Speckle autocorrelation spectroscopy and pulse transmission as probes of photon transport in strongly scattering random media

Abstract

Photon transport in strongly scattering random media has been examined by two experimental methods in order to search for departures from diffusive transport and the approach to strong photon localization. The first method measures the evolution in the transmitted shape of a short light pulse introduced into the interior of the random medium via an optical fiber. Comparison of the observed pulse shape with calculated solution to the diffusion equation, including loss, as the effective sample thickness is varied permits quantitative determination of both the elastic and inelastic photon mean-free-path lengths. Deviations from the predicted behavior are small and accounted for in terms of breakdown in the independent-scatterer approximation. The second method, laser speckle autocorrelation spectroscopy, involves measurements in the frequency rather than in the time domain. It is therefore suitable for samples with both smaller thicknesses and shorter inelastic mean free paths. In principle, both methods should provide the same information, and we present a theoretical analysis that connects the two. We find that the theoretical speckle results are surprisingly sensitive to the effects of absorption and sample geometry. This sensitivity has not previously been quantified or taken into account in making comparisons with experiment. Our analysis and experiments emphasize the difficulty in extracting precise photon-transport parameters from speckle autocorrelation spectra.