arXiv subjects

Gerhard G. Paulus

Publications and source records attributed to Gerhard G. Paulus.

At least 19 recordsLinked to original sources

arXiv2026

A New Bound on Light-by-Light Scattering in the Low-Energy Regime

The superposition principle at the heart of optics states that electromagnetic fields do not interact in vacuum. This principle is not universal, instead electromagnetic fields couple weakly via quantum vacuum fluctuations. Striking consequences are light-by-light scattering and vacuum birefringence. Experimental results from colliding X-ray free-electron and optical high-intensity lasers combined with dark-field detection and polarimetry for background suppression are reported. This approach yields the most stringent bound for light-by-light scattering in the low-energy regime achieved to date -- improving upon previous experimental limits by ten orders of magnitude -- and paves the way for precision measurements.

hep-ex↗

arXiv2026

Dark-field Search for Quantum Vacuum Signals in the Low-Energy Regime

We report on the experimental details of a dark-field search for light-by-light scattering signals performed at the High Energy Density (HED) instrument of the European XFEL (EuXFEL) in a pump-probe type experiment colliding the EuXFEL beam with the 300-TW-class near-infrared Relativistic Laser at XFEL (ReLaX) at a center-of-mass photon energy of $ω_\star = 116\,\mathrm{eV}$. Especially in combination with polarization filtering, the dark-field concept allows the signal-to-background ratio to be significantly enhanced and both polarization channels to be recorded. This work elaborates on the experimental setup and the diagnostics used, from the per-shot spatiotemporal-overlap diagnostics, via the polarizing beam splitter, to the characterization of the residual background. Moreover, it provides details on the start-to-end simulations of the setup used for analysis. Accumulating 2751 pump-probe collisions, no excess above background is observed, and an upper limit on the light-by-light scattering cross section of $7\times10^{4}$ times the QED prediction at a 95% confidence level is derived. This is the most stringent bound on low-energy light-by-light scattering achieved to date, representing an improvement by ten orders of magnitude upon previous experimental limits.

hep-ex↗

arXiv2026

Entanglement and photoelectron holography in dissociative photoionization: molecular quantum eraser

In a double-slit experiment with a bipartite system, the visibility of interference fringes depends on the availability of which-way information. Here, we report a Bell-like state of photoelectron and residual ion in the multiphoton dissociative ionization of the D$_2$ molecule. Evidence for entanglement is provided by the correlated emission directions of photoelectron and ion, which is observed using a COLTRIMS reaction microscope. In the presence of this correlation, the holographic interference fringes contained in the photoelectron momentum distributions are suppressed, indicating the existence of which-way information. The interference pattern persists when a single ionic state is populated and thus no which-way information is available. The experimental observations and conclusions are supported by the numerical solution of the electronic-nuclear time-dependent Schrödinger equation. Our work demonstrates that coincidence spectroscopy of ions and electrons is a powerful method for studying fundamental concepts of quantum information science within the context of ultrafast light-matter interactions.

quant-ph↗

arXiv2026

Single-shot sorting of Mössbauer time-domain data at X-ray free-electron lasers

Mössbauer spectroscopy is widely used to study structure and dynamics of matter with remarkably high energy resolution, provided by the narrow nuclear resonance line widths. However, the narrow width implies low count rates, such that experiments commonly average over extended measurement times or many x-ray pulses (``shots''). This averaging impedes the study of non-equilibrium phenomena. It has been suggested that X-ray free-electron lasers (XFELs) could enable Mössbauer single-shot measurements without averaging, and a proof-of-principle demonstration has been reported. However, so far, only a tiny fraction of all shots resulted in signal-photon numbers which are sufficiently high for a single-shot analysis. Here, we demonstrate coherent nuclear-forward-scattering of self-seeded XFEL radiation, with up to 900 signal-photons per shot. We develop a sorting approach which allows us to include all data on a single-shot level, independent of the signal content of the individual shots. It utilizes the presence of different dynamics classes, i.e. different nuclear evolutions after each excitation. Each shot is assigned to one of the classes, which can then be analyzed separately. Our approach determines the classes from the data without requiring theory modeling nor prior knowledge on the dynamics, making it also applicable to unknown phenomena. We envision that our approach opens up new grounds for Mössbauer science, enabling the study of out-of-equilibrium transient dynamics of the nuclei or their environment.

