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Ultra-intense lasers that ionize and accelerate electrons in solids to near the speed of light can lead to kinetic instabilities that alter the laser absorption and subsequent electron transport, isochoric heating, and ion acceleration. These instabilities can be difficult to characterize, but a novel approach using X-ray scattering at keV energies allows for their visualization with femtosecond temporal resolution on the few nanometer mesoscale. Our experiments on laser-driven flat silicon membranes show the development of structure with a dominant scale of $~60\unit{nm}$ in the plane of the laser axis and laser polarization, and $~95\unit{nm}$ in the vertical direction with a growth rate faster than $0.1/\mathrm{fs}$. Combining the XFEL experiments with simulations provides a complete picture of the structural evolution of ultra-fast laser-induced instability development, indicating the excitation of surface plasmons and the growth of a new type of filamentation instability. These findings provide new insight into the ultra-fast instability processes in solids under extreme conditions at the nanometer level with important implications for inertial confinement fusion and laboratory astrophysics.

Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) is a modern imaging technique used in material research to study nanoscale materials. Reconstruction of the parameters of an imaged object imposes an ill-posed inverse problem that is further complicated when only an in-plane GISAXS signal is available. Traditionally used inference algorithms such as Approximate Bayesian Computation (ABC) rely on computationally expensive scattering simulation software, rendering analysis highly time-consuming. We propose a simulation-based framework that combines variational auto-encoders and normalizing flows to estimate the posterior distribution of object parameters given its GISAXS data. We apply the inference pipeline to experimental data and demonstrate that our method reduces the inference cost by orders of magnitude while producing consistent results with ABC.

Observing ultrafast laser-induced structural changes in nanoscale systems is essential for understanding the dynamics of intense light-matter interactions. For laser intensities on the order of 1014W/cm2, highly collisional plasmas are generated at and below the surface. Subsequent transport processes such as heat conduction, electron-ion thermalization, surface ablation, and resolidification occur at picosecond and nanosecond timescales. Imaging methods, e.g., using x-ray free-electron lasers (XFEL), were hitherto unable to measure the depth-resolved subsurface dynamics of laser-solid interactions with appropriate temporal and spatial resolution. Here we demonstrate picosecond grazing-incidence small-angle x-ray scattering (GISAXS) from laser-produced plasmas using XFEL pulses. Using multilayer (ML) samples, both the surface ablation and subsurface density dynamics are measured with nanometer depth resolution. Our experimental data challenges the state-of-the-art modeling of matter under extreme conditions and opens new perspectives for laser material processing and high-energy density science.

Employing X-ray photon correlation spectroscopy we measure the kinetics and dynamics of a pressure-induced liquid-liquid phase separation (LLPS) in a water-lysozyme solution. Scattering invariants and kinetic information provide evidence that the system reaches the phase boundary upon pressure-induced LLPS with no sign of arrest. The coarsening slows down with increasing quench depths. The $g_2$-functions display a two-step decay with a gradually increasing non-ergodicity parameter typical for gelation. We observe fast superdiffusive ($\gamma \geq 3/2$) and slow subdiffusive ($\gamma < 0.6$) motion associated with fast viscoelastic fluctuations of the network and a slow viscous coarsening process, respectively. The dynamics age linear with time $\tau \propto t_\mathrm{w}$ and we observe the onset of viscoelastic relaxation for deeper quenches. Our results suggest that the protein solution gels upon reaching the phase boundary.

