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Peer-Review Publications


2022

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.

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

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

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

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

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

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

Z. Yang, Z. He, G. Xiong, K. Yao, Y. Yang, B. Wei, Y. Zou, Z. Wu, Z. Tian, Y. Ma, C. Wu, X. Gao, and Z. Hu
Apparent change of the 3C/3D line intensity ratio in neonlike ions
Optics Express 30, 25326 (2022)

Abstract: The resonance 3C ([(2p(5))(1/2)3d(3/2)](J=1)->[2p(6)](J=0)) to intercombination 3D ([(2p(5))(3/2) 3d(5/2)](J=1)->[2p(6)](J=0)) line intensity ratio of neonlike ions has been studied. The measured line intensity ratio for neonlike Xe44+ ions shows an apparent change, which is reproduced by the calculations using the relativistic configuration interaction plus many-body perturbation theory. It is clearly elucidated that the change in the 3C/3D line intensity ratio is caused by strong configuration mixing between the upper levels of the 3D and 3F ([(2p(5))(1/2)3s](J=1)->[2p(6)](J=0)) lines. The present measurement allows us to discuss the 3C/3D line intensity ratio for the highest-Z ions hitherto, which suggests that the experiment-theory discrepancy in the 3C/3D line intensity ratio of neonlike ions diminishes with increasing atomic number Z and further trends to vanish at higher-Z ions. Furthermore, the present study provides benefits to better understand configuration mixing effect in the radiative opacity of hot plasmas. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

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

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

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

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

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

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

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

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

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

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

S. Hosseini, A. Iliyasu, T. Akilan, A. Salama, E. Eftekhari-Zadeh, and K. Hirota
Accurate Flow Regime Classification and Void Fraction Measurement in Two-Phase Flowmeters Using Frequency-Domain Feature Extraction and Neural Networks
Separations 9, 160 (2022)

Abstract: Two-phase flow is very important in many areas of science, engineering, and industry. Two-phase flow comprising gas and liquid phases is a common occurrence in oil and gas related industries. This study considers three flow regimes, including homogeneous, annular, and stratified regimes ranging from 5-90% of void fractions simulated via the Mont Carlo N-Particle (MCNP) Code. In the proposed model, two NaI detectors were used for recording the emitted photons of a cesium 137 source that pass through the pipe. Following that, fast Fourier transform (FFT), which aims to transfer recorded signals to frequency domain, was adopted. By analyzing signals in the frequency domain, it is possible to extract some hidden features that are not visible in the time domain analysis. Four distinctive features of registered signals, including average value, the amplitude of dominant frequency, standard deviation (STD), and skewness were extracted. These features were compared to each other to determine the best feature that can offer the best separation. Furthermore, artificial neural network (ANN) was utilized to increase the efficiency of two-phase flowmeters. Additionally, two multi-layer perceptron (MLP) neural networks were adopted for classifying the considered regimes and estimating the volumetric percentages. Applying the proposed model, the outlined flow regimes were accurately classified, resulting in volumetric percentages with a low root mean square error (RMSE) of 1.1%.

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

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

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

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

S. Fritzsche
Photon Emission from Hollow Ions Near Surfaces
Atoms 10, 37 (2022)

Abstract: Ions with multiple inner-shell vacancies frequently arise due to their interaction with different targets, such as (intense) light pulses, atoms, clusters or bulk material. They are formed, in addition, if highly charged ions approach surfaces and capture electrons at rather large distances. To explore the interaction of such hollow ions and their subsequent relaxation, photon spectra in different frequency regions have been measured and compared to calculations. To support these and related measurements, we here show within the framework of the Jena Atomic Calculator (JAC) how (additional) electrons in outer shells modify photon emission and lead to characteristic shifts in the observed spectra. Further, for highly charged Ar ions in KLm (m = 1 ... 8) configurations, we analyze the mean relaxation time for their stabilization into the different ground configurations. These examples demonstrate how a powerful and flexible toolbox such as JAC will be useful (and necessary) in order to model the photon and electron emission of ions as they occur not only near surfaces but also in astro-, atomic and plasma physics.

