Second-harmonic generation in magnetic colloids by orientation of the nanoparticles

Abstract

We show that an optical second harmonic (SH) is generated in a magnetic colloid if a static magnetic field which breaks the fluid isotropy is applied. We propose a statistical model in which all the magnetic nanoparticles are supposed to be identical with a nonzero complex second-order polarizability tensor bound to their magnetic moment. These grains align under a static magnetic field according to Boltzmann’s statistics. The nonlinear second-order macroscopic electric susceptibility tensor, ruled by a Langevin-like model, is found to be zero without any applied magnetic field and to be an increasing function of its strength. The nonlinear susceptibility tensor of the colloidal solution exhibits an axial ${\mathit{C}}_{\mathrm{\infty }}$ (and not ${\mathit{C}}_{\mathrm{\infty }\mathit{v}}$) symmetry around the magnetic field. The measurements of the generated SH intensity as a function of the applied field strength, in all the independent input and output electric polarization directions, are in perfect agreement with our model and confirm the expected ${\mathit{C}}_{\mathrm{\infty }}$ symmetry. Under oblique incidence and without any applied magnetic field, the surface SH is generated by particles adsorbed on the glass cell walls and orientated normally to the interfaces. The complex values of the nonzero elements of the second-order polarizability tensor (leading to the observed SH wave ellipticity) are determined with a high precision, as well as that of the particle magnetic moment. The magnetic size found for the nanoparticle is in accordance with those given by other methods and could imply quantum confinement effects. A correlation between the ferrite particle atomic structure and the magnetic moment is found to be responsible of the noncentrosymmetry and of the chirality of the colloid under applied magnetic field. © 1996 The American Physical Society.