Laser-induced damage in transparent dielectrics: The relationship between surface damage and surface plasmas

A number of experiments are described in which entrance and exit surface laser-induced damage on sapphire samples are studied. Damage thresholds are measured as a function of beam size and divergence at the surfaces. Temporal evolution of surface plasmas is studied using a streaking camera, and spatial differentiation of surface plasmas is accomplished by allowing the light to strike the surfaces away from normal incidence. It is found that the entrance surface has two plasma components, an air plasma that is directed along the light beam and an "explosion" plasma that is directed normal to the surface. The exit surface has only the explosion plasma. It is concluded that surface plasmas are a result rather than a cause of surface damage.