  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.
