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Temporal fluctuations of waves in weakly nonlinear disordered media
1Laboratoire de Physique et Modélisation des Milieux Condensés, Université Joseph Fourier, Maison des Magistères-CNRS, Bôite Postale 166, 38042 Grenoble 9, France.
Summary
Multiple scattering in nonlinear disordered media becomes unstable above a threshold. This instability causes spontaneous fluctuations in the speckle pattern due to distributed feedback, regardless of nonlinearity sign.
Area of Science:
- Wave propagation
- Nonlinear optics
- Condensed matter physics
Background:
- Multiple scattering describes wave behavior in complex media.
- Kerr nonlinearity introduces intensity-dependent refractive index changes.
- Perturbation theory is limited for short correlation times in nonlinear systems.
Purpose of the Study:
- Investigate the temporal autocorrelation function of scattered waves in nonlinear disordered media.
- Determine the conditions for instability in the multiple-scattering speckle pattern.
- Understand the underlying mechanism of spontaneous fluctuations.
Main Methods:
- Employed self-consistent calculations beyond standard perturbation theory.
- Analyzed the temporal autocorrelation function of a scalar wave.
- Focused on systems with weak Kerr-type nonlinearity.
Main Results:
- Perturbation theory fails at short correlation times.
- A threshold nonlinearity value was identified for speckle pattern instability.
- Spontaneous fluctuations occur above this threshold, independent of nonlinearity sign.
- Instability arises from distributed feedback between the coherent wave and the nonlinear medium.
Conclusions:
- The multiple-scattering speckle pattern in nonlinear disordered media is prone to instability.
- Distributed feedback from multiple scattering drives spontaneous fluctuations.
- This phenomenon occurs above a critical nonlinearity threshold and is robust to the sign of nonlinearity.