Dielectric properties of crystalline BiOCl in the terahertz region

We perform terahertz (THz) transmission spectroscopy to investigate the refractive index of crystalline BiOCl, which appears as a dielectric by-product during a certain process of fabricating superconducting Bi-based THz oscillators. We find that the refractive index is 4.8 or more for THz polarization parallel to the (001) plane; this value is substantially larger than the optical refractive index for parallel light polarization and also than the square root of the static dielectric constant for perpendicular electric field. Our experimental results can be interpreted from a strong dielectric anisotropy of crystalline BiOCl at low frequencies as predicted previously by a few theoretical studies.