When logged in, additional information is available in some parts of the website.

Peer-Review Publications


2019

G. Becker, M. Schwab, R. Lötzsch, S. Tietze, D. Klöpfel, M. Rehwald, H.-P. Schlenvoigt, A. Sävert, U. Schramm, M. Zepf, and M. Kaluza
Characterization of laser-driven proton acceleration from water microdroplets
Scientific Reports 9, 17169 (2019)

Abstract: We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 mm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 mm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser's angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet's surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.

J. Polz, A. P. L. Robinson, A. Kalinin, G. A. Becker, R. Fraga, M. Hellwing, M. Hornung, S. Keppler, A. Kessler, D. Klöpfel, H. Liebetrau, F. Schorcht, J. Hein, M. Zepf, R. E. Grisenti, and M. C. Kaluza
Efficient Laser-Driven Proton Acceleration from a Cryogenic Solid Hydrogen Target
Scientific Reports 9, 16534 (2019)

Abstract: We report on the successful implementation and characterization of a cryogenic solid hydrogen target in experiments on high-power laser-driven proton acceleration. When irradiating a solid hydrogen filament of 10 mm diameter with 10-Terawatt laser pulses of 2.5 J energy, protons with kinetic energies in excess of 20?MeV exhibiting non-thermal features in their spectrum were observed. The protons were emitted into a large solid angle reaching a total conversion efficiency of several percent. Two-dimensional particle-in-cell simulations confirm our results indicating that the spectral modulations are caused by collisionless shocks launched from the surface of the the high-density filament into a low-density corona surrounding the target. The use of solid hydrogen targets may significantly improve the prospects of laser-accelerated proton pulses for future applications.

D. A. Glazov, F. Köhler-Langes, A. V. Volotka, K. Blaum, F. Heiße, G. Plunien, W. Quint, S. Rau, V. M. Shabaev, S. Sturm, and G. Werth
g Factor of Lithiumlike Silicon: New Challenge to Bound-State QED
Physical Review Letters 123, 173001 (2019)

Abstract: The recently established agreement between experiment and theory for the g factors of lithiumlike silicon and calcium ions manifests the most stringent test of the many-electron bound-state quantum electrodynamics (QED) effects in the presence of a magnetic field. In this Letter, we present a significant simultaneous improvement of both theoretical gth=2.000 889 894 4 (34) and experimental gexp=2.000 889 888 45 (14) values of the g factor of lithiumlike silicon 28Si11+. The theoretical precision now is limited by the many-electron two-loop contributions of the bound-state QED. The experimental value is accurate enough to test these contributions on a few percent level.

I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, T. Stöhlker, and D. A. Tumakov
How to Observe the Vacuum Decay in Low-Energy Heavy-Ion Collisions
Physical Review Letters 123, 113401 (2019)

Abstract: In slow collisions of two bare nuclei with the total charge larger than the critical value Zcr≈173, the initially neutral vacuum can spontaneously decay into the charged vacuum and two positrons. The detection of the spontaneous emission of positrons would be direct evidence of this fundamental phenomenon. However, the spontaneously produced particles are indistinguishable from the dynamical background in the positron spectra. We show that the vacuum decay can nevertheless be observed via impact-sensitive measurements of pair-production probabilities. The possibility of such an observation is demonstrated using numerical calculations of pair production in low-energy collisions of heavy nuclei.

F. Aumayr, K. Ueda, E. Sokell, S. Schippers, H. Sadeghpour, F. Merkt, T. F. Gallagher, F. B. Dunning, P. Scheier, O. Echt, T. Kirchner, S. Fritzsche, A. Surzhykov, X. Ma, R. Rivarola, O. Fojon, L. Tribedi, E. Lamour, J. R. C. Lopez-Urrutia, Y. A. Litvinov, V. Shabaev, H. Cederquist, H. Zettergren, M. Schleberger, R. A. Wilhelm, T. Azuma, P. Boduch, H. T. Schmidt, and T. Stöhlker
Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: with zero to relativistic speeds
Journal of Physics B: Atomic, Molecular and Optical Physics 52, 171003 (2019)

Abstract: We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field.

F. Karbstein, and E. A. Mosman
X-ray photon scattering at a focused high-intensity laser pulse
Physical Review D 100, 033002 (2019)

Abstract: We study x-ray photon scattering in the head-on collision of an XFEL pulse and a focused high-intensity laser pulse, described as paraxial Laguerre-Gaussian beam of arbitrary mode composition. For adequately chosen relative orientations of the polarization vectors of the colliding laser fields, this gives rise to a vacuum birefringence effect manifesting itself in polarization flipped signal photons. Throughout this article the XFEL is assumed to be mildly focused to a waist larger than that of the high-intensity laser beam. As previously demonstrated for the special case of a fundamental paraxial Gaussian beam, this scenario is generically accompanied by a scattering phenomenon of x-ray energy signal photons outside the forward cone of the XFEL beam, potentially assisting the detection of the effect in experiment. Here, we study the fate of the x-ray scattering signal under exemplary deformations of the transverse focus profile of the high-intensity pump.

