  Imaging the propagation of light in time and space is crucial in optics,
notably for the study of complex media. We here demonstrate the passive
measurement of time-dependent Green's functions between every point at the
surface of a strongly scattering medium by means of low coherence
interferometry. The experimental access to this Green's matrix is essential
since it contains all the information about the complex trajectories of light
within the medium. In particular, the spatio-temporal spreading of the
diffusive halo and the coherent backscattering effect can be locally
investigated in the vicinity of each point acting as a virtual source. On the
one hand, this approach allows a quantitative imaging of the diffusion constant
in the scattering medium with a spatial resolution of the order of a few
transport mean free paths. On the other hand, our approach is able to reveal
and quantify the anisotropy of light diffusion, which can be of great interest
for optical characterization purposes. This study opens important perspectives
both in optical diffuse tomography with potential applications to biomedical
imaging and in fundamental physics for the experimental investigation of
Anderson localization.

Optics