Observation of long-range intensity correlation in the transport of coherent light through a random medium

We have measured the correlation in frequency-dependent intensity fluctuations in the total transmission through random dielectric samples, using visible light in a (essentially) plane-wave geometry. The correlation function, of which the width at half maximum is proportional to ${\mathit{L}}^{\mathrm{-}2}$ (L is the thickness of the sample), decays as (Δω${)}^{\mathrm{-}1/2}$. This constitutes the experimental proof for the buildup of long-range intensity correlations in the propagation of classical waves by multiple scattering.