Peer-Review Publications

We derive analytical expressions for the photon polarization tensor in circularly polarized Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) beams, complementing the corresponding results for linearly polarized beams obtained recently. As they are based upon a locally constant field approximation of the one-loop Heisenberg–Euler effective Lagrangian for quantum electrodynamics (QED) in constant fields, our results are generically limited to slowly varying electromagnetic fields, varying on spatial (temporal) scales much larger than the Compton wavelength (time) of the electron.

A scheme has been developed permitting the spatial and temporal characterisation of ultrafast dynamics induced by laser driven proton bursts in transparent dielectrics. Advantage is taken of the high degree of synchronicity between the proton bursts generated during laser-foil target interactions and the probing laser to provide the basis for streaking of the dynamics. Relaxation times of electrons (<10⁻¹² s) are measured following swift excitation across the optical band gap for various glass samples. A temporal resolution of <500 fs is achieved demonstrating that these ultrafast dynamics can be characterized on a single-shot basis.

A tomographic gas-density diagnostic using a Single-Beam Wollaston Interferometer able to characterize non-symmetric density distributions in gas jets is presented. A real-time tomographic algorithm is able to reconstruct three-dimensional density distributions. A Maximum Likelihood-Expectation Maximization algorithm, an iterative method with good convergence properties compared to simple back projection, is used. With the use of graphical processing units, real-time computation and high resolution are achieved. Two different gas jets are characterized: a kHz, piezo-driven jet for lower densities and a solenoid valve-based jet producing higher densities. While the first jet is used for free electron laser photon beam characterization, the second jet is used in laser wake field acceleration experiments. In this latter application, well-tailored and non-symmetric density distributions produced by a supersonic shock front generated by a razor blade inserted laterally to the gas flow, which breaks cylindrical symmetry, need to be characterized.

It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.

A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.

We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.

We study all-optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum, using the collision of two focused high-intensity laser pulses as an example. The experimental signatures of quantum vacuum nonlinearities are encoded in signal photons, whose kinematic and polarization properties differ from the photons constituting the macroscopic laser fields. We implement an efficient numerical algorithm allowing for the theoretical investigation of such signatures in realistic field configurations accessible in experiment. This algorithm is based on a vacuum emission scheme and can readily be adapted to the collision of more laser beams or further involved field configurations. We solve the case of two colliding pulses in full 3+1-dimensional spacetime and identify experimental geometries and parameter regimes with improved signal-to-noise ratios.

We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4). By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 1.92 µm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4). The measured results match simulations of both the generalized and a hybrid nonlinear Schrödinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity. We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics.

We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30  MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500  MeV) with an intense laser pulse (a₀>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ϵ꜀ᵣᵢₜ>30  MeV.

In this work, we demonstrate experimentally the formation of 10 different structures consisting of bright beams with flat phase fronts in the focus of a lens (i.e., in the artificial far field). The basic structure used is a large, stable, square-shaped optical vortex (OV) array composed of vortices with alternating topological charges (TCs). The TCs of one individual OV, of a subarray of OVs, or of the complete OV lattice were erased/doubled in the cases of perfect superposition (on-site alignment) or are manipulated in phase in the cases of an offset between the vortices (off-site alignment). A dramatic reshaping of the beam is observed in the far field and shown to be in excellent agreement with numerical simulations.

We theoretically examine neon atoms in ultrashort and intense x-rays from free electron lasers and compare our results with data from experiments conducted at the Linac Coherent Light Source. For this purpose, we treat in detail the electronic structure in all possible nonrelativistic cationic configurations using a relativistic multiconfiguration approach. The interaction with the x-rays is described in rate-equation approximation. To understand the mechanisms of the interaction, a path analysis is devised which allows us to investigate what sequences of photoionization and decay processes lead to a specific configuration and with what probability. Thereby, we uncover a connection to the mathematics of graph theory and formal languages. In detail, we study the ion yields and find that plain rate equations do not provide a satisfactory description. We need to extend the rate equations for neon to incorporate double Auger decay of a K-shell vacancy and photoionization shake off for neutral neon. Shake off is included for valence and core ionization; the former has hitherto been overlooked but has important consequences for the ion yields from an x-ray energy below the core ionization threshold. Furthermore, we predict the photon yields from xuv and x-ray fluorescence; these allow one insights into the configurations populated by the interaction with the x-rays. Finally, we discover that inaccuracies in those Auger decay widths employed in previous studies have only a minor influence on ion and photon yields.

