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Referierte Publikationen


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

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

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

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

J. B. Ohland, U. Eisenbarth, B. Zielbauer, Y. Zobus, D. Posor, J. Hornung, D. Reemts, and V. Bagnoud
Ultra-compact post-compressor on-shot wavefront measurement for beam correction at PHELIX
High Power Laser Science and Engineering 10, 18 (2022)

Abstract: In order to reach the highest intensities, modern laser systems use adaptive optics to control their beam quality. Ideally, the focal spot is optimized after the compression stage of the system in order to avoid spatio-temporal couplings. This also requires a wavefront sensor after the compressor, which should be able to measure the wavefront on-shot. At PHELIX, we have developed an ultra-compact post-compressor beam diagnostic due to strict space constraints, measuring the wavefront over the full aperture of 28 cm. This system features all-reflective imaging beam transport and a high dynamic range in order to measure the wavefront in alignment mode as well as on shot.

A. de Beurs, L. Lötgering, M. Herczog, M. Du, K. Eikema, and S. Witte
aPIE: an angle calibration algorithm for reflection ptychography
Optics Letters 47, 1949 (2022)

Abstract: Reflection ptychography is a lensfree microscopy technique particularly promising in regions of the electromagnetic spectrum where imaging optics are inefficient or not available. This is the case in tabletop extreme ultraviolet microscopy and grazing incidence small angle x ray scattering experiments. Combining such experimental configurations with ptychography requires accurate knowledge of the relative tilt between the sample and the detector in non-coplanar scattering geometries. Here, we describe an algorithm for tilt estimation in reflection ptychography. The method is verified experimentally, enabling sample tilt determination within a fraction of a degree. Furthermore, the angle-estimation uncertainty and reconstruction quality are studied for both smooth and highly structured beams.

D. Budker, J. Berengut, V. Flambaum, M. Gorchtein, J. Jin, F. Karbstein, M. Krasny, Y. Litvinov, A. Pálffy, V. Pascalutsa, A. Petrenko, A. Surzhykov, P. Thirolf, M. Vanderhaeghen, H. Weidenmüller, and V. Zelevinsky
Expanding Nuclear Physics Horizons with the Gamma Factory
Annalen der Physik 534 (3), 2100284 (2022)

Abstract: Abstract The Gamma Factory (GF) is an ambitious proposal, currently explored within the CERN Physics Beyond Colliders program, for a source of photons with energies up to ≈400 MeV and photon fluxes (up to ≈1017 photons s-1) exceeding those of the currently available gamma sources by orders of magnitude. The high-energy (secondary) photons are produced via resonant scattering of the primary laser photons by highly relativistic partially-stripped ions circulating in the accelerator. The secondary photons are emitted in a narrow cone and the energy of the beam can be monochromatized, down to 10-3...10-6 level, via collimation, at the expense of the photon flux. This paper surveys the new opportunities that may be afforded by the GF in nuclear physics and related fields.

F. Karbstein
Vacuum Birefringence at the Gamma Factory
Annalen der Physik 534 (3), 2100137 (2022)

Abstract: Abstract The perspectives of studying vacuum birefringence at the Gamma Factory are explored. To this end, the parameter regime which can be reliably analyzed resorting to the leading contribution to the Heisenberg?Euler effective Lagrangian is assessed in detail. It is explicitly shown that?contrary to naive expectations?this approach allows for the accurate theoretical study of quantum vacuum signatures up to fairly large photon energies. The big advantage of this parameter regime is the possibility of studying the phenomenon in experimentally realistic, manifestly inhomogeneous pump and probe field configurations. Thereafter, two specific scenarios giving rise to a vacuum birefringence effect for traversing gamma probe photons are analyzed. In the first scenario the birefringence phenomenon is induced by a quasi-constant static magnetic field. In the second case it is driven by a counter-propagating high-intensity laser field.

S. Fritzsche
Level Structure and Properties of Open f-Shell Elements
Atoms 10, 7 (2022)

Abstract: Open f-shell elements still constitute a great challenge for atomic theory owing to their (very) rich fine-structure and strong correlations among the valence-shell electrons. For these medium and heavy elements, many atomic properties are sensitive to the correlated motion of electrons and, hence, require large-scale computations in order to deal consistently with all relativistic, correlation and rearrangement contributions to the electron density. Often, different concepts and notations need to be combined for just classifying the low-lying level structure of these elements. With JAC, the Jena Atomic Calculator, we here provide a toolbox that helps to explore and deal with such elements with open d- and f-shell structures. Based on Dirac\textquotesingle s equation, JAC is suitable for almost all atoms and ions across the periodic table. As an example, we demonstrate how reasonably accurate computations can be performed for the low-lying level structure, transition probabilities and lifetimes for Th2+ ions with a 5f6d ground configuration. Other, and more complex, shell structures are supported as well, though often for a trade-off between the size and accuracy of the computations. Owing to its simple use, however, JAC supports both quick estimates and detailed case studies on open d- or f-shell elements.

