No abstract

A summary of recent investigations of surface damage of transparent dielectrics is presented. Damage threshold measurements made on Owens-Illinois ED-2 laser glass at normal incidence and at Brewster's angle are reported. For 30-nsec pulses at normal incidence, exit surface damage thresholds are typically 100 J/cm(2) for ED-2 glass. The observed ratio between entrance and exit damage thresholds for the two geometries can be explained by considering the electric field strengths at the surfaces and including interference between incident and reflected light waves. A similar analysis is applied to surface damage that occurs during total internal reflection. Finally the morphology of surface damage of ED-2 laser glass is described, and a model based upon reflections from the laser induced plasma is proposed to explain the observations.

It is shown that interference between incident and reflected light waves at the surfaces of a transparent dielectric material significantly influences surface damage thresholds. For normal incidence the light that is reflected at the entrance face of a surface is 180° out of phase with the incident light, and the resulting interference causes a sizable reduction in the electric field at the entrance face. The light reflected at the exit face suffers no phase shift, and as a result its interference with the incident light slightly increases the electric fields experienced at the exit surface. The analysis predicts a definite ratio for the damage threshold of the entrance and exit surfaces. This and one other prediction are experimentally investigated for Owens-Illinois ED-2 laser glass.

The morphological difference between entrance and exit surface damage is examined in terms of Fresnel reflections at the interface of the dielectric and the plasma associated with damage. It is suggested that the standing wave formed at the exit surface by the incident wave and the wave reflected from the plasma-dielectric interface is responsible for the characteristic pits of exit surface damage, while entrance damage characteristics are due to thermal shock from the hot plasma contiguous to the surface.