna, E. E. McBride, D. McGonegle, G. Monaco, B. K. Ofori-Okai, C. A. J. Palmer, C. Plückthun, R. Redmer, C. Strohm, I. Thorpe, T. Tschentscher, I. Uschmann, J. S. Wark, T. G. White, K. Appel, G. Gregori, and U. Zastrau
High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the Free-Electron Laser
Review of Scientific Instruments 92, 013101 (2021)

Abstract: We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.

L. Bocklage, J. Gollwitzer, C. Strohm, C. Adolff, K. Schlage, I. Sergeev, O. Leupold, H.-C. Wille, G. Meier, and R. Röhlsberger
Coherent control of collective nuclear quantum states via transient magnons
Science Advances 7, 1 (2021)

Abstract: Ultrafast and precise control of quantum systems at x-ray energies involves photons with oscillation periods below 1 as. Coherent dynamic control of quantum systems at these energies is one of the major challenges in hard x-ray quantum optics. Here, we demonstrate that the phase of a quantum system embedded in a solid can be coherently controlled via a quasi-particle with subattosecond accuracy. In particular, we tune the quantum phase of a collectively excited nuclear state via transient magnons with a precision of 1 zs and a timing stability below 50 ys. These small temporal shifts are monitored interferometrically via quantum beats between different hyperfine-split levels.The experiment demonstrates zeptosecond interferometry and shows that transient quasi-particles enable accurate control of quantum systems embedded in condensed matter environments.


P. Wustelt, M. Kübel, G. Paulus, and A. Sayler
Strong-field laser-induced fragmentation of small molecules from fast to slow
Advances in Atomic, Molecular and Optical Physics 69, 67 (2020)

Abstract: The structure and dynamics of molecules are governed by the electric forces acting between electrons and nuclei. Intense, ultrashort laser pulses offer the possibility to manipulate these forces, on the time scales relevant for the motion of a molecule's constituents. Thus, laser fields can act, not only as a mechanism to trigger molecular dynamics, but also controlling them. The fragmentation patterns that result from the interaction testify to the laser-induced processes occurring in the molecule. In this review, we examine how a laser addresses the different degrees of freedom of a molecule, from electronic excitation to vibrations of nuclei, to rotations of the molecule. We will focus the discussion on the most fundamental systems, particularly H2+, H2, and HeH+. These simple systems allow for accurate theoretical analysis of experimental results, and extrapolation of the conclusions to more complex systems. Since some of the most fundamental molecules, such as HeH+ and H3+ do not exist in the neutral form, we put an emphasis on experiments starting from molecular ions, but do not restrict the discussion to these. Strong-field interactions of small molecules are a test ground, not only for experimental but also for theoretical methods. The joint effort of the two scientific disciplines have delivered deep insights into fundamental concepts of molecular science. The recent developments of novel laser sources with longer wavelength, higher peak power, or repetition rates, as well as more complex targets and detection schemes, promise that the field will remain highly relevant in the decades to come.

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.

R. Hollinger, D. Gupta, M. Zapf, M. Karst, R. Röder, I. Uschmann, U. Reislöhner, D. Kartashov, C. Ronning, and C. Spielmann
Polarization dependent multiphoton absorption in ZnO thin films
Journal of Physics D: Applied Physics 53, 055102 (2020)

Abstract: We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

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.