Search

Particle-in-cell (PIC) simulations are a superior tool to model kinetics-dominated plasmas in relativistic and ultrarelativistic laser-solid interactions (dimensionless vectorpotential $a_0 > 1$). The transition from relativistic to subrelativistic laser intensities ($a_0 \lesssim 1$), where correlated and collisional plasma physics become relevant, is reaching the limits of available modeling capabilities. This calls for theoretical and experimental benchmarks and the establishment of standardized testbeds. In this work, we develop such a suitable testbed to experimentally benchmark PIC simulations using a laser-irradiated micron-sized cryogenic hydrogen-jet target. Time-resolved optical shadowgraphy of the expanding plasma density, complemented by hydrodynamics and ray-tracing simulations, is used to determine the bulk-electron temperature evolution after laser irradiation. As a showcase, a study of isochoric heating of solid hydrogen induced by laser pulses with a dimensionless vectorpotential of $a_0 \approx 1$ is presented. The comparison of the bulk-electron temperature of the experiment with systematic scans of PIC simulations demostrates that, due to an interplay of vacuum heating and resonance heating of electrons, the initial surface-density gradient of the target is decisive to reach quantitative agreement at \SI{1}{\ps} after the interaction. The showcase demostrates the readiness of the testbed for controlled parameter scans at all laser intensities of $a_0 \lesssim 1$.

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.

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.

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