Investigation of photoionization processes in ultrashort laser induced damage in optical materials

Theoretical investigation is presented on femtosecond laser induced damage in optically transparent materials, of different energy band gaps, by a single shot of different pulse durations (<10 ps). A computer simulation model is developed based on the Fokker-Planck approximation of the Boltzmann equation and allows for electron production via electron impact ionization, tunnelling ionization and multiphoton ionization processes. The calculated damage thresholds, of SiO2, Ta2O5 and Si at similar to800 nm laser wavelength, are in reasonable quantitative agreement with the examined experimental measurements. The relative importance of each ionization process is also examined.