F. Karbstein, A. Blinne, H. Gies, and M. Zepf
Boosting Quantum Vacuum Signatures by Coherent Harmonic Focusing
Physical Review Letters 123, 091802 (2019)

Abstract: We show that coherent harmonic focusing provides an efficient mechanism to boost all-optical signatures of quantum vacuum nonlinearity in the collision of high-intensity laser fields, thereby offering a promising route to their first experimental detection. Assuming two laser pulses of given parameters at our disposal, we demonstrate a substantial increase of the number of signal photons measurable in experiments where one of the pulses undergoes coherent harmonic focusing before it collides with the fundamental-frequency pulse. Imposing a quantitative criterion to discern the signal photons from the background of the driving laser photons and accounting for the finite purity of polarization filtering, we find that signal photons arising from inelastic scattering processes constitute a promising signature. By contrast, quasielastic contributions which are conventionally assumed to form the most prospective signal remain background dominated. Our findings may result in a paradigm shift concerning which photonic signatures of quantum vacuum nonlinearity are accessible in experiment.

I. Tamer, S. Keppler, J. Körner, M. Hornung, M. Hellwing, F. Schorcht, J. Hein, and M. Kaluza
Modeling of the 3D spatio-temporal thermal profile of joule-class Yb³⁺-based laser amplifiers
High Power Laser Science and Engineering 7, E42 (2019)

Abstract: Thermal profile modification of an active material in a laser amplifier via optical pumping results in a change in the material’s refractive index, and causes thermal expansion and stress, eventually leading to spatial phase aberrations, or even permanent material damage. For this purpose, knowledge of the 3D spatio-temporal thermal profile, which can currently only be retrieved via numerical simulations, is critical for joule-class laser amplifiers to reveal potentially dangerous thermal features within the pumped active materials. In this investigation, a detailed, spatio-temporal numerical simulation was constructed and tested for accuracy against surface thermal measurements of various end-pumped Yb³⁺-doped laser-active materials. The measurements and simulations show an excellent agreement and the model was successfully applied to a joule-class Yb³⁺-based amplifier currently operating in the POLARIS laser system at the Friedrich-Schiller-University and Helmholtz-Institute Jena in Germany.

S. Kumar, W. Quint, S. Ringleb, C. P. Safvan, N. Stallkamp, T. Stöhlker, and M. Vogel
Properties of a cylindrical Penning trap with conical endcap openings
Physica Scripta 94, 075401 (2019)

Abstract: We describe the results of analytical calculations and numerical simulations of the confinement properties of a mechanically compensated cylindrical Penning trap which has conical endcap openings for large-solid-angle access for example with highly focused laser beams. While the analytical calculations show that under the common geometrical conditions the harmonicity of the confining fields near the centre of the trap does not change when a conical shape of the endcap electrodes is introduced, numerical simulations show significant changes when the opening angle of the cone exceeds a certain critical angle. We also show that these sharp features are due to the fringe-field effects above the critical angle, which are not described by the analytical calculations. These effects are also observed in a cylindrical Penning trap when the length of the endcap electrodes is reduced below a certain critical value.

M. Herdrich, A. Fleischmann, D. Hengstler, S. Allgeier, C. Enss, S. Trotsenko, T. Morgenroth, R. Schuch, G. Weber, and T. Stöhlker
High-precision X-ray spectroscopy of Fe ions in an EBIT using a micro-calorimeter detector: First results
X-Ray Spectrometry n/a, 1 (2019)

Abstract: A micro-calorimeter X-ray detector of the maXs-30 type was used to record the X-ray radiation from Fe ions, being produced in the S-EBIT-I electron beam ion trap at the site of GSI. The resulting spectra demonstrate the superior energy resolving power of micro-calorimeter detectors compared with conventional semiconductor detectors. The experiment serves as another proof of principle for the application of calorimeters as dedicated high-resolution X-ray spectrometers at an ion facility. Together with the development of an improved analysis algorithm for online readout, these results present a step towards the use of maXs-type detectors as standard instrumentation at GSI/FAIR.

O. Forstner, D. Bemmerer, T. Cowan, R. Dressler, A. Junghans, D. Schumann, T. Stöhlker, T. Szücs, A. Wagner, and K. Zuber
Opportunities for measurements of astrophysical-relevant alpha-capture reaction rates at CRYRING@ESR
X-Ray Spectrometry n/a, 1 (2019)

Abstract: The heavy-ion storage ring CRYRING@ESR has recently been installed and commissioned at GSI as one of the first installations of the upcoming Facility for Antiproton and Ion Research (FAIR). It is designed to store highly charged ions in the energy range between 300?keV/u and about 10?MeV/u. It will incorporate a gas-jet target providing high-density jets of, among other gases, hydrogen and helium. This will allow to study alpha-capture reaction rates of astrophysical interest in the energy range of the Gamow window for core-collapse supernovae. Special interest comes from the long-lived radio-isotope 44Ti (t1/2?=?58.9?years), which is supposed to be produced in the alpha-rich freeze-out during such an event. The nucleosynthesis of this isotope is of great interest, as the amount of material produced can be estimated by direct observation in remnants of recent supernovae. The disagreements between the observations and the estimations from astrophysical models show the need of more experimental data for the production and consumption reactions in the energy range of a core-collapse supernova. In this article, we will describe the proposed method of injecting beams of 44Ti into CRYRING@ESR and performing the actual reaction rate measurements.

