Systematic Analysis of the Coupling Effects within Supported Plasmonic Nanorod Antenna Arrays

Vertically aligned gold nanorod arrays (of aspect ratios from 3.0 to 18.0), supported on metal substrates were fabricated by electrochemical deposition within porous anodic aluminum oxide (AAO) templates. However, the coupling effects that occur within these supported arrays are complex, involving both particle-substrate and particle-particle coupling, and far from fully understood. We have performed a systematic investigation into these effects using finite element modeling (FEM) and correlated these results to experiment. We demonstrate that within the strong coupling regime, the optical properties of the arrays are predominantly governed by inter-rod spacing. Additionally, by supporting the arrays on metal films, the absorption efficiency is significantly enhanced. We explain these coupling effects in terms of plasmon hybridization theory and image charges. We demonstrate that the longitudinal mode may be tuned throughout the visible region and present resonant wavelength contour plots as a function of inter-rod spacing and aspect ratio (AR) as an aid for the design of plasmonic arrays in applications such as photovoltaics and photocatalysis. Finally, we show that coupling within unsupported and supported arrays can redistribute the electric field to either the center or base of the nanorods respectively whilst propagating along the inter-rod axis, which is potentially of interest for optical waveguide applications