Large third-order optical nonlinearity in coupled Au-Ni-Au composite nanorods

Ordered Au-Ni-Au composite nanorod arrays with rod diameter of 18 nm were grown in anodic aluminum oxide (AAO) templates. Strong longitudinal surface plasmon resonance (LSPR) with peak position around 800 nm was observed in the nanorods. Z-scan technique was used to investigate the third-order optical nonlinearities of the sample. Large third-order nonlinear absorption coefficient of −2.65×106cm/GW was obtained at LSPR wavelength. The excitation wavelength and incident angle dependent nonlinear absorptions in the Au-Ni-Au composite nanorods show that strong coupling of the longitudinal plasmon between nanorods can result in a large enhancement of the third-order optical nonlinearity. Comparison between the Au-Ni-Au composite nanorods and the pure Au nanorods demonstrates that further growing of a Au segment on the as-grown one separated by a Ni thin layer will lead to a large improvement of the third-order optical nonlinearities. These results are important for manipulation of the third-order optical nonlinearities in plasmonic devices.