Die Anregung von Festkörpern durch ultraschnelle, intensive Laser erzeugt hochdichte Plasmen, die für die Astrophysik, die Fusion unter Trägheitseinschluss, verschiedene praktische Anwendungen einschließlich Laserbearbeitung, die Realisierung kompakter Quellen für kohärente XUV- bis Röntgenstrahlung und für helle Teilchen einschließlich Ionen relevant sind. Es liegt auf der Hand, dass ein umfassendes Verständnis der Laserkopplung und des anschließenden Energietransports in dichte Plasmen von zentraler Bedeutung ist. Bislang hinderte uns der derzeitige Mangel an geeigneten Methoden zur Beobachtung der komplexen Dichtedynamik mit ausreichender räumlicher Auflösung daran, die zugrundeliegende Physik quantitativ zu verstehen und bestehende Modelle zu vergleichen. Diese Arbeit zeigt zum ersten Mal die Durchführbarkeit von Röntgendiffusionsstreuexperimenten an laserangeregter kondensierter Materie unter streifendem Einzelimpuls-Einfall mit einem XFEL-Puls von 7 fs Pulsdauer. Die Experimente zeigen die ultraschnelle Entwicklung der Elektronendichte in der Nähe der Oberfläche. Wir leiten die Dynamik der Dichte- und Oberflächenprofile auf ultraschnellen Zeitskalen ab. Die gewonnene Dichteinformation lässt auf die elektronische Wärmeleitgeschwindigkeit und die Dynamik des Oberflächenabtrags durch elektrostatische Ablation oder Elektron-Ionen-Kollisionen schließen. Da die Dynamik der Vielschichtprobe durch die Teilcheneindringung und den Druckausgleich zwischen benachbarten Schichten verursacht wird, wird unser neues Werkzeug eine zentrale Rolle beim Benchmarking verschiedener Modelle einschließlich Teilchenkollisionen und der Zustandsgleichung spielen. Es ist zu erwarten, dass die Ergebnisse für verschiedene Anwendungen nützlich sein werden, die auf der transienten Dynamik von Zuständen hoher Dichte beruhen: Materialbearbeitung, isochore Erwärmung, dynamische Laserkompression und relativistische Laser-Materie-Wechselwirkung. Unsere neue Technik wird eine wichtige Rolle beim lang erwarteten quantitativen Benchmark verschiedener Modelle spielen, die unter dem Mangel an präzisen experimentellen Daten gelitten haben. Zusätzlich liefert die Röntgenstreuung unter streifendem Einfall auch Informationen über laterale und vertikale Korrelation und Rauheitseigenschaften an der Oberfläche bei Laseranregung. Mit dieser Änderung der Oberflächenrauheit und der Welligkeit bei Laseranregung können die Dynamik der Instabilität des Oberflächenplasmas und die räumliche Homogenität der Schockwelle mit einer Auflösung von sub-μm untersucht werden. Die Zusammenführung von Röntgenstreuungstechniken und der Wissenschaft der Hochenergiedichte eröffnet ein völlig neues Feld für das Verständnis der grundlegenden Prozesse bei der Laser-Plasma-Wechselwirkung., The excitation of solids by ultrafast, intense lasers creates high-density plasmas relevant for astrophysics, inertial confinement fusion, various practical applications including laser processing, realization of compact sources of coherent XUV to X-ray radiation and bright particles including ions. Obviously, a comprehensive understanding of laser coupling and subsequent energy transport into dense-plasma is of key importance. Up to now, the current lack of appropriate methods observing complex density dynamics with sufficient spatial resolution prevented us from a quantitative understanding of the underlying physics and benchmarking existing models. This thesis demonstrates for the first time the feasibility of single pulse grazing incidence X-ray diffuse scattering experiments from laser excited condensed matter employing single XFEL pulse of 7 fs pulse duration. The experiments, performed at the Japanese FEL facility SACLA, reveal the ultrafast evolution of the electron density in the vicinity of the surface. We deduce the dynamics of the density and surface profiles on ultrafast time scales. The retrieved density information infers the electronic heat conduction velocity and surface ablation dynamics caused by electrostatic ablation or electron-ion collisions. As the dynamics of the multilayer sample is caused by particle penetration and pressure balance between adjacent layers, our new tool will play a central role in benchmarking various models including particle collisions and the equation of states. The results can be anticipated to be useful for various applications that rely on transient dynamics of high-density states: material processing, isochoric heating, laser dynamic compression and relativistic laser matter interaction. Our new technique will play an important role towards long-awaited quantitative benchmark of various models that have been suffered from lack of precise experimental data. Additionally, grazing-incidence X-ray scattering provides also information on lateral and vertical correlation and roughness properties at the surface upon laser excitation. With this surface roughness and ripples changing upon laser excitation, surface plasma instability dynamics and spatial homogeneity of shock wave with sub-μm resolution can be investigated. Bringing together X-ray scattering techniques and high energy density science opens a complete new field in understanding the fundamental processes in laser-plasma interaction.

Extreme states of matter exist throughout the universe e.g. inside planetary cores, stars or astrophysical jets. Such conditions are generated in the laboratory in the interaction of powerful lasers with solids, and their evolution can be probed with femtosecond precision using ultra-short X-ray pulses to study laboratory astrophysics, laser-fusion research or compact particle acceleration. X-ray scattering (SAXS) patterns and their asymmetries occurring at X-ray energies of atomic bound-bound transitions contain information on the volumetric nanoscopic distribution of density, ionization and temperature. Buried heavy ion structures in high intensity laser irradiated solids expand on the nanometer scale following heat diffusion, and are heated to more than 2 million Kelvin. These experiments demonstrate resonant SAXS with the aim to better characterize dynamic processes in extreme laboratory plasmas.

Observing ultrafast laser-induced structural changes in nanoscale systems is essential for understanding the dynamics of intense light-matter interactions. For laser intensities on the order of $10^{14} \, \rm W/cm^2$, highly-collisional plasmas are generated at and below the surface. Subsequent transport processes such as heat conduction, electron-ion thermalization, surface ablation and resolidification occur at picosecond and nanosecond time scales. Imaging methods, e.g. using x-ray free-electron lasers (XFEL), were hitherto unable to measure the depth-resolved subsurface dynamics of laser-solid interactions with appropriate temporal and spatial resolution. Here we demonstrate picosecond grazing-incidence small-angle x-ray scattering (GISAXS) from laser-produced plasmas using XFEL pulses. Using multi-layer (ML) samples, both the surface ablation and subsurface density dynamics are measured with nanometer depth resolution. Our experimental data challenges the state-of-the-art modeling of matter under extreme conditions and opens new perspectives for laser material processing and high-energy-density science., Comment: 16 pages, 4 figures. This is the version of the article before peer review, as submitted by authors. There is a Supplementary Information file in the Ancillary files directory