TIME EVOLUTION OF TRANSIENT GRATINGS IN A NONLINEAR FILM ABOVE THE SURFACE OF A DISORDERED MEDIUM

The time evolution of light intensity gratings formed under the influence of two short pulses of coherent radiation in a thin films located above the surface of a disordered medium is investigated. It is assumed that the light wavelength lambda << l and l << l(a) (l is the photon elastic scattering length, l(a) is the linear absorption length) and that the pulse duration is shorter than the mean free time of photons in the disordered medium. The peculiarities of the rise and decay of the transient gratings arising in the film due to the interference of coherent backscattered radiation are considered. It is found that in a film situated at distances \z\ >> l/theta (2-theta is a small angle between the incident beams) above the surface of a disordered medium the grating intensity has a maximum at times of the order of L2/D, where D is the effective diffusion coefficient for the photons on the scale L(z), and L(z) is the characteristic length of the diffusive-like propagation of those photons which give the mean contribution to the transient grating formation. We believe that this result opens up new possibilities for studying the scaling properties of the diffusion coefficient of photons in disordered media.