Abstract

The transport of photons in a random medium may be directly studied by studying the angular and temporal distribution of photons after they transverse a slab of random medium. We found that a coherent (ballistic) pulse coexists with an incoherent (diffuse) pulse when the ultrafast laser pulse transverses a slab of random medium that is less than 10 scattering mean-free-paths thick. The incoherent pulse shows large angular and temporal spreads. The temporal profile of the incoherent component can be approximated by diffusion theory if the thickness of the slab is greater than 10 transport mean-free-paths; e.g., for slab 10 mm thick. Photons are found to arrive significantly earlier than the arrival times predicted by diffusion theory when the thickness of the slab is less than 10 transport mean-free-paths. We computed the mean time of photon arrival from experimental data, and it compares favorably with the theory. Anisotropic scattering, as well as coherent interference among the multiple scattered waves, may account for the early arrival of photons.

© 1990 Optical Society of America