Peer-Review Publications

We investigate the process of nuclear excitation via a two-photon electron transition (NETP) for the case of the doubly charged thorium. The theory of the NETP process was originally devised for heavy-helium-like ions. In this work, we study this process in the nuclear clock isotope 229Th in the 2+ charge state. For this purpose we employ a combination of configuration interaction and many-body perturbation theory to calculate the probability of NETP in resonance approximation. The experimental scenario we propose for the excitation of the low-lying isomeric state in 229Th is a circular process starting with a two-step pumping stage followed by NETP. The ideal intermediate steps in this process depends on the supposed energy ℏωN of the nuclear isomeric state. For each of these energies, the best initial state for NETP is calculated. Special focus is put on the most recent experimental results for ℏωN.

We generate temporally modulated optical pulses with a beat frequency of 255 GHz, a duration of 360 ps, and a repetition rate of 2 MHz. The temporal envelope, beat frequency, and repetition rate are computer-programmable. A frequency comb serves as a phase and frequency reference for the locking of two laser lines. The system enables beat frequencies that are adjustable in steps of the frequency comb's repetition rate and exhibit Hz-level precision and accuracy. We expect the optical beat pulses to be well suited for versatile multi-cycle terahertz-wave generation with controllable carrier-envelope phase. We demonstrate that the inherent synchronization of the frequency comb's ultra-short pulse train and the synthesized optical beat (or later the multi-cycle terahertz) pulses enables rapid and phase-sensitive sampling of such pulses.

Temporal pulse profile characterization is necessary to ensure and quantify the quality of short pulse laser systems. Yet it remains challenging to measure the temporal behavior of a pulse in all of its comprehensiveness. In this manuscript we present results which encourage to perform more ambitious pulse characterizations with optimized scanning cross-correlators. Several temporal laser pulse profile measurements in multiple nanosecond time scale with high dynamic range are shown. The measurements were taken by our in-house third-order cross-correlator EICHEL (Schanz et al. in Opt Express 25:9252, 2017), which is able to resolve the intensity dynamics down to the level of amplified spontaneous emission. With this device we show for the first time the onset of the plateau of the amplified spontaneous emission in the laser profile and investigate the origin of several side-pulses created early in the laser system.

The LIBELLE experiment performed at the experimental storage ring at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, has successfully determined the ground state hyperfine (HFS) splittings in hydrogen-like (Bi-209(82+)) and lithium-like (Bi-209(80+)) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. Besides the transition wavelengths the time-resolved detection of fluorescence photons following the excitation of the ions by a pulsed laser system also allows the extraction of lifetimes of the upper HFS levels and g-factors of the bound 1s and 2s electrons for both charge states. While the lifetime of the upper HFS state in Bi-209(82+) has already been measured in earlier experiments, an experimental value for lifetime of this state in Bi-209(80+) is reported for the first time in this work.

The rapid heating of a thin titanium foil by a high intensity, subpicosecond laser is studied by using a 2D narrow-band x-ray imaging and x-ray spectroscopy. A novel monochromatic imaging diagnostic tuned to 4.51 keV Ti K alpha was used to successfully visualize a significantly ionized area (< Z > > 17 +/- 1) of the solid density plasma to be within a similar to 35 mu m diameter spot in the transverse direction and 2 mu m in depth. The measurements and a 2D collisional particle-in-cell simulation reveal that, in the fast isochoric heating of solid foil by an intense laser light, such a high ionization state in solid titanium is achieved by thermal diffusion from the hot preplasma in a few picoseconds after the pulse ends. The shift of K alpha and formation of a missing K alpha cannot be explained with the present atomic physics model. The measured K alpha image is reproduced only when a phenomenological model for the K alpha shift with a threshold ionization of < Z > = 17 is included. This work reveals how the ionization state and electron temperature of the isochorically heated nonequilibrium plasma are independently increased.

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.

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.

We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6—18 µm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 µm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-µm pulses in GaSe.

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.

We have studied the K-shell excitation of He-like uranium (U90+) in relativistic collisions with hydrogen and argon atoms. Performing measurements with different targets, as well as with different collision energies, enabled us to explore the proton- (nucleus-) impact excitation as well as the electron-impact excitation process for the heaviest He-like ion. The large fine-structure splitting in uranium allowed us to partially resolve excitation into different L-shell levels. State-of-the-art relativistic calculations which include excitation mechanisms due to the interaction with both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the electron-impact excitation process.

