Evidence of multipolar excitations in surface enhanced Raman scattering

Visible and near-infrared extinction spectra of regular arrays of gold nanorods display several bands depending on polarization of the electromagnetic incident field. When setting the incident field perpendicular to the nanorod axis ͑transverse polarization͒, a single band is observed, which is assigned to a dipolar plasmon mode. The several bands obtained under an incident polarization along the nanorod axis ͑longitudinal polarization͒ are attributed to multipolar plasmon resonances. Raman scattering experiments carried out under transverse and longitudinal laser excitations on methylene blue adsorbed on these arrays show that multipolar plasmon resonances can give rise to strongly enhanced spectra with an amplification per molecule similar to that obtained for dipolar plasmon excitation.