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Abstract

We report the infrared dielectric properties of α-quartz in the temperature range from 1.5K to 200K. Using an infrared free-electron laser, far-infrared reflectivity spectra of a single crystal y cut were acquired along both principal axes, under two different incidence angles, in S and P polarization. These experimental data have been fitted globally for each temperature with a multioscillator model, allowing one to extract frequencies and damping rates of the ordinary and extraordinary, transverse and longitudinal optic phonon modes, and hence the temperature-dependent dispersion of the infrared dielectric function. The results are in line with previous high-temperature studies, allowing for a parametrized description of all temperature-dependent phonon parameters and the resulting dielectric function from 1.5K up to the α−β-phase transition temperature, T_C=846K. Using these data, we predict remarkably high quality factors for polaritons in α-quartz's hyperbolic spectral region at low temperatures.