Terahertz field depolarization and absorption within composite media

Abstract

In this work, we pursue a deeper understanding of the expression of the inclusion morphology and index contrast in the refraction and absorption characteristics of composites within the terahertz (THz) spectrum. The composites are composed of SiO2 and Si nanoparticles as well as SiO2 and Si microparticles functioning as deeply subwavelength inclusions in a polydimethylsiloxane (PDMS) host. Terahertz time-domain spectroscopy is used for experimental characterization of the composites over a wide range of volumetric fractions, and the trends that emerge are contrasted to theoretical predictions from the Bruggeman model. It is found that the refraction characteristics have a heightened dependence on the inclusions' shape when their index contrast with respect to the host becomes sufficiently large. We attribute such a correlation to terahertz field depolarization that occurs within inclusions at high index contrasts and the dependence of the fields to the inclusions' shape—as defined by a depolarization factor in the generalized form of the Bruggeman model. Moreover, it is found that the absorption characteristics have a heightened dependence on the inclusions' size when that size becomes sufficiently small. We attribute this to the manifold of surface states that form in small inclusions, due to their high surface-to-volume ratio, which raises the absorption beyond that of the bulk. It is concluded that the Bruggeman model can accurately characterize the refraction and absorption of THz radiation within composites having deeply subwavelength inclusions if their (shape-dependent) polarization and (size-dependent) absorption are suitably defined.