Localized Raman Enhancement from a Double-Hole Nanostructure in a Metal Film

Abstract

An isolated double-hole indentation, with concentric rings, in a metal film was used to obtain highly localized surface-enhanced Raman scattering (SERS) from regions much smaller than the optical wavelength. The structure was created by a focused ion beam (FIB) milling partially through the 100 nm thick gold film to a depth of 50 nm. Significant SERS enhancement was observed for both oxazine 720 and rhodamine 6G. The SERS was polarization-dependent because of the biaxial symmetry of the double-hole at the apexes where the indentations overlap; these apexes were responsible for the strong subwavelength focusing. The finite-difference time-domain method was used to calculate the electromagnetic field of the nanostructure, and it showed strong polarization-dependent focusing, in agreement with the experimentally observed SERS enhancement. On the basis of these calculations, it is estimated that the 60% polarization-dependent SERS enhancement is the result of only ∼1300 molecules in the region of the apexes, and it is estimated that the limit of detection is 20 molecules for the best-case configuration. This work is an important step toward single-molecule SERS from tailored nanostructures designed for predictable field enhancement.