Measurement of the bound-electron g-factor difference in coupled ions - Sailer, T.; Debierre, V.; Harman, Z.; Hei{\"{s}}e, F.; Konig, C.; Morgner, J.; Tu, B.; Volotka, A.; Keitel, C.; Blaum, K.; Sturm, S. (Nature, 2022)
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 extquotesingle 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.
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Near K-edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions - Schippers, S.; Stock, S.; Buhr, T.; Perry-Sassmannshausen, A.; Reinwardt, S.; Martins, M.; Mueller, A.; Fritzsche, S. (Astrophysical Journal, 2022)
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.
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Photon Emission from Hollow Ions Near Surfaces - Fritzsche, S. (Atoms, 2022)
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.
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Quantum vacuum processes in the extremely intense light of relativistic plasma mirror sources - Sainte-Marie, A.; Fedeli, L.; Za{\"{i}}m, N.; Karbstein, F.; Vincenti, H. (New Journal of Physics, 2022)
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.
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Real-Time Observation of Temperature-Induced Surface Nanofaceting in M-Plane alpha-Al2O3 - Erb, D.; Perlich, J.; Roth, V.; R{\"{o}}hlsberger, R.; Schlage, K. (ACS Applied Materials { &} Interfaces, 2022)
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.
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Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate - Heuermann, T.; Wang, Z.; Lenski, M.; Gebhardt, M.; Gaida, C.; Abdelaal, M.; Buldt, J.; Mueller, M.; Klenke, A.; Limpert, J. (Optics letters, 2022)
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.
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Absolute calibration of Fujifilm BAS-TR image plate response to laser driven protons up to 40 MeV - Martin, P.; Ahmed, H.; Doria, D.; Alejo, A.; Clarke, R.; Ferguson, S.; Fernandez-Tobias, J.; Freeman, R. R.; Fuchs, J.; Green, A.; Green, J. S.; Gwynne, D.; et al. (Review of Scientific Instruments, 2022)
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.
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Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers - Steinkopff, A.; Aleshire, C.; Klenke, A.; Jauregui, C.; Limpert, J. (Optics Express, 2022)
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.
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Scanning high-sensitive x-ray polarization microscopy - Marx-Glowna, B.; Grabiger, B.; Loetzsch, R.; Uschmann, I.; Schmitt, A. T.; Schulze, K. S.; Last, A.; Roth, T.; Antipov, S.; Schlenvoigt, H.-P.; Sergueev, I.; Leupold, O.; et al. (New Journal of Physics, 2022)
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.
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X-ray emission associated with radiative recombination for Pb⁸²⁺ ions at threshold energies - Zhu, B.; Gumberidze, A.; Over, T.; Weber, G.; Andelkovic, Z.; Br{\"{a}uning-Demian, A. and Chen, R. J. and Dmytriiev, D. and Forstner, O. and Hahn, C. and Herfurth, F. and Herdrich, M. O. and Hillenbrand, P.-M. and Kalinin, A. and Kr{\"{o}}ger, F. M. and Lestinsky, M. and Litvinov, Yu. A. and Menz, E. B. and Middents, W. and Morgenroth, T. and Petridis, N. and Pf{\"{a}fflein, P. and Sanjari, M. S. and Sidhu, R. S. and Spillmann, U. and Schuch, R. and Schippers, S. and Trotsenko, S. and Varga, L. and Vorobyev, G. and St{\"{o}}hlker, T. (Physical Review A, 2022)
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.
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aPIE: an angle calibration algorithm for reflection ptychography - de Beurs, A.; L{\"{o}}tgering, L.; Herczog, M.; Du, M.; Eikema, K.; Witte, S. (Optics Letters, 2022)
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.
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High-energy Q-switched 16-core tapered rod-type fiber laser system - Aleshire, C.; Steinkopff, A.; Klenke, A.; Jauregui, C.; Kuhn, S.; Nold, J.; Haarlammert, N.; Schreiber, T.; Limpert, J. (Optics Letters, 2022)
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.
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Highly Charged Ions for High-Resolution Soft X-ray Grating Monochromator Optimisation - Hoesch, M.; Seltmann, J.; Trinter, F.; Kuhn, S.; Togawa, M.; Steinbr{\"{u}}gge, R.; Bernitt, S.; Lopez-Urrutia, J. R. C. (14th International Conference on Synchrotron Radiation Instrumentation, 2022)
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Material-specific high-resolution table-top extreme ultraviolet microscopy - Eschen, W.; Loetgering, L.; Schuster, V.; Klas, R.; Kirsche, A.; Berthold, L.; Steinert, M.; Pertsch, T.; Gross, H.; Krause, M.; Limpert, J.; Rothhardt, J. (Light: Science { &} Applications, 2022)
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.
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Ultra-compact post-compressor on-shot wavefront measurement for beam correction at PHELIX - Ohland, J. B.; Eisenbarth, U.; Zielbauer, B.; Zobus, Y.; Posor, D.; Hornung, J.; Reemts, D.; Bagnoud, V. (High Power Laser Science and Engineering, 2022)
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.
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A new benchmark of soft X-ray transition energies of Ne, CO2, and SF6: paving a pathway towards ppm accuracy - Stierhof, J.; Kuhn, S.; Winter, M.; Micke, P.; Steinbr{\"{u}}gge, R.; Shah, C.; Hell, N.; Bissinger, M.; Hirsch, M.; Ballhausen, R.; Lang, M.; Gr{\"{a}fe, C. and Wipf, S. and Cumbee, R. and Betancourt-Martinez, G. L. and Park, S. and Niskanen, J. and Chung, M. and Porter, F. S. and St{\"{o}}hlker, T. and Pfeifer, T. and Brown, G. V. and Bernitt, S. and Hansmann, P. and Wilms, J. and Crespo L{\'{o}}pez-Urrutia, J. R. and Leutenegger, M. A. (The European Physical Journal D, 2022)
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.
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Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses - Haedrich, S.; Shestaev, E.; Tschernajew, M.; Stutzki, F.; Walther, N.; Just, F.; Kienel, M.; Seres, I.; J{\'{o}}j{\'{a}}rt, P.; Bengery, Z.; Gilicze, B.; V{\'{a}}rallyay, Z.; et al. (Optics Letters, 2022)
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
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Demonstration of non-destructive and isotope-sensitive material analysis using a short-pulsed laser-driven epi-thermal neutron source - Zimmer, M.; Scheuren, S.; Kleinschmidt, A.; Mitura, N.; Tebartz, A.; Schaumann, G.; Abel, T.; Ebert, T.; Hesse, M.; Zaehter, S.; Vogel, S.; Merle, O.; et al. (Nature Communications, 2022)
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.
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Expanding Nuclear Physics Horizons with the Gamma Factory - Budker, D.; Berengut, J.; Flambaum, V.; Gorchtein, M.; Jin, J.; Karbstein, F.; Krasny, M.; Litvinov, Y.; P{\'{a}}lffy, A.; Pascalutsa, V.; Petrenko, A.; Surzhykov, A.; et al. (Annalen der Physik, 2022)
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.
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g Factor of Lithiumlike Silicon and Calcium: Resolving the Disagreement between Theory and Experiment - Kosheleva, V. P.; Volotka, A. V.; Glazov, D. A.; Zinenko, D. V.; Fritzsche, S. (Physical Review Letters, 2022)
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.
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Level Structure and Properties of Open f-Shell Elements - Fritzsche, S. (Atoms, 2022)
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 extquotesingle 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.
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Photoexcitation of atoms by cylindrically polarized Laguerre-Gaussian beams - Ramakrishna, S.; Hofbrucker, J.; Fritzsche, S. (Physical Review A, 2022)
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.
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Polarization of Photons Scattered by Ultra-Relativistic Ion Beams - Volotka, A.; Samoilenko, D.; Fritzsche, S.; Serbo, V.; Surzhykov, A. (Annalen Der Physik, 2022)
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 extasciigrave extasciigrave seenin 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 extasciigrave P1 -> nS0\ n S_0 \ rightarrow n<\^>\ \ prime \ P_1 \ rightarrow n S_0\ , nS1/2 -> n extasciigrave P1/2 -> nS1/2\ n S_\ 1/2\ \ rightarrow n<\^>\ \ prime \ P_\ 1/2\ \ rightarrow n S_\ 1/2\ \ , and nS1/2 -> n extasciigrave P3/2 -> nS1/2\ n S_\ 1/2\ \ rightarrow n<\^>\ \ prime \ P_\ 3/2\ \ rightarrow n S_\ 1/2\ \ 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.
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Towards perfectly linearly polarized x-rays - Schulze, K.; Grabiger, B.; Loetzsch, R.; Marx-Glowna, B.; Schmitt, A.; Garcia, A.; Hippler, W.; Huang, L.; Karbstein, F.; Konopkova, Z.; Schlenvoigt, H.-P.; Schwinkendorf, J.-P.; et al. (Physical Review Research, 2022)
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.
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Vacuum Birefringence at the Gamma Factory - Karbstein, F. (Annalen der Physik, 2022)
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.
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Background-free femtosecond autocorrelation in collinearly-aligned inverted field geometry using optical vortices - Dimitrov, N.; Zhekova, M.; Zhang, Y.; Paulus, G.; Dreischuh, A. (Optics Communications, 2022)
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.
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Experimental estimates of the photon background in a potential light-by-light scattering study - Doyle, L.; Khademi, P.; Hilz, P.; S{\"{a}vert, A. and Sch{\"{a}fer, G. and Schreiber, J. and Zepf, M. (New Journal of Physics, 2022)
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.
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High-Resolution Kinoform X-Ray Optics Printed via 405 nm 3D Laser Lithography - Sanli, U. T.; Messer, T.; Weigand, M.; L{\"{o}}tgering, L.; Sch{\"{u}}tz, G.; Wegener, M.; Kern, C.; Keskinbora, K. (Advanced Materials Technologies, 2022)
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Long-term stable, synchronizable, low-noise picosecond Ho:fiber NALM oscillator for Ho:YLF amplifier seeding - Mahnke, C.; Hua, Y.; Ma, Y.; Salman, S.; Lamb, T.; Schulz, S.; Heyl, C.; Cankaya, H.; Hartl, I. (Optics letters, 2022)
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.
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Multi-pass cells for post-compression of ultrashort laser pulses - Viotti, A.-L.; Seidel, M.; Escoto, E.; Rajhans, S.; Leemans, W.; Hartl, I.; Heyl, C. (Optica, 2022)
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.
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Photon counting of extreme ultraviolet high harmonics using a superconducting nanowire single-photon detector - Fuchs, S.; Abel, J.; Nathanael, J.; Reinhard, J.; Wiesner, F.; Wuensche, M.; Skruszewicz, S.; Roedel, C.; Born, D.; Schmidt, H.; Paulus, G. (Applied Physics B, 2022)
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.
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Relativistic calculations of two-color two-photon K-shell ionization - Fan, J.; Hofbrucker, J.; Volotka, A. V.; Fritzsche, S. (European Physical Journal D, 2022)
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.
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Single and double K-shell vacancy production in slow Xe⁵⁴⁺,⁵³⁺-Xe collisions - Hillenbrand, P.-M.; Hagmann, S.; Kozhedub, Y. S.; Benis, E. P.; Brandau, C.; Chen, R. J.; Dmytriiev, D.; Forstner, O.; Glorius, J.; Grisenti, R. E.; Gumberidze, A.; Lestinsky, M.; et al. (Physical Review A, 2022)
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.
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500 W rod-type 4 × 4 multicore ultrafast fiber laser - Klenke, A.; Steinkopff, A.; Aleshire, C.; Jauregui, C.; Kuhn, S.; Nold, J.; Hupel, C.; Hein, S.; Schulze, S.; Haarlammert, N.; Schreiber, T.; T{\"{u}}nnermann, A.; et al. (Optics Letters, 2022)
We present a coherently combined femtosecond fiber chirped-pulse-amplification system based on a rod-type, ytterbium-doped, multicore fiber with 4 × 4 cores. A high average power of up to 500 W (after combination and compression) could be achieved at 10 MHz repetition rate with excellent beam quality. Additionally, {ł}ess 500 fs pulses with up to 600 µJ of pulse energy were also realized with this setup. This architecture is intrinsically power scalable by increasing the number of cores in the fiber.
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Advances in laboratory-scale ptychography using high harmonic sources [Invited] - L{\"{o}}tgering, L.; Witte, S.; Rothhardt, J. (Optics Express, 2022)
Extreme ultraviolet microscopy and wavefront sensing are key elements for nextgeneration ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging.
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Focal beam structuring by triple mixing of optical vortex lattices - Stoyanov, L.; Maleshkov, G.; Stefanov, I.; Paulus, G.; Dreischuh, A. (Optical and Quantum Electronics, 2022)
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.
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High-energy bow tie multi-pass cells for nonlinear spectral broadening applications - Heyl, C.; Seidel, M.; Escoto, E.; Schoenberg, A.; Carlstroem, S.; Arisholm, G.; Lang, T.; Hartl, I. (Journal of Physics-Photonics, 2022)
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.
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Intensity scaling limitations of laser-driven proton acceleration in the TNSA-regime - Keppler, S.; Elkina, N.; Becker, G. A.; Hein, J.; Hornung, M.; M{\"{a}usezahl, M. and R{\"{o}}del, C. and Tamer, I. and Zepf, M. and Kaluza, M.C. (Physical Review Research, 2022)
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.
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Large N external-field quantum electrodynamics - Karbstein, F. (Journal of High Energy Physics, 2022)
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.
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Laser energy absorption and x-ray generation in nanowire arrays irradiated by relativistically intense ultra-high contrast femtosecond laser pulses - Eftekhari-Zadeh, E.; Bl{\"{u}}mcke, M. S.; Samsonova, Z.; Loetzsch, R.; Uschmann, I.; Zapf, M.; Ronning, C.; Rosmej, O. N.; Kartashov, D.; Spielmann, C. (Physics of Plasmas, 2022)
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.
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M{\"{o}}ssbauer spectroscopic camera for operando measurement of pattern formation processes - Kobayashi, M.; Hayakawa, K.; Yoshida, Y.; R{\"{o}}hlsberger, R. (Hyperfine Interactions, 2022)
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Plasma environment effects on K lines of astrophysical interest - Palmeri, P.; Deprince, J.; Bautista, M. A.; Fritzsche, S.; Garc{\'{i}}a, J. A.; Kallman, T. R.; Mendoza, C.; Quinet, P. (Astronomy and Astrophysics, 2022)
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.
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Proposal for complete characterization of attosecond pulses from relativistic plasmas - Wu, C.; Li, L.; Yeung, M.; Wu, S.; Cousens, S.; Tietze, S.; Dromey, B.; Zhou, C.; Ruan, S.; Zepf, M. (Optics Express, 2022)
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.
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Single particle detection system for strong-field QED experiments - Salgado, F. C.; Cavanagh, N.; Tamburini, M.; Storey, D. W.; Beyer, R.; Bucksbaum, P. H.; Chen, Z.; Di Piazza, A.; Gerstmayr, E.; Harsh, H.; Isele, E.; Junghans, A. R.; et al. (New Journal of Physics, 2022)
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.
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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 - Seidel, M.; Pressacco, F.; Akcaalan, O.; Binhammer, T.; Darvill, J.; Ekanayake, N.; Frede, M.; Grosse-Wortmann, U.; Heber, M.; Heyl, C.; Kutnyakhov, D.; Li, C.; et al. (Laser { &} Photonics Reviews, 2022)
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.