S. Ramakrishna, J. Hofbrucker, and S. Fritzsche
Photoexcitation of atoms by cylindrically polarized Laguerre-Gaussian beams
Physical Review A 105, 033103 (2022)

Abstract: We analyze the photoexcitation of atoms with a single valence electron by cylindrically polarized Laguerre-Gaussian beams. Theoretical analysis is performed within the framework of first-order perturbation theory and by expanding the vector potential of the Laguerre-Gaussian beam in terms of its multipole components. For cylindrically polarized Laguerre-Gaussian beams, we show that the (magnetic) sub-components of electric-quadrupole field vary significantly in the beam cross section with beam waist and radial distance from the beam axis. We discuss the influence of varying magnetic multipole components in the beam cross section on the sublevel population of a localized atomic target. In addition, we calculate the total excitation rate of electric-quadrupole transition (4s S-2(1/2) -> 3d D-2(5/2)) in a mesoscopic target of a Ca+ ion. These calculations shows that the total rate of excitation is sensitive to the beam waist and the distance between the center of the target and the beam axis. However, the excitation by a cylindrically polarized Laguerre-Gaussian beam is found more efficient in driving electric-quadrupole transition in the mesoscopic atomic target than the circularly polarized beams.

V. P. Kosheleva, A. V. Volotka, D. A. Glazov, D. V. Zinenko, and S. Fritzsche
g Factor of Lithiumlike Silicon and Calcium: Resolving the Disagreement between Theory and Experiment
Physical Review Letters 128, 103001 (2022)

Abstract: The bound-electron g factor is a stringent tool for tests of the standard model and the search for new physics. The comparison between an experiment on the g factor of lithiumlike silicon and the two recent theoretical values revealed the discrepancies of 1.7 sigma [Glazov et al. Phys. Rev. Lett. 123, 173001 (2019)] and 5.2 sigma [Yerokhin et al. Phys. Rev. A 102, 022815 (2020)]. To identify the reason for this disagreement, we accomplish large-scale high-precision computation of the interelectronic-interaction and many-electron QED corrections. The calculations are performed within the extended Furry picture of QED, and the dependence of the final values on the choice of the binding potential is carefully analyzed. As a result, we significantly improve the agreement between the theory and experiment for the g factor of lithiumlike silicon. We also report the most accurate theoretical prediction to date for lithiumlike calcium, which perfectly agrees with the experimental value.

K. Schulze, B. Grabiger, R. Loetzsch, B. Marx-Glowna, A. Schmitt, A. Garcia, W. Hippler, L. Huang, F. Karbstein, Z. Konopková, H.-P. Schlenvoigt, J.-P. Schwinkendorf, C. Strohm, T. Toncian, I. Uschmann, H.-C. Wille, U. Zastrau, R. Röhlsberger, T. Stöhlker, T. Cowan, and G. Paulus
Towards perfectly linearly polarized x-rays
Physical Review Research 4, 013220 (2022)

Abstract: In recent years, high-precision x-ray polarimeters have become a key method for the investigation of fundamental physical questions from solid-state physics to quantum optics. Here, we report on the verification of a polarization purity of better than 8×10−11 at an x-ray free-electron laser, which implies a suppression of the incoming photons to the noise level in the crossed polarizer setting. This purity provides exceptional sensitivity to tiny polarization changes and offers intriguing perspectives for fundamental tests of quantum electrodynamics.

S. Fuchs, J. Abel, J. Nathanael, J. Reinhard, F. Wiesner, M. Wuensche, S. Skruszewicz, C. Roedel, D. Born, H. Schmidt, and G. Paulus
Photon counting of extreme ultraviolet high harmonics using a superconducting nanowire single-photon detector
Applied Physics B 128, 26 (2022)

Abstract: Laser-driven light sources in the extreme ultraviolet range (EUV) enable nanoscopic imaging with unique label-free elemental contrast. However, to fully exploit the unique properties of these new sources, novel detection schemes need to be developed. Here, we show in a proof-of-concept experiment that superconducting nanowire single-photon detectors (SNSPD) can be utilized to enable photon counting of a laser-driven EUV source based on high harmonic generation (HHG). These detectors are dark-count free and accommodate very high count rates-a perfect match for high repetition rate HHG sources. In addition to the advantages of SNSPDs for classical imaging applications with laser-driven EUV sources, the ability to count single photons paves the way for very promising applications in quantum optics and quantum imaging with high energetic radiation like, e.g., quantum ghost imaging with nanoscale resolution.

J. Fan, J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Relativistic calculations of two-color two-photon K-shell ionization
European Physical Journal D 76, 18 (2022)

Abstract: We investigate the two-color two-photon K-shell ionization of neutral atoms based on the relativistic second-order perturbation theory and independent particle approximation. Analytical expressions for the relativistic and nonrelativistic total cross sections are derived in terms of radial transition amplitudes and Stokes parameters. Particular attention is paid especially to how the two-photon ionization total cross section depends on the energy sharing and polarization of the two incident photons. We construct the nonrelativistic expressions of cross section ratios for different polarization combinations of the two incident photons. The numerical results of total cross section and cross section ratios show that the energy sharing of the two incident photons plays an essential role in two-photon K-shell ionization. Particularly, if the energies of the two incident photons are identical, the total cross section and cross section ratios will reach the minimum or maximum value. Moreover, due to the strong screening effects, we find strong deviations of the cross section ratios near the two-photon ionization threshold of the Ne atom.

