Light diffusion and localization in 3D nonlinear disordered media

Using a 3D Finite-Difference Time-Domain parallel code, we report on the linear and nonlinear propagation of light pulses in a disordered assembly of scatterers, whose spatial distribution is generated by a Molecular Dynamics code; refractive index dispersion is also taken into account. We calculate the static and dynamical diffusion constant of light, while considering a pulsed excitation. Our results are in quantitative agreement with reported experiments, also furnishing evidence of a non-exponential decay of the transmitted pulse in the linear regime and in the presence of localized modes. By using an high power excitation, we numerically demonstrate the ``modulational instability random laser'': at high peak input powers energy is transferred to localized states from the input pulse, via third-order nonlinearity and optical parametric amplification, and this process is signed by a power-dependent non-exponential time-decay of the transmitted pulse.