Terahertz fingerprinting in presence of quasi-ballistic scattering

characteristic molecular vibrational and rotational modes in the terahertz spectral range (0.3‐3THz), enabling them to absorb terahertz (THz) radiation at specific frequencies. These absorption features are unique to every material that absorbs in the THz spectral range and therefore may be used as spectral fingerprints for their classification and identification. 5 However, variations in refractive index within the sample, caused by sample granularity and impurities, cause the THz radiation to scatter, which can significantly alter or obscure the spectral fingerprints of the material under study. Usually in the case of the solids, the material of interest is quite dense and causes multiple scattering of THz radiation within the sample. The response of a dense medium, as a consequence of multiple scattering, can be classified into three regimes: ballistic, quasi-ballistic, and diffusive transport. 6 While the THz time domain spectroscopy (TDS) technique is sensitive to both quasi-ballistic and diffusive scattering, the criteria to determine which scattering regime is dominant, depends on the scattering (ksc) and transport mean free path lengths (ktr) in the medium, given by, ksc ¼ c=2nix; ktr ¼ ksc=ð1 �h cosðhÞi;