Abstract

Second harmonic generation by a disordered array of superparamagnetic nanoparticles in a static magnetic field H is studied experimentally. For H = 0, only hyper-Rayleigh scattering takes place; i.e., the second-harmonic radiation is diffuse and unpolarized. For H not equal 0, a coherent (specular and linearly polarized) component appears against the hyper-Rayleigh scattering background. The phase of the coherent component changes by pi 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.