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Dielectronic recombination strengths and plasma rate coefficients of multiply charged ions - Fritzsche, S. (Astronomy and Astrophysics, 2021)
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Generation of elliptically polarized high-order harmonic radiation with bi-elliptical two-color laser beams - Weber, A.; B{\"{o}}ning, B.; Minneker, B.; Fritzsche, S. (Physical Review A, 2021)
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Hyperfine-induced effects on the K α_1 angular distribution following electron-impact excitation of heliumlike spin- 1/2 Tl⁷⁹⁺ ions - Wu, Z. W.; Tian, Z. Q.; Jiang, J.; Dong, C. Z.; Fritzsche, S. (Physical Review A, 2021)
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Possible Polarization Measurements in Elastic Scattering at the Gamma Factory Utilizing a 2D Sensitive Strip Detector as Dedicated Compton Polarimeter - Middents, W.; Weber, G.; Spillmann, U.; Krings, T.; Vockert, M.; Volotka, A.; Surzhykov, A.; St{\"{o}}hlker, T. (Annalen der Physik, 2021)
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Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies - Appi, E.; Papadopoulou, C. C.; Mapa, J. L.; Jusko, C.; Mosel, P.; Schoenberg, A.; Stock, J.; Feigl, T.; Ali{{s}}auskas, S.; Lang, T.; Heyl, C. M.; Manschwetus, B.; et al. (Review of Scientific Instruments, 2021)
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Time-resolved study of holeboring in realistic experimental conditions - Hornung, J.; Zobus, Y.; Roeder, S.; Kleinschmidt, A.; Bertini, D.; Zepf, M.; Bagnoud, V. (Nature Communications, 2021)
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A laser–plasma platform for photon–photon physics: the two photon Breit–Wheeler process - Kettle, B.; Hollatz, D.; Gerstmayr, E.; Samarin, G. M.; Alejo, A.; Astbury, S.; Baird, C.; Bohlen, S.; Campbell, M.; Colgan, C.; Dannheim, D.; Gregory, C.; et al. (New Journal of Physics, 2021)
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Multiple photodetachment of silicon anions via K -shell excitation and ionization - Perry-Sassmannshausen, A.; Buhr, T.; Martins, M.; Reinwardt, S.; Trinter, F.; M{\"{u}}ller, A.; Fritzsche, S.; Schippers, S. (Physical Review A, 2021)
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Table-top interferometry on extreme time and wavelength scales - Skruszewicz, S.; Przystawik, A.; Schwickert, D.; Sumfleth, M.; Namboodiri, M.; Hilbert, V.; Klas, R.; Gierschke, P.; Schuster, V.; Vorobiov, A.; Haunhorst, C.; Kip, D.; et al. (Optics Express, 2021)
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.
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Torus-knot angular momentum in twisted attosecond pulses from high-order harmonic generation - Minneker, B.; B{\"{o}}ning, B.; Weber, A.; Fritzsche, S. (Physical Review A, 2021)
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Alignment procedure for off-axis-parabolic telescopes in the context of high-intensity laser beam transport - Ohland, J.; Zobus, Y.; Eisenbarth, U.; Zielbauer, B.; Reemts, D.; Bagnoud, V. (Optics Express, 2021)
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.
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Analysis of laser-proton acceleration experiments for development of empirical scaling laws - Zimmer, M.; Scheuren, S.; Ebert, T.; Schaumann, G.; Schmitz, B.; Hornung, J.; Bagnoud, V.; R{\"{o}}del, C.; Roth, M. (Physical Review E, 2021)
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Angular coefficients for symmetry-adapted configuration states in jj-coupling - Gaigalas, G.; Fritzsche, S. (Computer Physics Communications, 2021)
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.
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Control and stabilization of the modal content of fiber amplifiers using traveling waves - Jauregui, C.; Stihler, C.; Kholaif, S.; Tu, Y.; Limpert, J. (Optics Express, 2021)
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.
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Delocalization of quantum information in long-range interacting systems - Wanisch, D.; Fritzsche, S. (Physical Review A, 2021)
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.
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Gas-plasma-based generation of broadband terahertz radiation with 640 mW average power - Buldt, J.; Stark, H.; Mueller, M.; Grebing, C.; Jauregui, C.; Limpert, J. (Optics Letters, 2021)
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.
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Modeling chromatic emittance growth in staged plasma wakefield acceleration to 1 TeV using nonlinear transfer matrices - Thomas, A.; Seipt, D. (Physical Review ST - Accelerators and Beams, 2021)
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Organic crystal-based THz source for complex refractive index measurements of window materials using single-shot THz spectroscopy - Du, L.; Roeder, F.; Li, Y.; Shalaby, M.; Beleites, B.; Ronneberger, F.; Gopal, A. (Applied Physics A, 2021)
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.
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Parametric Study of Proton Acceleration from Laser-Thin Foil Interaction - Almassarani, M.; Meng, S.; Beleites, B.; Ronneberger, F.; Paulus, G.; Gopal, A. (Plasma, 2021)
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.
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Towards pair production in the non-perturbative regime - Salgado, F. C.; Grafenstein, K.; Golub, A.; Dopp, A.; Eckey, A.; Hollatz, D.; M{\"{u}}ller, C.; Seidel, A.; Seipt, D.; Karsch, S.; Zepf, M. (New Journal of Physics, 2021)
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.
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Transverse ultrafast laser inscription in bulk silicon - Chambonneau, M.; Blothe, M.; Li, Q.; Fedorov, V.; Heuermann, T.; Gebhardt, M.; Gaida, C.; Tertelmann, S.; Sotier, F.; Limpert, J.; Tzortzakis, S.; Nolte, S. (Physical Review Research, 2021)
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.
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X-ray vacuum diffraction at finite spatiotemporal offset - Karbstein, F.; Oude Weernink, R. (Physical Review D, 2021)
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.
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Charge‐State Distributions of Highly Charged Lead Ions at Relativistic Collision Energies - Kr{\"{o}}ger, F.; Weber, G.; Hirlaender, S.; Alemany-Fernandez, R.; Krasny, M.; St{\"{o}}hlker, T.; Tolstikhina, I.; Shevelko, V. (Annalen der Physik, 2021)
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Conceptual design report for the LUXE experiment - Abramowicz, H.; Acosta, U.; Altarelli, M.; Assmann, R.; Bai, Z.; Behnke, T.; Benhammou, Y.; Blackburn, T.; Boogert, S.; Borysov, O.; Borysova, M.; Brinkmann, R.; et al. (European Physical Journal Special Topics, 2021)
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Derivative corrections to the Heisenberg-Euler effective action - Karbstein, F. (Journal of High Energy Physics, 2021)
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.
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Experimental study of the laser-induced ionization of heavy metal and metalloid ions: Au+ and Si2+ in intense and sculpted femtosecond laser fields - Ying, B.; Machalett, F.; Huth, V.; Kuebel, M.; Sayler, A.; Stoehlker, T.; Paulus, G.; Wustelt, P. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2021)
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.
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Role of L-shell single and double core-hole production and decay in m-fold (1 <= m <= 6) photoionization of the Ar+ ion - Mueller, A.; Martins, M.; Borovik, J.; Buhr, T.; Perry-Sassmannshausen, A.; Reinwardt, S.; Trinter, F.; Schippers, S.; Fritzsche, S.; Kheifets, A. S. (Physical Review A, 2021)
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.
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Steering of circular dichroism in biharmonic ionization of atoms - Volotka, A. V.; Hofbrucker, J.; Fritzsche, S. (Physical Review A, 2021)
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Symbolic Evaluation of Expressions from Racah s Algebra - Fritzsche, S. (Symmetry, 2021)
Based on the rotational symmetry of isolated quantum systems, Racah s algebra plays a significant role in nuclear, atomic and molecular physics, and at several places elsewhere. For N-particle (quantum) systems, for example, this algebra helps carry out the integration over the angular coordinates analytically and, thus, to reduce them to systems with only N (radial) coordinates. However, the use of Racah s algebra quickly leads to complex expressions, which are written in terms of generalized Clebsch-Gordan coefficients, Wigner n-j symbols, (tensor) spherical harmonics and/or rotation matrices. While the evaluation of these expressions is straightforward in principle, it often becomes laborious and prone to making errors in practice. We here expand Jac, the Jena Atomic Calculator, to facilitate the sum-rule evaluation of typical expressions from Racah s algebra. A set of new and revised functions supports the simplification and subsequent use of such expressions in daily research work or as part of lengthy derivations. A few examples below show the recoupling of angular momenta and demonstrate how Jac can be readily applied to find compact expressions for further numerical studies. The present extension makes Jac a more flexible and powerful toolbox in order to deal with atomic and quantum many-particle systems.
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Temporal pulse quality of a Yb:YAG burst-mode laser post-compressed in a multi-pass cell - Viotti, A.-L.; Ali{{s}}auskas, S.; Tuennermann, H.; Escoto, E.; Seidel, M.; Dudde, K.; Manschwetus, B.; Hartl, I.; Heyl, C. (Optics Letters, 2021)
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.
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Obtaining Intense Attosecond Pulses in the Far Field from Relativistic Laser-Plasma Interactions - Zhang, Y.; Zhong, C. L.; Zhu, S. P.; He, X. T.; Zepf, M.; Qiao, B. (Physical Review Applied, 2021)
In this paper, we show that the Gouy phase shift plays a key role in the far-field waveform evolution of the reflected harmonic radiations from plasma surfaces driven by a relativistic Gaussian laser. With a proper adjustment of laser focal position away from the plasma surface, the inherent separations between the peaks of different harmonic carrier waves as well as the fundamental wave due to different wavelengths can be cleared away when they propagate from near to far field, since they experience the same Gouy phase shift of pi/2. Using this method, intense attosecond pulses can be obtained in the far field with no need of any spectral filters. Three-dimensional particle-in-cell simulations show that far-field attosecond pulses with intensity of 4 x 1015 W/cm2 (2.56 x 1017 W/sr; 65 times increase) and duration of 76 as (50% decrease) can be obtained by lasers at intensities of 1021 W/cm2. Such brilliant pulses with fully reserved spectra significantly benefit applications in attosecond science.
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Proton-neutron pairing correlations in the self-conjugate nucleus Sc-42 - Koszorus, A.; Vormawah, L. J.; Beerwerth, R.; Bissell, M. L.; Campbell, P.; Cheal, B.; Devlin, C. S.; Eronen, T.; Fritzsche, S.; Geldhof, S.; Heylen, H.; Holt, J. D.; et al. (Physical Review B, 2021)
Collinear laser spectroscopy of the N = Z = 21 self-conjugate nucleus Sc-42 has been performed at the JYFL IGISOL IV facility in order to determine the change in nuclear mean-square charge radius between the I-pi = 0(+) ground state and the I-pi = 7(+) isomer via the measurement of the Sc-42g,Sc-42m isomer shift. New multi-configurational Dirac-Fock calculations for the atomic mass shift and field shift factors have enabled a recalibration of the charge radii of the Sc42-46 isotopes which were measured previously. While consistent with the treatment of proton-neutron, proton-proton and neutron-neutron pairing on an equal footing, the reduction in size for the isomer is observed to be of a significantly larger magnitude than that expected from both shell-model and ab-initio calculations. The measured nuclear magnetic dipole moment and electric quadruple moment, on the other hand, are in good agreement with simple empirical estimates and shell-model calculations.
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Relativistic modified Bessel-Gaussian beam generated from plasma-based beam braiding - Lei, B.; Seipt, D.; Shi, M.; Liu, B.; Wang, J.; Zepf, M.; Rykovanov, S. (Physical Review A, 2021)
We theoretically and numerically demonstrate the generation of a relativistic modified Bessel-Gaussian beam (MBGB) via plasma-based beam braiding. It is realized by injecting several intense Gaussian pulses with well-designed offsets and angles into an underdense plasma channel which acts as a laser-pulse combiner via refractive coupling. The MBGB propagates stably in the plasma channel with a well-controlled orbital angular momentum of large value, exciting a twisted plasma wave. After leaving the plasma, it becomes unguided and survives in vacuum for at least hundreds of femtoseconds. This method is insensitive to the initial laser injection conditions and thus should be robust for experimental implementation. It provides an alternative approach in generating high-quality tunable intense optical vortex beams which are desired for various applications.
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Scaling potential of beam-splitter-based coherent beam combination - Mueller, M.; Aleshire, C.; Buldt, J.; Stark, H.; Grebing, C.; Klenke, A.; Limpert, J. (Optics Express, 2021)
The impact of nonlinear refraction and residual absorption on the achievable peakand average power in beam-splitter-based coherent beam combination is analyzed theoretically. While the peak power remains limited only by the aperture size, a fundamental average power limit is given by the thermo-optical and thermo-mechanical properties of the beam splitter material and its coatings. Based on our analysis, 100 kW average power can be obtained with state-of-the-art optics at maintained high beam quality (M-2 <= 1.1) and at only 2% loss of combining efficiency. This result indicates that the power-scaling potential of today extquotesingle s beam-splitter-based coherent beam combination is far from being depleted. A potential scaling route to megawatt-level average power is discussed for optimized beam splitter geometry.
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Vacuum birefringence at x-ray free-electron lasers - Karbstein, F.; Sundqvist, C.; Schulze, K. S.; Uschmann, I.; Gies, H.; Paulus, G.G. (New Journal of Physics, 2021)
We study the perspectives of measuring the phenomenon of vacuum birefringence predicted by quantum electrodynamics using an x-ray free-electron laser (XFEL) alone. We devise an experimental scheme allowing two consecutive XFEL pulses to collide under a finite angle, and thus act as both pump and probe field for the effect. The signature of vacuum birefringence is encoded in polarization-flipped signal photons to be detected with high-purity x-ray polarimetry. Our findings for idealized scenarios underline that the discovery potential of solely XFEL-based setups can be comparable to those involving optical high-intensity lasers. For currently achievable scenarios, we identify several key details of the x-ray optical ingredients that exert a strong influence on the magnitude of the desired signatures.
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A pairwise nuclear fusion algorithm for particle-in-cell simulations: Weighted particles at relativistic energies - Wu, D.; Sheng, Z. M.; Yu, W.; Fritzsche, S.; He, X. T. (AIP Advances, 2021)
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A review of high‐resolution microscopic ghost imaging with a low‐dose pseudothermal light - Sun, Z.; Tuitje, F.; Spielmann, C. (Journal of Microscopy, 2021)
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Above-threshold ionization driven by Gaussian laser beams: beyond the electric dipole approximation - B{\"{o}}ning, B.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2021)
Strong-field atomic experiments have recently become sensitive to nondipole (magnetic) interactions. In particular, photoelectrons emitted in above-threshold ionization possess a nonzero momentum along the beam axis as a result of the Lorentz force. Here, we show how this longitudinal momentum can be theoretically calculated based on a nondipole strong-field approximation that accounts not only for the temporal but also the spatial dependence of the laser field in the photoelectron continuum. If the driving laser beam is approximated as a plane wave, the theoretical values differ from known experimental results by a constant offset. We demonstrate that this offset can successfully be removed if a realistic Gaussian beam profile is accounted for in the quantum description of ATI. We also discuss the influence of the size of the beam waist in the focus.
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Agile spectral tuning of high order harmonics by interference of two driving pulses - Schuster, V.; Hilbert, V.; Klas, R.; Liu, C.; Tschernajew, M.; Bernhardt, B.; Rothhardt, J.; Limpert, J. (Optics Express, 2021)
In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.
