Polarization- and wavelength-resolved near-field imaging of complex plasmonic modes in Archimedean nanospirals.

Asymmetric nanophotonic structures enable a wide range of opportunities in optical nanotechnology because they support efficient optical nonlinearities mediated by multiple plasmon resonances over a broad spectral range. The Archimedean nanospiral is a canonical example of a chiral plasmonic structure because it supports even-order nonlinearities that are not generally accessible in locally symmetric geometries. However, the complex spiral response makes nanoscale experimental characterization of the plasmonic near-field structure highly desirable. Here we employ high-efficiency, high-spatial-resolution cathodoluminescence imaging in a scanning transmission electron microscope to describe the spatial, spectral, and polarization response of plasmon modes in the nanospiral geometry.