TRANSIENT GRATINGS IN A FILM ABOVE THE SURFACE OF A DISORDERED MEDIUM

The contribution of scattered radiation to light intensity transient gratings in a thin film located above the surface of a disordered scattering medium is investigated. A new effect of weak localization is predicted. It is shown that at the distances \z\ >> l/THETA (THETA is a small angle between the incident coherent beams, l is the photon elastic scattering length) the main contribution to the grating formation in the film comes from the interference of beams of scattered radiation which inside the disordered medium propagate along the same random-walk trajectories but in opposite directions. It is shown also that under these conditions the most important are long trajectories with the length of the order of z-THETA >> l. Thus, varying the distance z the contributions from the random light trajectories of different lengths can be separated.