Our theoretical study of the third-order susceptibility (χ (3) ) for Ag dielectric composite reveals a critical role of saturation of optical transitions between discrete states of conduction electrons in metal quantum dots. The calculated size dependence of the χ (3) for Ag nanoparticles reproduces the published experimental results. Saturation effects lead to a decrease of the local field enhancement factor that is of particular importance for surface-enhanced phenomena, such as Raman scattering and nonlinear optical responses. Plasmonic nanomaterials have attracted much recent research interest because of their unique optical properties, such as nonlinear optical activity, 1 the chirality of plasmon modes, 2 and the quantum-size effect in two-photon excited luminescence. 3 Current state-of-the-art nanofabrication techniques allow the development of novel applications based on such properties. Of particular importance for applications are the large local-field enhancements for metal particle aggregates that lead to surface-enhanced Raman scattering (SERS) and a number of nonlinear optical phenomena, 4 including th
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