Magnetic-field-induced coherence in hyper-Rayleigh scattering

Second harmonic generation by a disordered array of superparamagnetic nanoparticles in a static magnetic field $\mathbf{H}$ is studied experimentally. For $\mathbf{H} = 0$, only hyper-Rayleigh scattering takes place; i.e., the second-harmonic radiation is diffuse and unpolarized. For $\mathbf{H}\neq 0$, a coherent (specular and linearly polarized) component appears against the hyper-Rayleigh scattering background. The phase of the coherent component changes by р when the direction of the magnetic field is reversed. It is shown that the effect can be explained by correlations in the spatial fluctuations of the nonlinear-optical and magnetic polarizabilities of the particles, taking into account the nonuniformity of the effective (local) static magnetic field.