Effects of scattering on THz spectra of granular solids

Experimental studies of granular solids have shown that significant scattering effects restrict the accurate determination of material absorption in the terahertz (THz) region. The present work investigates the grain size dependent scattering contribution on the extinction spectra of Ammonium Nitrate, flour and salt between 0.2 to 1.2 THz using THz time-domain spectroscopy. The scattering contribution can be estimated by applying Mie theory for spherical grains. The approach essentially separates the independent contributions of true absorption and scattering losses and thus determines the total extinction for different grain sizes of various materials. The separation of the intrinsic material absorption from scattering losses shows that the frequency dependence in weakly absorbing materials is predominantly particle size dependent. Consequently, that range of THz frequencies cannot be used to differentiate granular solids having no intrinsic absorption.