Abstract

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 ($n_2$) and the two-photon absorption coefficient ($\beta$) 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 $\beta =1.5\mathrm{cm}/\mathrm{GW}$ (without field) to $\beta =2.4\mathrm{cm}/\mathrm{GW}$ (2700 Oe). For the field direction perpendicular to the light polarization direction, $\beta$ decreases to $\beta =1.0\mathrm{cm}/\mathrm{GW}$ (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 ${\chi }^{(3)}$. The SAXS experiments revealed that when the field is applied, small linear aggregates are formed in the direction of $H$.

© 2018 Optical Society of America

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