Large refractive-index change in PbI_2 films by photolysis at 150–180 °C*

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Abstract
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.

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