Experimental study of laser penetration in overdense plasmas at relativistic intensities. II: Explosion of thin foils by laser driven fast electrons

Propagation of a high-contrast frequency-doubled subpicosecond (300 fs) relativistic ${\mathrm{(I\lambda }}^2$ up to ${\text{5×10}}^{18}$ ${\mathrm{W\cdot cm}}^{\mathrm{-2}}{\mathrm{\cdot \mu m}}^{\mathrm{2}})$ laser pulse through thin and initially solid foils is studied. Transmission values up to 10% are measured through targets with initial near solid densities. The strong intensity threshold observed for the transmitted energy is correlated with clear modifications of the transmitted and reflected spectra, electron generation, and beam imaging. Two-dimensional Cartesian particle-in-cell (PIC) simulations that qualitatively reproduce the experimental results suggest specific rapid heating of the thin targets by fast electrons, plasma expansion, and density decrease to relativistically transmissive conditions during the pulse.