Phenomenological theory of laser damage in insulators

A phenomenological theory of laser damage is presented. The physical mechanism for laser damage in insulators is presumed to be similar to the one for breakdown in semiconductors. A few starter electrons in the conduction band are excited by the external field to the point where an electron avalanche occurs. Heat is generated by electron relaxation by phonon emission, and damage occurs when the temperature of the irradiated volume reaches the melting point. The phenomenological theory contains two parameters: an average cross section for excited-state absorption, which for NaCl is 1.8×{}10^-17 cm² and an average relaxation rate by phonon emission, which is about 10¹⁴ sec^-1 for a 200-cm^-1 phonon in NaCl. These values overcome problems with previous models, which require very large electron relaxation rates (> 10¹⁵ sec^-1). The parameters of the theory are determined by fitting the results of calculations to experiment. With this theory, subthreshold properties such as the hot-electron distribution, rate of electron avalanche, and rate of heat generation can also be calculated. Ways of verifying the theory are discussed in detail.