DIFFUSING-LIGHT SPECTROSCOPIES BEYOND THE DIFFUSION LIMIT - THE ROLE OF BALLISTIC TRANSPORT AND ANISOTROPIC SCATTERING

Diffuse transmission and diffusing-wave spectroscopy (DWS) can be used to probe the structure and dynamics of opaque materials such as collo ids, foams, and sand. A crucial step is to model photon transport as a diffusion process. This approach is acceptable for optically thick sa mples, far into the limit of strong multiple scattering; however, it b ecomes increasingly inaccurate for thinner samples for several reasons . Here, we correct for two of these defects. By modeling photon propag ation by a telegrapher equation with suitable boundary conditions, we can account for the ballistic transport of photons at finite speed bet ween successive scattering events. By introducing a discontinuity in t he photon concentration at the source point, and then averaging over a range of penetration depths, we can account for the fact that photons usually scatter anisotropically into the forward direction, rather th an being completely randomized at each event. The accuracy of our appr oach is tested by comparison both with random walk computer simulation s and with experiments on specially designed suspensions of polystyren e spheres. We find that our predictions extend the utility of diffuse transmission to slabs of all thicknesses and of DWS to slabs down to a bout two transport mean free paths.