Observation of long-range correlations in temporal intensity fluctuations of light

We present evidence for long-range correlations $C_2\mathrm{}\left(t\right)\mathrm{}$ in temporal intensity fluctuations of multiply scattered visible light. The time autocorrelation function of the angular-averaged light intensity transmitted through a thin slab containing particles undergoing Brownian motion was determined for a series of different sample thicknesses, beam spot sizes at the sample surface, and optical transport mean free paths $l^{*}.$ The results for both the amplitude (i.e., the inverse conductance $1/g$) as well as the time $\mathrm{}\left(t\right)\mathrm{}$ dependence of the correlation function $C_2\mathrm{}\left(t\right)\mathrm{}$ are in good overall agreement with theory. Significant deviations are found, however, when correlations generated in a layer of ${1l}^{*}$ to ${2l}^{*}$ adjacent to the incoming surface become important. In particular the predicted $t^{-1/2}$ long-time tail of $C_2\mathrm{}\left(t\right)\mathrm{}$ was not observed. A physical explanation of these deviations is given based on the argument that at least one scattering event is required before long-range correlations $C_2\mathrm{}\left(t\right)\mathrm{}$ can develop.