Optical properties of arrays of five-pointed nanostars

Abstract

The optical properties of nanostructures control the performance of applications that are based on localized surface plasmon resonances. Here we use finite-difference time-domain calculations to explore the effect of geometry and material-of-construction on the transmission and near-field optical intensity of arrays of closely-spaced five-pointed nano-star shapes. We compared isolated solid star shapes to star-shaped nano-gaps set within a surrounding square metal shape. The materials investigated were silver, gold, copper and aluminum. The study showed that both the geometry and material chosen had a significant effect on the resulting transmittance spectra. Transmittance spectra of arrays of solid five-pointed nano-stars did not show any strong absorption peaks in the visible region whereas, in contrast, the arrays of star-shaped nano-gaps set within the metal squares did show strong absorption peaks. However, on closer examination it became obvious that the enhanced electric field of the latter was mostly on the corners of the square metal domains and not actually in or on the star-shaped nano-gaps. Therefore we deduce that arrays of simple metal squares will be more suitable as substrates for surface enhanced Raman spectroscopy than arrays of stars or star-shaped nano-gaps. Gold, silver and copper were suitable choices for the latter type of array. Aluminum was unsuitable, at least for applications in the visible part of the spectrum, because it was associated with relatively weak electric fields.