T. Sailer, V. Debierre, Z. Harman, F. Heisse, C. Konig, J. Morgner, B. Tu, A. Volotka, C. Keitel, K. Blaum, and S. Sturm
Measurement of the bound-electron g-factor difference in coupled ions
Nature 606, 479 (2022)

Abstract: Quantum electrodynamics (QED) is one of the most fundamental theories of physics and has been shown to be in excellent agreement with experimental results(1-5). In particular, measurements of the electron\textquotesingle s magnetic moment (orgfactor) of highly charged ions in Penning traps provide a stringent probe for QED, which allows testing of the standard model in the strongest electromagnetic fields(6). When studying the differences between isotopes, many common QED contributions cancel owing to the identical electron configuration, making it possible to resolve the intricate effects stemming from the nuclear differences. Experimentally, however, this quickly becomes limited, particularly by the precision of the ion masses or the magnetic field stability(7). Here we report on a measurement technique that overcomes these limitations by co-trapping two highly charged ions and measuring the difference in their g factors directly. We apply a dual Ramsey-type measurement scheme with the ions locked on a common magnetron orbit(8), separated by only a few hundred micrometres, to coherently extract the spin precession frequency difference. We have measured the isotopic shift of the bound-electrongfactor of the isotopes Ne-20(9+) and Ne-22(9+) to 0.56-parts-per-trillion (5.6 x 10(-13)) precision relative to their g factors, an improvement of about two orders of magnitude compared with state-of-the-art techniques(7). This resolves the QED contribution to the nuclear recoil, accurately validates the corresponding theory and offers an alternative approach to set constraints on new physics.

A. Sainte-Marie, L. Fedeli, N. Zaïm, F. Karbstein, and H. Vincenti
Quantum vacuum processes in the extremely intense light of relativistic plasma mirror sources
New Journal of Physics 24, 065005 (2022)

Abstract: The advent of petawatt-class laser systems allows generating electromagnetic fields of unprecedented strength in a controlled environment, driving increasingly more efforts to probe yet unobserved processes through their interaction with the quantum vacuum. Still, the lowest intensity scale governing these effects lies orders of magnitude beyond foreseen capabilities, so that such endeavor is expected to remain extremely challenging. In recent years, however, plasma mirrors have emerged as a promising bridge across this gap, by enabling the conversion of intense infrared laser pulses into coherently focused Doppler harmonic beams lying in the X-UV range. In this work, we present predictions on the quantum vacuum signatures produced when such beams are focused to intensities between 1024 and 1028 W cm−2, specifically photon–photon scattering and electron–positron pair creation. These signatures are computed via the stimulated vacuum formalism, combined with a model of perfectly focused beam built from PIC-generated harmonics spectra, and implemented on state-of-the-art massively parallel numerical tools. In view of identifying experimentally favorable configurations, we also consider the coupling of the focused harmonic beam with an auxiliary optical beam, and provide comparison with other established schemes. Our results show that a single coherently focused harmonic beam can produce as much scattered photons as two infrared pulses in head-on collision, and confirm that the coupling of the harmonic beam to an auxiliary beam gives rise to significant levels of inelastic scattering, and hence holds the potential to strongly improve the attainable signal to noise ratios in experiments.

T. Aidukas, L. Lötgering, and A. Harvey
Addressing phase-curvature in Fourier ptychography
Optics Express 30, 22421 (2022)

Abstract: In Fourier ptychography, multiple lowresolution images are captured and subsequently combined computationally into a high-resolution, large-field of view micrograph. A theoretical image-formation model based on the assumption of plane-wave illumination from various directions is commonly used, to stitch together the captured information into a high synthetic aperture. The underlying far-field (Fraunhofer) diffraction assumption connects the source, sample, and pupil planes by Fourier transforms. While computationally simple, this assumption neglects phase-curvature due to non-planar illumination from point sources as well as phasecurvature from finite-conjugate microscopes (e.g., using a single-lens for image-formation). We describe a simple, efficient, and accurate extension of Fourier ptychography by embedding the effect of phase-curvature into the underlying forward model. With the improved forward model proposed here, quantitative phase reconstruction is possible even for wide fields-of-views and without the need of image segmentation. Lastly, the proposed method is computationally efficient, requiring only two multiplications: prior and following the reconstruction.

T. Heuermann, Z. Wang, M. Lenski, M. Gebhardt, C. Gaida, M. Abdelaal, J. Buldt, M. Mueller, A. Klenke, and J. Limpert
Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate
Optics letters 47, 3095 (2022)

Abstract: High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.

