Role of many-photon and impact ionization in the breakdown of dielectrics by picosecond laser pulses

An analysis is made of the relative role of the many-photon and impact (avalanche) ionization mechanisms in the breakdown of transparent dielectrics by picosecond laser pulses. It is shown that both mechanisms have to be allowed for in the calculation of the breakdown thresholds. The dependence of the threshold field on the pulse duration is calculated for the nanosecond and picosecond ranges and it is shown to agree with the published experimental data.