Test of CaF₂ glass sub-surface damage using angle-resolved ellipsometry

Calcium fluoride (CaF₂), as a typical brittle optical material, has been applied in various fields due to its excellent optical properties. However, an existing challenge is how to non-destructively characterize the surface integrity of brittle materials, which may easily suffer from subsurface damage (SSD) during machining processes. This paper explores a novel snapshot surface-integrity test method based on single-exposure back-focal-plane (BFP) imaging of CaF₂ within ellipsometric parameter spectra corresponding to an extensive range of incident angles. The relationship between the layer thicknesses of surface roughness, SSD, and ellipsometric spectra was analyzed by establishing a four-layer-subsurface optical model combined with quasi-Brewster angle technology. Through polishing CaF₂ under different conditions and combining the measurement results of angle-resolved ellipsometry (ARE), it has been demonstrated that the study based on ARE is a successful non-destructive testing method for surface integrity, which could help to further elucidate the surface evolution mechanism of brittle material polishing processes.