W. Placzek, A. Abramov, S. Alden, R. Alemany Fernandez, P. Antsiferov, A. Apyan, H. Bartosik, E. Bessonov, N. Biancacci, J. Bieron, A. Bogacz, A. Bosco, R. Bruce, D. Budker, K. Cassou, F. Castelli, I. Chaikovska, C. Curatolo, P. Czodrowski, A. Derevianko, K. Dupraz, Y. Dutheil, K. Dzierzcega, V. Fedosseev, N. Fuster Martinez, S. Gibson, B. Goddard, A. Gorzawski, S. Hirlander, J. Jowett, R. Kersevan, M. Kowalska, M. Krasny, F. Kroeger, M. Lamont, T. Lefevre, D. Manglunki, B. Marsh, A. Martens, J. Molson, D. Nutarelli, L. Nevay, A. Petrenko, V. Petrillo, S. Radaelli, S. Pustelny, S. Rochester, M. Sapinski, M. Schaumann, L. Serafini, V. Shevelko, T. Stoehlker, A. Surzhikov, I. Tolstikhina, F. Velotti, G. Weber, Y. Wu, C. Yin-Vallgren, M. Zanetti, F. Zimmermann, M. Zolotorev, and F. Zomer
Gamma Factory at CERN - Novel Research Tools Made of Light
Acta Physica Polonica B 50, 1191 (2019)

Abstract: We discuss the possibility of creating novel research tools by producing and storing highly relativistic beams of highly ionised atoms in the CERN accelerator complex, and by exciting their atomic degrees of freedom with lasers to produce high-energy photon beams. Intensity of such photon beams would be by several orders of magnitude higher than offered by the presently operating light sources, in the particularly interesting gamma-ray energy domain of 0.1-400 MeV. In this energy range, the high-intensity photon beams can be used to produce secondary beams of polarised electrons, polarised positrons, polarised muons, neutrinos, neutrons and radioactive ions. New research opportunities in a wide domain of fundamental and applied physics can be opened by the Gamma Factory scientific programme based on the above primary and secondary beams.

D. A. Glazov, A. V. Volotka, O. V. Andreev, V. P. Kosheleva, S. Fritzsche, V. M. Shabaev, G. Plunien, and Th. Stöhlker
Ground-state hyperfine splitting of B-like ions in the high-Z region
Physical Review A 99, 062503 (2019)

Abstract: The hyperfine splitting of the ground state of selected B-like ions within the range of nuclear charge numbers Z=49–83 is investigated in detail. The rigorous QED approach together with the large-scale configuration-interaction Dirac-Fock-Sturm method are employed for the evaluation of the interelectronic-interaction contributions of first and higher orders in 1/Z. The screened QED corrections are evaluated to all orders in αZ by using an effective potential. The influence of nuclear magnetization distribution is taken into account within the single-particle nuclear model.

J. Rothhardt, M. Bilal, R. Beerwerth, A. Volotka, V. Hilbert, T. Stöhlker, S. Fritzsche, and J. Limpert
Lifetime measurements of ultrashort-lived excited states in Be-like ions
X-Ray Spectrometry 1, 1 (2019)

Abstract: We propose to measure the lifetime of short-lived excited states in highly charged ions by pump-probe experiments. Utilizing two synchronized and delayed Femtosecond pulses allows accessing these lifetimes with Femtosecond precision. Such measurements could provide sensitive tests of state-of-the art atomic structure calculations beyond the capabilities of established methods.

W. Nörtershäuser, J. Ullmann, L. V. Skripnikov, Z. Andelkovic, C. Brandau, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, F. Kraus, B. Kresse, Y. A. Litvinov, M. Lochmann, B. Maaß, J. Meisner, T. Murböck, A. F. Privalov, R. Sánchez, B. Scheibe, M. Schmidt, S. Schmidt, V. M. Shabaev, M. Steck, T. Stöhlker, R. C. Thompson, C. Trageser, M. Vogel, J. Vollbrecht, A. V. Volotka, and C. Weinheimer
The hyperfine puzzle of strong-field bound-state QED
Hyperfine Interactions 240, 51 (2019)

Abstract: The hyperfine splitting in heavy highly charged ions provide the means to test QED in extremely strong magnetic fields. In order to provide a meaningful test, the splitting has to be measured in H-like and Li-like ions to remove uncertainties from nuclear structure. This has been achieved at the experimental storage ring ESR but a discrepancy to the theoretical prediction of more than 7s was observed. We report on these measurements as well as on NMR measurements that were performed to solve this issue.

M. Vockert, G. Weber, H. Bräuning, A. Surzhykov, C. Brandau, S. Fritzsche, S. Geyer, S. Hagmann, S. Hess, C. Kozhuharov, R. Märtin, N. Petridis, R. Hess, S. Trotsenko, Yu. A. Litvinov, J. Glorius, A. Gumberidze, M. Steck, S. Litvinov, T. Gassner, P.-M. Hillenbrand, M. Lestinsky, F. Nolden, M. S. Sanjari, U. Popp, C. Trageser, D. F. A. Winters, U. Spillmann, T. Krings, and Th. Stöhlker
Radiative electron capture as a tunable source of highly linearly polarized x rays
Physical Review A 99, 052702 (2019)

Abstract: The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U92+. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.

G. Torgrimsson
Perturbative methods for assisted nonperturbative pair production
Physical Review D 99, 096002 (2019)

Abstract: In the dynamically assisted Schwinger mechanism, the pair production probability is significantly enhanced by including a weak, rapidly varying field in addition to a strong, slowly varying field. In a previous paper we showed that several features of dynamical assistance can be understood by a perturbative treatment of the weak field. Here we show how to calculate the prefactors of the higher-orders terms, which is important because the dominant contribution can come from higher orders. We give a new and independent derivation of the momentum spectrum using the worldline formalism, and extend our WKB approach to calculate the amplitude to higher orders. We show that these methods are also applicable to doubly assisted pair production.

