Influence of an external magnetic field in the two-photon absorption coefficient of magnetite nanoparticles in colloids and thin films

The Z-scan nonlinear optical and the small-angle x-ray scattering (SAXS) techniques are used to investigate the structure and nonlinear optical properties of magnetite nanoparticles dispersed in a colloid and trapped in thin films. The nonlinear refractive index (n2) and the two-photon absorption coefficient (β) were measured as a function of the intensity of an external applied magnetic field H. Different relative orientations of the field with respect to the light polarization direction were investigated. When the external magnetic field is applied to the colloidal sample (H parallel to the light polarization direction), the two-photon absorption coefficient increases with the field, ranging from β=1.5  cm/GW (without field) to β=2.4  cm/GW (2700 Oe). For the field direction perpendicular to the light polarization direction, β decreases to β=1.0  cm/GW (2700 Oe) and after remains constant. These values evidence a two-photon absorption anisotropy in magnetite, and allowed us to evaluate some elements of the third-order nonlinear optical susceptibility tensor χ(3). The SAXS experiments revealed that when the field is applied, small linear aggregates are formed in the direction of H.