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Application of Gamma Attenuation Technique and Artificial Intelligence to Detect Scale Thickness in Pipelines in Which Two-Phase Flows with Different Flow Regimes and Void Fractions Exist - Alamoudi, M.; Sattari, M.; Balubaid, M.; Eftekhari-Zadeh, E.; Nazemi, E.; Taylan, O.; Kalmoun, E. (Symmetry, 2021)
Scale deposits can reduce equipment efficiency in the oil and petrochemical industry. The gamma attenuation technique can be used as a non-invasive effective tool for detecting scale deposits in petroleum pipelines. The goal of this study is to propose a dual-energy gamma attenuation method with radial basis function neural network (RBFNN) to determine scale thickness in petroleum pipelines in which two-phase flows with different symmetrical flow regimes and void fractions exist. The detection system consists of a dual-energy gamma source, with Ba-133 and Cs-137 radioisotopes and two 2.54-cm x 2.54-cm sodium iodide (NaI) detectors to record photons. The first detector related to transmitted photons, and the second one to scattered photons. The transmission detector recorded two signals, which were the counts under photopeak of Ba-133 and Cs-137 with the energy of 356 keV and 662 keV, respectively. The one signal recorded in the scattering detector, total counts, was applied to RBFNN as the inputs, and scale thickness was assigned as the output.
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Compact, All-PM Fiber Integrated and Alignment-Free Ultrafast Yb:Fiber NALM Laser With Sub-Femtosecond Timing Jitter - Ma, Y.; Salman, S.; Li, C.; Mahnke, C.; Hua, Y.; Droste, S.; Fellinger, J.; Mayer, A.; Heckl, O.; Heyl, C.; Hartl, I. (Journal of Lightwave Technology, 2021)
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Connecting Fano Interference and the Jaynes-Cummings Model in Cavity Magnonics - Gollwitzer, J.; Bocklage, L.; R{\"{o}}hlsberger, R.; Meier, G. (NPJ Quantum Information, 2021)
We show that Fano interference can be realized in a macroscopic microwave cavity coupled to a spin ensemble at room temperature. Via a formalism developed from the linearized Jaynes-Cummings model of cavity electromagnonics, we show that generalized Fano interference emerges from the photon–magnon interaction at low cooperativity. In this regime, the reflectivity approximates the scattering cross-section derived from the Fano-Anderson model. Although asymmetric lineshapes in this system are often associated with the Fano formalism, we show that whilst Fano interference is actually present, an exact Fano form cannot be achieved from the linear Jaynes-Cummings model. In the Fano model an additional contribution arises, which is attributed to decoherence in other systems, and in this case is due to the resonant nature of the photonic mode. The formalism is experimentally verified and accounts for the asymmetric lineshapes arising from the interaction between magnon and photon channels. As the magnon–photon coupling strength is increased, these channels merge into hybridized magnon–photon modes and the generalized Fano interference picture breaks down. Our results are universally applicable to systems underlying the linearized Jaynes-Cummings Hamiltonian at low cooperativity and connect the microscopic parameters of the quantum optical model to generalized Fano lineshapes.
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Electric pulse heating device for the analysis of solid/solid phase transformations - Kaaden, T.; Tympel, V.; Kober, M.; Schmidl, F.; Rettenmayr, M.; Lippmann, S. (Review of Scientific Instruments, 2021)
Ohmic pulse heating is applied to investigate diffusion and interface controlled solid-state phase transformations. The developed device uses extensive solid-state electronics providing a high current, low voltage approach that overcomes the limitations of existing setups, most notably the use of sample geometries that allow for the reliable measurement of local temperatures and their assignment to microstructures. Power for heating is supplied by a capacitor array with adjustable voltage, and the process is controlled by microcontrollers and a solid-state relay, which allows for controlled pulses that are adjustable in microseconds. Electric currents of up to 22 kA at 90 V can be realized by the setup. Electric data are monitored and collected during the experiments, and temperature data are captured using a high-resolution infrared camera at high frame rates (1200 fps). The capabilities of the setup are demonstrated by rapid heating (10(6) K/s) and subsequent cooling of a brass sample. Two distinct areas of the sample are analyzed in detail, showing similar heating, but different cooling curves with rates of 10(4) and 10(2) K/s. Local microstructure analysis shows that different phase transformation mechanisms were dominant, and thus, the setup fulfills its purpose.
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Electron-loss-to-continuum cusp in collisions of U89+ with N-2 and Xe - Hillenbrand, P.-M.; Lyashchenko, K. N.; Hagmann, S.; Andreev, O.; Bana{ś}, D.; Benis, E. P.; Bondarev, I.; Brandau, C.; De Filippo, E.; Forstner, O.; Glorius, J.; Grisenti, R. E.; et al. (Physical Review A, 2021)
We study the electron-loss-to-continuum (ELC) cusp experimentally and theoretically by comparing the ionization of U89+ projectiles in collisions with N-2 and Xe targets, at a beam energy of 75.91 MeV/u. The coincidence measurement between the singly ionized projectile and the energy of the emitted electron is used to compare the shape of the ELC cusp at weak and strong perturbations. A significant energy shift for the centroid of the electron cusp is observed for the heavy target of Xe as compared to the light target of N-2. Our results provide a stringent test for fully relativistic calculations of double-differential cross sections performed in the first-order approximation and in the continuum-distorted-wave approach.
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Elliptical dichroism in biharmonic ionization of atoms - Hofbrucker, J.; B{\"{o}}ning, B.; Volotka, A. V.; Fritzsche, S. (Physical Review A, 2021)
In multiphoton ionization of atoms, elliptical dichroism may arise in the photoelectron angular distributions due to the interference of the possible ionization pathways. We here consider the interaction of atoms with an elliptically polarized biharmonic $({ø}mega + 2{ø}mega)$ field which simultaneously allows one- and two-photon ionization of the atoms. The interference between these two ionization pathways introduces contributions to the elliptical dichroism in addition to the dichroism that arises from the two-photon ionization alone. We show that these additional dichroism contributions can lead to a stronger dichroism in comparison to the one arising from two-photon ionization only. We present a relativistic analysis of the corresponding photoelectron angular distributions and discuss individual contributions to the dichroic phenomena. Detailed computations have been performed for biharmonic ionization of neutral helium atoms.
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Laser-Driven Anharmonic Oscillator: Ground-State Dissociation of the Helium Hydride Molecular Ion by Midinfrared Pulses - Wustelt, P.; Oppermann, F.; Mhatre, S.; Kuebel, M.; Sayler, A.; Lein, M.; Graefe, S.; Paulus, G. (Physical Review Letters, 2021)
The vibrational motion of molecules represents a fundamental example of an anharmonic oscillator. Using a prototype molecular system, HeH+, we demonstrate that appropriate laser pulses make it possible to drive the nuclear motion in the anharmonic potential of the electronic ground state, increasing its energy above the potential barrier and facilitating dissociation by purely vibrational excitation. We find excellent agreement between the frequency-dependent response of the helium hydride molecular cation to both classical and quantum mechanical simulations, thus removing any ambiguities through electronic excitation. Our results provide access to the rich dynamics of anharmonic quantum oscillator systems and pave the way to state-selective control schemes in ground-state chemistry by the adequate choice of the laser parameters.
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Novel Tunnel Magnetoresistive Sensor Functionalities via Oblique-Incidence Deposition - Willing, S.; Schlage, K.; Bocklage, L.; Moayed, M. M. R.; Gurieva, T.; Meier, G.; R{\"{o}}hlsberger, R. (ACS Applied Materials { &} Interfaces, 2021)
Controlling the magnetic properties of ultrathin films remains one of the main challenges to the further development of tunnel magnetoresistive (TMR) device applications. The magnetic response in such devices is mainly governed by extending the primary TMR trilayer with the use of suitable contact materials. The transfer of magnetic anisotropy to ferromagnetic electrodes consisting of CoFeB layers results in a field-dependent TMR response, which is determined by the magnetic properties of the CoFeB as well as the contact materials. We flexibly apply oblique-incidence deposition (OID) to introduce arbitrary intrinsic in-plane anisotropy profiles into the magnetic layers. The OID-induced anisotropy shapes the magnetic response and eliminates the requirement of additional magnetic contact materials. Functional control is achieved via an adjustable shape anisotropy that is selectively tailored for the ultrathin CoFeB layers. This approach circumvents previous limitations on TMR devices and allows for the design of new sensing functionalities, which can be precisely customized to a specific application, even in the high field regime. The resulting sensors maintain the typical TMR signal strength as well as a superb thermal stability of the tunnel junction, revealing a striking advantage in functional TMR design using anisotropic interfacial roughness.
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Role of free-carrier interaction in strong-field excitations in semiconductors - Hollinger, R.; Haddad, E.; Zapf, M.; Shumakova, V.; Herrmann, P.; Roeder, R.; Uschmann, I.; Reisloehner, U.; Pug{{z}}lys, A.; Baltuska, A.; L{\'{e}}gar{\'{e}}, F.; Zuerch, M.; et al. (Physical Review B, 2021)
The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 mu m (1 eV) and 10 mu m (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.
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Single-cycle versus multicycle nonsequential double ionization of argon - Liu, F.; Chen, Z.; Morishita, T.; Bartschat, K.; B{\"{o}}ning, B.; Fritzsche, S. (Physical Review A, 2021)
Using an improved quantitative rescattering model, we calculate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization of Ar exposed to intense laser pulses with a wavelength of 790 nm at a peak intensity of 1.0×10¹⁴ W/cm². We analyze the drastic variations in the CMDs that were observed by K{\"{u}}bel et al. [New J. Phys. 16, 033008 (2014)] in the transition from near-single-cycle to multicycle driving laser pulses. Our model reproduces their experimental data well. We also find that the transition from near-single-cycle to multicycle driving laser pulses depends strongly on the details of the pulse envelope. Special attention is paid to the mechanisms responsible for the cross-shaped structure observed experimentally with 4 fs pulses. Our analysis reveals that the cross-shaped structure in the carrier-envelope phase-averaged CMD for near-single-cycle pulses can be attributed to strong backward scattering of the recolliding electron as well as the narrow momentum distributions of the tunnel-ionized electrons compared to those for long pulses. This also explains why the cross-shaped distributions collapse to a rather structureless distribution when the pulse duration is increased to 8 fs.
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Towards attosecond imaging at the nanoscale using broadband holography-assisted coherent imaging in the extreme ultraviolet - Eschen, W.; Wang, S.; Liu, C.; Klas, R.; Steinert, M.; Yulin, S.; Meissner, H.; Bussmann, M.; Pertsch, T.; Limpert, J.; Rothhardt, J. (Communications Physics, 2021)
The inherently broad bandwidth of attosecond pulses conflicts with the coherence requirements of lensless imaging. Here, broadband holography-assisted coherent imaging is demonstrated with a resolution of less than 35 nm. Nanoscale coherent imaging has emerged as an indispensable modality, allowing to surpass the resolution limit given by classical imaging optics. At the same time, attosecond science has experienced enormous progress and has revealed the ultrafast dynamics in complex materials. Combining attosecond temporal resolution of pump-probe experiments with nanometer spatial resolution would allow studying ultrafast dynamics on the smallest spatio-temporal scales but has not been demonstrated yet. To date, the large bandwidth of attosecond pulses poses a major challenge to high-resolution coherent imaging. Here, we present broadband holography-enhanced coherent imaging, which enables the combination of high-resolution coherent imaging with a large spectral bandwidth. By implementing our method at a high harmonic source, we demonstrate a spatial resolution of 34 nm in combination with a spectral bandwidth of 5.5 eV at a central photon energy of 92 eV. The method is single-shot capable and retrieves the spectrum from the measured diffraction pattern.
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Ultraviolet dual comb spectroscopy: a roadmap - Schuster, V.; Liu, C.; Klas, R.; Dominguez, P.; Rothhardt, J.; Limpert, J.; Bernhardt, B. (Optics Express, 2021)
Dual Comb Spectroscopy proved its versatile capabilities in molecular fingerprinting in different spectral regions, but not yet in the ultraviolet (UV). Unlocking this spectral window would expand fingerprinting to the electronic energy structure of matter. This will access the prime triggers of photochemical reactions with unprecedented spectral resolution. In this research article, we discuss the milestones marking the way to the first UV dual comb spectrometer. We present experimental and simulated studies towards UV dual comb spectroscopy, directly applied to planned absorption measurements of formaldehyde (centered at 343 nm, 3.6 eV) and argon (80 nm, 16 eV). This will enable an unparalleled relative resolution of up to 10-9 - with a table-top UV source surpassing any synchrotron-linked spectrometer by at least two and any grating-based UV spectrometer by up to six orders of magnitude.
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Vacuum birefringence and diffraction at an x-ray free-electron laser: From analytical estimates to optimal parameters - Mosman, E.; Karbstein, F. (Physical Review D, 2021)
We study vacuum birefringence and x-ray photon scattering in the head-on collision of x-ray free electron and high-intensity laser pulses. Resorting to analytical approximations for the numbers of attainable signal photons, we analyze the behavior of the phenomenon under the variation of various experimental key-parameters and provide new analytical scalings. Our optimized approximations allow for quantitatively accurate results on the one-percent level. We in particular demonstrate that an appropriate choice of the x-ray focus and pulse duration can significantly improve the signal for given laser parameters, using the experimental parameters to be available at the Helmholtz International Beamline for Extreme Fields at the European XFEL as example. Our results are essential for the identification of the optimal choice of parameters in a discovery experiment of vacuum birefringence at the high-intensity frontier.
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Angular Distribution of Characteristic Radiation Following the Excitation of He-Like Uranium in Relativistic Collisions - Gumberidze, A.; Thorn, D.; Surzhykov, A.; Fontes, C.; Bana{ś}, D.; Beyer, H.; Chen, W.; Grisenti, R.; Hagmann, S.; Hess, R.; Hillenbrand, P.-M.; Indelicato, P.; et al. (Atoms, 2021)
In this paper, we present an experimental and theoretical study of excitation processes for the heaviest stable helium-like ion, that is, He-like uranium occurring in relativistic collisions with hydrogen and argon targets. In particular, we concentrate on angular distributions of the characteristic K alpha radiation following the K -> L excitation of He-like uranium. We pay special attention to the magnetic sub-level population of the excited 1s2l(j) states, which is directly related to the angular distribution of the characteristic K alpha radiation. We show that the experimental data can be well described by calculations taking into account the excitation by the target nucleus as well as by the target electrons. Moreover, we demonstrate for the first time an important influence of the electron-impact excitation process on the angular distributions of the K alpha radiation produced by excitation of He-like uranium in collisions with different targets.
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Comparison of two low-noise CEO frequency stabilization methods for an all-PM Yb:fiber NALM oscillator - Salman, S.; Ma, Y.; G{\"{u}}rel, K.; Schilt, S.; Li, C.; Pf{\"{a}fflein, P. and Mahnke, C. and Fellinger, J. and Droste, S. and Mayer, A. and Heckl, O. and S{\"{u}}dmeyer, T. and Heyl, C. and Hartl, I. (OSA Continuum, 2021)
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Curvature bound from gravitational catalysis in thermal backgrounds - Gies, H.; Salek, A. (Physical Review D, 2021)
We investigate the phenomenon of gravitational catalysis, i.e., curvature-induced chiral symmetry breaking and fermion mass generation, at finite temperature. Using a scale-dependent analysis, we derive a thermal bound on the curvature of local patches of spacetime. This bound quantifies regions in parameter space that remain unaffected by gravitational catalysis and thus are compatible with the existence of light fermions as observed in nature. While finite temperature generically relaxes the curvature bound, we observe a comparatively strong dependence of the phenomenon on the details of the curvature. Our bound can be applied to scenarios of quantum gravity, as any realistic candidate has to accommodate a sufficient number of light fermions. We argue that our bound therefore represents a test for quantum-gravity scenarios: A suitably averaged spacetime in the (trans-)Planckian regime that satisfies our curvature bound does not induce correspondingly large Planckian fermion masses by gravitational catalysis. The temperature dependence derived in this work facilitates to follow the fate of gravitational catalysis during the thermal history of the (quantum) Universe. In an application to the asymptotic-safety scenario of quantum gravity, our bound translates into a temperature-dependent upper bound on the number of fermion flavors.
