Multiple light scattering in anisotropic random media

TL;DR

This paper develops a formalism for light diffusion in anisotropic turbid media, specifically applying it to nematic liquid crystals, and relates diffusion constants to material properties and director fluctuations.

Contribution

It introduces a comprehensive theoretical framework for anisotropic light diffusion and provides explicit formulas for diffusion tensors in complex liquid crystalline systems.

Findings

Derived explicit formulas for diffusion tensor and absorption coefficient in anisotropic media.

Analyzed diffusion constants in nematic liquid crystals in relation to elastic and dielectric properties.

Linked dynamic absorption to director fluctuation viscosity.

Abstract

In the last decade Diffusing Wave Spectroscopy (DWS) has emerged as a powerful tool to study turbid media. In this article we develop the formalism to describe light diffusion in general anisotropic turbid media. We give explicit formulas to calculate the diffusion tensor and the dynamic absorption coefficient, measured in DWS experiments. We apply our theory to uniaxial systems, namely nematic liquid crystals, where light is scattered from thermal fluctuations of the local optical axis, called director. We perform a detailed analysis of the two essential diffusion constants, parallel and perpendicular to the director, in terms of Frank elastic constants, dielectric anisotropy, and applied magnetic field. We also point out the relevance of our results to different liquid crystalline systems, such as discotic nematics, smectic-A phases, and polymer liquid crystals. Finally, we show that the dynamic absorption coefficient is the angular average over the inverse viscosity, which governs the dynamics of director fluctuations.