Photon migration in random media: angle and time-resolved studies

Abstract

The transport. of photons in random medium were directly studied by measuring the angular and temporal distribution of photons after traversing through a slab of a random medium. The experiments were performed using an ultrafast laser pulse of 100 fs and streak camera detection with a temporal response of 8 ps. When the ultrafast laser pulse traverse through a slab of random medium of less than 10 scattering mean free path thickness, a coherent (ballistic) pulse was found to coexists with a incoherent (diffusive) pulse. The speed of the coherent component of the 100 fS laser pulse reduces as the concentration of the scatterer increases. This reduction in speed may arise from the coherent interference of the multiply scattered waves. The transport of photons was found to deviate from the diffusion approximation when the thickness of the slab is less than 10 transport mean free paths; photons were found to arrive significantly earlier than predicted by the diffusion theory. The theoretical results from the two-frequency coherence function in the Rytov approximation were in qualitative hut not in quantitative agreement with the experimental results. Finally, we show that the ratio of signal (coherent component) to noise (incoherent component) of the light propagating through a random medium can be significantly increased by introducing an absorbing dye. This absorption technique of increasing the signal to noise ratio can improve our capability to image through a random medium.