quant-ph↗

arXiv2025

Dark-fringe interferometer with dynamic phase control for Mössbauer science

Interference is a powerful tool for measuring and control. In Mössbauer science, interference effects are essential to most applications, due to the coherent scattering nature. However, Mössbauer interferometry remains challenging, due to stability requirements imposed by the short x-ray wavelength. Here, we put forward a ``dark fringe'' interferometer with vanishing transmission in the empty state, thereby facilitating sensitive measurements. The relative interferometer phase can dynamically be tuned by displacing a Mössbauer target. We experimentally demonstrate the tuning capabilities of this interferometer by controlling the transmitted x-ray intensity on nanosecond time scales. Then, we demonstrate sensitive measurements by observing the propagation of impulsively launched sound waves in the target over $\sim 10\,μs$. The interferometer concept opens avenues towards polarization-sensitive phase measurements, the generation of coherent multi-pulse sequences for controlling nuclear dynamics, and the implementation of feedback loops to adaptively optimize the interferometer, thereby fueling the further development of nuclear quantum optics.

quant-ph↗

arXiv2025

The Darkfield Approach to Measuring Vacuum Birefringence and Light-by-Light Couplings -- A Proof-of-Principle Experiment

Vacuum fluctuations give rise to effective nonlinear interactions between electromagnetic fields. These generically modify the characteristics of light traversing a strong-field region. X-ray free-electron lasers constitute a particularly promising probe, due to their brilliance, the possibility of precise control and favourable frequency scaling. However, the nonlinear vacuum response is very small even when probing a tightly focused high-intensity laser field with XFEL radiation and direct measurement of light-by-light scattering of real photons and the associated fundamental physics constants of the quantum vacuum has not been possible to date. Achieving a sufficiently good signal-to-background separation is key to a successful quantum vacuum experiment. To master this challenge, a darkfield detection concept has recently been proposed. Here we present the results of a proof-of-principle experiment validating this approach at the High Energy Density scientific instrument of the European X-Ray Free Electron Laser.

hep-ex↗

arXiv2025

Soft X-ray imaging with coherence tomography in the water window spectral range using high-harmonic generation

High-harmonic generation (HHG) is used as a source for various imaging applications in the extreme ultraviolet spectral range. It offers spatially coherent radiation and unique elemental contrast with the potential for attosecond time resolution. The unfavorable efficiency scaling to higher photon energies prevented the imaging application in the soft X-ray range so far. In this work we demonstrate the feasibility of using harmonics for imaging in the water window spectral region (284$\,$eV to 532$\,$eV). We achieve nondestructive depth profile imaging in a heterostructure by utilizing a broadband and noise-resistant technique called soft X-ray Coherence Tomography (SXCT) at a high-flux lab-scale HHG source. SXCT is derived from Optical Coherence Tomography, a Fourier based technique, that can use the full bandwidth of the source to reach an axial resolution of 12$\,$nm in this demonstration. The employed source covers the entire water window, with a photon flux exceeding 10$^\text{6}\,$photons/eV/s at a photon energy of 500$\,$eV. We show local cross sections of a sample consisting of Aluminium oxide and Platinum layers of varying thickness on a Zinc oxide substrate. We validate the findings with scanning and transmission electron microscopy after preparation with focused ion beam milling.

physics.optics↗

arXiv2024

Exploring Valence Electron Dynamics of Xenon through Laser-Induced Electron Diffraction

Strong-field ionization can induce electron motion in both the continuum and the valence shell of the parent ion. Here, we explore their interplay by studying laser-induced electron diffraction (LIED) patterns arising from interaction with the potentials of two-hole states of the xenon cation. The quantitative rescattering theory is used to calculate the corresponding photoelectron momentum distributions, providing evidence that the spin-orbit dynamics could be detected by LIED. We identify the contribution of these time-evolving hole states to the angular distribution of the rescattered electrons, particularly noting a distinct change along the backward scattering angles. We benchmark numerical results with experiments using ultrabroad and femtosecond laser pulses centered at \SI{3100}{nm}.

