Laser heating of solid matter by light pressure-driven shocks at ultra-relativistic intensities

Heating by irradiation of a solid surface in vacuum with 5x10$^{20}$ Wcm$^{-2}$, 0.8 ps, 1.05 micron wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo and V. A surface layer is heated to $\sim $5 keV with an axial temperature gradient of 0.6 $\mu $m scale length. Images of Ni Ly$\alpha $ show the hot region has a $\sim $25 $\mu $m diameter. These data are consistent with collisional particle-in-cell simulations using pre-formed plasma density profiles from hydrodynamic modeling, which show that the more than 100 \textit{Gbar }light pressure compresses the preformed plasma and drives a shock into the solid heating a thin layer.