The refractive index of evaporated PbI2 films, in the red region of the visible spectrum, can be reduced up to 18% by exposure to 488 nm light at film temperatures of 150–180°C. Absorption in the same region remains very small throughout the process, until a temperature-dependent exposure threshold is reached, then rises rapidly due to scattering by photolysis-produced Pb agglomerates. For exposures below this threshold, localized band-gap states and/or bounded-plasma effects in the small Pb agglomerates give rise to an absorption band at 520 < λ < 580 nm, which causes the large refractive-index change. The photolysis in the polycrystalline films is uniform through the film rather than concentrated near the free surface, as in single crystals.
A. B. Buckman, N. H. Hong, and D. Wilson, "Large refractive-index change in PbI2 films by photolysis at 150–180°C," J. Opt. Soc. Am. 65, 914-918 (1975)