Spatio-temporal imaging of light transport in strongly scattering media

We report on the passive measurement of time-dependent Green's functions in the optical frequency domain with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the mutual coherence function of a broadband and incoherent wave-field can directly yield the Green's functions between scatterers of a complex medium. Both the ballistic and multiple scattering components of the Green's function are retrieved. This simple and powerful approach directly yields a wealth of information about the medium under investigation. In particular, it allows to investigate locally the growth of the diffusive halo within the scattering medium. Local measurements of transport parameters can thus be performed and allow to image a strongly scattering layer with a unprecedented resolution of a few transport mean free paths. This constitutes a major breakthrough compared to state-of-the-art techniques such as optical diffuse tomography.