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.