Angle dependence of the frequency correlation in random photonic media: Diffusive regime and its breakdown near localization

The frequency correlations of light in complex photonic media are of interest as a tool for characterizing the dynamical aspects of light diffusion. We demonstrate here that the frequency correlation shows a pronounced angle dependence both in transmission and in reflection geometries. Using a broadband white-light supercontinuum, this angle dependence is characterized and explained theoretically by a combination of propagation effects outside the medium and coherent backscattering. We report a strong dependence of the coherent backscattering contribution on the scattering strength that cannot be explained by the diffusion theory. Our results indicate that coherent backscattering of the frequency correlation forms a sensitive probe for the breakdown of the diffusive regime near localization.