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Generation of straight and curved hollow plasma channels by laser-generated nonlinear wakefields and studies of ultra-intense laser pulse guiding - Ma, Y.; Seipt, D.; Krushelnick, K.; Thomas, A. G. R. (Physics of Plasmas, 2021)
We report on a study of highly controllable, quasi-static hollow plasma channels generated by ion motion following a laser–plasma interaction via numerical simulations. These channels are generated by ion motion in a nonlinear wakefield, which has an asymmetry between focusing and defocusing periods. Such plasma channels, driven by a fs duration, 100 TW level laser pulse, have the capability to guide more powerful laser pulses. Moreover, we show that curved plasma channels can be generated in a transverse density gradient plasma and all-optical guiding of a second, significantly more powerful laser pulse in such a curved plasma channel. This mechanism may be useful for creating plasma optics for multi-stage TeV laser plasma accelerators and compact synchrotron radiation sources.
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Justifying the thin-crystal approximation in spontaneous parametric down-conversion for collinear phase matching - Baghdasaryan, B.; Steinlechner, F.; Fritzsche, S. (Physical Review A, 2021)
Spatially engineered photons from spontaneous parametric down-conversion (SPDC) are a valuable tool for studying and applying photonic entanglement. An advantage of SPDC is that simple expressions for the two-photon state can be obtained using justified approximations. In particular, the thin-crystal approximation has often been invoked in the engineering of high-dimensional entangled states. Knowledge of the conditions under which the thin-crystal approximation remains valid is essential for the realization of experimental setups. We provide a quantitative guideline on the validity of the thin-crystal approximation in calculating the two-photon spatial state. In particular, we show that the applicability of this regime is related to the focusing parameter (w) over barp = w(p)/root lambda(p) L, where w(p) and lambda(p) are the beam waist and wavelength of the pump beam, respectively, and L is the length of the nonlinear crystal. Additionally, the validity of the thin-crystal regime is investigated concerning the size of a subspace in the Laguerre Gaussian basis, into which the two-photon state can be projected in a given experiment.
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Many-Electron QED with Redefined Vacuum Approach - Soguel, R.; Volotka, A.; Glazov, D.; Fritzsche, S. (Symmetry, 2021)
The redefined vacuum approach, which is frequently employed in the many-body perturbation theory, proved to be a powerful tool for formula derivation. Here, we elaborate this approach within the bound-state QED perturbation theory. In addition to general formulation, we consider the particular example of a single particle (electron or vacancy) excitation with respect to the redefined vacuum. Starting with simple one-electron QED diagrams, we deduce first- and second-order many-electron contributions: screened self-energy, screened vacuum polarization, one-photon exchange, and two-photon exchange. The redefined vacuum approach provides a straightforward and streamlined derivation and facilitates its application to any electronic configuration. Moreover, based on the gauge invariance of the one-electron diagrams, we can identify various gauge-invariant subsets within derived many-electron QED contributions.
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Multipass cell for high-power few-cycle compression - Mueller, M.; Buldt, J.; Stark, H.; Grebing, C.; Limpert, J. (Optics Letters, 2021)
A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 mu J pulse energy is obtained at 82% cell transmission. A high output beam quality (M-2 < 1.2) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.
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The effects of laser polarization and wavelength on injection dynamics of a laser wakefield accelerator - Ma, Y.; Seipt, D.; Hussein, A. E.; Hakimi, S.; Beier, N. F.; Hansen, S. B.; Hinojosa, J.; Maksimchuk, A.; Nees, J.; Krushelnick, K.; Thomas, A. G. R.; Dollar, F. (Physics of Plasmas, 2021)
Here, we investigate the effects of laser polarization and wavelength on electron injection dynamics in a laser wakefield accelerator. During the ionization process, electrons gain residual momentum and kinetic energy via above threshold ionization, which has a strong dependence on laser polarization. A circularly polarized laser pulse results in a much higher residual momentum and kinetic energy gain for the ionized electrons compared with the linearly polarized case. This residual momentum results in particle injection because of the sensitivity of particle trapping to the initial conditions and enhanced the total injected beam charge in both experiments and particle-in-cell simulations. Due to the strong correlation of above threshold ionization with laser wavelength, in this work we extended the investigation to long wavelength (up to 20 μm) drive pulses using particle-in-cell simulations. Owing to the gain in kinetic energy, it may be expected that the charge trapped would consistently increase for circular polarization with increasing laser wavelength, but this was not observed. Instead, there are oscillations with wavelength in the relative trapped charge between linear and circular polarization cases, which arise because of ionization and heating effects on the plasma. Our studies highlight the complex interplay between several different physical effects, including injection regimes{–}above threshold ionization assisted injection, wave-breaking injection by carrier-envelope-phase effects and ionization injection{–}ionization gradient induced laser pulse evolution, and thermal modifications to the wake structure that need considering when extrapolating laser wakefield acceleration to different wavelength regimes.
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Unification of topological invariants in Dirac models - von Gersdorff, G.; Panahiyan, S.; Chen, W. (Physical Review B, 2021)
Topological phases of materials are characterized by topological invariants that are conventionally calculated by different means according to the dimension and symmetry class of the system. For topological materials described by Dirac models, we introduce a wrapping number as a unified approach to obtain the topological invariants in arbitrary dimensions and symmetry classes. Given a unit vector that parametrizes the momentum dependence of the Dirac model, the wrapping number describes the degree of the map from the Brillouin zone torus to the sphere formed by the unit vector that we call the Dirac sphere. This method is gauge-invariant and originates from the intrinsic features of the Dirac model and moreover places all known topological invariants, such as the Chern number, winding number, Pfaffian, etc, on equal footing.
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Approximate Atomic Green Functions - Fritzsche, S.; Surzhykov, A. (Molecules, 2021)
In atomic and many-particle physics, Green functions often occur as propagators to formally represent the (integration over the) complete spectrum of the underlying Hamiltonian. However, while these functions are very crucial to describing many second- and higher-order perturbation processes, they have hardly been considered and classified for complex atoms. Here, we show how relativistic (many-electron) Green functions can be approximated and systematically improved for few- and many-electron atoms and ions. The representation of these functions is based on classes of virtual excitations, or so-called excitation schemes, with regard to given bound-state reference configurations, and by applying a multi-configuration Dirac-Hartree-Fock expansion of all atomic states involved. A first implementation of these approximate Green functions has been realized in the framework of Jac, the Jena Atomic Calculator, and will facilitate the study of various multi-photon and/or multiple electron (emission) processes.
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Compact, diode-pumped, unstable cavity Yb:YAG laser and its application in laser shock peening - K{\"{o}}rner, J.; Zulic, S.; Reiter, J.; Lenski, M.; Hein, J.; B{\"{o}}defeld, R.; Rostohar, D.; Mocek, T.; Kaluza, M.C. (Optics Express, 2021)
We present the setup of a compact, q-switched, cryogenically cooled Yb:YAG laser, which is capable of producing over 1 J output energy in a 10 ns pulse at 10 Hz. The system’s design is based on the recently published unstable cavity layout with gain shaping of the spatial intra-cavity intensity distribution. Using a hexagonal homogenized pump beam, the laser generated an according hexagonal output beam profile. The suitability of such laser properties for the intended use in a laser shock peening process is demonstrated. In the experiment an aluminum plate was treated and the generated residual stresses in the sample subsequently measured. Other applications of this laser system like laser pumping or surface cleaning are conceivable.
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Polarized QED cascades - Seipt, D.; Ridgers, C. P.; Del Sorbo, D.; Thomas, A. G. R. (New Journal of Physics, 2021)
By taking the spin and polarization of the electrons, positrons and photons into account in the strong-field QED processes of nonlinear Compton emission and pair production, we find that the growth rate of QED cascades in ultra-intense laser fields can be substantially reduced. While this means that fewer particles are produced, we also found them to be highly polarized. We further find that the high-energy tail of the particle spectra is polarized opposite to that expected from Sokolov–Ternov theory, which cannot be explained by just taking into account spin-asymmetries in the pair production process, but results significantly from -straggling’. We employ a kinetic equation approach for the electron, positron and photon distributions, each of them spin/polarization-resolved, with the QED effects of photon emission and pair production modelled by a spin/polarization dependent Boltzmann-type collision operator. For photon-seeded cascades, depending on the photon polarization, we find an excess or a shortage of particle production in the early stages of cascade development, which provides a path towards a controlled experiment. Throughout this paper we focus on rotating electric field configuration, which represent an idealized model and allows for a straightforward interpretation of the observed effects.
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Signatures of the Carrier Envelope Phase in Nonlinear Thomson Scattering - Ruijter, M.; Petrillo, V.; Teter, T.; Valialshchikov, M.; Rykovanov, S. (Crystals, 2021)
High-energy radiation can be generated by colliding a relativistic electron bunch with a high-intensity laser pulse-a process known as Thomson scattering. In the nonlinear regime the emitted radiation contains harmonics. For a laser pulse whose length is comparable to its wavelength, the carrier envelope phase changes the behavior of the motion of the electron and therefore the radiation spectrum. Here we show theoretically and numerically the dependency of the spectrum on the intensity of the laser and the carrier envelope phase. Additionally, we also discuss what experimental parameters are required to measure the effects for a beamed pulse.
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Table-top extreme ultraviolet second harmonic generation - Helk, T.; Berger, E.; Jamnuch, S.; Hoffmann, L.; Kabacinski, A.; Gautier, J.; Tissandier, F.; Goddet, J.-P.; Chang, H.-T.; Oh, J.; Das Pemmaraju, C.; Pascal, T.; et al. (Science Advances, 2021)
The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers ( FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the TiM2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.
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Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads - Gaida, C.; Gebhardt, M.; Heuermann, T.; Wang, Z.; Jauregui, C.; Limpert, J. (Optics Express, 2021)
We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 mu m, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.
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Coherence tomography with broad bandwidth extreme ultraviolet and soft X-ray radiation - Skruszewicz, S.; Fuchs, S.; Abel, J. J.; Nathanael, J.; Reinhard, J.; R{\"{o}}del, C.; Wiesner, F.; Wuensche, M.; Wachulak, P.; Bartnik, A.; Janulewicz, K.; Fiedorowicz, H.; et al. (Applied Physics B, 2021)
We present an overview of recent results on optical coherence tomography with the use of extreme ultraviolet and soft X-ray radiation (XCT). XCT is a cross-sectional imaging method that has emerged as a derivative of optical coherence tomography (OCT). In contrast to OCT, which typically uses near-infrared light, XCT utilizes broad bandwidth extreme ultraviolet (XUV) and soft X-ray (SXR) radiation (Fuchs et al in Sci Rep 6:20658, 2016). As in OCT, XCT extquotesingle s axial resolution only scales with the coherence length of the light source. Thus, an axial resolution down to the nanometer range can be achieved. This is an improvement of up to three orders of magnitude in comparison to OCT. XCT measures the reflected spectrum in a common-path interferometric setup to retrieve the axial structure of nanometer-sized samples. The technique has been demonstrated with broad bandwidth XUV/SXR radiation from synchrotron facilities and recently with compact laboratory-based laser-driven sources. Axial resolutions down to 2.2 nm have been achieved experimentally. XCT has potential applications in three-dimensional imaging of silicon-based semiconductors, lithography masks, and layered structures like XUV mirrors and solar cells.
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High-Repetition Rate Optical Pump-Nuclear Resonance Probe Experiments Identify Transient Molecular Vibrations after Photoexcitation of a Spin Crossover Material - Sadashivaiah, S.; Wolny, J.; Scherthan, L.; Jenni, K.; Omlor, A.; Mueller, C.; Sergueev, I.; Herlitschke, M.; Leupold, O.; Wille, H.-C.; R{\"{o}}hlsberger, R.; Schuenemann, V. (Journal of Physical Chemistry Letters, 2021)
Phonon modes play a vital role in the cooperative phenomenon of light-induced spin transitions in spin crossover (SCO) molecular complexes. Although the cooperative vibrations, which occur over several hundreds of picoseconds to nanoseconds after photoexcitation, are understood to play a crucial role in this phase transition, they have not been precisely identified. Therefore, we have performed a novel optical laser pump-nuclear resonance probe experiment to identify the Fe-projected vibrational density of states (pDOS) during the first few nanoseconds after laser excitation of the mononuclear Fe(II) SCO complex [Fe(PM-BiA)(2)(NCS)(2)]. Evaluation of the so obtained nanosecond-resolved pDOS yields an excitation of similar to 8% of the total volume of the complex from the low-spin to high-spin state. Density functional theory calculations allow simulation of the observed changes in the pDOS and thus identification of the transient inter- and intramolecular vibrational modes at nanosecond time scales.
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Laser spectroscopy of the 2S1/2-2P1/2, 2P3/2 transitions in stored and cooled relativistic C³⁺ ions - Winzen, D.; Hannen, V.; Bussmann, M.; Bu{\"{s}}, A.; Egelkamp, C.; Eidam, L.; Huang, Z.; Kiefer, D.; Klammes, S.; K{\"{u}}hl, T.; Loeser, M.; Ma, X.; et al. (Scientific Reports, 2021)
The 2S1/2−2P1/2 and 2S1/2−2P3/2 transitions in Li-like carbon ions stored and cooled at a velocity of beta=0.47 in the experimental storage ring (ESR) at the GSI Helmholtz Centre in Darmstadt have been investigated in a laser spectroscopy experiment. Resonance wavelengths were obtained using a new continuous-wave UV laser system and a novel extreme UV (XUV) detection system to detect forward emitted fluorescence photons. The results obtained for the two transitions are compared to existing experimental and theoretical data. A discrepancy found in an earlier laser spectroscopy measurement at the ESR with results from plasma spectroscopy and interferometry has been resolved and agreement between experiment and theory is confirmed.
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Quantum vacuum signatures in multicolor laser pulse collisions - Gies, H.; Karbstein, F.; Klar, L. (Physical Review D, 2021)
Quantum vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. A prominent signature of quantum vacuum nonlinearities driven by macroscopic fields are signal photons differing in characteristic properties such as frequency, propagation direction and polarization from the driving fields. We devise a strategy for the efficient tracing of the various vacuum-fluctuation-mediated interaction processes in order to identify the most prospective signal photon channels. As an example, we study the collision of up to four optical laser pulses and pay attention to sum and difference frequency generation. We demonstrate how this information can be used to enhance the signal photon yield in laser pulse collisions for a given total laser energy.
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Redefined vacuum approach and gauge-invariant subsets in two-photon-exchange diagrams for a closed-shell system with a valence electron - Soguel, R. N.; Volotka, A. V.; Tryapitsyna, V.; Glazov, D. A.; Kosheleva, V. P.; Fritzsche, S. (Physical Review A, 2021)
CThe two-photon-exchange diagrams for atoms with single valence electrons are investigated. Calculation formulas are derived for an arbitrary state within the rigorous bound-state QED framework utilizing the redefined vacuum formalism. In contrast to other methods, the redefined vacuum approach enables the identification of eight gauge-invariant subsets and, thus, efficiently checks the consistency of the obtained results. The gauge invariance of found subsets is demonstrated both analytically (for an arbitrary state) as well as numerically for 2s, 2p(1/2), and 2p(3/2) valence electrons in Li-like ions. Identifying gauge-invariant subsets in the framework of the proposed approach opens a way to tackle more complex diagrams, e.g., three-photon exchange, where the fragmentation on simpler subsets is crucial for its successful calculation.
