X-ray absorption analysis of KDP optics

Abstract

Potassium dihydrogen phosphate (KDP) is a non-linear optical material used for laser frequency conversion and optical switches. Unfortunately, when KDP crystals are coated with a porous silica anti-reflection coating (Thomas, Appl. Opt. 1986;25(9):1481) and then exposed to ambient humidity, they develop dissolution pits (Wheeler et al., SPIE 1999;3902:451–459; Wheeler et al., Mater. Res. Soc. Conf., San Francisco, 2000). Previous investigations (Wheeler et al., SPIE 1999;3902:451–459) have shown that thermal annealing renders KDP optics less susceptible to pitting, suggesting that a modification of surface chemistry has occurred. X-ray absorption was used to characterize changes in the composition and structure of KDP optics as a function of process parameters. KDP native crystals were also analyzed to provide a standard basis for interpretation. Surface sensitive total electron yield and bulk sensitive fluorescence yield from the K2p, P2p (L2,3-edge) and O1s (K-edge) absorption edges were measured at each process step. Results indicate that annealing at 160°C dehydrates the surface of KDP resulting in a metaphosphate surface composition with K:P:O=1:1:3.