A recent measurement of the hyperfine splitting in the ground state of Li-like 208Bi80+ has established a "hyperfine puzzle" - the experimental result exhibits a 7σ deviation from the theoretical prediction. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment (μI) of 209Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of μI(209Bi) and combine it with nuclear magnetic resonance measurements of Bi(NO3)3 in nitric acid solutions and of the hexafluoridobismuthate(V) BiF−6 ion in acetonitrile. The result clearly reveals that μI(209Bi) is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.

Ultrafast processes in matter can be captured and even controlled by using sequences of few-cycle optical pulses, which need to be well characterized, both in amplitude and phase. The same degree of control has not yet been achieved for few-cycle extreme ultraviolet pulses generated by high-order harmonic generation (HHG) in gases, with duration in the attosecond range. Here, we show that by varying the spectral phase and carrier-envelope phase (CEP) of a high-repetition rate laser, using dispersion in glass, we achieve a high degree of control of the relative phase and CEP between consecutive attosecond pulses. The experimental results are supported by a detailed theoretical analysis based upon the semi-classical three-step model for HHG.

The Dirac-Heisenberg-Wigner formalism is employed to investigate electron-positron pair production in cylindrically symmetric but otherwise spatially inhomogeneous, oscillating electric fields. The oscillation frequencies are hereby tuned to obtain multiphoton pair production in the nonperturbative threshold regime. An effective mass, as well as a trajectory-based semiclassical analysis, is introduced in order to interpret the numerical results for the distribution functions as well as for the particle yields and spectra. The results, including the asymptotic particle spectra, display clear signatures of ponderomotive forces.

We investigate the quantum interference induced shifts between energetically close states in highly charged ions, with the energy structure being observed by laser spectroscopy. In this work, we focus on hyperfine states of lithiumlike heavy-Z isotopes and quantify how much quantum interference changes the observed transition frequencies. The process of photon excitation and subsequent photon decay for the transition 2s→2p→2s is implemented with fully relativistic and full-multipole frameworks, which are relevant for such relativistic atomic systems. We consider the isotopes 207Pb79+ and 209Bi80+ due to experimental interest, as well as other examples of isotopes with lower Z, namely 141Pr56+ and 165Ho64+. We conclude that quantum interference can induce shifts up to 11% of the linewidth in the measurable resonances of the considered isotopes, if interference between resonances is neglected. The inclusion of relativity decreases the cross section by 35%, mainly due to the complete retardation form of the electric dipole multipole. However, the contribution of the next higher multipoles (e.g., magnetic quadrupole) to the cross section is negligible. This makes the contribution of relativity and higher-order multipoles to the quantum interference induced shifts a minor effect, even for heavy-Z elements.

The elastic Rayleigh scattering of twisted light and, in particular, the polarization (transfer) of the scattered photons have been analyzed within the framework of second-order perturbation theory and Dirac's relativistic equation. Special attention was paid hereby to the scattering on three different atomic targets: single atoms, a mesoscopic (small) target, and a macroscopic (large) target, which are all centered with regard to the beam axis. Detailed calculations of the polarization Stokes parameters were performed for C5+ ions and for twisted Bessel beams. It is shown that the polarization of scattered photons is sensitive to the size of an atomic target and to the helicity, the opening angle, and the projection of the total angular momentum of the incident Bessel beam. These computations indicate more that the Stokes parameters of the (Rayleigh) scattered twisted light may significantly differ from their behavior for an incident plane-wave radiation.

Abstract A comprehensive spatio-temporal characterization is presented describing the pump-induced wavefront aberrations in Yb3 + -doped YAG, CaF2, and fluorophosphate glass. Time-resolved interferometric measurements were performed to reveal the profiles of the total optical path differences (OPDs), which are described by the spatio-temporal superposition of thermal as well as electronic contributions, across the free aperture of the considered diode-pumped active materials. These contributions were individually determined by a COMSOL-based thermal profile model along with a detailed characterization of the electronic changes by measuring the single-pass gain and the spatial fluorescence profile. Due to the low quantum defect, the amplitude of the electronic component becomes comparable for all three materials and, in the case of Yb:CaF2, almost completely compensates the thermal component resulting from a pump pulse during the time frame of laser pulse amplification. Finally, all relevant material constants – such as the photoelastic constant and the polarizability difference – could be determined during this investigation, allowing the accurate modeling of the total pump-induced wavefront aberrations and subsequent optimization for laser systems worldwide employing these Yb3 + -doped materials.

