Temperature-Dependent Dielectric Function of Calcium Fluoride
The optical properties of calcium fluoride (fluorspar, CaF2) are mainly determined by a strongly temperature-dependent giant infrared (IR) peak, and a series of nearly temperature-independent ultraviolet (UV) peaks. We find that the temperature dependence of the IR peak can be modeled, to good accuracy, by a radiation-reaction improved coupled-oscillator model (RRCO model), with temperature-dependent parameters. For the UV peaks, we find a convenient functional form which covers both the real as well as the imaginary parts of the dielectric function and provide a comparison to first-principles calculations based on time-dependent density-functional theory (TDDFT). The result is a compact functional form for the dielectric function of undoped CaF2 applicable to wide frequency and temperature ranges (0 < hbar omega < 60 eV, 22C < T < 500C). With the help of the temperature-dependent dielectric function, we obtain temperature-dependent values of the short-range and long-range asymptotics of atom-surface interactions with CaF2, for hydrogen, as well as ground-state and metastable helium. The giant IR absorption peak of CaF2 is shown to lead to a delayed onset of the fully retarded Casimir--Polder limit in the long-range interaction regime. We present arguments supporting a more general applicability of the RRCO model to materials of general interest.