Dark plasmons in hot spot generation and polarization in interelectrode nanoscale junctions

TL;DR

This paper reveals that in nanoscale gold junctions, the strongest surface-enhanced Raman signals occur when light is polarized across the gap, due to a unique plasmon mode coupling with dark multipolar modes, leading to significant enhancement.

Contribution

It uncovers a counterintuitive polarization dependence in hot spot generation, highlighting the role of transverse plasmon modes and dark multipolar modes in nanoscale junctions.

Findings

Transverse polarization yields stronger SERS enhancement than longitudinal.

Dark multipolar modes significantly contribute to hot spot formation.

Reproducible SERS enhancements are at least ten times larger with transverse polarization.

Abstract

Nanoscale gaps between adjacent metallic nanostructures give rise to extraordinarily large field enhancements, known as "hot spots", upon illumination. Incident light with the electric field polarized across the gap (along the interparticle axis) is generally known to induce the strongest surface enhanced Raman spectroscopy (SERS) enhancements. However, here we show that for a nanogap located within a nanowire linking extended Au electrodes, the greatest enhancement and resulting SERS emission occurs when the electric field of the incident light is polarized along the gap (transverse to the interelectrode axis). This surprising and counterintuitive polarization dependence results from a strong dipolar plasmon mode that resonates transversely across the nanowire, coupling with dark multipolar modes arising from subtle intrinsic asymmetries in the nanogap. These modes give rise to highly reproducible SERS enhancements at least an order of magnitude larger than the longitudinal modes in these structures.