Influence of reflecting boundaries and finite interfacial thickness on the coherent backscattering cone.

Abstract

We investigate the effect of reflecting boundary conditions on the shape of the coherent backscattering (CB) cone of light by measuring the CB cone from a suspension of microspheres as a function of the thickness of the glass window containing the sample. The reflection from the air-glass interface is observed to narrow the cone, as previously predicted and observed; however, the finite thickness of the window introduces a slope discontinuity in the CB cone. The dominant contribution to the cone for angles less than the kink is from reflected light, while the cone at angles higher than the kink arises from light that directly leaves the sample. Thus the study of the CB cone as a function of window thickness offers an experimental method of separating the component of the cone due to interfacial reflectivity from the directly backscattered component. Monte Carlo simulations of the path length distribution of multiply scattered light were performed and compared well with both measurement and theories incorporating reflecting interfaces. It was found that under certain circumstances an interface of finite width lowered the CB cone height. Additionally, high quality measurements of the CB cone from Mie scatterers revealed an anisotropy in the cone similar to the case of Rayleigh spheres. Finally, the influence of reflecting boundaries on the decay of the temporal autocorrelation function and the height of the CB cone were investigated.