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

We present evidence for long-range correlations C2(t) 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 (t) dependence of the correlation function C2(t) 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 long-time tail of C2(t) 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 C2(t) can develop. @S0163-1829~97!08341-0#