  Temporal fluctuations of the speckle pattern formed upon backscattering of a
laser beam from an interface between gold and nonlinear polymer film have been
observed as a function of optical power. The instability can be explained by
coupling of laser light to surface plasmons and other guided modes, which
experience multiple scattering while propagating in the film along the
interface. The speckle pattern produced in this process is extremely sensitive
to fluctuations of the scattering potential near the interface.