C. Kohlfürst
Spin states in multiphoton pair production for circularly polarized light
Physical Review D 99, 096017 (2019)

Abstract: Scalar and fermionic particle pair production in rotating electric fields is investigated in the nonperturbative multiphoton regime. Angular momentum distribution functions in above-threshold pair production processes are calculated numerically within quantum kinetic theory and discussed on the basis of a photon absorption model. The particle spectra can be understood if the spin states of the particle-antiparticle pair are taken into account.

G. Torgrimsson
Thermally versus dynamically assisted Schwinger pair production
Physical Review D 99, 096007 (2019)

Abstract: We study electron-positron pair production by the combination of a strong, constant electric field and a thermal background. We show that this process is similar to dynamically assisted Schwinger pair production, where the strong field is instead assisted by another coherent field, which is weaker but faster. We treat the interaction with the photons from the thermal background perturbatively, while the interaction with the electric field is nonperturbative (i.e., a Furry picture expansion in α). At O(α2) we have ordinary perturbative Breit-Wheeler pair production assisted nonperturbatively by the electric field. Already at this order we recover the same exponential part of the probability as previous studies, which did not expand in α. This means that we do not have to consider higher orders, so our approach allows us to calculate the preexponential part of the probability, which has not been obtained before in this regime. Although the prefactor is in general subdominant compared to the exponential part, in this case it can be important because it scales as α2≪1 and is therefore much smaller than the prefactor at O(α0) (pure Schwinger pair production). We show that, because of the exponential enhancement, O(α2) still gives the dominant contribution for temperatures above a certain threshold, but, because of the small prefactor, the threshold is higher than what the exponential alone would suggest.

F. Karbstein
All-Loop Result for the Strong Magnetic Field Limit of the Heisenberg-Euler Effective Lagrangian
Physical Review Letters 122, 211602 (2019)

Abstract: We provide an explicit expression for the strong magnetic field limit of the Heisenberg-Euler effective Lagrangian for both scalar and spinor quantum electrodynamics. To this end, we show that the strong magnetic field behavior is fully determined by one-particle reducible contributions discovered only recently. The latter can efficiently be constructed in an essentially algebraic procedure from lower-order one-particle reducible diagrams. Remarkably, the leading strong magnetic field behavior of the all-loop Heisenberg-Euler effective Lagrangian only requires input from the one-loop Lagrangian. Our result revises previous findings based exclusively on one-particle irreducible contributions. In addition, we briefly discuss the strong electric field limit and comment on external field QED in the large N limit.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
Photon-Photon Scattering at the High-Intensity Frontier: Paraxial Beams
Journal of Physics: Conference Series 1206, 012016 (2019)

Abstract: Our goal is to study optical signatures of quantum vacuum nonlinearities in strong macroscopic electromagnetic fields provided by high-intensity laser beams. The vacuum emission scheme is perfectly suited for this task as it naturally distinguishes between incident laser beams, described as classical electromagnetic fields driving the effect, and emitted signal photons encoding the signature of quantum vacuum nonlinearity. Using the Heisenberg-Euler effective action, our approach allows for a reliable study of photonic signatures of QED vacuum nonlinearity in the parameter regimes accessible by all-optical high-intensity laser experiments. To this end, we employ an efficient, flexible numerical algorithm, which allows for a detailed study of the signal photons emerging in the collision of focused paraxial high-intensity laser pulses. Due to the high accuracy of our numerical solutions we predict the total number of signal photons, but also have full access to the signal photons’ characteristics, including their spectrum, propagation directions and polarizations. We discuss setups offering an excellent background-to-noise ratio, thus providing an important step towards the experimental verification of quantum vacuum nonlinearities.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
The Vacuum Emission Picture Beyond Paraxial Approximation
Journal of Physics: Conference Series 1206, 012017 (2019)

Abstract: Optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum can be conveniently described in terms of stimulated photon emission processes induced by strong classical, space-time dependent electromagnetic fields. Recent studies have adopted this approach to study collisions of Gaussian laser pulses in paraxial approximation. The present study extends these investigations beyond the paraxial approximation by using an efficient numerical solver for the classical input fields. This new numerical code allows for a consistent theoretical description of optical signatures of QED vacuum nonlinearities in generic electromagnetic fields governed by Maxwell’s equations in the vacuum, such as manifestly non-paraxial laser pulses. Our code is based on a locally constant field approximation of the Heisenberg-Euler effective Lagrangian. As this approximation is applicable for essentially all optical high-intensity laser experiments, our code is capable of calculating signal photon emission amplitudes in completely generic input field configurations, limited only by numerical cost.

J. Glorius, C. Langer, Z. Slavkovská, L. Bott, C. Brandau, B. Brückner, K. Blaum, X. Chen, S. Dababneh, T. Davinson, P. Erbacher, S. Fiebiger, T. Gaßner, K. Göbel, M. Groothuis, A. Gumberidze, G. Gyürky, M. Heil, R. Hess, R. Hensch, P. Hillmann, P.-M. Hillenbrand, O. Hinrichs, B. Jurado, T. Kausch, A. Khodaparast, T. Kisselbach, N. Klapper, C. Kozhuharov, D. Kurtulgil, G. Lane, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Yu. A. Litvinov, B. Löher, F. Nolden, N. Petridis, U. Popp, T. Rauscher, M. Reed, R. Reifarth, M. S. Sanjari, D. Savran, H. Simon, U. Spillmann, M. Steck, T. Stöhlker, J. Stumm, A. Surzhykov, T. Szücs, T. T. Nguyen, A. Taremi Zadeh, B. Thomas, S. Yu. Torilov, H. Törnqvist, M. Träger, C. Trageser, S. Trotsenko, L. Varga, M. Volknandt, H. Weick, M. Weigand, C. Wolf, P. J. Woods, and Y. M. Xing
Approaching the Gamow Window with Stored Ions: Direct Measurement of ¹²⁴Xe(p,γ) in the ESR Storage Ring
Physical Review Letters 122, 092701 (2019)

Abstract: We report the first measurement of low-energy proton-capture cross sections of 124Xe in a heavy-ion storage ring. 124Xe^54+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The 125Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.

