Frequency dependent optical and dielectric properties of zinc sulfide in Terahertz regime

•Comparison of various grades of ZnS samples using terahertz time-domain and millimeter wave spectroscopy systems.•Good agreement of vibrational modes exhibited by ZnS samples at 0.78THz and 2.20THz with literature.•Comparison of frequency dependent dielectric properties of the ZnS samples using both techniques.•Comparison of frequency dependent refractive index of the ZnS samples using undamped harmonic oscillator model. Frequency dependent optical and dielectric properties for several grades of chemical vapor deposited (CVD) zinc sulfide (standard, elemental, and multi-spectral) was performed using a terahertz time-domain spectroscopy (THz-TDS) system in the frequency range from 0.15THz to 2.5THz. Zinc sulfide exhibits low frequency vibrational modes characterized by the THz-TDS. Two low-frequency phonon resonance lines were revealed at 0.78THz and 2.20THz. These samples were also characterized in the GHz range using a backward wave oscillator (BWO) source quasi-optical spectrometer, and the data obtained by both approaches were compared. Experimental data were also compared with an undamped harmonic oscillator model. These results compare well with the literature values obtained using other methods.