Scattering of Coherent Light by a Statistical Medium

Abstract

By the joint use of a coherent light source and of photon counting statistics, we have verified the statistical properties of Brownian motion. The light source is a helium-neon laser emitting at 6328 \AA{}. The scattering media are dilute solutions of monodisperse polystyrene spherical particles with diameters ranging from 0.088 to 1.06 \ensuremath{\mu}. Three types of results are presented. First, by working at a density of scattering centers such that neither particle correlations nor multiple scattering processes have any influence, we measure the angular distributions of the field scattered by single particles, and verify the Mie theory. Second, by frequency measurements, we show the Lorentzian character of the scattered light spectrum, as one would expect from the diffusion approximation. Third, by use of photocount distributions, we measure the ensemble distribution of the scattered field within a coherence area and time, and show that the field distribution is Gaussian. The last two results imply that the field scattered by Brownian particles is accurately described as a stochastic Markov process. The techniques here used open the way to similar measurements on more complicated scattering media.