Y. S. You, J. Lu, E. F. Cunningham, C. Rödel, and S. Ghimire
Crystal orientation-dependent polarization state of high-order harmonics
Optics Letters 44, 530 (2019)

Abstract: We analyze the crystal orientation-dependent polarization state of extreme ultraviolet high-order harmonics from bulk magnesium oxide crystals subjected to intense linearly polarized laser fields. We find that only along high-symmetry directions do high-order harmonics follow the polarization direction of the laser field. In general, there are strong deviations that depend on harmonic order, strength of the laser field, and crystal orientation. We use a real-space electron trajectory picture to understand the origin of polarization deviations. These results have implications in all-optical probing of electronic band structure in momentum space and valence charge distributions in real space, and in producing attosecond pulses with time-dependent polarization in compact setups.

A. Blinne, S. Kuschel, S. Tietze, and M. Zepf
Efficient retrieval of phase information from real-valued electromagnetic field data
Journal of Computational Physics: X 1, 100019 (2019)

Abstract: While analytic calculations may give access to complex-valued electromagnetic field data which allow trivial access to envelope and phase information, the majority of numeric codes uses a real-valued representation. This typically increases the performance and reduces the memory footprint, albeit at a price: In the real-valued case it is much more difficult to extract envelope and phase information, even more so if counter propagating waves are spatially superposed. A novel method for the analysis of real-valued electromagnetic field data is presented in this paper. We show that, by combining the real-valued electric and magnetic field at a single point in time, we can directly reconstruct the full information of the electromagnetic fields in the form of complex-valued spectral coefficients (k→-space) at a low computational cost of only three Fourier transforms. The method allows for counter propagating plane waves to be accurately distinguished as well as their complex spectral coefficients, i.e. spectral amplitudes and spectral phase to be calculated. From these amplitudes, the complex-valued electromagnetic fields and also the complex-valued vector potential can be calculated from which information about spatiotemporal phase and amplitude is readily available. Additionally, the complex fields allow for efficient vacuum propagation allowing to calculate far field data or boundary input data from near field data. An implementation of the new method is available as part of PostPic1, a data analysis toolkit written in the Python programming language.

A. Blinne, H. Gies, F. Karbstein, C. Kohlfürst, and M. Zepf
All-optical signatures of quantum vacuum nonlinearities in generic laser fields
Physical Review D 99, 016006 (2019)

Abstract: All-optical experiments at the high-intensity frontier offer a promising route to unprecedented precision tests of quantum electrodynamics in strong macroscopic electromagnetic fields. So far, most theoretical studies of all-optical signatures of quantum vacuum nonlinearity are based on simplifying approximations of the beam profiles and pulse shapes of the driving laser fields. Since precision tests require accurate quantitative theoretical predictions, we introduce an efficient numerical tool facilitating the quantitative theoretical study of all-optical signatures of quantum vacuum nonlinearity in generic laser fields. Our approach is based on the vacuum emission picture, and makes use of the fact that the dynamics of the driving laser fields are to an excellent approximation governed by classical Maxwell theory in vacuum. In combination with a Maxwell solver, which self-consistently propagates any given laser field configuration, this allows for accurate theoretical predictions of photonic signatures of vacuum nonlinearity in high-intensity laser experiments from first principles. We employ our method to simulate photonic signatures of quantum vacuum nonlinearity in laser pulse collisions involving a few-cycle pulse, and show that the angular and spectral distributions of the emitted signal photons deviate from those of the driving laser beams.

T. Helk, M. Zürch, and C. Spielmann
Perspective: Towards single shot time-resolved microscopy using short wavelength table-top light sources
Structural Dynamics 6, 010902 (2019)

Abstract: Time-resolved imaging allows revealing the interaction mechanisms in the microcosm of both inorganic and biological objects. While X-ray microscopy has proven its advantages for resolving objects beyond what can be achieved using optical microscopes, dynamic studies using full-field imaging at the nanometer scale are still in their infancy. In this perspective, we present the current state of the art techniques for full-field imaging in the extreme-ultraviolet- and soft X-ray-regime which are suitable for single exposure applications as they are paramount for studying dynamics in nanoscale systems. We evaluate the performance of currently available table-top sources, with special emphasis on applications, photon flux, and coherence. Examples for applications of single shot imaging in physics, biology, and industrial applications are discussed.

2018

K. S. Schulze
Fundamental limitations of the polarization purity of x rays
APL Photonics 3, 126106 (2018)

Abstract: For a few years, x-ray polarimeters have been discussed and even used as a key method for the investigation of fundamental physical questions, from quantum electrodynamics to solid state physics. However, the sensitivity of optical instruments is limited. In the case of x-ray polarimeters, this limitation is connected with the polarization purity. This article quantifies two fundamental effects which lead to a limited polarization purity and, thus, to a limited sensitivity: the divergence of the source and multiple-wave diffraction inside the polarizer crystals. A comparison shows that the current best polarization purities realized in the x-ray range are limited by these effects. The quantitative knowledge of their influence, however, can improve the purity by two orders of magnitude in future polarimetric experiments.

