Refractive index changes in photochromic SbPO4–WO3 glass by exposure to band-gap radiation

We report photoinduced photo-darkening in SbPO4–WO3 glass by exposure to 532 nm light with a power density of 143 mW/cm2. The time of exposure was varied between 0 and 256 min following which visible photo-darkening, peaking at 850 nm was observed. Spectrophotometer measurement of absorption was performed for both treated and untreated regions of the sample. Time exposure to below band-gap light results in a single exponent Gaussian absorption function over an exceptionally wide range of wavelengths (500 nm–1600 nm), with a 1/e width of 647.5 nm. Kramers–Kronig transform of the change in the absorption indicates a negative local change in the refractive index. The dispersed refractive index change at 1550 nm, ∆n, is calculated to be ∼−5 × 10− 8. The peak absorption increases with time of exposure and the photo-darkening remains irreversible at room temperature.