Intensity-dependent reversal of nonlinearity sign in a gold nanoparticle array.

The optical nonlinearities of a Au nanoparticle array were determined by performing Z-scan measurements with a femtosecond laser (800 nm, 50 fs). As the exciting energy increased, the nonlinear absorption of the Au nanoparticle array changed from reverse-saturable absorption to saturable absorption, while the nonlinear refraction changed from self-defocusing to self-focusing, which implies competition between ground-state bleaching and two-photon absorption. One- and two-photon figures of merit of W=5.1 and T=0.11, respectively, were obtained that satisfy the demand, W>1 and T<1, for all-optical switching.