Two-photon rayleigh scattering from isolated and aggregated hollow gold nanospheres

We report the optical quadratic nonlinearity of hollow gold nanospheres (HGNs). The two-photon Rayleigh scattering (TPRS) technique has been used to measure the first hyperpolarizabilities (β) of the nanospheres. The measured β values are extremely large for the HGNs and, more interestingly, larger than similar-sized solid gold nanospheres (SGNs). Larger β values in HGNs over SGNs are, in part, due to a larger surface area, local electromagnetic field enhancement due to surface morphology, and one-photon resonance enhancements that occurred with large aspect ratio structures that had near-infrared surface plasmon resonances. Our results advocate for the utilization of HGNs over SGNs in various practical applications based on nonlinear optical (NLO) techniques or processes. In the case of HGN aggregates, the TPRS response has been found to depend strongly on the interfacial structure of the aggregates and the resulting interparticle interaction mechanisms. This finding illustrates the sensitivity of NLO techniques, such as TPRS, for probing nanoscale structure-dependent optical properties, as well as the nature of internanoparticle interactions.