E. Eftekhari-Zadeh, A. Bensalama, G. Roshani, A. Salama, C. Spielmann, and A. Iliyasu
Enhanced Gamma-Ray Attenuation-Based Detection System Using an Artificial Neural Network
Photonics 9, 382 (2022)

Abstract: Scale deposition is the accumulation of various materials in the walls of transmission lines and unwanted parts in the oil and gas production system. It is a leading moot point in all transmission lines, tanks, and petroleum equipment. Scale deposition leads to drastic detrimental problems, reduced permeability, pressure and production losses, and direct financial losses due to the failure of some equipment. The accumulation of oil and gas leads to clogged pores and obstruction of fluid flow. Considering the passage of a two-phase flow, our study determines the thickness of the scale, and the flow regime is detected with the help of two Multilayer Perceptron (MLP) networks. First, the diagnostic system consisting of a dual-energy source, a steel pipe, and a NaI detector was implemented, using the Monte Carlo N Particle Code (MCNP). Subsequently, the received signals were processed, and properties were extracted using the wavelet transform technique. These features were considered as inputs of an Artificial Neural Network (ANN) model used to determine the type of flow regimes and predict the scale thickness. By accurately classifying the flow regimes and determining the scale inside the pipe, our proposed method provides a platform that could enhance many areas of the oil industry.

D. Schwickert, M. Ruberti, P. Kolorenc, S. Usenko, A. Przystawik, K. Baev, I. Baev, M. Braune, L. Bocklage, M. Czwalinna, S. Deinert, S. Duesterer, A. Hans, G. Hartmann, C. Haunhorst, M. Kuhlmann, S. Palutke, R. Röhlsberger, J. Roensch-Schulenburg, P. Schmidt, S. Toleikis, J. Viefhaus, M. Martins, A. Knie, D. Kip, V. Averbukh, J. Marangos, and T. Laarmann
Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time
Science Advances 8, 11 (2022)

Abstract: Here, we use x-rays to create and probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 to 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. In the kinematically complete x-ray absorption measurement, we monitor its dynamics for a period of 175 fs and observe an evolving modulation that may implicate the coupling of electronic to vibronic coherence at longer time scales. Our experiment provides a direct support for the existence of long-lived electronic coherence in photoionized biomolecules.

B. Marx-Glowna, B. Grabiger, R. Loetzsch, I. Uschmann, A. T. Schmitt, K. S. Schulze, A. Last, T. Roth, S. Antipov, H.-P. Schlenvoigt, I. Sergueev, O. Leupold, R. Roehlsberger, and G. G. Paulus
Scanning high-sensitive x-ray polarization microscopy
New Journal of Physics 24, 053051 (2022)

Abstract: We report on the realization of an extremely sensitive x-ray polarization microscope, allowing to detect tiniest polarization changes of 1 in 100 billion (10(-11)) with a mu m-size focused beam. The extreme degree of polarization purity places the most stringent requirements on the orientation of the polarizer and analyzer crystals as well as the composition and the form fidelity of the lenses, which must not exhibit any birefringence. The results show that these requirements are currently only met by polymer lenses. Highly sensitive scanning x-ray polarization microscopy thus is established as a new method. It can provide new insights in a wide range of applications ranging from quantum electrodynamics and quantum optics to x-ray spectroscopy, materials research, and laser physics.

A. Steinkopff, C. Aleshire, A. Klenke, A. Klenke, C. Jauregui, J. Limpert, J. Limpert, and J. Limpert
Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers
Optics Express 30, 16896 (2022)

Abstract: Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

B. Zhu, A. Gumberidze, T. Over, G. Weber, Z. Andelkovic, A. Bräuning-Demian, R. J. Chen, D. Dmytriiev, O. Forstner, C. Hahn, F. Herfurth, M. O. Herdrich, P.-M. Hillenbrand, A. Kalinin, F. M. Kröger, M. Lestinsky, Yu. A. Litvinov, E. B. Menz, W. Middents, T. Morgenroth, N. Petridis, Ph. Pfäfflein, M. S. Sanjari, R. S. Sidhu, U. Spillmann, R. Schuch, S. Schippers, S. Trotsenko, L. Varga, G. Vorobyev, and Th. Stöhlker
X-ray emission associated with radiative recombination for Pb⁸²⁺ ions at threshold energies
Physical Review A 105, 052804 (2022)