L. Doyle, P. Khademi, P. Hilz, A. Sävert, G. Schäfer, J. Schreiber, and M. Zepf
Experimental estimates of the photon background in a potential light-by-light scattering study
New Journal of Physics 24, 025003 (2022)

Abstract: High power short pulse lasers provide a promising route to study the strong field effects of the quantum vacuum, for example by direct photon-photon scattering in the all-optical regime. Theoretical predictions based on realistic laser parameters achievable today or in the near future predict scattering of a few photons with colliding Petawatt laser pulses, requiring single photon sensitive detection schemes and very good spatio-temporal filtering and background suppression. In this article, we present experimental investigations of this photon background by employing only a single high power laser pulse tightly focused in residual gas of a vacuum chamber. The focal region was imaged onto a single-photon sensitive, time gated camera. As no detectable quantum vacuum signature was expected in our case, the setup allowed for characterization and first mitigation of background contributions. For the setup employed, scattering off surfaces of imperfect optics dominated below residual gas pressures of 1 x 10(-4) mbar. Extrapolation of the findings to intensities relevant for photon-photon scattering studies is discussed.

A.-L. Viotti, M. Seidel, E. Escoto, S. Rajhans, W. Leemans, I. Hartl, and C. Heyl
Multi-pass cells for post-compression of ultrashort laser pulses
Optica 9, 197 (2022)

Abstract: Ultrafast lasers reaching extremely high powers within short fractions of time enable a plethora of applications. They grant advanced material processing capabilities, are effective drivers for secondary photon and particle sources, and reveal extreme light-matter interactions. They also supply platforms for compact accelerator technologies, with great application prospects for tumor therapy or medical diagnostics. Many of these scientific cases benefit from sources with higher average and peak powers. Following mode-locked dye and titanium-doped sapphire lasers, broadband optical parametric amplifiers have emerged as high peak- and average power ultrashort pulse lasers. A much more power-efficient alternative is provided by direct post-compression of high-power diode-pumped ytterbium lasers-a route that advanced to another level with the invention of a novel spectral broadening approach, the multi-pass cell technique. The method has enabled benchmark results yielding sub-50-fs pules at average powers exceeding 1 kW, has facilitated femtosecond post-compression at pulse energies above 100 mJ with large compression ratios, and supports picosecond to few-cycle pulses with compact setups. The striking progress of the technique in the past five years puts light sources with tens to hundreds of TW peak and multiple kW of average power in sight-an entirely new parameter regime for ultrafast lasers. In this review, we introduce the underlying concepts and give brief guidelines for multi-pass cell design and implementation. We then present an overview of the achieved performances with both bulk and gas-filled multipass cells. Moreover, we discuss prospective advances enabled by this method, in particular including opportunities for applications demanding ultrahigh peak-power, high repetition rate lasers such as plasma accelerators and laser-driven extreme ultraviolet sources.

N. Dimitrov, M. Zhekova, Y. Zhang, G. Paulus, and A. Dreischuh
Background-free femtosecond autocorrelation in collinearly-aligned inverted field geometry using optical vortices
OPTICS COMMUNICATIONS 504, 127493 (2022)

Abstract: Among the existing techniques for measuring ultrashort pulse durations, the two classical second-order methods - interferometric and the background-free autocorrelation - are distinguished due to their simplicity and reliability. In this work we report on a technique that allows realignment-free switching between these two modes of autocorrelation. It is based on a collinearly aligned inverted-field interferometer and an optical vortex plate that is added/removed in front of the device in order to switch between both modes. Experiment and theoretical modeling confirm the effectiveness of the technique down to the 10-fs range.

C. Mahnke, Y. Hua, Y. Ma, S. Salman, T. Lamb, S. Schulz, C. Heyl, H. Cankaya, and I. Hartl
Long-term stable, synchronizable, low-noise picosecond Ho:fiber NALM oscillator for Ho:YLF amplifier seeding
Optics letters 47, 822 (2022)

Abstract: We demonstrate a 41.6 MHz, 1.3 ps, 140 pJ Ho:fiber oscillator using a nonlinear amplifying loop mirror (NALM) as saturable absorber. The oscillator is constructed entirely with polarization-maintaining (PM) fibers, is tunable with a center wavelength between 2035 nm and 2075 nm, and can be synchronized to an external RF reference. For our application of Ho:YLF amplifier seeding for dielectric electron acceleration, the laser is tuned to 2050 nm and synchronized to a stable RF reference with 45 fs rms timing jitter in the integration interval [10 Hz, 1 MHz]. We show long term synchronized operation and characterize the relative intensity noise (RIN) and timing jitter of the oscillator for two different Tm-fiber pump lasers.

F. Karbstein
Large N external-field quantum electrodynamics
Journal of High Energy Physics 01, 057 (2022)

Abstract: We advocate the study of external-field quantum electrodynamics with N charged particle flavors. Our main focus is on the Heisenberg-Euler effective action for this theory in the large N limit which receives contributions from all loop orders. The contributions beyond one loop stem from one-particle reducible diagrams. We show that specifically in constant electromagnetic fields the latter are generated by the one-loop Heisenberg-Euler effective Lagrangian. Hence, in this case the large N Heisenberg-Euler effective action can be determined explicitly at any desired loop order. We demonstrate that further analytical insights are possible for electric-and magnetic-like field configurations characterized by the vanishing of one of the secular invariants of the electromagnetic field and work out the all-orders strong field limit of the theory.

