Referierte Publikationen


S. Hendi, B. Panah, S. Panahiyan, and M. Momennia
Magnetic solutions in Einstein-massive gravity with linear and nonlinear fields
Eur. Phys. J. C, 78 :432 (May 2018)
The solutions of U(1) gauge-gravity coupling is one of the interesting models for analyzing the semi-classical nature of spacetime. In this regard, different well-known singular and nonsingular solutions have been taken into account. The paper at hand investigates the geometrical properties of the magnetic solutions by considering Maxwell and power Maxwell invariant (PMI) nonlinear electromagnetic fields in the context of massive gravity. These solutions are free of curvature singularity, but have a conic one which leads to presence of deficit/surplus angle. The emphasize is on modifications that these generalizations impose on deficit angle which determine the total geometrical structure of the solutions, hence, physical/gravitational properties. It will be shown that depending on the background spacetime [being anti de Sitter (AdS) or de Sitter (dS)], these generalizations present different effects and modify the total structure of the solutions differently.
C. Stihler, C. Jauregui, A. Tünnermann, and J. Limpert
Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers
Light Sci. Appl., 7 :59 (May 2018)
Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.
C. Kohlfürst
Phase-space analysis of the Schwinger effect in inhomogeneous electromagnetic fields
Eur. Phys. J. Plus, 133 :191 (May 2018)
Schwinger pair production in spatially and temporally inhomogeneous electric and magnetic fields is studied. The focus is on the particle phase-space distribution within a high-intensity few-cycle pulse. Accurate numerical solutions of a quantum kinetic theory (DHW formalism) are presented in momentum space and, with the aid of coarse-graining techniques, in a mixed spatial-momentum representation. Additionally, signatures of the carrier-envelope phase as well as spin-field interactions are discussed on the basis of a trajectory-based model taking into account instantaneous pair production and relativistic single-particle dynamics. Although our simple semi-classical single-particle model cannot describe every aspect of the particle production process (quantum interferences), essential features such as spin-field interactions are captured.
M. Vockert, G. Weber, U. Spillmann, T. Krings, and T. Stöhlker
Polarization reconstruction algorithm for a Compton polarimeter
J. Phys.: Conf. Ser., 1024 :012041 (May 2018)
We present the technique of Compton polarimetry using X-ray detectors based on double-sided segmented semiconductor crystals that were developed within the SPARC collaboration. In addition, we discuss the polarization reconstruction algorithm with particular emphasis on systematic deviations between the observed detector response and our model function for the Compton scattering distribution inside the detector.
G. Torgrimsson, C. Schneider, and R. Schützhold
Sauter-Schwinger pair creation dynamically assisted by a plane wave
Phys. Rev. D, 97 :096004 (May 2018)
We study electron-positron pair creation by a strong and constant electric field superimposed with a weaker transversal plane wave which is incident perpendicularly (or under some angle). Comparing the fully nonperturbative approach based on the world-line instanton method with a perturbative expansion into powers of the strength of the weaker plane wave, we find good agreement—provided that the latter is carried out to sufficiently high orders. As usual for the dynamically assisted Sauter-Schwinger effect, the additional plane wave induces an exponential enhancement of the pair-creation probability if the combined Keldysh parameter exceeds a certain threshold.
S. Breitkopf, N. Lilienfein, T. Achtnich, C. Zwyssig, A. Tünnermann, I. Pupeza, and J. Limpert
Velocity- and pointing-error measurements of a 300 000-r/min self-bearing permanent-magnet motor for optical applications
Rev. Sci. Instrum., 89 :063110 (May 2018)
Compact, ultra-high-speed self-bearing permanent-magnet motors enable a wide scope of applications including an increasing number of optical ones. For implementation in an optical setup, the rotors have to satisfy high demands regarding their velocity and pointing errors. Only a restricted number of measurements of these parameters exist and only at relatively low velocities. This manuscript presents the measurement of the velocity and pointing errors at rotation frequencies up to 5 kHz. The acquired data allow us to identify the rotor drive as the main source of velocity variations with fast fluctuations of up to 3.4 ns (RMS) and slow drifts of 23 ns (RMS) over ∼120 revolutions at 5 kHz in vacuum. At the same rotation frequency, the pointing fluctuated by 12 μrad (RMS) and 33 μrad (peak-to-peak) over ∼10 000 round trips. To our best knowledge, this states the first measurement of velocity and pointing errors at multi-kHz rotation frequencies and will allow potential adopters to evaluate the feasibility of such rotor drives for their application.
Y. Hua, W. Liu, M. Hemmer, L. E. Zapata, G. Zhou, D. N. Schimpf, T. Eidam, J. Limpert, A. Tünnermann, F. X. Kärtner, and G. Chang
87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb: yttrium lithium fluoride amplifier
Opt. Lett., 43 :1686 (April 2018)
We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-µJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers.
