Backscattering effects for discrete random media

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 actively studied. When the incident light is unpolarized, the opposition intensity peak can 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 significantly depress the photometric opposition peak 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.