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Ultra-short-pulse high-average-power megahertz-repetition-rate coherent extreme-ultraviolet light source - Klas, R.; Kirsche, A.; Gebhardt, M.; Buldt, J.; Stark, H.; H{\"{a}drich, S. and Rothhardt, J. and Limpert, J. (PhotoniX, 2021)
High harmonic generation (HHG) enables coherent extreme-ultraviolet (XUV) radiation with ultra-short pulse duration in a table-top setup. This has already enabled a plethora of applications. Nearly all of these applications would benefit from a high photon flux to increase the signal-to-noise ratio and decrease measurement times. In addition, shortest pulses are desired to investigate fastest dynamics in fields as diverse as physics, biology, chemistry and material sciences. In this work, the up-to-date most powerful table-top XUV source with 12.9 ± 3.9 mW in a single harmonic line at 26.5 eV is demonstrated via HHG of a frequency-doubled and post-compressed fibre laser. At the same time the spectrum supports a Fourier-limited pulse duration of sub-6 fs in the XUV, which allows accessing ultrafast dynamics with an order of magnitude higher photon flux than previously demonstrated. This concept will greatly advance and facilitate applications of XUV radiation in science and technology and enable photon-hungry ultrafast studies.
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1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system - Stark, H.; Buldt, J.; Mueller, M.; Klenke, A.; Limpert, J. (Optics Letters, 2021)
An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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Atomic Cascade Computations - Fritzsche, S.; Palmeri, P.; Schippers, S. (Symmetry, 2021)
Atomic cascades are ubiquitous in nature and they have been explored within very different scenarios, from precision measurements to the modeling of astrophysical spectra, and up to the radiation damage in biological matter. However, up to the present, a quantitative analysis of these cascades often failed because of their inherent complexity. Apart from utilizing the rotational symmetry of atoms and a proper distinction of different physical schemes, a hierarchy of useful approaches is therefore needed in order to keep cascade computations feasible. We here suggest a classification of atomic cascades and demonstrate how they can be modeled within the framework of the Jena Atomic Calculator. As an example, we shall compute within a configuration-average approach the stepwise decay cascade of atomic magnesium, following a 1s inner-shell ionization, and simulate the corresponding (final) ion distribution. Our classification of physical scenarios (schemes) and the hierarchy of computational approaches are both flexible to further refinements as well as to complex shell structures of the atoms and ions, for which the excitation and decay dynamics need to be modeled in good detail.
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Diode-pumped, electro-optically Q -switched, cryogenic Tm:YAG laser operating at 1.88 μm - K{\"{o}}rner, J.; Jambunathan, V.; Yue, F.; Reiter, J.; Slez{\'{a}}k, O.; Navr{\'{a}}til, P.; David, S.; Lucianetti, A.; Hein, J.; Mocek, T.; Kaluza, M. (High Power Laser Science and Engineering, 2021)
We present a diode-pumped, electro-optically Q-switched Tm:YAG laser with a cryogenically cooled laser crystal at 120 K. Output pulses of up to 2.55 mJ and 650 ns duration were demonstrated in an actively Q-switched configuration with a repetition rate of 1 Hz. By using cavity dumping the pulse duration was shortened to 18 ns with only a slightly lower output energy of 2.22 mJ. Furthermore, using a simplified rate equation model, we discuss design constraints on the pump fluence in a pulse pump approach for Tm:YAG to maximize the energy storage capability at a given pump power.
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Frustrated double ionization of atoms in circularly polarized laser fields - Kang, H.; Chen, S.; Chen, J.; Paulus, G. (New Journal of Physics, 2021)
We theoretically study frustrated double ionization (FDI) of atoms subjected to intense circularly polarized laser pulses using a three-dimensional classical model. We find a extasciigrave knee extquotesingle structure of FDI probability as a function of intensity, which is similar to the intensity dependence of nonsequential double ionization probability. The observation of FDI is more favourable when using targets with low ionization potentials and short driving laser wavelengths. This is attributed to the crucial role of recollision therein, which can be experimentally inferred from the photoelectron momentum distribution generated by FDI. This work provides novel physical insights into FDI dynamics with circular polarization.
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High-Order Phase-Dependent Asymmetry in the Above-Threshold Ionization Plateau - K{\"{u}}bel, M.; Wustelt, P.; Zhang, Y.; Skruszewicz, S.; Hoff, D.; W{\"{u}}rzler, D.; Kang, H.; Zille, D.; Adolph, D.; Paulus, G.; Sayler, A.; Dumergue, M.; et al. (Physical Review Letters, 2021)
Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1 μm wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from 0 to 2π. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.
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Improving the Contrast of Pseudothermal Ghost Images Based on the Measured Signal Distribution of Speckle Fields - Sun, Z.; Tuitje, F.; Spielmann, C. (Applied Sciences, 2021)
In this study, we examine the quality of microscale ghost images as a function of the measured histographic signal distribution of the speckle fields from a nonuniform pseudothermal light source. This research shows that the distribution of the detected signal level on each pixel of the camera plays a significant role in improving the contrast-to-noise ratio (CNR) of pseudothermal ghost imaging. To our knowledge, the scaling of CNR with different pixel intensity distributions of the speckle fields is observed for the first time in the field of pseudothermal microscale ghost imaging. The experimental observations are in very good agreement with numerical analysis. Based on these findings, we can predict the settings for light sources that will maximize the CNR of microscale ghost images.
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Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries - Gebhardt, M.; Amuah, E.; Klas, R.; Stark, H.; Buldt, J.; Steinkopff, A.; Limpert, J. (Optics Express, 2021)
Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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Long-range quasi-non-diffracting Gauss-Bessel beams in a few-cycle laser field - Stoyanov, L.; Zhang, Y.; Dreischuh, A.; Paulus, G. (Optics Express, 2021)
Many applications ranging from nonlinear optics to material processing would benefit from pulsed ultrashort (quasi-)non-diffracting Gauss-Bessel beams (GBBs). Here we demonstrate a straightforward yet efficient method for generating such zeroth- and first-order GBBs using a single reflective spatial light modulator. Even in the sub-8-fs range there are no noticeable consequences for the measured pulse duration. The only effect is a weak coloring of the outer-lying satellite rings of the beams due to the spectrum spanning over more than 300 nm. The obtained beams have diffraction half-angles below 40 mu rad and reach propagation distances in excess of 1.5 m.
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Real-time phase-retrieval and wavefront sensing enabled by an artificial neural network - White, J.; Wang, S.; Eschen, W.; Rothhardt, J. (Optics Express, 2021)
In this manuscript we demonstrate a method to reconstruct the wavefront of focused beams from a measured diffraction pattern behind a diffracting mask in real-time. The phase problem is solved by means of a neural network, which is trained with simulated data and verified with experimental data. The neural network allows live reconstructions within a few milliseconds, which previously with iterative phase retrieval took several seconds, thus allowing the adjustment of complex systems and correction by adaptive optics in real time. The neural network additionally outperforms iterative phase retrieval with high noise diffraction patterns.
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State-selective single-electron capture in intermediate-energy C4+ + He collisions - Guo, D. L.; Gao, J. W.; Zhang, S. F.; Zhu, X. L.; Gao, Y.; Zhao, D. M.; Zhang, R. T.; Wu, Y.; Wang, J. G.; Dubois, A.; Ma, X. (Physical Review A, 2021)
A combined experimental and theoretical study on single capture in 15–50keV/uC4+ + He collisions was performed. State-selective single-electron capture cross sections and projectile scattering angle distributions were obtained by using a reaction microscope. A comparison of the state-selective cross sections with theoretical calculations based on the two-active-electron semiclassical atomic-orbital close-coupling method showed an excellent agreement for the considered impact energies. For the angular differential cross sections, an overall agreement was also obtained between the present experimental and theoretical results. Simulations performed using an extended Fraunhofer-type diffraction model suggest that the undulatory structures observed at small scattering angles for capture to C3+(1s22s) and C3+(1s22p0) can be interpreted as the diffraction pattern of the incident projectile de Broglie wave confined by an aperture defined as the region around the target where the considered capture process is likely. The strong competition between single-electron capture to C3+(1s22p0) and C3+(1s22p1) is also discussed to interpret the differences observed between their respective differential cross sections.
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Application Driven Optimization of Cryogenic Current Comparators (CCC) for Beam Storage Rings - Stapelfeld, M.; Schmidl, F.; Seidel, P.; St{\"{u}}ck, S.; Tympel, V.; St{\"{o}}hlker, T.; Haider, D.; Schwickert, M.; Sieber, T.; Schmelz, M.; Sch{\"{o}}nau, T.; Stolz, R. (IEEE Transactions on Applied Superconductivity, 2021)
Non-destructive measurements of nA beam currents in particle beam storage rings by detecting the azimuthal magnetic field generated by moving charged particles with a Cryogenic Current Comparator (CCC) are well established. The detection of beam currents with small amplitudes with a CCC in a storage ring demands a high slew rate which is caused by the rapid change of the beam current exceeding the operational limit of the SQUID in flux-locked loop mode. Previous solutions to increase the slew rate used a LCR first-order low-pass filter were a small resistor, unfortunately, dominated the current noise of the CCC. In this work we present a novel take by adding a second resonator into the CCC which in turn allows for higher resistances of the LCR low-pass filter and therefore lower thermal current noise. A second challenge connected with this CCC approach is the residual magnetization of the highly permeable magnetic core and the resulting shielding currents in the superconducting circuits of the CCC. The timing of a storage ring in the range of minutes opens a way to reduce these DC currents using a LR high-pass filter. Using serial sub-micro ohm resistors, time constants in the hour range can be achieved to improve the stability and performance of the CCC system.
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Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre - Gebhardt, M.; Heuermann, T.; Klas, R.; Liu, C.; Kirsche, A.; Lenski, M.; Wang, Z.; Gaida, C.; Antonio-Lopez, J. E.; Schulzgen, A.; Amezcua-Correa, R.; Rothhardt, J.; et al. (Light: Science { &} Applications, 2021)
Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.
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Bulk, cascaded pulse compression scheme and it s application to spin emitter characterization - Calendron, A.-L.; Meier, J.; Kueny, E.; Velten, S.; Bocklage, L.; R{\"{o}}hlsberger, R.; Kaertner, F. (Applied Optics, 2021)
The 35-fs-long pulses of a commercial Ti:sapphire amplifier are compressed to similar to 20 fs via self-phase modulation in bulk glass substrates. The cascading of both nonlinear broadening and dispersion compensation stages makes use of the increasing peak power in the successive nonlinear stages. As an application example, the compressed pulses are used for electro-optical sampling of terahertz waves created by optically pumped thin-film spin emitters. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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Coherent X-ray-optical control of nuclear excitons - Heeg, K.; Kaldun, A.; Strohm, C.; Ott, C.; Subramanian, R.; Lentrodt, D.; Haber, J.; Wille, H.-C.; Goerttler, S.; R{\"{u}}ffer, R.; Keitel, C.; R{\"{o}}hlsberger, R.; et al. (Nature, 2021)
Coherent control of quantum dynamics is key to a multitude of fundamental studies and applications1. In the visible or longer-wavelength domains, near-resonant light fields have become the primary tool with which to control electron dynamics2. Recently, coherent control in the extreme-ultraviolet range was demonstrated3, with a few-attosecond temporal resolution of the phase control. At hard-X-ray energies (above 5{–}10 kiloelectronvolts), M{\"{o}}ssbauer nuclei feature narrow nuclear resonances due to their recoilless absorption and emission of light, and spectroscopy of these resonances is widely used to study the magnetic, structural and dynamical properties of matter4,5. It has been shown that the power and scope of M{\"{o}}ssbauer spectroscopy can be greatly improved using various control techniques6{–}16. However, coherent control of atomic nuclei using suitably shaped near-resonant X-ray fields remains an open challenge. Here we demonstrate such control, and use the tunable phase between two X-ray pulses to switch the nuclear exciton dynamics between coherent enhanced excitation and coherent enhanced emission. We present a method of shaping single pulses delivered by state-of-the-art X-ray facilities into tunable double pulses, and demonstrate a temporal stability of the phase control on the few-zeptosecond timescale. Our results unlock coherent optical control for nuclei, and pave the way for nuclear Ramsey spectroscopy17 and spin-echo-like techniques, which should not only advance nuclear quantum optics18, but also help to realize X-ray clocks and frequency standards19. In the long term, we envision time-resolved studies of nuclear out-of-equilibrium dynamics, which is a long-standing challenge in M{\"{o}}ssbauer science20.
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Decreasing the bandwidth of linear and nonlinear Thomson scattering radiation for electron bunches with a finite energy spread - Ruijter, M.; Petrillo, V.; Zepf, M. (Physical Review ST - Accelerators and Beams, 2021)
Relative narrow bandwidth-high energy radiation can be produced through Thomson scattering, where highly relativistic electrons collide with a laser pulse. The bandwidth of such a source is determined, among others factors, by the bandwidth of the laser pulse and the energy spread of the electrons. Here we investigate how the bandwidth of such a source can be minimized, with a particular emphasis on electron bunches with a correlated energy spread of several percent, that are typical for plasma based accelerator schemes. We show that by introducing a chirp on the laser pulse it is possible to compensate the broadening effect due to the energy spread of the electrons, and obtain the same bandwidth as a quasi-monochromatic plane wave laser pulse colliding with a monoenergetic electron bunch. Ultimately, the bandwidth of a Thomson source is limited by the acceptance angle and the initial transverse momentum of electrons (emittance).
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Generation of long range low-divergent Gauss-Bessel beams by annihilating optical vortices - Stoyanov, L.; Zhekova, M.; Stefanov, A.; Ivanov, B.; Stefanov, I.; Paulus, G.; Dreischuh, A. (Optics Communications, 2021)
Bessel beams are remarkable since they do not diverge. Accordingly, they have numerous applications ranging from precision laser micro-machining to laser particle acceleration. We demonstrate a novel approach for generating long-range Gauss-Bessel beams. A ring-shaped beam is produced by imprinting a vortex with high topological charge in a Gaussian beam. The phase singularities are thereafter removed and the ring-shaped beam focused/Fourier-transformed by a thin lens. This results in a remarkably good realization of a Gauss- Bessel beam. Divergence angles in the microradian range and Gauss-Bessel beam lengths up to 2.5 m behind the focal plane of the lens are demonstrated.
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Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography - Wiesner, F.; W{\"{u}}nsche, M.; Reinhard, J.; Abel, J.; Nathanael, J.; Skruszewicz, S.; R{\"{o}}del, C.; Yulin, S.; Gawlik, A.; Schmidl, G.; Huebner, U.; Plentz, J.; et al. (Optica, 2021)
Scientific and technological progress depend substantially on the ability to image on the nanoscale. In order to investigate complex, functional, nanoscopic structures like, e.g., semiconductor devices, multilayer optics, or stacks of 2D materials, the imaging techniques not only have to provide images but should also provide quantitative information. We report the material-specific characterization of nanoscopic buried structures with extreme ultraviolet coherence tomography. The method is demonstrated at a laser-driven broadband extreme ultraviolet radiation source, based on high-harmonic generation. We show that, besides nanoscopic axial resolution, the spectral reflectivity of all layers in a sample can be obtained using algorithmic phase reconstruction. This provides localized, spectroscopic, material-specific information of the sample. The method can be applied in, e.g., semiconductor production, lithographic mask inspection, or quality control of multilayer fabrication. Moreover, it paves the way for the investigation of ultrafast nanoscopic effects at functional buried interfaces. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.
