Polarization-controlled gap-mode surface-enhanced Raman scattering with a single nanoparticle

Metallic nanoparticle-film gaps are widely used as high-efficiency surface-enhanced Raman scattering (SERS) substrates owing to the plasmonic hybridization of the propagating surface plasmon polaritons on the films and the localized surface plasmons on the nanoparticles. Here, based on both theoretical and experimental studies, we reveal the enhancement mechanism of gap-mode SERS in a single nanoparticle-film gap by comparing the SERS intensity under different incident polarizations. The results demonstrate that the SERS signal can be significantly enhanced under illumination with radial polarization and can be significantly suppressed under azimuthal polarization (as compared with under traditional linear polarization). This is attributed to the distinct longitudinal electric field distributions of the excited surface plasmon polaritons. The results of our work will be of great interest for single-particle SERS research and quantitative molecular detection.