physics.atom-ph↗

arXiv2023

Non-destructive depth reconstruction of Al-Al$_2$Cu layer structure with nanometer resolution using extreme ultraviolet coherence tomography

Non-destructive cross-sectional characterization of materials systems with a resolution in the nanometer range and the ability to allow for time-resolved in-situ studies is of great importance in material science. Here, we present such a measurements method, extreme ultraviolet coherence tomography (XCT). The method is non-destructive during sample preparation as well as during the measurement, which is distinguished by a negligible thermal load as compared to electron microscopy methods. Laser-generated radiation in the extreme ultraviolet (XUV) and soft x-ray range is used for characterization. The measurement principle is interferometric and the signal evaluation is performed via an iterative Fourier analysis. The method is demonstrated on the metallic material system Al-Al$_2$Cu and compared to electron and atomic force microscopy measurements. We also present advanced reconstruction methods for XCT which even allow for the determination of the roughness of outer and inner interfaces.

cond-mat.mtrl-sci↗

arXiv2023

Intensity-resolved measurement of above-threshold ionization of Ar-H$_2$O

Above-treshold ionization (ATI) by femtosecond laser pulses centered at 515\,nm is studied for a gas mixture containing the Van-der-Waals complex Ar-H$_2$O. By detecting photoions and -electrons in coincidence, the ATI spectra for Ar, Ar$_2$, \HHO, and Ar-\HHO are discerned and measured simultaneously. Using an intensity-scanning technique, we observe the red-shift of the ATI spectra as a function of the laser intensity. The intensity-dependent shift of the ATI peak positions observed for Ar-H$_2$O and H$_2$O match but significantly differ from the ones measured for Ar and Ar$_2$. This indicates that the photoelectron is emitted from the \HHO site of the complex and the vertical ionization potential of Ar-H$_2$O is determined as $(12.4 \pm 0.1)$\,eV. For resacttered electrons, however, an enhancement of high-order ATI is observed for Ar-H$_2$O, as compared to H$_2$O, suggesting that the relatively large Ar atom acts as a scattering center, which influences the ionization dynamics.

physics.atom-ph↗

arXiv2023

Laboratory-Based Correlative Soft X-ray and Fluorescence Microscopy in an Integrated Setup

Correlative microscopy is a powerful technique that combines the advantages of multiple imaging modalities to achieve a comprehensive understanding of investigated samples. For example, fluorescence microscopy provides unique functional contrast by imaging only specifically labeled components, especially in biological samples. However, the achievable structural information on the sample in its full complexity is limited. Here, the intrinsic label-free carbon contrast of water window soft X-ray microscopy can complement fluorescence images in a correlative approach ultimately combining nanoscale structural resolution with functional contrast. However, soft X-ray microscopes are complex and elaborate, and typically require a large-scale synchrotron radiation source due to the demanding photon flux requirements. Yet, with modern high-power lasers it has become possible to generate sufficient photon flux from laser-produced plasmas, thus enabling laboratory-based setups. Here, we present a compact table-top soft X-ray microscope with an integrated epifluorescence modality for 'in-situ' correlative imaging. Samples remain in place when switching between modalities, ensuring identical measurement conditions and avoiding sample alteration or destruction. We demonstrate our new method by multimodal images of several exemplary samples ranging from nanoparticles to various multicolor labeled cell types. A structural resolution of down to 50 nm was reached.

physics.ins-det↗

arXiv2021

Vacuum birefringence at x-ray free-electron lasers

We study the perspectives of measuring the phenomenon of vacuum birefringence predicted by quantum electrodynamics using an x-ray free-electron laser (XFEL) alone. We devise an experimental scheme allowing the XFEL beam to collide with itself under a finite angle, and thus act as both pump and probe field for the effect. The signature of vacuum birefringence is encoded in polarization-flipped signal photons to be detected with high-purity x-ray polarimetry. Our findings for idealized scenarios underline that the discovery potential of solely XFEL-based setups can be comparable to those involving optical high-intensity lasers. For currently achievable scenarios, we identify several key details of the x-ray optical ingredients that exert a strong influence on the magnitude of the desired signatures.