V. A. Zaytsev, A. S. Surzhykov, V. M. Shabaev, and T. Stöhlker
Analysis of angular momentum properties of photons emitted in fundamental atomic processes
Phys. Rev. A, 97 :043808 (April 2018)
Many atomic processes result in the emission of photons. Analysis of the properties of emitted photons, such as energy and angular distribution as well as polarization, is regarded as a powerful tool for gaining more insight into the physics of corresponding processes. Another characteristic of light is the projection of its angular momentum upon propagation direction. This property has attracted a special attention over the past decades due to studies of twisted (or vortex) light beams. Measurements being sensitive to this projection may provide valuable information about the role of angular momentum in the fundamental atomic processes. Here we describe a simple theoretical method for determination of the angular momentum properties of the photons emitted in various atomic processes. This method is based on the evaluation of expectation value of the total angular momentum projection operator. To illustrate the method, we apply it to the textbook examples of plane-wave, spherical-wave, and Bessel light. Moreover, we investigate the projection of angular momentum for the photons emitted in the process of the radiative recombination with ionic targets. It is found that the recombination photons do carry a nonzero projection of the orbital angular momentum.
A. Klenke, M. Müller, H. Stark, F. Stutzki, C. Hupel, T. Schreiber, A. Tünnermann, and J. Limpert
Coherently combined 16-channel multicore fiber laser system
Opt. Lett., 43 :1519 (April 2018)
We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled-aperture beam splitting and combination, together with temporal phasing, is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses is achieved with combination efficiencies around 80%.
H. Gies, and R. Martini
Curvature bound from gravitational catalysis
Phys. Rev. D, 97 :085017 (April 2018)
We determine bounds on the curvature of local patches of spacetime from the requirement of intact long-range chiral symmetry. The bounds arise from a scale-dependent analysis of gravitational catalysis and its influence on the effective potential for the chiral order parameter, as induced by fermionic fluctuations on a curved spacetime with local hyperbolic properties. The bound is expressed in terms of the local curvature scalar measured in units of a gauge-invariant coarse-graining scale. We argue that any effective field theory of quantum gravity obeying this curvature bound is safe from chiral symmetry breaking through gravitational catalysis and thus compatible with the simultaneous existence of chiral fermions in the low-energy spectrum. With increasing number of dimensions, the curvature bound in terms of the hyperbolic scale parameter becomes stronger. Applying the curvature bound to the asymptotic safety scenario for quantum gravity in four spacetime dimensions translates into bounds on the matter content of particle physics models.
A. S. Varentsova, V. A. Agababaev, D. A. Glazov, A. M. Volchkova, A. V. Volotka, V. M. Shabaev, and G. Plunien
Interelectronic-interaction contribution to the nonlinear Zeeman effect in boronlike ions
Phys. Rev. A, 97 :043402 (April 2018)
Relativistic calculations of the second- and third-order contributions in magnetic field to the Zeeman splitting in boronlike ions are presented for the wide range of nuclear charge numbers Z=6−92. The interelectronic-interaction correction of the first order in 1/Z is evaluated to all orders in αZ. The higher order corrections in 1/Z are taken into account approximately by means of effective screening potentials. The obtained results are important for interpretation of experimental data on the Zeeman splitting in boronlike ions, in particular, for the ARTEMIS experiment presently implemented at GSI.
L. J. Vormawah, M. Vilén, R. Beerwerth, P. Campbell, B. Cheal, A. Dicker, T. Eronen, S. Fritzsche, S. Geldhof, A. Jokinen, S. Kelly, I. D. Moore, M. Reponen, S. Rinta-Antila, S. O. Stock, and A. Voss
Isotope shifts from collinear laser spectroscopy of doubly charged yttrium isotopes
Phys. Rev. A, 97 :042504 (April 2018)
Collinear laser spectroscopy has been performed on doubly charged ions of radioactive yttrium in order to study the isotope shifts of the 294.6-nm 5s2S1/2→5p2P1/2 line. The potential of such an alkali-metal-like transition to improve the reliability of atomic-field-shift and mass-shift factor calculations, and hence the extraction of nuclear mean-square radii, is discussed. Production of yttrium ion beams for such studies is available at the IGISOL IV Accelerator Laboratory, Jyväskylä, Finland. This newly recommissioned facility is described here in relation to the on-line study of accelerator-produced short-lived isotopes using collinear laser spectroscopy and application of the technique to doubly charged ions.