D. Wu, W. Yu, Y. Zhao, S. Fritzsche, and X. He
Characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids: The role of bremsstrahlung and radiation reactions
Matter and Radiation at Extremes 3, 293 (2018)

Abstract: In this work, characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids are investigated by means of a newly developed particle-in-cell (PIC) simulation code. The PIC code takes advantage of the recently developed ionization and collision dynamics models, which make it possible to model different types of materials based on their intrinsic atomic properties. Within the simulations, both bremsstrahlung and nonlinear Compton scatterings have been included. Different target materials and laser intensities are considered for studying the parameter-dependent features of X/γ-ray radiations. The relative strength and angular distributions of X/γ ray productions from bremsstrahlung and nonlinear Compton scatterings are compared to each other. The threshold under which the nonlinear Compton scatterings become dominant over bremsstrahlung is also outlined.

L. Obst-Huebl, T. Ziegler, F.-E. Brack, J. Branco, M. Bussmann, T. E. Cowan, C. B. Curry, F. Fiuza, M. Garten, M. Gauthier, S. Göde, S. H. Glenzer, A. Huebl, A. Irman, J. B. Kim, T. Kluge, S. D. Kraft, F. Kroll, J. Metzkes-Ng, R. Pausch, I. Prencipe, M. Rehwald, C. Rödel, H.-P. Schlenvoigt, U. Schramm, and K. Zeil
All-optical structuring of laser-driven proton beam profiles
Nature Communications 9, 5292 (2018)

Abstract: Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

M. Müller, A. Klenke, A. Steinkopff, H. Stark, A. Tünnermann, and J. Limpert
3.5 kW coherently combined ultrafast fiber laser
Optics Letters 43, 6037 (2018)

Abstract: An ultrafast laser based on the coherent beam combination of four ytterbium-doped step-index fiber amplifiers is presented. The system delivers an average power of 3.5 kW and a pulse duration of 430 fs at an 80 MHz repetition rate. The beam quality is excellent (M2 < 1.24·1.10), and the relative intensity noise is as low as 1% in the frequency span from 1 Hz to 1 MHz. The system is turn-key operable, as it features an automated spatial and temporal alignment of the interferometric amplification channels.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, and J. Limpert
Ultrafast thulium fiber laser system emitting more than 1  kW of average power
Optics Letters 43, 5853 (2018)

Abstract: In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.

I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, and Th. Stöhlker
Electron-positron pair production in slow collisions of heavy nuclei beyond the monopole approximation
Physical Review A 98, 062709 (2018)

Abstract: Electron-positron pair production in low-energy collisions of heavy nuclei is considered beyond the monopole approximation. The calculation method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. Bound-free and free-free pair-production probabilities as well as the energy spectra of the emitted positrons are calculated for the collisions of bare uranium nuclei. The calculations are performed for collision energy near the Coulomb barrier for different values of the impact parameter. The obtained results are compared with the corresponding values calculated in the monopole approximation.

F. Karbstein, M. Wagner, and M. Weber
Determination of ΛMSbar (nf=2) and analytic parametrization of the static quark-antiquark potential
Physical Review D 98, 114506 (2018)

Abstract: While lattice QCD allows for reliable results at small momentum transfers (large quark separations), perturbative QCD is restricted to large momentum transfers (small quark separations). The latter is determined up to a reference momentum scale Λ, which is to be provided from outside, e.g., from experiment or lattice QCD simulations. In this article, we extract ΛMSbar for QCD with nf=2 dynamical quark flavors by matching the perturbative static quark-antiquark potential in momentum space to lattice results in the intermediate momentum regime, where both approaches are expected to be applicable. In a second step, we combine the lattice and the perturbative results to provide a complete analytic parametrization of the static quark-antiquark potential in position space up to the string breaking scale. As an exemplary phenomenological application of our all-distances potential, we compute the bottomonium spectrum in the static limit.

A. H. Woldegeorgis, B. Beleites, F. Ronneberger, R. Grosse, and A. Gopal
Investigating the influence of incident laser wavelength and polarization on particle acceleration and terahertz generation
Physical Review E 98, 061201 (2018)

Abstract: The interaction of a high-power laser pulse with a thin foil can generate energetic, broadband terahertz radiation. Here, we report an experimental investigation on the influence of incident laser polarization and wavelength on the terahertz emission and maximum proton energy from the target rear surface. For similar incident laser intensities, the characteristics of the particle beams and the terahertz radiation show a wavelength dependence. The results fit well with the established scaling laws for the terahertz yield and the maximum proton energy as a function of the incident laser irradiance.

V. Zakosarenko, M. Schmelz, T. Schönau, S. Anders, J. Kunert, V. Tympel, R. Neubert, F. Schmidl, P. Seidel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, and R. Stolz
Coreless SQUID-based cryogenic current comparator for non-destructive intensity diagnostics of charged particle beams
Superconductor Science and Technology 32, 014002 (2018)

Abstract: We report on a novel concept and prototype development of a coreless SQUID-based charged-particle beam monitor as a non-destructive diagnostic tool for accelerator facilities. Omitting the typically used pickup coil with a high magnetic permeability core leads to a significant improvement in low-frequency noise performance. Moreover, a revised shielding geometry allows for very compact and rather lightweight device designs. Based on highly sensitive SQUIDs featuring sub-micron cross-type Josephson tunnel junctions, our prototype device exhibits a current sensitivity of about 6 pA Hz^(−1/2) in the white noise region. Together with a measured shielding factor of about 135 dB this opens up the way for its widespread use in modern accelerator facilities.

