Strongly coupled surface plasmon resonance modes at both FF and SH in clusters of symmetrically distributed metallic nanospheres

Abstract

Abstract Plasmonic nanoclusters play an important role in the rapid development of application of nanophotonics. In this article both the fundamental-frequency (FF) and second-harmonic (SH) responses of the complex nanoclusters (tetramer, hexamer, and octamer), constructed by coupled spherical plasmonic nanoparticles, are theoretically investigated. By using the multiple scattering algorithm, we present the comprehensive numerical analysis of the physical mechanisms pertaining to the linear and nonlinear coupled-plasmon modes, including ‘hot spot’ modes (HSMs) and ‘whispering-gallery’ modes (WGMs). Different from the symmetric coupling of plasmonic eigenmodes excited by the nanodimer, our analysis reveals that such coupled modes from the nanomultimers are the result of the ‘hybridization’ of symmetric and asymmetric coupling between adjacent dipole or higher-order multipole moments. Also, the remarkable enhancement as large as 3–4 orders of magnitude is achieved for the SH intensity when employing the plasmonic HSMs or WGMs at the FF. Interestingly, the nonlinear plasmonic HSMs and WGMs (the maximum Q up to 4800) can also be obtained by elaborately designing the system geometry. In addition, the resonant wavelengths of such coupled-plasmon modes can be linearly modulated within a broad range. These numerical results have potential applications in surface optical microscopy, plasmonic sensor and nanolaser.