OBSERVATION OF LONG-RANGE CORRELATIONS IN TEMPORAL INTENSITY FLUCTUATIONS OF LIGHT

We present evidence for long-range correlations C-2(t) in temporal int ensity fluctuations of multiply scattered visible light. The time auto correlation function of the angular-averaged light intensity transmitt ed through a thin slab containing particles undergoing Brownian motion was determined for a series of different sample thicknesses, beam spo t 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 C-2 (t) are in good overall agreement with theory. Significant deviations are Found, however, when correlations generated in a layer of 1l to 2 l adjacent to the incoming surface become important. In particular th e predicted t(-1/2) long-time tail of C-2(t) was not observed. A physi cal explanation of these deviations is given based on the argument tha t at least one scattering event is required before long-range correlat ions C-2(t) can develop. [S0163-1829(97)08341-0].