The hyperfine structure splitting in the 6p3S3/24→6p27sP1/24 transition at 307 nm in atomic 208Bi was measured with collinear laser spectroscopy at ISOLDE, CERN. The hyperfine A and B factors of both states were determined with an order of magnitude improved accuracy. Based on these measurements, theoretical input for the hyperfine structure anomaly, and results from hyperfine measurements on hydrogen-like and lithium-like 209Bi80+,82+, the nuclear magnetic moment of 208Bi has been determined to μ(Bi208)=+4.570(10) μN. Using this value, the transition energy of the ground-state hyperfine splitting in hydrogen-like and lithium-like 208Bi80+,82+ and their specific difference of −67.491(5)(148) meV are predicted. This provides a means for an experimental confirmation of the cancellation of nuclear structure effects in the specific difference in order to exclude such contributions as the cause of the hyperfine puzzle, the recently reported 7-σ discrepancy between experiment and bound-state strong-field QED calculations of the specific difference in the hyperfine structure splitting of 209Bi80+,82+.

We study the trident process in inhomogeneous plane-wave background fields. We obtain compact analytical expressions for all terms in the probability, including the exchange part, for an arbitrarily shaped plane wave. We evaluate the probability numerically using complex deformation of light-front time integrals and derive various analytical approximations. Our results provide insights into the importance of the one-step and exchange parts of the probability relative to the two-step process, and into the convergence to the locally constant field approximation.

Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump–probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.

The feasibility of an experiment for the observation of parity nonconserving effects using He-like highly charged ions in storage rings is discussed theoretically. The basic idea is the observation of an asymmetry in the emission of the hyperfine quenched transition (1s2s)¹S₀\to (1s)²¹S₀+γ with respect to the direction of a beam of ions with polarized nuclei. It will be shown, that for such an experiment 151Eu^61+ ions with nuclear spin I=5/2 in the excited electronic state (1s2s)¹S₀ with zero total electron angular momentum and polarized nuclei are the best available candidates. The nuclei can be polarized if H-like Eu^62+ ions capture an electron from a polarized electron beam which overlays the ion beam on some part of the ring and travels with nearly the same velocity. It is suggested, to monitor steadily the degree of the nuclear polarization by the observation of the selective laser excitation and subsequent decay of the Zeeman sublevels of the excited hyperfine states of the ionic ground state. Estimates for the observation time aiming at an accuracy of PNC measurement of about 0.1 % are given.

A description of the spectral and angular distributions of Compton scattered light in collisions of intense laser pulses with high-energy electrons is unwieldy and usually requires numerical simulations. However, due to the large number of parameters affecting the spectra such numerical investigations can become computationally expensive. Using methods of catastrophe theory we predict higher-dimensional caustics in the spectra of the Compton scattered light, which are associated with bright narrow-band spectral lines, and in the simplest case can be controlled by the value of the linear chirp of the pulse. These findings require no full-scale calculations and have direct consequences for the photon yield enhancement of future nonlinear Compton scattering x-ray or gamma-ray sources.

Often, the interpretation of experiments concerning the manipulation of the energy distribution of laser-accelerated ion bunches is complicated by the multitude of competing dynamic processes simultaneously contributing to recorded ion signals. Here we demonstrate experimentally the acceleration of a clean proton bunch. This was achieved with a microscopic and three-dimensionally confined near critical density plasma, which evolves from a 1 µm diameter plastic sphere, which is levitated and positioned with micrometer precision in the focus of a Petawatt laser pulse. The emitted proton bunch is reproducibly observed with central energies between 20 and 40 MeV and narrow energy spread (down to 25%) showing almost no low-energetic background. Together with three-dimensional particle-in-cell simulations we track the complete acceleration process, evidencing the transition from organized acceleration to Coulomb repulsion. This reveals limitations of current high power lasers and viable paths to optimize laser-driven ion sources.