S. Kraft-Bermuth, D. Hengstler, P. Egelhof, C. Enss, A. Fleischmann, M. Keller, and T. Stöhlker
Microcalorimeters for X-Ray Spectroscopy of Highly Charged Ions at Storage Rings
Atoms 6, 59 (2018)

Abstract: X-ray spectroscopy of highly charged heavy ions is an important tool for the investigation of many topics in atomic physics. Such highly charged ions, in particular hydrogen-like uranium, are investigated at heavy ion storage rings, where high charge states can be produced in large quantities, stored for long times and cooled to low momentum spread of the ion beam. One prominent example is the determination of the 1s Lamb Shift in hydrogen-like heavy ions, which has been investigated at the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research. Due to the large electron binding energies, the energies of the corresponding photon transitions are located in the X-ray regime. To determine the transition energies with high accuracy, highly resolving X-ray spectrometers are needed. One concept of such spectrometers is the concept of microcalorimeters, which, in contrast to semiconductor detectors, uses the detection of heat rather than charge to detect energy. Such detectors have been developed and successfully applied in experiments at the ESR. For experiments at the Facility for Antiproton and Ion Research (FAIR), the Stored Particles and Atoms Collaboration (SPARC) pursues the development of new microcalorimeter concepts and larger detector arrays. Next to fundamental investigations on quantum electrodynamics such as the 1s Lamb Shift or electron–electron interactions in two- and three-electron systems, X-ray spectroscopy may be extended towards nuclear physics investigations like the determination of nuclear charge radii.

V. A. Agababaev, D. A. Glazov, A. V. Volotka, D. V. Zinenko, V. M. Shabaev, and G. Plunien
Ground-state g factor of middle-Z boronlike ions
Journal of Physics: Conference Series 1138, 012003 (2018)

Abstract: Theoretical calculations of the interelectronic-interaction and QED corrections to the g factor of the ground state of boronlike ions are presented. The first-order interelectronic-interaction and the self-energy corrections are evaluated within the rigorous QED approach in the effective screening potential. The second-order interelectronic interaction is considered within the Breit approximation. The nuclear recoil effect is also taken into account. The results for the ground-state g factor of boronlike ions in the range Z = 10-20 are presented and compared to the previous calculations.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, J. Antonio-Lopez, A. Schülzgen, R. Amezcua-Correa, A. Tünnermann, I. Pupeza, and J. Limpert
Watt-scale super-octave mid-infrared intrapulse difference frequency generation
Light: Science & Applications 7, 94 (2018)

Abstract: The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.

D. Jahn, D. Schumacher, C. Brabetz, J. Ding, S. Weih, F. Kroll, F. Brack, U. Schramm, A. Blažević, and M. Roth
First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 173 (2018)

Abstract: In the last two decades, the generation of intense ion beams based on laser-driven sources has become an extensively investigated field. The LIGHT collaboration combines a laser-driven intense ion source with conventional accelerator technology based on the expertise of laser, plasma and accelerator physicists. Our collaboration has installed a laser-driven multi-MeV ion beamline at the GSI Helmholtzzentrum für Schwerionenforschung delivering intense proton bunches in the subnanosecond regime. We investigate possible applications for this beamline, especially in this report we focus on the imaging capabilities. We report on our proton beam homogenization and on first imaging results of a solid target.

J. Ding, D. Schumacher, D. Jahn, A. Blažević, and M. Roth
Simulation studies on generation, handling and transport of laser-accelerated carbon ions
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 168 (2018)

Abstract: To this day the interaction of high-intensity lasers with matter is considered to be a possible candidate for next generation particle accelerators. Within the LIGHT collaboration crucial work for the merging of a high-intensity laser driven ion source with conventional accelerator technology has been done in the past years. The simulation studies we report about are an important step in providing short and intense mid-Z heavy ion beams for future applications.

A. Bernhard, V. A. Rodríguez, S. Kuschel, M. Leier, P. Peiffer, A. Sävert, M. Schwab, W. Werner, C. Widmann, A. Will, A.-S. Müller, and M. Kaluza
Progress on experiments towards LWFA-driven transverse gradient undulator-based FELs
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 391 (2018)

Abstract: Free Electron Lasers (FEL) are commonly regarded as the potential key application of laser wakefield accelerators (LWFA). It has been found that electron bunches exiting from state-of-the-art LWFAs exhibit a normalized 6-dimensional beam brightness comparable to those in conventional linear accelerators. Effectively exploiting this beneficial beam property for LWFA-based FELs is challenging due to the extreme initial conditions particularly in terms of beam divergence and energy spread. Several different approaches for capturing, reshaping and matching LWFA beams to suited undulators, such as bunch decompression or transverse-gradient undulator schemes, are currently being explored. In this article the transverse gradient undulator concept will be discussed with a focus on recent experimental achievements.

