## Abstract

By applying the Maxwell Garnett model to gold nanoparticles in water we deduce a value of $\mathrm{Im}{\chi}_{i}^{(3)}=1.1$ $\times {10}^{-7}\mathrm{esu}$ for the imaginary part of the cubic susceptibility for gold corresponding to a Fermi smearing mechanism. We also demonstrate a sign reversal in the nonlinear absorption for gold particles in $1,{1}^{\prime},3,3,{3}^{\prime},{3}^{\prime}$-hexamethylindotricarbocyanine iodide. Although the imaginary part of ${\chi}^{(3)}$ is positive for each component by itself, remarkably the imaginary part of ${\chi}^{(3)}$ is negative for the colloid as a whole. We show that the nonlinearity of the host must be considered and that the sign reversal in ${\chi}^{(3)}$ is a result of the fact that at the surface plasmon resonance the local field factor has an imaginary component that arises from a phase shift between the applied field and the local field inside the particle.

© 1997 Optical Society of America

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