Mechanism of damage of the surface of a transparent dielectric during illumination with short light pulses

New experimental data were obtained on the laser damage of the surfaces of transparent dielectrics. It was found that illumination with high-power light pulses of 10^–8–10^–7 sec duration caused surfaces to emit primarily positive ions. The dependences of the photo-emission current on the intensity of the light flux were obtained for sapphire and K8 glass with surfaces treated in different ways. The spark appearance threshold was independent of the initial temperature of the surface and was a nonmonotonic function of the wavelength of the incident light. The optical strength of the surface of carefully polished calcite was found to be strongly anisotropic. The spark threshold at the glass surfaces depended weakly on the chemical composition, with the exception of flint glass. All the results obtained and some earlier observations can be explained satisfactorily by the generation of free electrons in the surface layer of a dielectric under the influence of a strong optical field and of heating of this layer as a result of its premelting "metallization," evaporation, and formation of a plasma above the illuminated area ("electron-thermal" hypothesis).