Optical constants of CaF₂ at 300 K from 0.03 to 6.5 eV

We describe the optical properties of CaF₂, an insulator with an ultrawide band gap of 12 eV and a large exciton binding energy of 1 eV. The range of transparency (125 meV-10 eV) makes CaF₂ a prime substrate for optical devices, e.g., tunable filters using phase change memory materials. The optical constants of CaF₂ were studied in the 1960s. With modern ellipsometry equipment, we revisited the optical constants of CaF₂ (100) and (111) substrates. CaF₂ has a Raman-active T_2g mode and an infrared-active T_2u mode, split into a TO doublet and a LO singlet. The T_2u mode can be seen with FTIR ellipsometry and described by a Lorentzian. The energies are TO=261 cm^-1 and LO=477 cm^-1, with an amplitude A=4.1, a broadening of 4 cm^-1, and a high-frequency dielectric constant of 1.98. A dip in the reststrahlen band is due to two-phonon absorption described by an anharmonically broadened Lorentzian. In the visible and near UV, normal dispersion can be described by a pole located at 7.48 eV and a Tauc-Lorentz oscillator at 20 eV. The imaginary part of the pseudodielectric function <ε₂> is negative above 3 eV, indicating a surface layer of 2-5 nm thickness with a larger refractive index than the bulk. We apply the CaF₂ optical constants to find the thickness of a SiO₂ layer on CaF₂.