A simple model of micro-damage mechanism of SiO2 under short pulse laser irradiation

Based on the rate equation of electron generation, a simple model of evolution of conduction electron under short pulse laser irradiation is employed to predict the laser damage thresholds of SiO2. The rates of impact ionization and photoionization are calculated and compared with each other. The threshold energy densities are obtained as function of pulse duration from 0.1 ps to 10 ns. The results show that for long pulse, the damage is dominated by impact ionization, and the thresholds are in proportion to the square root of pulse duration. While in the cases of short pulse, the damage is also governed by impact ionization but with photoionization supplying the seed electrons, and with decreasing pulse duration, the threshold first rises，then drops．