We investigated the high-pressure behavior of polyethylene (CH2) by probing dynamically-compressed samples with X-ray diffraction. At pressures up to 200 GPa, comparable to those present inside icy giant planets (Uranus, Neptune), shock-compressed polyethylene retains a polymer crystal structure, from which we infer the presence of significant covalent bonding. The A2/m structure which we observe has previously been seen at significantly lower pressures, and the equation of state measured agrees with our findings. This result appears to contrast with recent data from shock-compressed polystyrene (CH) at higher temperatures, which demonstrated demixing and recrystallization into a diamond lattice, implying the breaking of the original chemical bonds. As such chemical processes have significant implications for the structure and energy transfer within ice giants, our results highlight the need for a deeper understanding of the chemistry of high pressure hydrocarbons, and the importance of better constraining planetary temperature profiles.

We perform a multi-dimensional parameter scan in the generation of high-order harmonics, with the main purpose to find the macroscopic conditions that optimize the harmonic yield in a specific spectral domain, around 40 eV for this particular case. The scanned parameters are the laser pulse energy, gas pressure, interaction cell position relative to focus and the cell length, while the fixed parameters are chosen to model a loose focusing configuration which is used in many existing laboratories. We performed the simulations with a 3D non-adiabatic model complemented by a detailed analysis of the phase matching mechanisms involved in an efficient harmonic generation. Based on the results we identify a range of parameter combinations that lead to a high yield in the specified spectral domain The method and results presented here can be the framework for the design and construction of high flux high-order harmonic generation beamlines.

Aims. In the context of black-hole accretion disks, the main goal of the present study is to estimate the plasma environment effects on the atomic structure and radiative parameters associated with the K-vacancy states in ions of the oxygen isonuclear sequence. Methods. We used a time-averaged Debye–Hückel potential for both the electron–nucleus and the electron–electron interactions implemented in the fully relativistic multiconfiguration Dirac–Fock (MCDF) method. Results. Modified ionization potentials, K thresholds, Auger widths, and radiative transition wavelengths and rates are reported for O I–O VII in plasma environments with electron temperature and density ranges 105−107 K and 1018−1022 cm−3.

X-ray phase-contrast imaging (XPCI) is a versatile technique with applications in many fields, including fundamental physics, biology and medicine. Where X-ray absorption radiography requires high density ratios for effective imaging, the image contrast for XPCI is a function of the density gradient. In this letter, we apply XPCI to the study of laser-driven shock waves. Our experiment was conducted at the Petawatt High-Energy Laser for Heavy Ion EXperiments (PHELIX) at GSI. Two laser beams were used: one to launch a shock wave and the other to generate an X-ray source for phase-contrast imaging. Our results suggest that this technique is suitable for the study of warm dense matter (WDM), inertial confinement fusion (ICF) and laboratory astrophysics.

In this paper, we investigate laser emission at 3.4μm in heavily-erbium-doped fluoride fibers using dual-wavelength pumping. To this extent, a monolithic 7 mol% erbium-doped fluoride fiber laser bounded by intracore fiber Bragg gratings at 3.42 μm is used to demonstrate a record efficiency of 38.6 % with respect to the 1976 nm pump. Through numerical modeling, we show that similar laser performances at 3.4 μm can be expected in fluoride fibers with erbium concentrations ranging between 1 – 7 mol%, although power scaling should rely on lightly-doped fibers to mitigate the heat load. Moreover, this work studies transverse mode-beating of the 1976 nm core pump and its role in the generation of a periodic luminescent grating and in the trapping of excitation in the metastable energy levels of the erbium system. Finally, we also report on the bistability of the 3.42 μm output power of the 7 mol% erbium-doped fluoride fiber laser.

We present a modular extreme ultraviolet (XUV) spectrometer system optimized for a broad spectral range of 12-41 nm (30-99 eV) with a high spectral resolution of lambda/Delta lambda greater than or similar to 784 +/- 89. The spectrometer system has several operation modes for (1) XUV beam inspection, (2) angular spectral analysis, and (3) imaging spectroscopy. These options allow for a versatile use in high harmonic spectroscopy and XUV beam analysis. The high performance of the spectrometer is demonstrated using a novel cross-sectional imaging method called XUV coherence tomography.

