Scattering Effects in Terahertz Wave Spectroscopy of Granulated Solids

Applications of terahertz spectroscopy methods to granulated solids are substantially constrained by the effects of scattering of the incident terahertz waves by spatial heterogeneities of a sample. The experimental results obtained by means of terahertz time-domain spectroscopy are reported for the samples of granulated 2, 4-dinitrotoluene and alpha-lactose without any special filling. We show that in samples of the same substance, but prepared in a different manner, the overall increase of the background extinction with frequency and appearance of additional extinction maxima can obscure the true absorption features of the material. In samples with large grain size, of order of 400 μm, a complicated irregular spectral structure is observed instead of a fingerprint maxima at 1.37 THz for alpha-lactose and at 1.08 THz for 2,4-dinitrotoluene. At higher frequencies no true spectral fingerprints are clearly seen in the raw experimental spectra of all the samples with the grain sizes of 130 μm and more. The influence of grain size, air-filling factor and the dispersion properties of the materials is studied. Theoretical analysis is made with the use of Mie theory in simplifying approximations and taking into account interference between the waves transmitted through a porous sample. It has been shown that the scattering-induced masking effects can be properly simulated even for the air-filled objects with initially unknown spatial and dispersion parameters. The procedure of eliminating the influence of scattering effects from the raw spectral data is proposed. Detection of true high-frequency spectral fingerprints is demonstrated using this procedure for various grain sizes up to 400 μm.