Optical antenna structures for surface enhanced scattering

Surface enhanced Raman scattering (SERS) is considered on two types of optical antenna substrate, using crystal violet as a test molecule. The first is a quasi-ordered array of gold nanorods grown in a porous alumina template and subsequently exposed by etch-back of the alumina and the second is a random array of silver-dressed, vertically aligned, multi-walled carbon nanotubes (MWNTs). In each case it is important to understand the contribution of the antenna structures and of ‘hot-spots’ within and between such structures - this is approached by modelling the electric field profiles associated with the respective structures.

SERS spectra from the gold nanorod arrays exhibit a variation in enhancement factor (EF) as a function of excitation wavelength that displays little correlation with the elastic optical properties. This turns out to be a hot-spot dominated system and the key to understanding this lack of correlation and to obtaining agreement between experimental and calculated EF spectra lies with consideration of randomly distributed, sub-10 nm gaps between the nanorods forming the substrate. Intense fields in these enhancement ‘hotspots’ make a dominant contribution to the Raman scattering, but have a very different spectral profile to that of the elastic optical response.

When it comes to the MWNT-based system the highly granular Ag dressing means that we have to address the electric field enhancement associated with the detailed structure within the nanostructures (optical antenna). To achieve this the modelling is performed using the surface integral equation technique. The field enhancement that underpins the SERS response is due to the overall antenna response of the nanoposts acting in combination with that due to more localised resonances associated with a) small silver grains of the nanopost sidewalls, b) highly confined inter-grain regions or c) small gaps between neighbouring nanoposts that lean towards each other.