Abstract: For decelerated bare lead ions at a low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at threshold energies has been studied at the electron cooler of CRYRING@ESR at GSI, Darmstadt. In our experiment, we observed the full x-ray emission pattern by utilizing dedicated x-ray detection chambers installed at 0∘ and 180∘ observation geometry. Most remarkably, no line distortion effects due to delayed emission are present in the well-defined x-ray spectra, spanning a wide range of x-ray energies (from about 5 to 100 keV), which enables us to identify fine-structure resolved Lyman, Balmer, and Paschen x-ray lines along with the RR transitions into the K, L, and M shells of the ions. For comparison with theory, an elaborate theoretical model is established taking into account the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number n=165 in combination with time-dependent feeding transitions. Within the statistical accuracy, the experimental data are in very good agreement with the results of rigorous relativistic predictions. Most notably, this comparison sheds light on the contribution of prompt and delayed x-ray emission (up to 70 ns) to the observed x-ray spectra, originating in particular from yrast transitions into inner shells.

P. Martin, H. Ahmed, D. Doria, A. Alejo, R. Clarke, S. Ferguson, J. Fernandez-Tobias, R. R. Freeman, J. Fuchs, A. Green, J. S. Green, D. Gwynne, F. Hanton, J. Jarrett, D. Jung, K. F. Kakolee, A. G. Krygier, C. L. S. Lewis, A. McIlvenny, P. McKenna, J. T. Morrison, Z. Najmudin, K. Naughton, G. Nersisyan, P. Norreys, M. Notley, M. Roth, J. A. Ruiz, C. Scullion, M. Zepf, S. Zhai, M. Borghesi, and S. Kar
Absolute calibration of Fujifilm BAS-TR image plate response to laser driven protons up to 40 MeV
Review of Scientific Instruments 93, 053303 (2022)

Abstract: Image plates (IPs) are a popular detector in the field of laser driven ion acceleration, owing to their high dynamic range and reusability. An absolute calibration of these detectors to laser-driven protons in the routinely produced tens of MeV energy range is, therefore, essential. In this paper, the response of Fujifilm BAS-TR IPs to 1-40 MeV protons is calibrated by employing the detectors in high resolution Thomson parabola spectrometers in conjunction with a CR-39 nuclear track detector to determine absolute proton numbers. While CR-39 was placed in front of the image plate for lower energy protons, it was placed behind the image plate for energies above 10 MeV using suitable metal filters sandwiched between the image plate and CR-39 to select specific energies. The measured response agrees well with previously reported calibrations as well as standard models of IP response, providing, for the first time, an absolute calibration over a large range of proton energies of relevance to current experiments.

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.

W. Eschen, L. Loetgering, V. Schuster, R. Klas, A. Kirsche, L. Berthold, M. Steinert, T. Pertsch, H. Gross, M. Krause, J. Limpert, and J. Rothhardt
Material-specific high-resolution table-top extreme ultraviolet microscopy
Light: Science & Applications 11, 2678 (2022)

Abstract: Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

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.

C. Aleshire, A. Steinkopff, A. Klenke, A. Klenke, C. Jáuregui, S. Kuhn, J. Nold, N. Haarlammert, T. Schreiber, J. Limpert, J. Limpert, and J. Limpert
High-energy Q-switched 16-core tapered rod-type fiber laser system
Optics Letters 47, 1725 (2022)

Abstract: High-energy Q-switched master oscillator power amplifier systems based on rod-type 4 × 4 multicore fibers are demonstrated, achieving energy up to 49 mJ in ns-class pulses. A tapered fiber geometry is tested that maintains low mode order in large multimode output cores, improving beam quality in comparison to a similar fiber with no taper. The tapered fiber design can be scaled both in the number of amplifying cores and in the dimensions of the cores themselves, providing a potential route toward joule-class fiber lasers systems.

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.

