Surface charging in laser damage to dielectric surfaces and thin films.

We investigated the relationship between laser damage and surface charging of the damaged area on dielectric thin film coatings of HfO(2) and M(g)F(2) and on bare polished surfaces of fused silica and M(g)F(2). A noncontacting Kelvin probe was developed which was sensitive to surface charge and surface potential changes and had a patial resolution of 1 mm. With this probe, surface charge density changes due to laser irradiation could be mapped over relatively large areas with great accuracy in a nonintrusive manner. All the samples tested showed the accumulation of negative charge in areas more than 10 times larger in diameter than the laser beam spot or damage area. This initial charge density was observed to decay on a time scale of 1 h to a lower fixed value associated with permanent damage to the surface.