Recovery of Polaritonic Stopband of Pressed Polycrystalline Calcium Fluoride Powder in Terahertz Frequency Region

By measuring and comparing the reflection spectra of single crystal and pressed polycrystalline powder of calcium fluoride (CaF₂) in the terahertz range, we have demonstrated recovery of polaritonic stopband in the powder. By using a simplified model to fit our data, we have deduced the longitudinal-optical and transverse-optical phonon frequencies and damping parameter for the polariton resonance. While the two phonon frequencies for the powder sample differ from those deduced for the single-crystal sample by less than 2%, the value for the deduced damping parameter is significantly increased. Due to small sizes of CaF₂ powder particles, we have attributed such a significant increase to damping of polaritonic waves at the particle surfaces.