A. Volotka, D. Samoilenko, S. Fritzsche, V. Serbo, and A. Surzhykov
Polarization of Photons Scattered by Ultra-Relativistic Ion Beams
Annalen Der Physik 534, 2100252 (2022)

Abstract: A theoretical investigation of the elastic resonant scattering of photons by ultra-relativistic and partially stripped ions is presented. Particular attention in the study is given to the angular distribution and polarization of scattered photons as \textasciigrave \textasciigrave seen'' in both the ion-rest and laboratory reference frames. In order to evaluate these angular and polarization properties, the irreducible polarization tensor approach is combined with the density matrix theory. If, furthermore, the ion-photon coupling is treated within the electric dipole approximation, this framework enables one to obtain simple analytical expressions for both the emission pattern and the polarization Stokes parameters of the outgoing radiation. These (analytical) expressions for the nS0 -> n \textasciigrave P1 -> nS0\textbackslash \textdollar n S\_0 \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P\_1 \textbackslash rightarrow n S\_0\textbackslash \textdollar , nS1/2 -> n \textasciigrave P1/2 -> nS1/2\textbackslash \textdollar n S\_\textbackslash 1/2\textbackslash \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P\_\textbackslash 1/2\textbackslash \textbackslash rightarrow n S\_\textbackslash 1/2\textbackslash \textbackslash \textdollar , and nS1/2 -> n \textasciigrave P3/2 -> nS1/2\textbackslash \textdollar n S\_\textbackslash 1/2\textbackslash \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P\_\textbackslash 3/2\textbackslash \textbackslash rightarrow n S\_\textbackslash 1/2\textbackslash \textbackslash \textdollar transitions are displayed and analyzed , that are of interest for the Gamma Factory project and whose realization is currently under discussion at CERN. Based on the performed analysis, it is demonstrated that the resonantly scattered photons can be strongly (linearly or circularly) polarized, and that this polarization can be well controlled by adjusting either the emission angle and/or the polarization state of the incident radiation. Moreover, the potential of the photon scattering for measuring the spin-polarization of ion beams is also discussed in detail.

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.

M. Zimmer, S. Scheuren, A. Kleinschmidt, N. Mitura, A. Tebartz, G. Schaumann, T. Abel, T. Ebert, M. Hesse, S. Zaehter, S. Vogel, O. Merle, R.-J. Ahlers, S. Pinto, M. Peschke, T. Kroell, V. Bagnoud, C. Roedel, and M. Roth
Demonstration of non-destructive and isotope-sensitive material analysis using a short-pulsed laser-driven epi-thermal neutron source
Nature Communications 13, 2051 (2022)

Abstract: High-power laser beams can be used to accelerate neutron beams. Here the authors demonstrate the application of laser-driven neutron beams to neutron resonance spectroscopy and neutron resonance imaging. Neutrons are a valuable tool for non-destructive material investigation as their interaction cross sections with matter are isotope sensitive and can be used complementary to x-rays. So far, most neutron applications have been limited to large-scale facilities such as nuclear research reactors, spallation sources, and accelerator-driven neutron sources. Here we show the design and optimization of a laser-driven neutron source in the epi-thermal and thermal energy range, which is used for non-invasive material analysis. Neutron resonance spectroscopy, neutron radiography, and neutron resonance imaging with moderated neutrons are demonstrated for investigating samples in terms of isotope composition and thickness. The experimental results encourage applications in non-destructive and isotope-sensitive material analysis and pave the way for compact laser-driven neutron sources with high application potential.

S. Haedrich, E. Shestaev, M. Tschernajew, F. Stutzki, N. Walther, F. Just, M. Kienel, I. Seres, P. Jojart, Z. Bengery, B. Gilicze, Z. Varallyay, A. Borzsonyi, M. Mueller, C. Grebing, A. Klenke, D. Hoff, G. Paulus, T. Eidam, and J. Limpert
Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses
Optics Letters 47, 1537 (2022)

Abstract: Two-stage multipass-cell compression of a fiber-chirpedpulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107W average power, 1.07 mJ pulses at 100kHz centered at 1030nm with excellent spatial beam quality (M-2 =1.1, Strehl ratio S = 0.98), pointing stability (2.3 mu rad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360mrad in the frequency range from 10Hz to 50kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation

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.

