  Observing ultrafast structural changes in nanoscale systems is essential for
understanding the dynamics of intense light-matter interactions, which play a
pivotal role in material processing, ultrafast phase transitions and diagnosis
of matter under extreme conditions. One of the most relevant applications of
femtosecond laser plasma interactions is laser machining and surface
structuring. For laser intensities on the order of $10^{14} \, \rm W/cm^2$,
collisional plasmas are generated at the surface and subsequent transport
processes such as electron-ion thermalization, melting and resolidification
occur at picosecond and nanosecond time scales. Experimental techniques with
nanometer spatial and femtosecond temporal resolution are required to test
physical models and obtain quantitative measurements of the subsurface plasma
dynamics. Here, we demonstrate grazing-incidence small-angle x-ray scattering
(GISAXS) using x-ray free electron laser (XFEL) pulses allowing the in situ
visualization of subsurface plasma dynamics with nanometer depth resolution. We
measure the surface ablation and plasma density perturbation of multilayer
samples following the femtosecond laser pulse interaction. This new methodology
opens new possibilities for accurate characterization of subsurface dynamics in
various applications of high-intensity laser-solid interactions including laser
ablation, creation of warm dense matter, dynamic compression, and relativistic
laser plasmas.
