Size-related third-order optical nonlinearities of Au nanoparticle arrays.

We report a systematic study of the size-related nonlinear optical properties of triangular Au particles. The triangular Au nanoparticle arrays of four sizes (37 nm, 70 nm, 140 nm and 190 nm) were fabricated on quartz substrates using nanosphere lithography. By performing the Z-scan method with femtosecond laser (800 nm, 50 fs), the optical nonlinearities of Au nanoparticle arrays were determined. The results showed a size-related competition between two mechanisms of groundstate bleaching and two-photon absorption. As the size increased, the nonlinear absorption changed from two-photon absorption to saturated absorption, while the nonlinear refraction changed from self-defocusing to self-focusing. These size-tunable nonlinearities make it possible to optimize the one- and two-photon figures of merit, W and T, for all-optical switching.