Observation of coherent back-scattering and its dynamics in a transverse 2D photonic disorder : from weak to strong localization

We report the first observation of coherent back-scattering (CBS) of light in a transverse photonic disorder. The CBS peak is recorded in the far-field, at a fixed propagation time set by our crystal length, and displays a contrast approaching the ideal value of 1, which proves good coherence of transport in our system. We study its dynamics for increasing disorder strength, and find a non-monotonous evolution. For weak disorder, the CBS signal increases, and the asymmetry of the momentum distribution becomes inverted compared to the initial condition. For stronger disorder, we observe a resymmetrization of the momentum distribution, confirmed by numerical simulations, and compatible with the onset of strong (Anderson) localization.