Subsurface damage and polishing compound affect the 355-nm laser damage threshold of fused silica surfaces

Abstract

Subsurface damage, namely grinding and polishing fractures or scratches hidden by the polishing redeposition layer, may lower laser damage threshold by reducing fracture strength, providing sites for absorbing contaminants, and modulating the electromagnetic field. Polishing compound may affect 355-nm laser damage by absorption and scratching. Fused silica surfaces were finished with different grinding and polishing removal depths to leave different extents of subsurface damage from grinding. Three surfaces were polished with ceria and three with zirconia. The surfaces were 355-nm laser damage tested using an automated 'R- on-1' protocol, and also characterized for surface roughness, subsurface damage as revealed by etching, and total internal reflectance microscopy (TIRM). The zirconia polishing produced the highest laser thresholds, especially when the surface was free of polishing and grinding damage. The zirconia polishing was prone to damaging the surface, which significantly reduced laser thresholds at some sites. the ceria polishing left very little polishing damage, and showed uniform damage thresholds when there was no subsurface damage from grinding present. Residual Blanchard fractures lowered thresholds dramatically. Subsurface fractures from fine loose grinding sometimes reduced threshold dramatically and sometimes very little. TIRM images correlate fairly well with microphotographs of subsurface damage revealed by etching.