hep-ph↗

arXiv2021

Disentangling X-ray dichroism and birefringence via high-purity polarimetry

High-brilliance synchrotron radiation sources have opened new avenues for X-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear X-ray polarizers in a crossed setting polarization extinction ratios down to 10$^{-10}$ can be achieved. This renders the method sensitive to probe tiniest optical anisotropies that would occur, for example, in strong-field QED due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La$_2$CuO$_4$ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and X-ray lasers, where polarization extinction ratios of 10$^{-12}$ can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.

physics.ins-det↗

arXiv2020

Frustrated double ionization of atoms in circularly polarized laser fields

We theoretically study frustrated double ionization (FDI) of atoms subjected to intense circularly polarized laser pulses using a three-dimensional classical model. We find a novel "knee" structure of FDI probability as a function of intensity, which is similar to the intensity dependence of nonsequential double ionization probability. The observation of FDI is more favourable when using targets with low ionization potentials and short driving laser wavelengths. This is attributed to the crucial role of recollision therein, which can be experimentally inferred from the photoelectron momentum distribution generated by FDI. This work provides novel physical insights into FDI dynamics with circular polarization.

physics.atom-ph↗

arXiv2020

Revealing charge anisotropies in metal compounds via high-purity x-ray polarimetry

Linear polarization analysis of hard x-rays is employed to probe electronic anisotropies in metal-containing complexes with very high selectivity. We use the pronounced linear dichroism of nuclear resonant x-ray scattering to determine electric field gradients in an iron(II) containing compound as they evolve during a temperature-dependent high-spin/low-spin phase transition. This method constitutes a novel approach to analyze changes in the electronic structure of metal-containing molecules as function of external parameters or stimuli. The polarization selectivity of the technique allows us to monitor defect concentrations of electronic valence states across phase transitions. This opens new avenues to trace electronic changes and their precursors that are connected to structural and electronic dynamics in the class of metal compounds ranging from simple molecular solids to biological molecules.

cond-mat.mtrl-sci↗

arXiv2020

Strong field frustrated double ionization of argon atoms

Using a three-dimensional semiclassical method, we theoretically investigate frustrated double ionization (FDI) of Ar atoms subjected to strong laser fields. The double-hump photoelectron momentum distribution generated from FDI observed in a recent experiment [S. Larimian et al., Phys. Rev. Research 2, 013021 (2020)] is reproduced by our simulation. We confirm that the observed spectrum is due to recollision. The laser intensity dependence of FDI is investigated. We reveal that the doubly excited states of Ar atoms and excited states of Ar+ are the dominant pathways for producing FDI at relatively low and high intensities, respectively. Our work demonstrates that at modest intensities, FDI is a general strong-field physical process accompanied with nonsequential double ionization and it is an important consequence of recollision.

physics.atom-ph↗

arXiv2019

Experimental Separation of Sub-Cycle Ionization Bursts in Strong-Field Double Ionization of H$_2$

We report on the unambiguous observation of the sub-cycle ionization bursts in sequential strong-field double ionization of H$_2$ and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase in combination with multi-particle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state specific Coulomb-distortions on emitted electrons in polyatomic molecules.

physics.chem-ph↗

arXiv2018

Strong-Field Polarizability-Enhanced Dissociative Ionization

We investigate dissociative single and double ionization of HeH+ induced by intense femtosecond laser pulses. By employing a semi-classical model with nuclear trajectories moving on field-dressed surfaces and ionization events treated as stochastical jumps, we identify a strong-field mechanism wherein the molecules dynamically align along the laser polarization axis and stretch towards a critical internuclear distance before getting dissociative ionized. As the tunnel-ionization rate is greater for larger internuclear distance and for aligned samples, ionization is enhanced. The strong dynamical rotation is traced back to a maximum in the parallel component of the internuclear-distance-dependent polarizability tensor. Qualitative agreement with our experimental observations is found. Finally the criteria for observing the isotope effect for the ion angular distribution is discussed.

physics.atom-ph↗