Controlling the optical response of regular arrays of gold particles for surface-enhanced Raman scattering

This paper is intended to show how the control of the optical response of regular arrays of gold nanoparticles allows to improve our understanding of surface-enhanced Raman scattering (SERS). Regular particle arrays, designed by electron-beam lithography, exhibit remarkable optical properties and appear to be suitable substrates for the deepened study of the mechanisms at the origin of the SERS effect. Indeed, the resonance of the surface plasmons localized on the particles, which are at the origin of visible to near-infrared extinction spectra and the SERS effect, can be tuned to almost any desirable wavelength by varying the particle shape, size, and spacing, thus optimizing the Raman amplification. The optical extinction spectrum of various gratings was calculated in order to gain insight into their physical sense. The SERS study of trans-1,2-bis (4-pyridyl) ethylene (BPE) adsorbed on these arrays enabled us to determine the enhancement factors G of four bands in the BPE Raman spectrum. The G values thus deduced were found to be of the same order of magnitude as those calculated using a phenomenological relation derived from the electromagnetic theory. Furthermore, a photon scanning tunneling microscope enabled us to acquire near-field optical images of the arrays and to estimate the electric near-field enhancement resulting from plasmon excitation; the Raman enhancement factor thus obtained is of the same order of magnitude as that found from Raman experiments.