  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.
