Determination of infrared refractive index of ZnS and YbF3 thin films by spectroscopy

The refractive index of the infrared thin films was calculated by a relatively simple and accurate spectroscopy method. Using the Sellmeier dispersion model, the refractive index and thickness of the Zinc sulfide(ZnS) thin film were obtained by fitting the transmittance in the range of 2.5 μm-11 μm. At the same time, the refractive index and thickness of the ZnS thin films were also measured by VASE ellipsometer. The results show that the refractive index deviation between the values fitted by the transmittance and that measured by the VASE ellipsometer is<0.02, and the relative deviation of the thickness is <1%.The YbF3/ZnS bilayer coatings were deposited on the CVD Zinc selenide(ZnSe) substrate to obtain the refractive index of the ytterbium fluoride(YbF3) thin films wrapped in the coatings. Using the Sellmeier dispersion model, the refractive index of the YbF3 thin film wrapped in the coatings was obtained by fitting the transmittance of the YbF3/ZnS bilayer coatings in the range of 2.5 μm-11 μm. The results show that there are significant differences in the refractive index of the YbF3 thin films wrapped in coatings and that exposed to the atmosphere.The refractive index of the YbF3 thin film exposed to the atmosphere is abrupt because the YbF3 thin films adsorbs water vapor, while that of the YbF3 thin film wrapped in the coatings is no mutation.