Temperature-dependent optical properties of monocrystalline CaF ₂, BaF ₂, and MgF ₂

Abstract

CaF ₂, BaF ₂, and MgF ₂ are low-index, infrared-transparent materials that are extensively used in optical systems. Despite their technological importance, a systematic investigation into the temperature dependence of their optical properties is lacking. In this study, spectroscopic ellipsometry was used to obtain the refractive index of monocrystalline CaF ₂, BaF ₂, and MgF ₂ for wavelengths between 220 nm and 1700 nm, and for temperatures between 21 °C and 368 °C. The raw ellipsometric data was fit to a Sellmeier model with temperature-dependent oscillator terms to extract the real part of the refractive index of each material. The refractive index of CaF ₂ and BaF ₂ was observed to decrease linearly with increasing temperature, which can be largely attributed to a reduction in the mass density due to thermal expansion. In contrast, the refractive index of MgF ₂ was found to vary nonlinearly with temperature, which suggests competing effects from the material’s electronic polarizability. The temperature-dependent refractive index data reported here provide a finely-resolved mapping of the thermo-optic coefficient for these three materials, which could inform the development of optical devices operating at elevated or unsteady temperatures.