Temporal fluctuations of waves in weakly nonlinear disordered media.
We consider the multiple scattering of a scalar wave in a disordered medium with a weak nonlinearity of Kerr type. The perturbation theory, developed to calculate the temporal autocorrelation function of scattered wave, fails at short correlation times. A self-consistent calculation shows that for nonlinearities exceeding a certain threshold value, the multiple-scattering speckle pattern becomes unstable and exhibits spontaneous fluctuations even in the absence of scatterer motion. The instability is due to a distributed feedback in the system "coherent wave + nonlinear disordered medium." The feedback is provided by the multiple scattering. The development of instability is independent of the sign of nonlinearity.
Skipetrov
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Laboratoire de Physique et Modélisation des Milieux Condensés, Université Joseph Fourier, Maison des Magistères-CNRS, Bôite Postale 166, 38042 Grenoble 9, France.