F. C. Salgado, N. Cavanagh, M. Tamburini, D. W. Storey, R. Beyer, P. H. Bucksbaum, Z. Chen, A. Di Piazza, E. Gerstmayr, . Harsh, E. Isele, A. R. Junghans, C. H. Keitel, S. Kuschel, C. F. Nielsen, D. A. Reis, C. Roedel, G. Sarri, A. Seidel, C. Schneider, I. Uggerhoj, J. Wulff, V. Yakimenko, C. Zepter, S. Meuren, and M. Zepf
Single particle detection system for strong-field QED experiments
New Journal of Physics 24, 015002 (2022)

Abstract: Measuring signatures of strong-field quantum electrodynamics (SF-QED) processes in an intense laser field is an experimental challenge: it requires detectors to be highly sensitive to single electrons and positrons in the presence of the typically very strong x-ray and gamma-photon background levels. In this paper, we describe a particle detector capable of diagnosing single leptons from SF-QED interactions and discuss the background level simulations for the upcoming Experiment-320 at FACET-II (SLAC National Accelerator Laboratory). The single particle detection system described here combines pixelated scintillation LYSO screens and a Cherenkov calorimeter. We detail the performance of the system using simulations and a calibration of the Cherenkov detector at the ELBE accelerator. Single 3 GeV leptons are expected to produce approximately 537 detectable photons in a single calorimeter channel. This signal is compared to Monte-Carlo simulations of the experiment. A signal-to-noise ratio of 18 in a single Cherenkov calorimeter detector is expected and a spectral resolution of 2% is achieved using the pixelated LYSO screens.

L. Stoyanov, G. Maleshkov, I. Stefanov, G. Paulus, and A. Dreischuh
Focal beam structuring by triple mixing of optical vortex lattices
Optical and Quantum Electronics 54, 34 (2022)

Abstract: On-demand generation and reshaping of arrays of focused laser beams is highly desired in many areas of science and technology. In this work, we present a versatile approach for laser beam structuring in the focal plane of a lens by triple mixing of square and/or hexagonal optical vortex lattices (OVLs). In the artificial far field the input Gaussian beam is reshaped into ordered arrays of bright beams with flat phase profiles. This is remarkable, since the bright focal peaks are surrounded by hundreds of OVs with their dark cores and two-dimensional phase dislocations. Numerical simulations and experimental evidences for this are shown, including a broad discussion of some of the possible scenarios for such mixing: triple mixing of square-shaped OVLs, triple mixing of hexagonal OVLs, as well as the two combined cases of mixing square-hexagonal-hexagonal and square-square-hexagonal OVLs. The particular ordering of the input phase distributions of the OV lattices on the used spatial light modulators is found to affect the orientation of the structures ruled by the hexagonal OVL. Reliable control parameters for the creation of the desired focal beam structures are the respective lattice node spacings. The presented approach is flexible, easily realizable by using a single spatial light modulator, and thus accessible in many laboratories.

C. Wu, L. Li, M. Yeung, S. Wu, S. Cousens, S. Tietze, B. Dromey, C. Zhou, S. Ruan, and M. Zepf
Proposal for complete characterization of attosecond pulses from relativistic plasmas
Optics Express 30, 389 (2022)

Abstract: In this study, we propose two full-optical-setup and single-shot measurable approaches for complete characterization of attosecond pulses from surface high harmonic generation (SHHG): SHHG-SPIDER (spectral phase interferometry for direct electric field reconstruction) and SHHG-SEA-SPIDER (spatially encoded arrangement for SPIDER). 1D- and 2D-EPOCH PIC (particle-in-cell) simulations were performed to generate the attosecond pulses from relativistic plasmas under different conditions. Pulse trains dominated by single isolated peak as well as complex pulse train structures are extensively discussed for both methods, which showed excellent accuracy in the complete reconstruction of the attosecond field with respect to the direct Fourier transformed result. Kirchhoff integral theorem has been used for the near-to-far-field transformation. This far-field propagation method allows us to relate these results to potential experimental implementations of the scheme. The impact of comprehensive experimental parameters for both apparatus, such as spectral shear, spatial shear, cross-angle, time delay, and intensity ratio between the two replicas has been investigated thoroughly. These methods are applicable to complete characterization for SHHG attosecond pulses driven by a few to hundreds of terawatts femtosecond laser systems.

S. Keppler, N. Elkina, G. A. Becker, J. Hein, M. Hornung, M. Mäusezahl, C. Rodel, I. Tamer, M. Zepf, and M. C. Kaluza
Intensity scaling limitations of laser-driven proton acceleration in the TNSA-regime
Physical Review Research 4, 013065 (2022)

Abstract: We report on experimental results on laser-driven proton acceleration using high-intensity laser pulses. We present power law scalings of the maximum proton energy with laser pulse energy and show that the scaling exponent 4 strongly depends on the scale length of the preplasma, which is affected by the temporal intensity contrast. At lower laser intensities, a shortening of the scale length leads to a transition from a square root toward a linear scaling. Above a certain threshold, however, a significant deviation from this scaling is observed. Two-dimensional particle-in-cell simulations show that, in this case, the electric field accelerating the ions is generated earlier and has a higher amplitude. However, since the acceleration process starts earlier as well, the fastest protons outrun the region of highest field strength, ultimately rendering the acceleration less effective. Our investigations thus point to a principle limitation of the proton energy in the target normal sheath acceleration regime, which would explain why a significant increase of the maximum proton energy above the limit of 100 MeV has not yet been achieved.

