## Abstract

The anisotropic nonlinear third-order susceptibility of nanocomposites consisting of aligned ellipsoidal metallic nanoparticles embedded in a dielectric matrix is modeled from the generalized Maxwell-Garnett equation involving depolarization factors. Depolarization factors take into account different anisotropic particle geometries such as flat disks, rods, or ellipsoids. The equations traditionally used to model third-order susceptibility of nanocomposites are valid only for very low metal volume fractions. Modified equations that allow metal volume fractions up to the limit of validity of the Maxwell-Garnett equation are used. The effect of the different model parameters, namely, the metal volume fraction, the real and imaginary parts of the metal dielectric constant, the matrix dielectric constant, and, finally, the ratio of the real and imaginary parts of the metal third-order susceptibility were investigated using the model gold/silica nanocomposite system. As previously reported in the literature for the isotropic particle case, counterintuitive effects such as sign reversal between the bulk metal and composite nonlinear susceptibilities have been observed. The calculations were applied to the case of gold nanorods embedded in silica that were experimentally found to exhibit anisotropic saturable absorption.

© 2008 Optical Society of America

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