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Near L-edge Single and Multiple Photoionization of Doubly Charged Iron Ions - Schippers, S.; Beerwerth, R.; Bari, S.; Buhr, T.; Holste, K.; Kilcoyne, A.; Perry-Sassmannshausen, A.; Phaneuf, R.; Reinwardt, S.; Savin, D.; Schubert, K.; Fritzsche, S.; et al. (The Astrophysical Journal, 2021)
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Polarization-dependent disappearance of a resonance signal: Indication for optical pumping in a storage ring? - N{\"{o}}rtersh{\"{a}user, W. and Surzhykov, A. and Sanchez, R. and Botermann, B. and Gwinner, G. and Huber, G. and Karpuk, S. and K{\"{u}}hl, T. and Novotny, C. and Reinhardt, S. and Saathoff, G. and St{\"{o}}hlker, T. and Wolf, A. (Physical Review Accelerators and Beams, 2021)
We report on laser spectroscopic measurements on Li+ ions in the experimental storage ring ESR at the GSI Helmholtz Centre for Heavy Ion Research. Driving the 2s 3S1(F=3/2)↔2p 3P2(F=5/2)↔2s 3S1(F=5/2) Λ-transition in 7Li+ with two superimposed laser beams it was found that the use of circularly polarized light leads to a disappearance of the resonance structure in the fluorescence signal. This can be explained by optical pumping into a dark state of polarized ions. We present a detailed theoretical analysis of this process that supports the interpretation of optical pumping and demonstrates that the polarization induced by the laser light must then be at least partially maintained during the round trip of the ions in the storage ring. Such polarized ion beams in storage rings will provide opportunities for new experiments, especially on parity violation.
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Polarized electron Mott scattering model for the Geant4 Monte Carlo toolkit - Dr{{a}}gowski, M.; Adamus, M.; Weber, G.; Wlodarczyk, M. (Nuclear Instruments { &} Methods in Physics Research Section B-Beam Interactions with Materials and Atoms, 2021)
A new method for simulation of polarized electron interactions with matter, based on the Geant4 Monte Carlo toolkit, is presented. The extension consists of a Mott scattering model taking into account the polarization dependence of the cross section, as well as the change of electron polarization in the scattering. The results regarding azimuthal asymmetry in Mott scattering of polarized electron beams off gold and lead targets are compared to available experimental data for energies up to 14 MeV.
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60 fs, 1030nm FEL pump{–}probe laser based on a multi-pass post-compressed Yb:YAG source - Viotti, A.-L.; Ali{{s}}auskas, S.; Bin Wahid, A.; Balla, P.; Schirmel, N.; Manschwetus, B.; Hartl, I.; Heyl, C. (Journal of Synchrotron Radiation, 2021)
This paper reports on nonlinear spectral broadening of 1.1ps pulses in a gas-filled multi-pass cell to generate sub-100fs optical pulses at 1030nm and 515nm at pulse energies of 0.8mJ and 225µ J, respectively, for pump{–}probe experiments at the free-electron laser FLASH. Combining a 100kHz Yb:YAG laser with~ 180W in-burst average power and a post-compression platform enables reaching simultaneously high average powers and short pulse durations for high-repetition-rate FEL pump{–}probe experiments.
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A strong-field approach with realistic wave functions to the above-threshold ionization of Ba+ - B{\"{o}}ning, B.; Abele, P.; Paufler, W.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2021)
We study the above-threshold ionization of atoms in intense circularly polarized laser pulses. In order to compute photoelectron energy spectra, we apply the strong-field approximation with different models for the initial state wave function. Specifically, we compare the spectra for singly ionized Barium (Ba^+) using hydrogenic wave functions and realistic one-particle wave functions obtained by multiconfiguration Dirac–Hartree–Fock computations, respectively. As a particular example, we discuss the dependence of the photoelectron spectra on the magnetic quantum number m of the initial state and we reproduce the well known m-selectivity in strong-field ionization. Here, we show that the photoelectron spectra exhibit noticeable differences for the two models of the initial state and that the m-selectivity is enhanced when realistic wave functions are used. We conclude that the description of strong-field processes within the strong-field approximation will benefit from a realistic description of the initial atomic state.
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Advanced X-ray polarimeter design for nuclear resonant scattering - Marx-Glowna, B.; Uschmann, I.; Schulze, K.; Marschner, H.; Wille, H.-C.; Schlage, K.; St{\"{o}}hlker, T.; R{\"{o}}hlsberger, R.; Paulus, G. (Journal of Synchrotron Radiation, 2021)
This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of Fe-57. By using four asymmetric reflections at asymmetry angles of alpha(1) = -28 degrees, alpha(2) = 28 degrees, alpha(3) = -28 degrees and alpha(4) = 28 degrees, the degree of polarization purity could be improved to 2.2 x 10(-9). For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.
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Characterizing ultralow emittance electron beams using structured light fields - Seidel, A.; Osterhoff, J.; Zepf, M. (Physical Review Accelerators and Beams, 2021)
Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Current methods for the characterization of low emittance electron beams such as pepperpot measurements or beam focus scanning are limited in their capability to resolve emittances in the sub 0.1 mm mrad regime. Here we propose a novel, highly sensitive method for the single shot characterization of the beam waist and emittance using interfering laser beams. In this scheme, two laser pulses are focused under an angle creating a gratinglike interference pattern. When the electron beam interacts with the structured laser field, the phase space of the electron beam becomes modulated by the laser ponderomotive force and results in a modulated beam profile after further electron beam phase advance, which allows for the characterization of ultralow emittance beams. 2D PIC simulations show the effectiveness of the technique for normalized emittances in the range of epsilon(n) = 1/20.01; 1] mm mrad.
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Coherent control of collective nuclear quantum states via transient magnons - Bocklage, L.; Gollwitzer, J.; Strohm, C.; Adolff, C.; Schlage, K.; Sergeev, I.; Leupold, O.; Wille, H.-C.; Meier, G.; R{\"{o}}hlsberger, R. (Science Advances, 2021)
Ultrafast and precise control of quantum systems at x-ray energies involves photons with oscillation periods below 1 as. Coherent dynamic control of quantum systems at these energies is one of the major challenges in hard x-ray quantum optics. Here, we demonstrate that the phase of a quantum system embedded in a solid can be coherently controlled via a quasi-particle with subattosecond accuracy. In particular, we tune the quantum phase of a collectively excited nuclear state via transient magnons with a precision of 1 zs and a timing stability below 50 ys. These small temporal shifts are monitored interferometrically via quantum beats between different hyperfine-split levels.The experiment demonstrates zeptosecond interferometry and shows that transient quasi-particles enable accurate control of quantum systems embedded in condensed matter environments.
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Disentangling x-ray dichroism and birefringence via high-purity polarimetry - Schmitt, A. T.; Joly, Y.; Schulze, K. S.; Marx-Glowna, B.; Uschmann, I.; Grabiger, B.; Bernhardt, H.; L{\"{o}}tzsch, R.; Juhin, A.; Debray, J.; Wille, H.-C.; Yava{{s}}, H.; et al. (Optica, 2021)
High-brilliance synchrotron radiation sources have opened new avenues for x-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear x-ray polarizers in a crossed setting, polarization extinction ratios down to 10⁻¹⁰ can be achieved. This renders the method sensitive to probe the tiniest optical anisotropies that would occur, for example, in strong-field quantum electrodynamics due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La₂CuO₄ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and x-ray lasers, where polarization extinction ratios of 10⁻¹² can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.
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High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the Free-Electron Laser - Wollenweber, L.; Preston, T. R.; Descamps, A.; Cerantola, V.; Comley, A.; Eggert, J. H.; Fletcher, L. B.; Geloni, G.; Gericke, D. O.; Glenzer, S. H.; Goede, S.; Hastings, J.; et al. (Review of Scientific Instruments, 2021)
We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.
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Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO - Hollinger, R.; Herrmann, P.; Korolev, V.; Zapf, M.; Shumakova, V.; Roeder, R.; Uschmann, I.; Pug{{z}}lys, A.; Baltuska, A.; Z{\"{u}}rch, M.; Ronning, C.; Spielmann, C.; et al. (Nanomaterials, 2021)
The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 mu m. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.
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Polarization studies on Rayleigh scattering of hard x rays by closed-shell atoms - Strnat, S.; Yerokhin, V. A.; Volotka, A. V.; Weber, G.; Fritzsche, S.; M{\"{u}}ller, R. A.; Surzhykov, A. (Physical Review A, 2021)
We present a theoretical study on the elastic Rayleigh scattering of x-ray photons by closed-shell atoms. Special attention is paid to the transfer of linear polarization from the incident to the outgoing photons. To study this process, we apply the density-matrix formalism combined with the relativistic perturbation theory. This formalism enables us to find general relations between the Stokes parameters of the incident and scattered photons. By using these expressions, we revisit the recent proposal to use Rayleigh scattering for the analysis of the polarization purity of synchrotron radiation. We show that this analysis can be performed without any need for the theoretically calculated scattering amplitudes, if the linear polarization of the scattered light is measured simultaneously at the azimuthal angles 0 degrees and 45 degrees with respect to the plane of the synchrotron. To illustrate our approach, we present detailed calculations for scattering of 145 keV photons by lead atoms.
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Toward simulation of topological phenomena with one-, two-, and three-dimensional quantum walks - Panahiyan, S.; Fritzsche, S. (Physical Review A, 2021)
We study the simulation of the topological phases in three subsequent dimensions with quantum walks. We focus mainly on the completion of a table for the protocols of the quantum walk that could simulate different families of the topological phases in one, two, and three dimensions. We also highlight the possible boundary states that can be observed for each protocol in different dimensions and extract the conditions for their emergences. To further enrich the simulation of the topological phenomena, we include step-dependent coins in the evolution operators of the quantum walks. This leads to step dependence of the simulated topological phenomena and their properties which introduces dynamicity as a feature of simulated topological phases and boundary states. This dynamicity provides the step number of the quantum walk as a means to control and engineer the numbers of topological phases and boundary states, their numbers, types, and even occurrences.
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X-ray dichroism in polyimide caused by non-resonant scattering - Schulze, K. S.; L{\"{o}}tzsch, R.; R{\"{u}}ffer, R.; Uschmann, I.; R{\"{o}}hlsberger, R.; Paulus, G.G. (Journal of Synchrotron Radiation, 2021)
Dichroism is one of the most important optical effects in both the visible and the X-ray range. Besides absorption, scattering can also contribute to dichroism. This paper demonstrates that, based on the example of polyimide, materials can show tiny dichroism even far from electronic resonances due to scattering. Although the effect is small, it can lead to a measurable polarization change and might have influence on highly sensitive polarimetric experiments.
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Carrier-envelope offset stable, coherently combined ytterbium-doped fiber CPA delivering 1 kW of average power - Shestaev, E.; Haedrich, S.; Walther, N.; Eidam, T.; Klenke, A.; Seres, I.; Bengery, Z.; J{\'{o}}j{\'{a}}rt, P.; V{\'{a}}rallyay, Z.; Borzsonyi, A.; Limpert, J. (Optics Letters, 2020)
We present a carrier-envelope offset (CEO) stable ytterbium-doped fiber chirped-pulse amplification system employing the technology of coherent beam combining and delivering more than 1 kW of average power at a pulse repetition rate of 80 MHz. The CEO stability of the system is 220 mrad rms, characterized out-of-loop with an f-to-2f interferometer in a frequency offset range of 10 Hz to 20 MHz. The high-power amplification system boosts the average power of the CEO stable oscillator by five orders of magnitude while increasing the phase noise by only 100 mrad. No evidence of CEO noise deterioration due to coherent beam combining is found. Low-frequency CEO fluctuations at the chirped-pulse amplifier are suppressed by a slow loop feedback. To the best of our knowledge, this is the first demonstration of a coherently combined laser system delivering an outstanding average power and high CEO stability at the same time.
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Characterization and application of nonlinear plastic materials for post-CPA pulse compression - Tamer, I.; Hornung, M.; Lukas, L.; Hellwing, M.; Keppler, S.; Van Hull, R.; Hein, J.; Zepf, M.; Kaluza, M.C. (Optics Letters, 2020)
We demonstrate the three-fold post-chirped-pulse-amplification (post-CPA) pulse compression of a high peak power laser pulse using ally) diglycol carbonate (CR39), which was selected as the optimal material for near-field self-phase modulation out of a set of various nonlinear plastic materials, each characterized with respect to its nonlinear refractive index and optical transmission. The investigated materials could be applied for further pulse compression at high peak powers, as well as for gain narrowing compensation within millijoule-class amplifiers. The post-CPA pulse compression technique was tested directly after the first CPA stage within the POLARIS laser system, with the compact setup containing a single 1 mm thick plastic sample and a chirped mirror pair, which enabled a substantial shortening of the compressed pulse duration and, hence, a significant increase in the laser peak power without any additional modifications to the existing CPA chain.
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EuPRAXIA Conceptual Design Report - Assmann, R. W.; Weikum, M. K.; Akhter, T.; Alesini, D.; Alexandrova, A. S.; Anania, M. P.; Andreev, N. E.; Andriyash, I.; Artioli, M.; Aschikhin, A.; Audet, T.; Bacci, A.; et al. (European Physical Journal Special Topics, 2020)
This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science {–} through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8{–}10 years.
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High-Precision Determination of Oxygen Kα Transition Energy Excludes Incongruent Motion of Interstellar Oxygen - Leutenegger, M. A.; Kuhn, S.; Micke, P.; Steinbr{\"{u}}gge, R.; Stierhof, J.; Shah, C.; Hell, N.; Bissinger, M.; Hirsch, M.; Ballhausen, R.; Lang, M.; Gr{\"{a}fe, C. and Wipf, S. and Cumbee, R. and Betancourt-Martinez, G. L. and Park, S. and Yerokhin, V. A. and Surzhykov, A. and Stolte, W. C. and Niskanen, J. and Chung, M. and Porter, F. S. and St{\"{o}}hlker, T. and Pfeifer, T. and Wilms, J. and Brown, G. V. and Lopez-Urrutia, J. R. C. and Bernitt, S. (Physical Review Letters, 2020)
We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the K-shell Rydberg spectrum of molecular O2 with 8 meV uncertainty. We reveal a systematic ∼450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O2 literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.
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Impact of thermo-optical effects in coherently combined multicore fiber amplifiers - Steinkopff, A.; Jauregui, C.; Aleshire, C.; Klenke, A.; Limpert, J. (Optics Express, 2020)
In this work we analyze the power scaling potential of amplifying multicore fibers (MCFs) used in coherently combined systems. In particular, in this study we exemplarily consider rod-type MCFs with 2 × 2 up to 10 × 10 ytterbium-doped cores arranged in a squared pattern. We will show that, even though increasing the number of active cores will lead to higher output powers, particular attention has to be paid to arising thermal effects, which potentially degrade the performance of these systems. Additionally, we analyze the influence of the core dimensions on the extractable and combinable output power and pulse energy. This includes a detailed study on the thermal effects that influence the propagating transverse modes and, in turn, the amplification efficiency, the combining efficiency, the onset of nonlinear effect, as well as differences in the optical path lengths between the cores. Considering all these effects under rather extreme conditions, the study predicts that average output powers higher than 10 kW from a single 1 m long ytterbium-doped MCF are feasible and femtosecond pulses with energies higher than 400 mJ can be extracted and efficiently recombined in a filled-aperture scheme.
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Laser-driven x-ray and proton micro-source and application to simultaneous single-shot bi-modal radiographic imaging - Ostermayr, T. M.; Kreuzer, C.; Englbrecht, F. S.; Gebhard, J.; Hartmann, J.; Huebl, A.; Haffa, D.; Hilz, P.; Parodi, K.; Wenz, J.; Donovan, M. E.; Dyer, G.; et al. (Nature Communications, 2020)
Radiographic imaging with x-rays and protons is an omnipresent tool in basic research and applications in industry, material science and medical diagnostics. The information contained in both modalities can often be valuable in principle, but difficult to access simultaneously. Laser-driven solid-density plasma-sources deliver both kinds of radiation, but mostly single modalities have been explored for applications. Their potential for bi-modal radiographic imaging has never been fully realized, due to problems in generating appropriate sources and separating image modalities. Here, we report on the generation of proton and x-ray micro-sources in laser-plasma interactions of the focused Texas Petawatt laser with solid-density, micrometer-sized tungsten needles. We apply them for bi-modal radiographic imaging of biological and technological objects in a single laser shot. Thereby, advantages of laser-driven sources could be enriched beyond their small footprint by embracing their additional unique properties, including the spectral bandwidth, small source size and multi-mode emission. Here the authors show a synchronized single-shot bi-modal x-ray and proton source based on laser-generated plasma. This source can be useful for radiographic and tomographic imaging.