E. Eftekhari-Zadeh, M. S. Blümcke, Z. Samsonova, R. Loetzsch, I. Uschmann, M. Zapf, C. Ronning, O. N. Rosmej, D. Kartashov, and C. Spielmann
Laser energy absorption and x-ray generation in nanowire arrays irradiated by relativistically intense ultra-high contrast femtosecond laser pulses
Physics of Plasmas 29, 013301 (2022)

Abstract: We report here on the results of comparative experimental measurements of laser energy absorption in a bulk and different morphology nanowire arrays interacting with relativistically intense, ultra-high temporal contrast femtosecond laser pulses. We compare polished, flat bulk samples with vertically and randomly oriented nanowires made of ZnO semiconductor material. The optical absorption of the 45° incident laser pulses of ∼40 fs duration with a central wavelength of 400 nm at intensities above 1019Wcm2 was determined using an integrating Ulbricht sphere. We demonstrate an almost twofold enhancement of absorption in both nanowire morphologies with an average of (79.6±1.9)% in comparison to the flat bulk sample of (45.8±1.9)%. The observed substantially enhanced absorption in nanowire arrays is also confirmed by high-resolution x-ray emission spectroscopy. The spectral analysis of the K-shell x-ray emission lines revealed that the He-like resonance line emission from highly ionized Zn (Zn28+) is only present in the case of nanowire arrays, whereas, for the flat bulk samples, only neutral and low charge states were observed. Our numerical simulations, based on radiative-collisional kinetic code FLYCHK, well reproduce the measured He-like emission spectrum and suggest that high charge state observed in nanowire arrays is due to substantially higher plasma temperature. Our results, which were measured for the first time with femtosecond laser pulses, can be used to benchmark theoretical models and numerical codes for the relativistic interaction of ultrashort laser pulses with nanowires.


W. Middents, G. Weber, U. Spillmann, T. Krings, M. Vockert, A. Volotka, A. Surzhykov, and T. Stöhlker
Possible Polarization Measurements in Elastic Scattering at the Gamma Factory Utilizing a 2D Sensitive Strip Detector as Dedicated Compton Polarimeter
Annalen der Physik 2100285, 2100285 (2021)
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S. Fritzsche
Dielectronic recombination strengths and plasma rate coefficients of multiply charged ions
Astronomy and Astrophysics 656, A163 (2021)
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J. Hornung, Y. Zobus, S. Roeder, A. Kleinschmidt, D. Bertini, M. Zepf, and V. Bagnoud
Time-resolved study of holeboring in realistic experimental conditions
Nature Communications 12, 6999 (2021)
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A. Weber, B. Böning, B. Minneker, and S. Fritzsche
Generation of elliptically polarized high-order harmonic radiation with bi-elliptical two-color laser beams
Physical Review A 104, 063118 (2021)
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Z. W. Wu, Z. Q. Tian, J. Jiang, C. Z. Dong, and S. Fritzsche
Hyperfine-induced effects on the K {α_1} angular distribution following electron-impact excitation of heliumlike spin- 1/2 Tl⁷⁹⁺ ions
Physical Review A 104, 062814 (2021)
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E. Appi, C. C. Papadopoulou, J. L. Mapa, C. Jusko, P. Mosel, A. Schoenberg, J. Stock, T. Feigl, S. Ališauskas, T. Lang, C. M. Heyl, B. Manschwetus, M. Brachmanski, M. Braune, H. Lindenblatt, F. Trost, S. Meister, P. Schoch, A. Trabattoni, F. Calegari, R. Treusch, R. Moshammer, I. Hartl, U. Morgner, and M. Kovacev
Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies
Review of Scientific Instruments 92, 123004 (2021)
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B. Kettle, D. Hollatz, E. Gerstmayr, G. M. Samarin, A. Alejo, S. Astbury, C. Baird, S. Bohlen, M. Campbell, C. Colgan, D. Dannheim, C. Gregory, H. Harsh, P. Hatfield, J. Hinojosa, Y. Katzir, J. Morton, C. D. Murphy, A. Nurnberg, J. Osterhoff, G. Pérez-Callejo, K. Põder, P. P. Rajeev, C. Roedel, F. Roeder, F. C. Salgado, G. Sarri, A. Seidel, S. Spannagel, C. Spindloe, S. Steinke, M. J. V. Streeter, A. G. R. Thomas, C. Underwood, R. Watt, M. Zepf, S. J. Rose, and S. P. D. Mangles
A laser–plasma platform for photon–photon physics: the two photon Breit–Wheeler process
New Journal of Physics 23, 115006 (2021)
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S. Skruszewicz, A. Przystawik, D. Schwickert, M. Sumfleth, M. Namboodiri, V. Hilbert, R. Klas, P. Gierschke, V. Schuster, A. Vorobiov, C. Haunhorst, D. Kip, J. Limpert, J. Rothhardt, and T. Laarmann
Table-top interferometry on extreme time and wavelength scales
Optics Express 29, 40333 (2021)

Abstract: High-resolution Fourier-transform spectroscopy using table-top sources in the extreme ultraviolet (XUV) spectral range is still in its infancy. In this contribution a significant advance is presented based on a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm - 1 µ m) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision resulting in a resolution of E/Δ E=2000. Its performance for spectroscopic applications is demonstrated by characterizing a very sharp electronic transition at 26.6 eV in Ar gas induced by the 11th harmonic of a frequency-doubled Yb-fiber laser leading to the characteristic 3s3p⁶4p¹P¹ Fano-resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.

