On the mechanism of the absorption of femtosecond laser pulses in the melting and ablation of Si and GaAs

The melting and ablation thresholds have been measured for Si and GaAs irradiated by 1240-nm femtosecond pulses of a chromium-forsterite laser, i.e., when the photon energy is lower than the bandgap width. A small difference of these quantities from the respective melting and ablation thresholds measured for the action of the 620-nm second-harmonic pulses with a photon energy higher than the bandgap width cannot be explained using available theoretical models. A new approach to the mechanism of the appearance of the electron-hole plasma and the formation of a thin, strongly absorbing surface layer in semiconductors irradiated by femtosecond laser pulses of the visible and infrared spectral ranges has been proposed on the basis of the avalanche mechanism of the filling of the conduction band.