Diffusing-wave spectroscopy and multiple scattering of light in correlated random media

We discuss the diffusing-wave spectroscopy technique for multiple scattering of light as introduced in recent experiments. This technique has proven to be useful in probing the self-diffusion of scattering particles in suspension by measuring the light-intensity autocorrelation function. We show how the autocorrelation function depends upon the dynamic structure factor of the medium in the presence of correlations between the scattering particles. There is a simple generalization of the result obtained for uncorrelated media to include the dynamic structure factor. Previous theoretical work has employed the white-noise model, valid for small uncorrelated scattering centers. The results of this model are valid only so long as the propagation of the light can be regarded as diffusive. On short length scales, however, the propagation becomes increasingly ballistic in nature. We present an exact formal solution of the transport equation capable of describing the crossover from ballistic to diffusive propagation. The resulting transport kernel provides a simple correction to the diffusion approximation for large scattering particles, which substantially improves the agreement with measured autocorrelation functions. We also discuss the polarization dependence of the autocorrelation function for both small as well as large scattering particles.