Z2-Yukawa-QCD models are a minimalistic model class with a Yukawa and a QCD-like gauge sector that exhibits a regime with asymptotic freedom in all its marginal couplings in standard perturbation theory. We discover the existence of further asymptotically free trajectories for these models by exploiting generalized boundary conditions. We construct such trajectories as quasi-fixed points for the Higgs potential within different approximation schemes. We substantiate our findings first in an effective-field-theory approach, and obtain a comprehensive picture using the functional renormalization group. We infer the existence of scaling solutions also by means of a weak-Yukawa-coupling expansion in the ultraviolet. In the same regime, we discuss the stability of the quasi-fixed point solutions for large field amplitudes. We provide further evidence for such asymptotically free theories by numerical studies using pseudo-spectral and shooting methods.

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.

We theoretically investigate the two-color above-threshold ionization of atoms and ions by twisted XUV Bessel and Laguerre-Gaussian (LG) beams in the presence of a strong circularly polarized near-infrared (NIR) laser field. The presence of the NIR field modifies the continuum states accessible to the photoelectron. Based on the strong-field approximation, we explore the resulting energy and angular distributions of photoelectron as a function of the beam parameters. In particular, we analyze dichroism signals that arise due to the twisted nature of the XUV beam and the helicity of the NIR field. We focus on the comparison between LG beams and Bessel beams in the paraxial approximation. Here, we find that both beams yield similar results when the paraxial regime is valid. For localized targets, the dichroism signals strongly depend on the size and position of the atoms relative to the beam axis. Moreover, the dichroism signal tends to zero when the XUV LG beam is linear polarized. Detailed computations of the dichroism are performed and discussed for the 4s valence-shell photoionization of Ca⁺ ions.

We present measurements of the hard x-ray emission from targets irradiated at relativistic laser intensities, with the objective of comprehensively characterizing source properties relevant to x-ray radiography backlighting in high energy density experiments. Thin gold foil and tungsten wire targets were irradiated at peak laser intensities varying between 10¹⁸ and 10²¹ W/cm², with laser pulse energies >100 J. We have measured the absolute x-ray yield in the spectral range between 20 and 200 keV, angularly resolved over a large range of emission angles with respect to the incident laser. In addition, we have determined the x-ray source sizes for the two target types in the direction both along and across the target. The results are compared with the predictions of a simple model for the hot electron propagation, x-ray generation, collisional stopping, and expansion cooling. Based on this model, our measurements allow extraction of the laser to hot electron conversion over the wide range of intensities covered by our experiment.

In this Letter, we present, to the best of our knowledge, the largest effective single-mode fiber reported to date. The employed waveguide is a passive large pitch fiber (LPF), which shows the core area scaling potential of such a fiber structure. In particular, we achieved stable single-transverse mode transmission at a wavelength of 1.03 µm through a straight passive LPF with a pitch of 140 µm, resulting in a measured mode-field diameter of 205 µm.

Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table- top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 łambda (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

In this paper, we investigate thermodynamical structure of dyonic black holes in the presence of gravity's rainbow. We confirm that for super magnetized and highly pressurized scenarios, the number of black holes' phases is reduced to a single phase. In addition, due to specific coupling of rainbow functions, it is possible to track the effects of temporal and spatial parts of our setup on thermodynamical quantities/behaviors including equilibrium point, existence of multiple phases, possible phase transitions and conditions for having a uniform stable structure.

The Facility for Antiproton and Ion Research (FAIR) will be the accelerator-based flagship research facility in many basic sciences and their applications in Europe for the coming decades. FAIR will open up unprecedented research opportunities in hadron and nuclear physics, in atomic physics and nuclear astrophysics as well as in applied sciences like materials research, plasma physics and radiation biophysics with applications towards novel medical treatments and space science. FAIR is currently under construction as an international facility at the campus of the GSI Helmholtzzentrum for Heavy-Ion Research in Darmstadt, Germany. While the full science potential of FAIR can only be harvested once the new suite of accelerators and storage rings is completed and operational, some of the experimental detectors and instrumentation are already available and will be used starting in summer 2018 in a dedicated research program at GSI, exploiting also the significantly upgraded GSI accelerator chain. The current manuscript summarizes how FAIR will advance our knowledge in various research fields ranging from a deeper understanding of the fundamental interactions and symmetries in nature to a better understanding of the evolution of the Universe and the objects within.

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.