A. Woldegeorgis, T. Kurihara, M. Almassarani, B. Beleites, R. Grosse, F. Ronneberger, and A. Gopal
Multi-MV/cm longitudinally polarized terahertz pulses from laser–thin foil interaction
Optica 5, 1474 (2018)

Abstract: Longitudinally polarized terahertz radiation offers access to the elementary excitations and particles that cannot be addressed by transverse waves. While transverse electric fields exceeding 1 MV/cm are widely utilized for nonlinear terahertz spectroscopy, longitudinally polarized terahertz waves at this field strength are yet to be realized. In this paper, we experimentally demonstrate that by focusing radially polarized terahertz fields generated from laser–thin metallic foil interaction, longitudinally polarized terahertz with record-breaking field strength above 1.5 MV/cm can be obtained. Furthermore, we also traced the evolution of the geometric phase of the longitudinal component as it propagates through focus. A novel scheme based on noncollinear electro-optic detection has been utilized to unambiguously measure the polarization states. Our result will scale up the nonlinear spectroscopy of solid materials and particle acceleration experiments where on-axis polarization plays a crucial role.

C. Gaida, T. Heuermann, M. Gebhardt, E. Shestaev, T. P. Butler, D. Gerz, N. Lilienfein, P. Sulzer, M. Fischer, R. Holzwarth, A. Leitenstorfer, I. Pupeza, and J. Limpert
High-power frequency comb at 2  μm wavelength emitted by a Tm-doped fiber laser system
Optics Letters 43, 5178 (2018)

Abstract: We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30  fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz–30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.

A. Surzhykov, V. A. Yerokhin, S. Fritzsche, and A. V. Volotka
Diagnostics of polarization purity of x rays by means of Rayleigh scattering
Physical Review A 98, 053403 (2018)

Abstract: Synchrotron radiation is commonly known to be completely linearly polarized when observed in the orbital plane of the synchrotron motion. Under actual experimental conditions, however, the degree of polarization of the synchrotron radiation may be lower than the ideal 100%. We demonstrate that even tiny impurities of polarization of the incident radiation can drastically affect the polarization of the elastically scattered light. We propose to use this effect as a precision tool for the diagnostics of the polarization purity of the synchrotron radiation. Two variants of the diagnostics method are proposed. The first one is based on the polarization measurements of the scattered radiation and relies on theoretical calculations of the transition amplitudes. The second one involves simultaneous measurements of the polarization and the cross sections of the scattered radiation and is independent of theoretical amplitudes.

J. Wang, W. Yu, M. Y. Yu, S. Rykovanov, J. Ju, S. Luan, K. Li, Y. Leng, R. Li, and Z.-M. Sheng
Very-long distance propagation of high-energy laser pulse in air
Physics of Plasmas 25, 113111 (2018)

Abstract: Long distance propagation of an energetic laser pulse with intensity slightly below that for multi-photon ionization in air is considered analytically, by noting that in the process, it is mainly the peak region of the pulse that interacts with the air molecules. Similar to that of much shorter femtosecond laser pulses of similar intensity, the affected air becomes slightly ionized and self-consistently forms a co-propagating thin and low-density plasma filament along the axis. It is found that a hundred-Joule-level laser pulse with a relatively large spot radius and pulse duration can propagate (also in the form of a self-consistent filament) tens of kilometers through the atmosphere. Such laser propagation properties should have applications in many areas.

R. Battesti, J. Beard, S. Böser, N. Bruyant, D. Budker, S. A. Crooker, E. J. Daw, V. V. Flambaum, T. Inada, I. G. Irastorza, F. Karbstein, D. L. Kim, M. G. Kozlov, Z. Melhem, A. Phipps, P. Pugnat, G. Rikken, C. Rizzo, M. Schott, Y. K. Semertzidis, H. H. t. Kate, and G. Zavattini
High magnetic fields for fundamental physics
Physics Reports 765-766, 1 (2018)

Abstract: Various fundamental-physics experiments such as measurement of the magnetic birefringence of the vacuum, searches for ultralight dark-matter particles (e.g., axions), and precision spectroscopy of complex systems (including exotic atoms containing antimatter constituents) are enabled by high-field magnets. We give an overview of current and future experiments and discuss the state-of-the-art DC- and pulsed-magnet technologies and prospects for future developments.

C. M. Heyl, S. B. Schoun, G. Porat, H. Green, and J. Ye
A nozzle for high-density supersonic gas jets at elevated temperatures
Review of Scientific Instruments 89, 113114 (2018)

Abstract: We present the development of a gas nozzle providing high-density gas at elevated temperaturesinside a vacuum environment. Fused silica is used as the nozzle material to allow the placement ofthe nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk ofsputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases thegas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction regionbetween consecutive laser shots. The nozzle accommodates a 50μm opening hole from which asupersonic gas jet emerges. Heater wires are used to bring the nozzle temperature up to 730 °C, whilea cooling unit ensures that the nozzle mount and the glued nozzle-to-mount connection is kept at atemperature below 50 °C. The presented nozzle design is used for high-order harmonic generationin hot gases using gas backing pressures of up to 124 bars.

K. T. Behm, J. M. Cole, A. S. Joglekar, E. Gerstmayr, J. C. Wood, C. D. Baird, T. G. Blackburn, M. Duff, C. Harvey, A. Ilderton, S. Kuschel, S. P. D. Mangles, M. Marklund, P. McKenna, C. D. Murphy, Z. Najmudin, K. Poder, C. P. Ridgers, G. Sarri, G. M. Samarin, D. Symes, J. Warwick, M. Zepf, K. Krushelnick, and A. G. R. Thomas
A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV
Review of Scientific Instruments 89, 113303 (2018)

Abstract: We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

N. Jayakumar, R. Sollapur, A. Hoffmann, T. Grigorova, A. Hartung, A. Schwuchow, J. Bierlich, J. Kobelke, M. A. Schmidt, and C. Spielmann
Polarization evolution in single-ring antiresonant hollow-core fibers
Applied Optics 57, 8529 (2018)

Abstract: Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.