A. Perry-Sassmannshausen, T. Buhr, M. Martins, S. Reinwardt, F. Trinter, A. Müller, S. Fritzsche, and S. Schippers
Multiple photodetachment of silicon anions via K -shell excitation and ionization
Physical Review A 104, 053107 (2021)
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B. Minneker, B. Böning, A. Weber, and S. Fritzsche
Torus-knot angular momentum in twisted attosecond pulses from high-order harmonic generation
Physical Review A 104, 053116 (2021)
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L. Du, F. Roeder, Y. Li, M. Shalaby, B. Beleites, F. Ronneberger, and A. Gopal
Organic crystal-based THz source for complex refractive index measurements of window materials using single-shot THz spectroscopy
Applied Physics A 127, 846 (2021)

Abstract: We employed N-benzyl-2-methyl-4-nitroaniline (BNA) crystals bonded on substrates of different thermal conductivity to generate THz radiation by pumping with 800 nm laser pulses. Crystals bonded on sapphire substrate provided four times more THz yield than glass substrate. A pyrodetector and a single-shot electro-optic (EO) diagnostic were employed for measuring the energy and temporal characterisation of the THz pulse. Systematic studies were carried out for the selection of a suitable EO crystal, which allowed accurate determination of the emitted THz spectrum from both substrates. Subsequently, the THz source and single-shot electro-optic detection scheme were employed to measure the complex refractive index of window materials in the THz range.

G. Gaigalas, and S. Fritzsche
Angular coefficients for symmetry-adapted configuration states in jj-coupling
Computer Physics Communications 267, 108086 (2021)

Abstract: In atomic structure and collision theory, the efficient spin-angular integration is known to be crucial and often decides, how accurate the properties and behavior of atoms can be predicted numerically. Various methods have been developed in the past to keep the computation (and implementation) of the spin-angular integration feasible for complex shell structures, including open d- and f-shell elements. To support such computations, we here provide a new implementation of the angular coefficients for jjcoupled and symmetry-adapted configuration states that is entirely built upon the quasi-spin formalism. The moduleSpinAngularis based on Julia, a new programming language for scientific computing, and supports a simple access to all (completely) reduced tensors, coefficients of fractional parentage for subshells with j <= 9/2 as well as the re-coupling coefficients from this formalism. Moreover, this module has been worked out for multiple purposes, including 1) the accurate calculation of atomic properties, 2) further studies on spin-angular integration theory, 3) the development of new or existing computer programs as well as 4) the manipulation of reduced matrix elements from this theory. The present implementation will therefore help advance the algebraic evaluation of many-electron (transition) amplitudes and to apply the theory to newly emerging research areas.

F. C. Salgado, K. Grafenstein, A. Golub, A. Dopp, A. Eckey, D. Hollatz, C. Muller, A. Seidel, D. Seipt, S. Karsch, and M. Zepf
Towards pair production in the non-perturbative regime
New Journal of Physics 23, 105002 (2021)

Abstract: The interaction of light with the quantum-vacuum is predicted to give rise to some of the most fundamental and exotic processes in modern physics, which remain untested in the laboratory to date. Electron-positron pair production from a pure vacuum target, which has yet to be observed experimentally, is possibly the most iconic. The advent of ultra-intense lasers and laser accelerated GeV electron beams provide an ideal platform for the experimental realisation. Collisions of high energy gamma-ray photons derived from the GeV electrons and intense laser fields result in detectable pair production rates at field strengths that approach and exceed the Schwinger limit in the centre-of-momentum frame. A detailed experiment has been designed to be implemented at the ATLAS laser at the centre of advanced laser applications. We show full calculations of the expected backgrounds and beam parameters which suggest that single pair events can be reliably generated and detected.

J. Ohland, Y. Zobus, U. Eisenbarth, B. Zielbauer, D. Reemts, and V. Bagnoud
Alignment procedure for off-axis-parabolic telescopes in the context of high-intensity laser beam transport
Optics Express 29, 34378 (2021)

Abstract: Off-axis parabolic telescopes are rarely used in high-intensity, high-energy lasers, despite their favorable properties for beam transport such as achromatism, low aberrations and the ability to handle high peak intensities. One of the major reasons for this is the alignment procedure which is commonly viewed as complicated and time consuming. In this article, we revisit off-axis parabolic telescopes in the context of beam transport in high-intensity laser systems and present a corresponding analytical model. Based on that, we propose a suitable setup that enables fast and repeatable alignment for everyday operation.

C. Jauregui, C. Stihler, S. Kholaif, Y. Tu, and J. Limpert
Control and stabilization of the modal content of fiber amplifiers using traveling waves
Optics Express 29, 34452 (2021)

Abstract: In this work we present a novel way to manipulate the effect of transverse mode instability by inducing traveling waves in a high-power fiber system. What sets this technique apart is the fact that it allows controlling the direction of the modal energy flow, for the first time to the best of our knowledge. Thus, using the method proposed in this work it will be possible to transfer energy from the higher-order mode into the fundamental mode of the fiber, which mitigates the effect of transverse mode instability, but also to transfer energy from the fundamental mode into the higher-order mode. Our simulations indicate that this approach will work both below and above the threshold of transverse mode instability. In fact, our model reveals that it can be used to force a nearly pure fundamental mode output in the fiber laser system almost independently of the input coupling conditions. In this context, this technique represents the first attempt to exploit the physics behind the effect of transverse mode instability to increase the performance of fiber laser systems.

