Surface Roughness Arising from Ultrafast Laser Techniques Performed on Glass for Fabricating X-Ray Optics

Abstract

X-ray astronomy uses grazing incidence optics that require sub-nanometer surface roughness and nanometer-level surface figure accuracy before and after fabrication. To maintain these precise surface characteristics, flexures can be incorporated into the mirror geometry, allowing controlled deformation of the reflecting region. Achieving the complex geometry of flexures on glass substrates is done with ultrafast laser assisted chemical etching. This thesis tests how chemical etching affects the surface roughness of glass mirror substrates (fused silica and Corning ULE) in hopes that chemical etching does not degrade the surface further than the initial surface roughness. It was determined that chemical etching did not significantly degrade the surface roughness of the glass substrate. The structure of an x-ray telescope system requires high reflectance on the mirror side, while having low reflectance on the back surface. To measure the mirror surface through the back surface, an anti-reflection (AR) coating can be helpful to provide low reflectance on the back surface. Laser-induced modifications on the surface of a transparent substrate, dubbed nanopillars, are a potential solution to create a long-term stable AR coating. Due to nanopillars on the substrate surface being hundreds of nanometers tall, they could be a viable solution to provide low reflectance for the x-ray band and visible light. This thesis evaluates the formation of nanopillars on fused silica and ULE glass to find optimal parameters for creating an AR coating. Using low pulse energy and multiple passes over the same laser-modified area achieved the desired nanopillar structures about 300 nm tall.