Spectroscopic analysis of photochromic films

Abstract

Sol-gel processing was used to dope photochromic dyes into three matrices: metal alkoxide, metal alkoxide-polymer, and pure polymeric materials. The films on silicon and quartz substrates were examined, with and without ultraviolet (UV) irradiation, by ellipsometry, UV-visible transmission spectroscopy, and Fourier transform infrared (FTIR) transmission spectroscopy. Variable angle spectroscopic ellipsometry (VASE) data were analyzed for film thickness and optical constants, and in some cases thickness nonuniformity. A modified Cauchy and a combined Cauchy–Lorentz model were used to fit the unirradiated and irradiated film optical constants, respectively. The optical constants of the films showed significant changes upon irradiation. This means that the absorption coefficient and hence the emissivity of the films is being modulated with UV irradiation. The VASE-fitted thicknesses of the films were in the range of 1–6 μm. In the FTIR spectra, the spiropyran doped samples have shown IR transmission changes in the two spectral regions (6–7 and 7.5–8.5 μm) where changes are expected due to bond opening. The transmission ratio for UV irradiated and unirradiated samples decreased by as much as about 24% at a particular IR wavelength.