Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

We present ultrafast pump-probe reflectivity and Doppler spectrometry of a silicon target at relativistic laser intensity. We observe an unexpected rise in reflectivity to a peak approximately $\sim$9 ps after the main pulse interaction with the target. This occurs after the reflectivity has fallen off from the initially high "plasma-mirror" phase. Simultaneously measured time-dependent Doppler shift data show an increase in blue shift at the same time. Numerical simulations show that the aforementioned trends in the experimental measurements correspond to a strong shock wave propagating back towards the laser. The relativistic laser-plasma interaction indirectly heats the cool-dense ($n_{e}\geq10^{23} cm^{-3}$ and $T_{e} \sim 10 eV$) target material adjacent to the corona, by hot electron induced return current heating, raising its temperature to around 150eV and causing it to explode violently. The increase in reflectivity is caused by the transient steepening of the plasma density gradient at the probe critical surface due to this explosive behaviour.