J. Stierhof, S. Kühn, M. Winter, P. Micke, R. Steinbrügge, C. Shah, N. Hell, M. Bissinger, M. Hirsch, R. Ballhausen, M. Lang, C. Gräfe, S. Wipf, R. Cumbee, G. L. Betancourt-Martinez, S. Park, J. Niskanen, M. Chung, F. S. Porter, T. Stöhlker, T. Pfeifer, G. V. Brown, S. Bernitt, P. Hansmann, J. Wilms, J. R. Crespo López-Urrutia, and M. A. Leutenegger
A new benchmark of soft X-ray transition energies of Ne, CO2, and SF6: paving a pathway towards ppm accuracy
The European Physical Journal D 76, 38 (2022)

Abstract: A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of Ne, CO2, and SF6 gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the CO2 result agrees well with previous measurements, the SF6 spectrum appears shifted by ∼0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to ∼40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV.

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.

P.-M. Hillenbrand, S. Hagmann, Y. S. Kozhedub, E. P. Benis, C. Brandau, R. J. Chen, D. Dmytriiev, O. Forstner, J. Glorius, R. E. Grisenti, A. Gumberidze, M. Lestinsky, Yu. A. Litvinov, E. B. Menz, T. Morgenroth, S. Nanos, N. Petridis, Ph. Pfäfflein, H. Rothard, M. S. Sanjari, R. S. Sidhu, U. Spillmann, S. Trotsenko, I. I. Tupitsyn, L. Varga, and Th. Stöhlker
Single and double K-shell vacancy production in slow Xe⁵⁴⁺,⁵³⁺-Xe collisions
Physical Review A 105, 022810 (2022)

Abstract: We present an experimental and theoretical study of symmetric Xe54++Xe collisions at 50, 30, and 15 MeV/u, corresponding to strong perturbations with vK/vp=1.20, 1.55, and 2.20, respectively (vK is the classical K-shell orbital velocity and vp is the projectile velocity), as well as Xe53++Xe collisions at 15 MeV/u. For each of these systems, x-ray spectra are measured under a forward angle of 35∘ with respect to the projectile beam. Target satellite and hypersatellite radiation Kαs2,1 and Kαhs2,1, respectively, are analyzed and used to derive cross-section ratios for double-to-single target K-shell vacancy production. We compare our experimental results to relativistic time-dependent two-center calculations.

P. Palmeri, J. Deprince, M. A. Bautista, S. Fritzsche, J. A. Garcia, T. R. Kallman, C. Mendoza, and P. Quinet
Plasma environment effects on K lines of astrophysical interest
Astronomy and Astrophysics 657, 11 (2022)

Abstract: Aims. We calculate the plasma environment effects on the ionization potentials (IPs) and K-thresholds used in the modeling of K lines for all the ions belonging to the isonuclear sequences of abundant elements apart from oxygen and iron, namely: carbon, silicon, calcium, chromium, and nickel. These calculations are used to extend the data points for the fits of the universal formulae, first proposed in our fourth paper of this series, to predict the IP and K-threshold lowerings in any elemental ion. Methods. We used the fully relativistic multi-configuration Dirac-Rock method and approximated the plasma electron-nucleus and electron-electron screenings with a time-averaged Debye-Huckel potential. Results. We report the modified ionization potentials and K-threshold energies for plasmas characterized by electron temperatures and densities in the ranges of 10(5)-10(2) K and 10(18)-10(22) cm(-3). In addition, the improved universal fitting formulae are obtained. Conclusions. We conclude that since explicit calculations of the atomic structures for each ion of each element under different plasma conditions is impractical, the use of these universal formulae for predicting the IP and K-threshold lowerings in plasma modeling codes is still recommended. However, their comparatively moderate to low accuracies may affect the predicted opacities with regard to certain cases under extreme plasma conditions that are characterized by a plasma screening parameter of mu > 0.2 a.u., especially for the K-thresholds.

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.

C. Heyl, M. Seidel, E. Escoto, A. Schoenberg, S. Carlstroem, G. Arisholm, T. Lang, and I. Hartl
High-energy bow tie multi-pass cells for nonlinear spectral broadening applications
Journal of Physics-Photonics 4, 014002 (2022)

Abstract: Multi-pass cells (MPCs) have emerged as very attractive tools for spectral broadening and post-compression applications. We discuss pulse energy limitations of standard MPCs considering basic geometrical scaling principles and introduce a novel energy scaling method using a MPC arranged in a bow tie geometry. Employing nonlinear pulse propagation simulations, we numerically demonstrate the compression of 125 mJ, 1 ps pulses to 50 fs using a compact 2 m long setup and outline routes to extend our approach into the Joule-regime.

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.