Third-order nonlinear properties of Au clusters containing dielectric thin films

We show experimentally and theoretically that very large enhancement of chi((3)) can be obtained in gold-cluster-containing dielectric thin films. At a high volume fraction of gold clusters, the concentration dependence of chi((3)) cannot be accounted for by the Maxwell Garnett theory. To go beyond the MG theory, we have derived the spectral densities of different microgeometries with related formulae for calculating the effective dielectric constant and the enhancement factors for third-order nonlinearities. It is shown that the largest enhancement source is the anomalous dispersion, realized by the MG and the Sheng theories. Substantially improved agreement between theory and experiment is obtained. Even larger enhancement is predicted as possible by the theory.