V. Shevelko, Yu. A. Litvinov, T. Stöhlker, and I. Yu. Tolstikhina
Lifetimes of relativistic heavy-ion beams in the High Energy Storage Ring of FAIR
Nucl. Instr. Meth. Phys. Res. B, 421 :45 (April 2018)
The High Energy Storage Ring, HESR, will be constructed at the Facility for Antiproton and Ion Research, FAIR, Darmstadt. For the first time, it will be possible to perform experiments with cooled high-intensity stable and radioactive heavy ions at highly relativistic energies. To design experiments at the HESR, realistic estimations of beam lifetimes are indispensable. Here we report calculated cross sections and lifetimes for typical U88+, U90+, U92+, Sn49+ and Sn50+ ions in the energy range E = 400 MeV/u–5 GeV/u, relevant for the HESR. Interactions with the residual gas and with internal gas-jet targets are also considered.
H. Gies, F. Karbstein, C. Kohlfürst, and N. Seegert
Photon-photon scattering at the high-intensity frontier
Phys. Rev. D, 97 :076002 (April 2018)
The tremendous progress in high-intensity laser technology and the establishment of dedicated high-field laboratories in recent years have paved the way towards a first observation of quantum vacuum nonlinearities at the high-intensity frontier. We advocate a particularly prospective scenario, where three synchronized high-intensity laser pulses are brought into collision, giving rise to signal photons, whose frequency and propagation direction differ from the driving laser pulses, thus providing various means to achieve an excellent signal to background separation. Based on the theoretical concept of vacuum emission, we employ an efficient numerical algorithm which allows us to model the collision of focused high-intensity laser pulses in unprecedented detail. We provide accurate predictions for the numbers of signal photons accessible in experiment. Our study is the first to predict the precise angular spread of the signal photons, and paves the way for a first verification of quantum vacuum nonlinearity in a well-controlled laboratory experiment at one of the many high-intensity laser facilities currently coming online.
M. Reichert, A. Eichhorn, H. Gies, J. M. Pawlowski, T. Plehn, and M. M. Scherer
Probing baryogenesis through the Higgs boson self-coupling
Phys. Rev. D, 97 :075008 (April 2018)
The link between a modified Higgs self-coupling and the strong first-order phase transition necessary for baryogenesis is well explored for polynomial extensions of the Higgs potential. We broaden this argument beyond leading polynomial expansions of the Higgs potential to higher polynomial terms and to nonpolynomial Higgs potentials. For our quantitative analysis we resort to the functional renormalization group, which allows us to evolve the full Higgs potential to higher scales and finite temperature. In all cases we find that a strong first-order phase transition manifests itself in an enhancement of the Higgs self-coupling by at least 50%, implying that such modified Higgs potentials should be accessible at the LHC.
C. Jauregui, C. Stihler, A. Tünnermann, and J. Limpert
Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold
Opt. Express, 26 :10691 (April 2018)
A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of \~600W, which is a factor of 2 above the mode instability threshold.
G. A. Becker, S. Tietze, S. Keppler, J. Reislöhner, J. H. Bin, L. Bock, F.-E. Brack, J. Hein, M. Hellwing, P. Hilz, M. Hornung, A. Kessler, S. D. Kraft, S. Kuschel, H. Liebetrau, W. Ma, J. Polz, H.-P. Schlenvoigt, F. Schorcht, M. B. Schwab, A. Seidel, K. Zeil, U. Schramm, M. Zepf, J. Schreiber, S. Rykovanov, and M.C. Kaluza
Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime
Plasma Phys. Contr. F., 60 :055010 (April 2018)
The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.
A. Klenke, M. Müller, H. Stark, A. Tünnermann, and J. Limpert
Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays
Opt. Express, 26 :12072 (April 2018)
We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.
W. Paufler, B. Böning, and S. Fritzsche
Strong-field ionization with twisted laser pulses
Phys. Rev. A, 97 :043418 (April 2018)
We apply quantum trajectory Monte Carlo computations in order to model strong-field ionization of atoms by twisted Bessel pulses and calculate photoelectron momentum distributions (PEMD). Since Bessel beams can be considered as an infinite superposition of circularly polarized plane waves with the same helicity, whose wave vectors lie on a cone, we compared the PEMD of such Bessel pulses to those of a circularly polarized pulse. We focus on the momentum distributions in propagation direction of the pulse and show how these momentum distributions are affected by experimental accessible parameters, such as the opening angle of the beam or the impact parameter of the atom with regard to the beam axis. In particular, we show that we can find higher momenta of the photoelectrons, if the opening angle is increased.
