Backscattering effects for discrete random media: Theoretical results

The effect of enhanced backscattering of light from discrete random media, often referred to as the coherent photometric opposition effect (or weak photon localization), is a remarkable optical phenomenon that is being studied actively. When the incident light is unpolarized, the opposition intensity peak may be accompanied by the so-called opposition polarization effect, which manifests itself as a sharp asymmetric negative-polarization feature at small phase. angles. The optical phenomenon that causes these effects is the constructive interference of multiply scattered waves propagating along the same light-scattering paths in a medium but in opposite directions. The theoretical description of multiple scattering becomes more complicated for closely packed media because of potentially significant near-field effects that can depress the photometric opposition peak significantly and increase the depth of the negative-polarization. feature. In this chapter, we discuss the opposition effects, for semi-infinite sparse scattering media and study their dependence on concentration and microphysical properties of the constituent scatterers. Manifestations of the near-field interactions are illustrated by theoretical calculations for randomly oriented clusters of spherical particles.