J. Buldt, H. Stark, M. Mueller, C. Grebing, C. Jauregui, and J. Limpert
Gas-plasma-based generation of broadband terahertz radiation with 640 mW average power
Optics Letters 46, 5256 (2021)

Abstract: We present a high-power source of broadband terahertz (THz) radiation covering the whole THz spectral region (0.1-30 THz). The two-color gas plasma generation process is driven by a state-of-the-art ytterbium fiber chirped pulse amplification system based on coherent combination of 16 rod-type amplifiers. Prior to the THz generation, the pulses are spectrally broadened in a multipass cell and compressed to 37 fs with a pulse energy of 1.3 mJ at a repetition rate of 500 kHz. A gas-jet scheme has been employed for the THz generation, increasing the efficiency of the process to 0.1%. The air-biased coherent detection scheme is implemented to characterize the full bandwidth of the generated radiation. A THz average power of 640 mW is generated, which is the highest THz average power achieved to date. This makes this source suitable for a variety of applications, e.g., spectroscopy of strongly absorbing samples or driving nonlinear effects for the studies of material properties.

D. Wanisch, and S. Fritzsche
Delocalization of quantum information in long-range interacting systems
Physical Review A 104, 042409 (2021)

Abstract: We investigate the delocalization of quantum information in the nonequilibrium dynamics of the XY spin chain with asymptotically decaying interactions similar to 1/r(alpha). As a figure of merit, we employ the tripartite mutual information (TMI), the sign of which indicates if quantum information is predominantly shared globally. Interestingly, the sign of the TMI distinguishes regimes of the exponent a that are known for different behaviors of information propagation. While an effective causal region bounds the propagation of information, if interactions decay sufficiently fast, this information is mainly delocalized, which leads to the necessity of global measurements. Furthermore, the results indicate that mutual information is monogamous for all possible partitionings in this case, implying that quantum entanglement is the dominant correlation. If interactions decay sufficiently slow, though information can propagate (quasi-)instantaneously, it is mainly accessible by local measurements at early times. Furthermore, it takes some finite time until correlations start to become monogamous, which suggests that entanglement is not the dominant correlation at early times. Our findings give new insights into the dynamics and structure of quantum information in many-body systems with long-range interactions, and might get verified on state-of-the-art experimental platforms.

F. Karbstein, and R. Oude Weernink
X-ray vacuum diffraction at finite spatiotemporal offset
Physical Review D 104, 076015 (2021)

Abstract: We study the nonlinear QED signature of x-ray vacuum diffraction in the head-on collision of optical high-intensity and x-ray free-electron laser pulses at finite spatiotemporal offsets between the laser foci. The high-intensity laser driven scattering of signal photons outside the forward cone of the x-ray probe constitutes a prospective experimental signature of quantum vacuum nonlinearity. Resorting to a simplified phenomenological ad hoc model, it was recently argued that the angular distribution of the signal in the far-field is sensitive to the wavefront curvature of the probe beam in the interaction region with the high-intensity pump. In this work, we model both the pump and probe fields as pulsed paraxial Gaussian beams and reanalyze this effect from first principles. We focus on vacuum diffraction both as an individual signature of quantum vacuum nonlinearity and as a potential means to improve the signal-to-background separation in vacuum birefringence experiments.

M. Zimmer, S. Scheuren, T. Ebert, G. Schaumann, B. Schmitz, J. Hornung, V. Bagnoud, C. Rödel, and M. Roth
Analysis of laser-proton acceleration experiments for development of empirical scaling laws
Physical Review E 104, 045210 (2021)
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M. Chambonneau, M. Blothe, Q. Li, V. Fedorov, T. Heuermann, M. Gebhardt, C. Gaida, S. Tertelmann, F. Sotier, J. Limpert, S. Tzortzakis, and S. Nolte
Transverse ultrafast laser inscription in bulk silicon
Physical Review Research 3, 043037 (2021)

Abstract: In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the production of modifications. By using advantageous spectral, temporal, and spatial conditions, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on irradiation at approximate to 2 mu m wavelength with temporally distorted femtosecond pulses. These pulses are focused in a way that spherical aberrations of different origins mutually balance, as predicted by point spread function analyses and in good agreement with nonlinear propagation simulations. We also establish the laws governing modification growth on a pulse-to-pulse basis, which allows us to demonstrate transverse inscription inside silicon with various line morphologies depending on the irradiation conditions. We finally show that the production of single-pulse repeatable modifications is a necessary condition for reliable transverse inscription inside silicon.

A. Thomas, and D. Seipt
Modeling chromatic emittance growth in staged plasma wakefield acceleration to 1 TeV using nonlinear transfer matrices
Physical Review ST - Accelerators and Beams 24, 104602 (2021)
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M. Almassarani, S. Meng, B. Beleites, F. Ronneberger, G. Paulus, and A. Gopal
Parametric Study of Proton Acceleration from Laser-Thin Foil Interaction
Plasma 4, 670 (2021)

Abstract: We experimentally investigated the accelerated proton beam characteristics such as maximum energy and number by varying the incident laser parameters. For this purpose, we varied the laser energy, focal spot size, polarization, and pulse duration. The proton spectra were recorded using a single-shot Thomson parabola spectrometer equipped with a microchannel plate and a high-resolution charge-coupled device with a wide detection range from a few tens of keV to several MeV. The outcome of the experimental findings is discussed in detail and compared to other theoretical works.

