Pulsed laser damage resistance of nanostructured high reflectors for 355nm

Abstract

An investigation into the potential for increased laser damage resistance was made for a random distribution and regular array of nanometer scale surface relief structures integrated with dielectric thin-films to create 355nm wavelength selective high reflectors (HR). First, Random Anti-Reflection (RAR) nanostructures were fabricated in a thick silica cap layer deposited on top of a conventional 30-layer HR stack designed as 45 degree turning mirrors. Surface absorption scans of these RAR nano-texture enhanced HR stacks showed a slight decrease in absorption with no impact on performance, however standardized pulsed laser damage threshold testing found no improvement in damage resistance over non-textured silica-cap HR stacks. Second, polarization and wavelength selective nanostructure resonant (NSR) array filters designed to be embedded within thick high damage resistance silica layers were modeled using rigorous coupled wave analysis. Prototypes were fabricated of one NSR design consisting of a low-aspect ratio grating defined in a fused silica substrate with a single thin layer of hafnium oxide over-coat. The performance of NSR prototypes was limited due to multiple fabrication and testing issues. Initial 355nm wavelength, 5ns pulse, s-on-1 laser damage testing yielded a damage threshold in the 3 to 4 J/cm2 range, comparable to that obtained for the multi-layer HR stacks. Despite these modest early results, it appears that with further fabrication improvements, nano-structure array resonators embedded within silica layers could yield significant increases in the laser damage resistance of HR optics.