Energy Transfer during Silicon Irradiation by Femtosecond Laser Pulse

Measurements of the total energy reflected from crystalline and amorphous silicon illuminated by a femtosecond laser pulse are reported, for wavelengths of 0.62 and 0.31 μm. The results are interpreted by use of a highly nonlinear regime of light propagation in the dense plasma of electrons and holes ($e-h$). Densities higher than ${10}^{22}$ ${\mathrm{cm}}^{-3\mathrm{}}$ are reached. We demonstrate that free-carrier absorption is dominated by $e-h$ collisions.