F. Kröger, G. Weber, S. Hirlaender, R. Alemany-Fernandez, M. Krasny, T. Stöhlker, I. Tolstikhina, and V. Shevelko
Charge‐State Distributions of Highly Charged Lead Ions at Relativistic Collision Energies
Annalen der Physik -, 2100245 (2021)
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H. Abramowicz, U. Acosta, M. Altarelli, R. Aßmann, Z. Bai, T. Behnke, Y. Benhammou, T. Blackburn, S. Boogert, O. Borysov, M. Borysova, R. Brinkmann, M. Bruschi, F. Burkart, K. Büßer, N. Cavanagh, O. Davidi, W. Decking, U. Dosselli, N. Elkina, A. Fedotov, M. Firlej, T. Fiutowski, K. Fleck, M. Gostkin, C. Grojean, J. Hallford, H. Harsh, A. Hartin, B. Heinemann, T. Heinzl, L. Helary, M. Hoffmann, S. Huang, X. Huang, M. Idzik, A. Ilderton, R. Jacobs, B. Kämpfer, B. King, H. Lahno, A. Levanon, A. Levy, I. Levy, J. List, W. Lohmann, T. Ma, A. J. Macleod, V. Malka, F. Meloni, A. Mironov, M. Morandin, J. Moron, E. Negodin, G. Perez, I. Pomerantz, R. Pöschl, R. Prasad, F. Quéré, A. Ringwald, C. Rödel, S. Rykovanov, F. Salgado, A. Santra, G. Sarri, A. Sävert, A. Sbrizzi, S. Schmitt, U. Schramm, S. Schuwalow, D. Seipt, L. Shaimerdenova, M. Shchedrolosiev, M. Skakunov, Y. Soreq, M. Streeter, K. Swientek, N. Hod, S. Tang, T. Teter, D. Thoden, A. I. Titov, O. Tolbanov, G. Torgrimsson, A. Tyazhev, M. Wing, M. Zanetti, A. Zarubin, K. Zeil, M. Zepf, and A. Zhemchukov
Conceptual design report for the LUXE experiment
European Physical Journal Special Topics 230, 2445 (2021)
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F. Karbstein
Derivative corrections to the Heisenberg-Euler effective action
Journal of High Energy Physics 09, 070 (2021)

Abstract: We show that the leading derivative corrections to the Heisenberg-Euler effective action can be determined efficiently from the vacuum polarization tensor evaluated in a homogeneous constant background field. After deriving the explicit parameter-integral representation for the leading derivative corrections in generic electromagnetic fields at one loop, we specialize to the cases of magnetic- and electric-like field configurations characterized by the vanishing of one of the secular invariants of the electromagnetic field. In these cases, closed-form results and the associated all-orders weak-and strong-field expansions can be worked out. One immediate application is the leading derivative correction to the renowned Schwinger-formula describing the decay of the quantum vacuum via electron-positron pair production in slowly-varying electric fields.

B. Ying, F. Machalett, V. Huth, M. Kuebel, A. Sayler, T. Stoehlker, G. Paulus, and P. Wustelt
Experimental study of the laser-induced ionization of heavy metal and metalloid ions: Au+ and Si2+ in intense and sculpted femtosecond laser fields
Journal of Physics B: Atomic, Molecular and Optical Physics 54, 174002 (2021)

Abstract: We implement a liquid metal ion source in a 3D coincidence momentum spectroscopy setup for studying the interaction of ionic targets with intense laser pulses. Laser intensities of up to 4 . 10(16) W cm(-2) allow for the observation of up to ten-fold ionization of Au+-ions and double ionization of Si2+-ions. Further, by utilizing two-color sculpted laser fields to control the ionization process on the attosecond time scale, we demonstrate the capability to resolve the recoil ion momenta of heavy metal atoms. Simulations based on a semiclassical model assuming purely sequential ionization reproduce the experimental data well. This work opens up the use of a range of metallic and metalloid ions, which have hardly been investigated in strong-field laser physics so far.

A.-L. Viotti, S. Alisauskas, H. Tuennermann, E. Escoto, M. Seidel, K. Dudde, B. Manschwetus, I. Hartl, and C. Heyl
Temporal pulse quality of a Yb:YAG burst-mode laser post-compressed in a multi-pass cell
Optics Letters 46, 4686 (2021)

Abstract: Nonlinear pulse post-compression represents an efficient method for ultrashort, high-quality laser pulse production. The temporal pulse quality is, however, limited by amplitude and phase modulations intrinsic to post-compression. We here characterize in frequency and time domain with high dynamic range individual post-compressed pulses within laser bursts comprising 100-kHz-rate pulse trains. We spectrally broaden 730 fs, 3.2 mJ pulses from a Yb:YAG laser in a gas-filled multi-pass cell and post-compress them to 56 fs. The pulses exhibit a nearly constant energy content of 78% in the main peak over the burst plateau, which is close to the theoretical limit. Our results demonstrate attractive pulse characteristics, making multi-pass post-compressed lasers very applicable for pump-probe spectroscopy at, e.g., free-electron lasers or as efficient drivers for secondary frequency conversion stages.

A. Mueller, M. Martins, J. Borovik, T. Buhr, A. Perry-Sassmannshausen, S. Reinwardt, F. Trinter, S. Schippers, S. Fritzsche, and A. S. Kheifets
Role of L-shell single and double core-hole production and decay in m-fold (1 <= m <= 6) photoionization of the Ar+ ion
Physical Review A 104, 033105 (2021)

Abstract: Multiple ionization of the Ar+(3s(2)3p(5)) ion by a single photon has been investigated in the photon-energy range 250-1800 eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all Ar(1+m)+ product-ion channels with 1 <= m <= 6 covering a size range from several tens of Mb down to a few b. Narrow 2p-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52 meV. Double excitations involving a 2p and a 3s or 3p electron were also studied at high resolution and the measurements of the broad 2s excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the 2s subshell being mainly responsible for the production of Ar7+. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionizationmechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net m-fold ionization with m >= 4.