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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.
[1
]Department
of Physics and Astronomy, ‡Department of Electrical and Computer Engineering,
§Laboratory for Nanophotonics, ∥Department of Chemistry, Rice University, 6100 Main
Street, Houston,
Texas, 77005, United States