Diffusing-wave spectroscopy : dynamic light scattering in the multiple scattering limit

Dynamic light scattering is extended to optically thick (opaque) media which exhibit a very high degree of multiple scattering. This new technique, called diffusing-wave spectroscopy (DWS), exploits the diffusive nature of the transport of light in strongly scattering media to relate the temporal fluctuations of the multiply scattered light to the motion of the scatterers. A simple theory of DWS, based on the diffusion approximation for the transport of light, is developed to calculate the temporal electric field autocorrelation functions of the multiply scattered light. Two important scattering geometries are treated : transmission and backscattering. The theory is compared to experimental measurements of Brownian motion of submicron-diameter polystyrene spheres in aqueous suspension. The agreement between theory and experiment is excellent. The limitations of the photon diffusion approximation and the polarization dependence of the autocorrelation functions are discussed for the backscattering measurements. The effects of absorption of light and particle polydispersity are also incorporated into the theory and verified experimentally. It is also shown how DWS can be used to obtain information about the mean size of the particles which scatter light.