E. Gelfer, N. Elkina, and A. Fedotov
Unexpected impact of radiation friction: enhancing production of longitudinal plasma waves
Sci. Rep., 8 :6478 (April 2018)
We study the penetration of ultra-intense (intensity I ≃ 10^(23–24) W/cm2) circularly polarized laser pulses into a thick subcritical plasma layer with accounting for radiation friction. We show that radiation pressure is enhanced due to radiation friction in the direction transverse to the laser pulse propagation, and that for stronger and longer laser pulses this mechanism dominates over the ordinary ponderomotive pressure, thus resulting in a substantionaly stronger charge separation than anticipated previously. We give estimates of the effect and compare them with the results of one and two dimensional particle-in-cell simulations. This effect can be important for laser-based acceleration schemes.
Y. Lin, Y. Yuan, F. Fang, and Z. Tan
A study of electric field distribution in Benjamin type proportional counter using finite element method
Appl. Radiat. Isot., 135 :142 (March 2018)
Tissue equivalent proportional counters (TEPCs) are commonly based on the Benjamin type of concept. Initially the electric field is optimized by pulse height measurement methods and only one optimum solution was established at that time. In this paper, the electric field distribution is analyzed and optimized using a three-dimensional finite element method. The calculations show that the characteristics of the radial electric field distribution of this type of counters can be equated to cylindrical counters using a pair of appropriate field shaping electrode. Furthermore, the paper analyzes the axial electric field distribution and the possibility of achieving a uniform electric field along its anode while reducing the size of Benjamin type proportional counter design down to 1/10 of currently feasible values with respect to the thinnest available anode wire diameters.
A. Blinne, D. Schinkel, S. Kuschel, N. Elkina, S. G. Rykovanov, and M. Zepf
A systematic approach to numerical dispersion in Maxwell solvers
Comput. Phys. Commun., 224 :273 (March 2018)
The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.
S. H. Hendi, B. Eslam Panah, and S. Panahiyan
Black Hole Solutions in Gauss-Bonnet-Massive Gravity in the Presence of Power-Maxwell Field
Fortschr. Phys., 66 :1800005 (March 2018)
Motivated by recent progresses in the field of massive gravity, the paper at hand investigates the thermodynamical structure of black holes with three specific generalizations: i) Gauss-Bonnet gravity which is motivated from string theory ii) PMI nonlinear electromagnetic field which is motivated from perspective of the QED correction iii) massive gravity which is motivated by obtaining the modification of standard general relativity. The exact solutions of this setup are extracted which are interpreted as black holes. In addition, thermodynamical quantities of the solutions are calculated and their critical behavior are studied. It will be shown that although massive and Gauss-Bonnet gravities are both generalizations in gravitational sector, they show opposing effects regarding the critical behavior of the black holes. Furthermore, a periodic effect on number of the phase transition is reported for variation of the nonlinearity parameter and it will be shown that for super charged black holes, system is restricted in a manner that prevents it to reach the critical point and to acquire phase transition. In addition, the effects of geometrical structure on thermodynamical phase transition will be highlighted.
O. Novak, R. Kholodov, A. Surzhykov, A. N. Artemyev, and T. Stöhlker
K-shell ionization of heavy hydrogenlike ions
Phys. Rev. A, 97 :032518 (March 2018)
A theoretical study of the K-shell ionization of hydrogenlike ions, colliding with bare nuclei, is performed within the framework of the time-dependent Dirac equation. Special emphasis is placed on the ionization probability that is investigated as a function of impact parameter, collision energy, and nuclear charge. To evaluate this probability in a wide range of collisional parameters we propose a simple analytical expression for the transition amplitude. This expression contains three fitting parameters that are determined from the numerical calculations, based on the adiabatic approximation. In contrast to previous studies, our analytical expression for the transition amplitude and ionization probability accounts for the full multipole expansion of the two-center potential and allows accurate description of nonsymmetric collisions of nuclei with different atomic numbers Z1≠Z2. The calculations performed for both symmetric and asymmetric collisions indicate that the ionization probability is reduced when the difference between the atomic numbers of ions increases.
A. M. Sayler, E. Eckner, J. McKenna, B. D. Esry, K. D. Carnes, I. Ben-Itzhak, and G.G. Paulus
Nonunique and nonuniform mapping in few-body Coulomb-explosion imaging
Phys. Rev. A, 97 :033412 (March 2018)
Much of our knowledge of molecular geometry and interaction dynamics comes from indirect measurements of the molecular fragments following breakup. This technique—Coulomb-explosion imaging (CEI), i.e., determining the initial molecular configuration of a system from the momenta of the resulting fragments using knowledge of the particle interactions—is one of the fundamental tools of molecular physics. Moreover, CEI has been a staple of molecular studies for decades. Here we show that one often cannot assign a unique initial configuration to the few-body breakup of a polyatomic molecule given the measurement of the resulting fragments' momenta. Specifically, multiple initial configurations can result in identical momenta for a molecule breaking into three or more parts. Further, the nonunique and nonuniform mapping from the initial configuration to the measured momenta also significantly complicates the determination of molecular alignment at the time of breakup.