Optical breakdown in alkali halides

A theory of optical breakdown in dielectrics is presented and applied to NaCl and NaBr for wavelengths $0.172\le \lambda \le 1.0642$ μm. It is based on multiphoton carrier generation and energy transfer from the radiation field to the lattice via polaron-photon absorption. This mechanism represents a clear alternative to avalanche breakdown models as no lattice impact ionization is involved. Breakdown field strengths calculated for various laser pulse lengths and frequencies, are fully corrected for the nonlinear refractive index $n_2$ as well as for contributions to the refractive index resulting from a sharp increase in polaron density during a damaging pulse. The results are in good agreement with experimental data obtained with nanosecond laser pulses.