| Linear and second-order nonlinear optical (NLO) properties of metal nanoparticles and nanoparticle-based assemblies are studied experimentally and computationally. NLO responses are determined by hyper-Rayleigh scattering (HRS). For solution species, HRS reports directly on the magnitude of the first hyperpolarizability, which quantifies a material's second-order NLO (i.e. frequency doubling) response. Impressive visible-region frequency doubling responses are observed from solutions of nanometer-sized noble metal particles that possess intense surface plasmon absorption, such as gold, silver, and copper particles. Metal particles lacking such absorption, including noble metal particles that are too small to exhibit a sharp surface plasmon absorption peak, are ineffective as frequency doublers.; Results of HRS experiments on core-shell nanoparticles, in which a thin layer of a second material is deposited onto a core gold nanoparticle, are also reported. Depending on the identity and thickness of the shell, the gold particle frequency doubling response can be increased or decreased by orders of magnitude.; Several types of aggregates of gold nanoparticles have been studied. In general, HRS intensities increase as nanoparticles aggregate, but Rayleigh scattering intensities are found to be much more sensitive to aggregation. Systematic studies of the dependences on aggregate size and interparticle distance within the aggregates are reported.; When possible and appropriate, the experimental findings are accompanied by predictions from existing theories of nanoparticle optics. Linear optical properties are modeled using Mie theory, NLO properties are calculated using the theory of Agarwal and Jha. The Agarwal-Jha theory predicts more efficient frequency doubling under conditions of two-photon resonance, matching experimental observations. |