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Nonlinear ionization dynamics of hot dense plasma observed in a laser-plasma amplifier - Tuitje, F.; Martinez Gil, P.; Helk, T.; Gautier, J.; Tissandier, F.; Goddet, J. -P.; Guggenmos, A.; Kleineberg, U.; Sebban, S.; Oliva, E.; Spielmann, C.; Zuerch, M. (Light: Science { &} Applications, 2020)
Understanding the behaviour of matter under conditions of extreme temperature, pressure, density and electromagnetic fields has profound effects on our understanding of cosmologic objects and the formation of the universe. Lacking direct access to such objects, our interpretation of observed data mainly relies on theoretical models. However, such models, which need to encompass nuclear physics, atomic physics and plasma physics over a huge dynamic range in the dimensions of energy and time, can only provide reliable information if we can benchmark them to experiments under well-defined laboratory conditions. Due to the plethora of effects occurring in this kind of highly excited matter, characterizing isolated dynamics or obtaining direct insight remains challenging. High-density plasmas are turbulent and opaque for radiation below the plasma frequency and allow only near-surface insight into ionization processes with visible wavelengths. Here, the output of a high-harmonic seeded laser-plasma amplifier using eight-fold ionized krypton as the gain medium operating at a 32.8 nm wavelength is ptychographically imaged. A complex-valued wavefront is observed in the extreme ultraviolet (XUV) beam with high resolution. Ab initio spatio-temporal Maxwell-Bloch simulations show excellent agreement with the experimental observations, revealing overionization of krypton in the plasma channel due to nonlinear laser-plasma interactions, successfully validating this four-dimensional multiscale model. This constitutes the first experimental observation of the laser ion abundance reshaping a laser-plasma amplifier. The presented approach shows the possibility of directly modelling light-plasma interactions in extreme conditions, such as those present during the early times of the universe, with direct experimental verification.
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Nonlinear photon trident versus double Compton scattering and resummation of one-step terms - Torgrimsson, G. (Physical Review D, 2020)
We study the photon trident process, where an initial photon turns into an electron-positron pair and a final photon under a nonlinear interaction with a strong plane-wave background field. We show that this process is very similar to double Compton scattering, where an electron interacts with the background field and emits two photons. We also show how the one-step terms can be obtained by resumming the small- and large -x expansions. We consider a couple of different resummation methods and also propose new resummations (involving Meijer-G functions) which have the correct type of expansions at both small and large x . These new resummations require relatively few terms to give good precision.
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Relativistic slingshot: A source for single circularly polarized attosecond x-ray pulses - Wang, J.; Bulanov, V.; Chen, M.; Lei, B.; Zhang, Y. X.; Zagidullin, R.; Zorina, V.; Yu, W.; Leng, Y.; Li, R.; Zepf, M.; Rykovanov, S. (Physical Review E, 2020)
We propose a mechanism to generate a single intense circularly polarized attosecond x-ray pulse from the interaction of a circularly polarized relativistic few-cycle laser pulse with an ultrathin foil at normal incidence. Analytical modeling and particle-in-cell simulation demonstrate that a huge charge-separation field can be produced when all the electrons are displaced from the target by the incident laser, resulting in a high-quality relativistic electron mirror that propagates against the tail of the laser pulse. The latter is efficiently reflected as well as compressed into an attosecond pulse that is also circularly polarized.
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Resonant two-photon ionization of atoms by twisted and plane-wave light - Kosheleva, V. P.; Zaytsev, V. A.; M{\"{u}}ller, R. A.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2020)
We study the resonant two-photon ionization of neutral atoms by a combination of twisted and plane-wave light within a fully relativistic framework. In particular, the ionization of an isotropic ensemble of neutral sodium atoms (Z = 11) from their ground 3 S-2(1/2) state via the 3 P-2(3/2) level is considered. We investigate in details the influence of the kinematic parameters of incoming twisted radiation on the photoelectron angular distribution and the circular dichroism. Moreover, we study the influence of the geometry of the process on these quantities. This is done by changing the propagation directions of the incoming twisted and plane-wave light. It is found that the dependence on the kinematic parameters of the twisted photon is the strongest if the plane-wave and twisted light beams are perpendicular to each other.
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Zeroth- and first-order long range non-diffracting Gauss-Bessel beams generated by annihilating multiple-charged optical vortices - Stoyanov, L.; Zhekova, M.; Stefanov, A.; Stefanov, I.; Paulus, G.; Dreischuh, A. (Scientific Reports, 2020)
We demonstrate an alternative approach for generating zeroth- and first-order long range non-diffracting Gauss-Bessel beams (GBBs). Starting from a Gaussian beam, the key point is the creation of a bright ring-shaped beam with a large radius-to-width ratio, which is subsequently Fourier-transformed by a thin lens. The phase profile required for creating zeroth-order GBBs is flat and helical for first-order GBBs with unit topological charge (TC). Both the ring-shaped beam and the required phase profile can be realized by creating highly charged optical vortices by a spatial light modulator and annihilating them by using a second modulator of the same type. The generated long-range GBBs are proven to have negligible transverse evolution up to 2 m and can be regarded as non-diffracting. The influences of the charge state of the TCs, the propagation distance behind the focusing lens, and the GBB profiles on the relative intensities of the peak/rings are discussed. The method is much more efficient as compared to this using annular slits in the back focal plane of lenses. Moreover, at large propagation distances the quality of the generated GBBs significantly surpasses this of GBBs created by low angle axicons. The developed analytical model reproduces the experimental data. The presented method is flexible, easily realizable by using a spatial light modulator, does not require any special optical elements and, thus, is accessible in many laboratories.
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A highly sensitive imaging polarimeter in the x-ray regime - Grabiger, B.; Marx-Glowna, B.; Uschmann, I.; Loetzsch, R.; Paulus, G.; Schulze, K. (Applied Physics Letters, 2020)
We report on the development of a highly sensitive imaging polarimeter that allows for the investigation of polarization changing properties of materials in the x-ray regime. By combining a microfocus rotating anode, collimating multilayer mirrors, and two germanium polarizer crystals, we achieved a polarization purity of the two orthogonal linear polarization states of 8 × 10−8. This enables the detection of an ellipticity on the same order or a rotation of the polarization plane of 6 arcsec. The high sensitivity combined with the imaging techniques allows us to study the microcrystalline structure of materials. As an example, we investigated beryllium sheets of different grades, which are commonly used for fabricating x-ray lenses, with a spatial resolution of 200 μm, and observed a strong degradation of the polarization purity due to the polycrystalline nature of beryllium. This makes x-ray lenses made of beryllium unsuitable for imaging polarimeter with higher spatial resolution. The results are important for the development of x-ray optical instruments that combine high spatial resolution and high sensitivity to polarization.
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Controllable simulation of topological phases and edge states with quantum walk - Panahiyan, S.; Fritzsche, S. (Physics Letters A, 2020)
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Enhanced entanglement from Ince-Gaussian pump beams in spontaneous parametric down-conversion - Baghdasaryan, B.; Fritzsche, S. (Physical Review A, 2020)
Spontaneous parametric down-conversion (SPDC) has been a reliable process for the generation of entangled photon pairs. In this process, a nonlinear quadratic crystal is pumped by a laser field in order to convert (high-energy) photons into correlated photon pairs whose efficient control plays an essential role in various applications of quantum information processing. In particular, the amount of entanglement has been successfully controlled by adjusting the spatial structure of the incident pump field. Here, we theoretically analyze how the entanglement of the down-converted two-photon state can be further enhanced by using Ince-Gaussian beams with well-defined ellipticity epsilon, i.e., solutions of the paraxial wave equation in elliptical coordinates. These spatially structured beams are quite universal as they include both the Laguerre-Gaussian beams for epsilon -> 0 as well as the Hermite-Gaussian beams for epsilon -> infinity. We demonstrate that the entanglement of the generated photon pairs in SPDC can be maximized by a proper choice of epsilon and that such an enhanced entanglement can be observed experimentally in terms of the Schmidt number.
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Nonlinear trident in the high-energy limit: Nonlocality, Coulomb field, and resummations - Torgrimsson, G. (Physical Review D, 2020)
We study nonlinear trident in laser pulses in the high-energy limit, where the initial electron experiences, in its rest frame, an electromagnetic field strength above Schwinger s critical field. At lower energies the dominant contribution comes from the two-step' part, but in the high-energy limit the dominant contribution comes instead from the one-step term. We obtain new approximations that explain the relation between the high-energy limit of trident and pair production by a Coulomb field, as well as the role of the Weizsacker-Williams approximation and why it does not agree with the high-chi limit of the locally-constant-field approximation. We also show that the next-to-leading order in the large-a(0) expansion is, in the high-energy limit, nonlocal and is numerically very important even for quite large a(0). We show that the small-a(0) perturbation series has a finite radius of convergence, but using Pade-conformal methods we obtain resummations that go beyond the radius of convergence and have a large numerical overlap with the large-a(0) approximation. We use Borel-Pade-conformal methods to resum the small-chi expansion and obtain a high precision up to very large chi. We also use newer resummation methods based on hypergeometric/Meijer-G and confluent hypergeometric functions.
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Observation of strong two-electron–one-photon transitions in few-electron ions - Togawa, M.; Kuhn, S.; Shah, C.; Amaro, P.; Steinbr{\"{u}}gge, R.; Stierhof, J.; Hell, N.; Rosner, M.; Fujii, K.; Bissinger, M.; Ballhausen, R.; Hoesch, M.; et al. (Physical Review A, 2020)
We resonantly excite the K series of O5+ and O6+ up to principal quantum number n=11 with monochromatic x rays, producing K-shell holes, and observe their relaxation by soft-x-ray emission. Some photoabsorption resonances of O5+ reveal strong two-electron–one-photon (TEOP) transitions. We find that for the [(1s2s)15p3/2]3/2;1/2 states, TEOP relaxation is by far stronger than the radiative decay and competes with the usually much faster Auger decay path. This enhanced TEOP decay arises from a strong correlation with the near-degenerate upper states [(1s2p3/2)14s]3/2;1/2 of a Li-like satellite blend of the He-like Kα transition. Even in three-electron systems, TEOP transitions can play a dominant role, and the present results should guide further research on the ubiquitous and abundant many-electron ions where electronic energy degeneracies are far more common and configuration mixing is stronger.
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Partial-wave representation of the strong-field approximation - B{\"{o}}ning, B.; Fritzsche, S. (Physical Review A, 2020)
The strong-field approximation (SFA) has been widely applied to model ionization processes in short and intense laser pulses. Several approaches have been suggested in order to overcome certain limitations of the original SFA formulation with regard to the representation of the initial bound and final continuum states of the emitted electron as well as a suitable description of the driving laser pulse. We here present a reformulation of the SFA in terms of partial waves and spherical tensor operators that supports a quite simple implementation and the comparison of different treatments of the active (photo)electron and the laser pulses. In particular, this reformulation helps to adapt the SFA to experimental setups, and it paves the way to extend the strong-field theory toward the study of nondipole contributions in light-atom interactions as well as of many-particle correlations in strong-field ionization processes. A series of detailed computations have been carried out in order to confirm the validity of the reformulation and to show how the representation of the bound and continuum states affects the predicted above-threshold ionization spectra and related observables.
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Plasma environment effects on K lines of astrophysical interest IV. IPs, K thresholds, radiative rates, and Auger widths in Fe ii-Fe viii - Deprince, J.; Bautista, M. A.; Fritzsche, S.; Garc{\'{i}}a, J. A.; Kallman, T. R.; Mendoza, C.; Palmeri, P.; Quinet, P. (Astronomy and Astrophysics, 2020)
Aims. Within the framework of compact-object accretion disks, we calculate plasma environment effects on the atomic structure and decay parameters used in the modeling of K lines in lowly charged iron ions, namely FeII-FeVIII.Methods. For this study, we used the fully relativistic multiconfiguration Dirac-Fock method approximating the plasma electron-nucleus and electron-electron screenings with a time-averaged Debye-Huckel potential.Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10(5)-10(7) K and 10(18)-10(22) cm(-3). In addition, we propose two universal fitting formulae to predict the IP and K-threshold lowerings in any elemental ion.Conclusions. We conclude that the high-resolution X-ray spectrometers onboard the future XRISM and ATHENA space missions will be able to detect the lowering of the K edges of these Fe ions due to the extreme plasma conditions occurring in the accretion disks around compact objects.
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Proton beams from intense laser-solid interaction: Effects of the target materials - Geng, Y. X.; Wu, D.; Yu, W.; Sheng, Z. M.; Fritzsche, S.; Liao, Q.; Wu, M. J.; Xu, X. H.; Li, D. Y.; Ma, W. J.; Lu, H. Y.; Zhao, Y. Y.; et al. (Matter and Radiation at Extremes, 2020)
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Spin- and polarization-dependent locally-constant-field-approximation rates for nonlinear Compton and Breit-Wheeler processes - Seipt, D.; King, B. (Physical Review A, 2020)
In this paper we derive and discuss the completely spin- and photon-polarization-dependent probability rates for nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. The locally constant field approximation, which is essential for applications in plasma-QED simulation codes, is rigorously derived from the strong-field QED matrix elements in the Furry picture for a general plane-wave background field. We discuss important polarization correlation effects in the spectra of both processes. Asymptotic limits for both small and large values of ${}hi$ are derived and their spin and polarization dependence is discussed.
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THz generation by optical rectification of intense near-infrared pulses in organic crystal BNA - Roeder, F.; Shalaby, M.; Beleites, B.; Ronneberger, F.; Gopal, A. (Optics Express, 2020)
Generation of terahertz radiation by optical rectification of intense near-infrared laser pulses in N-benzyl-2-methyl-4-nitroaniline (BNA) is investigated in detail by carrying out a complete characterization of the terahertz radiation. We studied the scaling of THz yield with pump pulse repetition rate and fluence which enabled us to predict the optimal operating conditions for BNA crystals at room temperature for 800 nm pump wavelength. Furthermore, recording the transmitted laser spectrum allowed us to calculate the nonlinear refractive index of BNA at 800 nm.
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Direct measurements for the fine-structure splitting of S VIII and Cl IX - Lu, Q.; Yan, C. L.; Xu, G. Q.; Fu, N.; Yang, Y.; Zou, Y.; Volotka, A. V.; Xiao, J.; Nakamura, N.; Hutton, R. (Physical Review A, 2020)
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Electron capture of Xe54+ in collisions with H2 molecules in the energy range between 5.5 and 30.9 MeV/u - Kr{\"{o}}ger, F. M.; Weber, G.; Herdrich, M. O.; Glorius, J.; Langer, C.; Slavkovsk{\'{a}}, Z.; Bott, L.; Brandau, C.; Br{\"{u}}ckner, B.; Blaum, K.; Chen, X.; Dababneh, S.; et al. (Physical Review A, 2020)
The electron-capture process was studied for Xe54+ colliding with H2 molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.
