Far-infrared optical and dielectric response of ZnS measured by terahertz time-domain spectroscopy

The optoelectronic technique of terahertz time-domain spectroscopy (THz-TDS) has been applied to measure the frequency-dependent optical and dielectric properties of ZnS in the frequency range extending from 0.3 to 3.5 THz. THz-TDS clearly reveals the low-frequency phonon resonance features in both the single- and polycrystalline ZnS. These phonons account for the increased absorption as indicated by the resonance lines in the spectra. The measured index of refraction is found to be dominated by the transverse optical-phonon resonance, which is verified by a theoretical fit using the relation for the dielectric response of the damped harmonic oscillator.