Laser pulse shaping due to self-induced relativistic transparency in laser -- nanofoil interactions

Shapes of the laser pulses transmitted through thin nanofoils due to self-induced relativistic transparency in intense, high contrast laser (80J, $\sim $600 fs, $>$10$^{20}$ W/cm$^{2}$ and 10$^{-10}$ contrast) -- nanofoil interactions were measured using a single shot second harmonic frequency resolved optical gating (FROG) system [Palaniyappan, et al., RSI, 81, 1 (2010)]. The FROG measurements show asymmetric pulse shapes, pulse shortening up to a factor of 2 and faster rise times on the leading edge than the falling edge. Part of the incident pulse is reflected/absorbed by the over dense plasma until it becomes relativistically under dense and transparent to the rest of the pulse. The measurements are qualitatively in agreement with a 1-D PIC simulation. Transmitted laser pulse shapes through 3 nm thick foil show large variations due to early target expansion.