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Enhanced polarization transfer to the characteristic L alpha x-ray lines near the nonlinear Cooper minimum of two-photon ionization - Hofbrucker, J.; Volotka, A. V.; Szlachetko, J.; Fritzsche, S. (Physical Review A, 2020)
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Fidelity susceptibility near topological phase transitions in quantum walks - Panahiyan, S.; Chen, W.; Fritzsche, S. (Physical Review B, 2020)
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First on-line detection of radioactive fission isotopes produced by laser-accelerated protons - Boller, P.; Zylstra, A.; Neumayer, P.; Bernstein, L.; Brabetz, C.; Despotopulos, J.; Glorius, J.; Hellmund, J.; Henry, E.; Hornung, J.; Jeet, J.; Khuyagbaatar, J.; et al. (Scientific Reports, 2020)
The on-going developments in laser acceleration of protons and light ions, as well as the production of strong bursts of neutrons and multi-MeV photons by secondary processes now provide a basis for novel high-flux nuclear physics experiments. While the maximum energy of protons resulting from Target Normal Sheath Acceleration is presently still limited to around 100MeV, the generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. This paper consists of two parts covering the scientific motivation and relevance of such experiments and a first proof-of-principle demonstration. In the presented experiment pulses of 200J at ≈500fs duration from the PHELIX laser produced more than 10 12 protons with energies above 15MeV in a bunch of sub-nanosecond duration. They were used to induce fission in foil targets made of natural uranium. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by Coulomb explosion and the velocity differences between ions of different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This was mitigated by utilizing fast transport of the fission products by a gas flow to a carbon filter, where the γ -rays were registered. The identified nuclides include those that have half-lives down to 39s. These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction. The approach promotes research towards relevant nuclear astrophysical studies at conditions currently only accessible at nuclear high energy density laser facilities.
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How to access QED at a supercritical Coulomb field - Popov, V.; Shabaev, V. M.; Telnov, D. A.; Tupitsyn, I. I.; Maltsev, I. A.; Kozhedub, Y. S.; Bondarev, I.; Kozin, V.; Ma, X.; Plunien, G.; St{\"{o}}hlker, T.; Tumakov, D. A.; et al. (Physical Review D, 2020)
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Hybridizing Multi-pass and Multi-plate Bulk Compression - Seidel, M.; Balla, P.; Binhammer, T.; Frede, M.; Arisholm, G.; Winkelmann, L.; Hartl, I.; Heyl, C. (9th EPS-QEOD Europhoton Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources (EUROPHOTON 2020), 2020)
With the advent of ultrafast Yb-ion based disk, slab and fibre lasers, nonlinear pulse compression methods became indispensable for the generation of high average power sub-100 fs pulses. In particular, spectral broadening in Herriott-type multi-pass cells (MPC) has been established as a novel tool for pulse compression in the 5 – 100 MW peak power range where neither solid-core fibre nor hollow-core capillaries work efficiently. The operation in the critical self-focusing regime becomes feasible through repetitive refocusing of the cell mirrors. Analogously, the multi-plate approach , which has mainly been used for broadband continuum generation, relies on the nonlinear refocusing of the Kerr media . By introducing the hybridization of both methods, we demonstrate compression factors up to 14. To the best of our knowledge this exceeds all single-stage compression factors achieved with bulk-based spectral broadening to date.
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Hyperfine-induced effects on angular emission of the magnetic-quadrupole line 1s2p(3/2) P-3(2) -> 1s(2) S-1(0) following electron-impact excitation of Tl79+ ions - Wu, Z. W.; Tian, Z. Q.; Jiang, J.; Dong, C. Z.; Fritzsche, S. (Physical Review A, 2020)
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Post-compression of high average power picosecond pulses for few cycle generation and FEL pump-probe experiments - Viotti, A.-L.; Ali{{s}}auskas, S.; Balla, P.; Wahid, A.; Sytcevich, I.; Guo, C.; Silletti, l.; Cartella, A.; Tavakol, H.; Grosse-Wortmann, U.; Sch{\"{o}}nberg, A.; Seidel, M.; et al. (9th EPS-QEOD Europhoton Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources (EUROPHOTON 2020), 2020)
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Rayleigh scattering of linearly polarized light: Scenario of the complete experiment - Volotka, A. V.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2020)
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Software defined radio for Schottky analysis in storage rings - Dmytriiev, D.; Sanjari, M. S.; Litvinov, Yu. A.; St{\"{o}}hlker, T. (Journal of Physics: Conference Series, 2020)
Resonant Schottky cavity pickups have applications in the measurements of beam parameters in a storage ring. Apart from that they can be used in nondestructive in-ring decay studies of radioactive ion beams. In order to obtain the results of an experiment suitable Data Acquisition System (DAQ) is necessary. Several DAQs were used at Experimental Storage Ring (ESR) at GSI based on the different hardware and software solutions such as TCAP or NTCAP. The goal of this work is to design a prototype of a DAQ using open hardware and open source software defined radio (SDR) and conduct the test measurements at ESR.
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Ultrafast dynamics and evolution of ion-induced opacity in transparent dielectrics - Coughlan, M.; Donnelly, H.; Breslin, N.; Arthur, C.; Nersisyan, G.; Yeung, M.; Villagomez-Bernabe, B.; Afshari, M.; Currell, F.; Zepf, M.; Dromey, B. (New Journal of Physics, 2020)
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Vacuum polarization and finite-nuclear-size effects in the two-photon decay of hydrogenlike ions - Sommerfeldt, J.; Mueller, R. A.; Volotka, A. V.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2020)
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An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser - Descamps, A.; Ofori-Okai, B.; Appel, K.; Cerantola, V.; Comley, A.; Eggert, J.; Fletcher, L.; Gericke, D.; G{\"{o}}de, S.; Humphries, O.; Karnbach, O.; Lazicki, A.; et al. (Scientific Reports, 2020)
We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8 % for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.
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Driven spin chains as high-quality quantum routers - Wanisch, D.; Fritzsche, S. (Physical Review A, 2020)
We propose a setup, based on a periodically driven spin chain, that can realize a high-quality quantum router. We present two protocols, which utilize this setup, that can either generate highly entangled two-qubit states over an arbitrary distance or transfer single-qubit states with high fidelity to any desired location on the chain. In addition, we can execute several protocols at the same time and also store quantum states on the spin chain. Our protocols exploit the effect of coherent destruction of tunneling to control, which spins on the chain couple to each other. This control is acquired by suitably shaping the external driving field. The success of our protocols does not depend on the values of the couplings between the spins as long as they are finite and much smaller than the driving frequency. Our setup is scalable, robust against errors, and may be of practical use for future quantum information technologies.
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Hyperfine structure study of Tc-97,Tc-98,Tc-99 in a new laser ion source for high-resolution laser spectroscopy - Kron, T.; Beerwerth, R.; Raeder, S.; Fritzsche, S.; Heinke, R.; Schoenberg, P.; Truemper, M.; Wendt, K. (Physical Review C, 2020)
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Measurement of the P1/2 2 - P3/2 2 fine-structure splitting in fluorinelike Kr, W, Re, Os, and Ir - O'Neil, G.; Sanders, S.; Szypryt, P.; Dipti, .; Gall, A.; Yang, Y.; Brewer, S.; Doriese, R.; Fowler, J.; Naing, A.; Swetz, D.; Tan, J.; et al. (Physical Review A, 2020)
Quantum electrodynamics (QED) is currently considered to be one of the most accurate theories of fundamental interactions. As its extraordinary precision offers unique scientific opportunities, e.g., search for new physics, stringent experimental tests of QED continue to be of high importance. To this end, highly charged ions represent an exceptional test-bed due to enhanced QED effects. Recently, forbidden transitions in F-like ions have been analyzed to few ppm precision, resolving previous discrepancies between theory and experiment. Here we further test the accuracy of QED calculations with three new (Re, Os, Ir), and two improved (Kr, W) measurements of the P1/22-P3/22 transition energy in F-like ions using the NIST electron-beam ion trap and extreme-ultraviolet and x-ray spectrometers. Good agreement between theoretical and experimental energies is found for all considered elements.
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One-dimensional quantum walks driven by two-entangled-qubit coins - Panahiyan, S.; Fritzsche, S. (Physics Letters A, 2020)
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Particle-in-cell simulation method for macroscopic degenerate plasmas - Wu, D.; Yu, W.; Fritzsche, S.; He, X. T. (Physical Review E, 2020)
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Photoelectron Angular Distributions of Nonresonant Two-Photon Atomic Ionization Near Nonlinear Cooper Minima - Hofbrucker, J.; Eiri, L.; Volotka, A. V.; Fritzsche, S. (Atoms, 2020)
Photoelectron angular distributions of the two-photon ionization of neutral atoms are theoretically investigated. Numerical calculations of two-photon ionization cross sections and asymmetry parameters are carried out within the independent-particle approximation and relativistic second-order perturbation theory. The dependence of the asymmetry parameters on the polarization and energy of the incident light as well as on the angular momentum properties of the ionized electron are investigated. While dynamic variations of the angular distributions at photon energies near intermediate level resonances are expected, we demonstrate that equally strong variations occur near the nonlinear Cooper minimum. The described phenomena is demonstrated on the example of two-photon ionization of magnesium atom.
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Propagation effects in multipass high harmonic generation from plasma surfaces - Tietze, S.; Zepf, M.; Rykovanov, S.; Yeung, M. (New Journal of Physics, 2020)
Multipass high harmonic generation from plasma surfaces is a promising technique to enhance the efficiency of the generation process. In this paper it is shown that there is an optimal distance between two targets where the efficiency is maximized, depending on the laser and plasma parameters. This can be explained by the Gouy phase shift, which leads to the relative phase between the colours being changed with propagation in free space. A simple model is used to mimic the propagation of light from one target to another and to observe this effect in 1D particle-in-cell (PIC) simulations. The results are also verified using 2D PIC simulations.
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Temperature dependent absorption and emission spectra of Tm:CaF2 - Reiter, J.; K{\"{o}}rner, J.; Pejchal, J.; Yoshikawa, A.; Hein, J.; Kaluza, M.C. (Optical Materials Express, 2020)
We present Emission and absorption cross sections of thulium doped calcium fluoride (Tm:CaF2) in the visible to short wave infrared (SWIR) wavelength range for temperatures between 80 K and 300 K. For spectral regions of high and low absorption the McCumber relation and the Fuchtbauer–Ladenburg equation have been used to give reliable results. Furthermore, an estimation for the cross relaxation efficiency is derived from the emission spectra as a function of doping concentration and temperature. In addition, nearly re-absorption-free fluorescence lifetimes for various doping concentrations were studied. It was found that a double exponential fit model is better suited than a migration model to represent the fluorescence decay curves. The measurement results are interpreted in the light of the application of Tm:CaF2 as an efficient active medium in high-energy class diode-pumped solid state lasers.
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A 410 MHz resonant cavity pickup for heavy ion storage rings - Sanjari, M.; Dmytriiev, D.; Litvinov, Y.; Gumenyuk, O.; Hess, R.; Joseph, R.; Litvinov, S.; Steck, M.; St{\"{o}}hlker, T. (Review of Scientific Instruments, 2020)
An improved design of a longitudinally sensitive resonant Schottky cavity pickup for the heavy ion storage rings of the Facility for Antiproton and Ion Research in Europe (FAIR) project is reported. The new detector has a higher measured Q value of ∼3000 and a higher simulated shunt impedance of 473.3 kω. It is possible to vary the sensitivity of the cavity with a motorized mechanism by inserting a dissipative blade during the operation based on experimental needs. Apart from a lower price tag, the new design features a more robust and production-friendly mechanical structure suitable for a series production in the future FAIR project. The manufactured cavity was built temporarily into the experimental storage ring and had delivered its first results using stored heavy ion beams. The structure, simulation results, and performance of this cavity are presented in this work.
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Critical reflections on asymptotically safe gravity - Bonanno, A.; Eichhorn, A.; Gies, H.; Pawlowski, J.; Percacci, R.; Reuter, M.; Saueressig, F.; Paolo Vacca, G. (Frontiers in Physics, 2020)
Asymptotic safety is a theoretical proposal for the ultraviolet completion of quantum field theories, in particular for quantum gravity. Significant progress on this program has led to a first characterization of the Reuter fixed point. Further advancement in our understanding of the nature of quantumspacetime requires addressing a number of open questions and challenges. Here, we aim at providing a critical reflection on the state of the art in the asymptotic safety program, specifying and elaborating on open questions of both technical and conceptual nature. We also point out systematic pathways, in various stages of practical implementation, toward answering them. Finally, we also take the opportunity to clarify some common misunderstandings regarding the program.
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Differential pumping unit for windowless coupling of laser beams to ultra high vacuum - Tschernajew, M.; Gierschke, P.; Lin, H.; Hilbert, V.; Kurdal, J.; Stancalie, A.; Limpert, J.; Rothhardt, J. (Vacuum, 2020)
Current laser technology enables table-top high flux XUV sources with photon energies from several tens to several hundreds of eV via high harmonic generation in noble gases. Here we present a compact versatile coupling unit to establish windowless, and thus lossless coupling of such light sources to ultra high vacuum (UHV). The particular coupling unit has been developed to couple a XUV laser source to a heavy ion storage ring. Three-stage differential pumping allows to reduce the input pressure of 10−6 mbar down to the 10−12 mbar range at the output. The unit particularly reduces the partial pressure of argon, which is used to generate the XUV radiation, by 6 orders of magnitude. Measurements of the pressure distribution inside the different chambers agree well with theoretical simulations. In principle, this unit can also serve for other purposes, where a windowless vacuum coupling is needed, with a transition from High Vacuum (HV) levels to deep UHV, such as coupling to cryogenically cooled detectors, ion traps or to photoelectron emission spectroscopy experiments.
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Multiple photodetachment of atomic anions via single and double core-hole creation - Schippers, S.; Perry-Sassmannshausen, A.; Buhr, T.; Martins, M.; Fritzsche, S.; M{\"{u}}ller, A. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2020)
We review the recent experimental and theoretical progress in K-shell detachment studies of atomic anions. On the experimental side, this field has largely benefitted from technical advances at 3rd generation synchrotron radiation sources. For multiple detachment of C-, O-, and F- ions, recent results were obtained at the photon-ion merged-beams setup PIPE which is a permanent end station at beamline P04 of the PETRA III synchrotron light source in Hamburg, Germany. In addition to a much increased photon flux as compared to what was available previously, the PIPE setup has an extraordinary detection sensitivity for heavy charged reaction products that allows one to study detachment processes with extremely low cross sections in the kilobarn range, e.g., for processes involving the simultaneous creation of two core-holes by a single photon as observed in the net triple detachment of F- and the net five-fold detachment of C-. Moreover, hitherto disregarded photodetachment resonances have been discovered, which exhibit a variety of line shapes. For O- the core-hole lifetime could be determined precisely from a high-resolution measurement of a photodetachment resonance. These experimental findings pose new challenges for state-of-the-art atomic theory and require calculations combining photoexcitation (ionization) with decay cascade processes that follow after initial core-hole production.
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Noncollinear Enhancement Cavity for Record-High Out-coupling Efficiency of an Extreme-UV Frequency Comb - Zhang, C.; Schoun, S.; Heyl, C.; Porat, G.; Gaarde, M.; Ye, J. (Physical Review Letters, 2020)
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Simulation of novel cell-like topological structures with quantum walk - Panahiyan, S.; Fritzsche, S. (European Physical Journal Plus, 2020)
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Single-shot characterization of strongly focused coherent XUV and soft X-ray beams - Eschen, W.; Tadesse, G.; Peng, Y.; Steinert, M.; Pertsch, T.; Limpert, J.; Rothhardt, J. (Optics Letters, 2020)
In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than ${ł}ambda /70$. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.
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Species-dependent tunneling ionization of weakly bound atoms in the short-wave infrared regime - Zille, D.; Adolph, D.; Skruszewicz, S.; Sayler, A. M.; Paulus, G.G. (New Journal of Physics, 2020)
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