Influence of local inhomogeneity of refractive index on laser damage

Resonant effect connected with nonlinear light propagation through dielectric with small non-absorbing microinclusion is considered. It can play important role in initiating of laser-induced damage of transparent materials through local increasing of laser intensity inside and near the microinclusion. The effect is developing of field instability in nonabsorbing microinclusions resulting from excitation of resonant mode through inducing of variations of refractive index. It is shown that transparent microinclusion can initiate about 10 times local increase of laser intensity accompanied by positive feedback. If the electric field strength exceeds damage threshold during nonlinear evolution in the inclusion then developing of field instability results in damage of micro-inclusion before electric field is stabilized at certain upper level determined by ionization processes. There estimated threshold of field instability is about several MV/cm. Its dependence on material and radiation parameters is studied.