  Understanding the near-field electromagnetic interactions that produce
optical orbital angular momentum (OAM) is central to the integration of twisted
light into nanotechnology. Here, we examine the cathodoluminescence (CL) of
plasmonic vortices carrying OAM generated in spiral nanostructures through
scanning transmission electron microscopy (STEM). The nanospiral geometry
defines the photonic local density of states (LDOS) sampled by STEM-CL, which
provides access to the phase and amplitude of the plasmonic vortex with
nanometer spatial and meV spectral resolution. We map the full spectral
dispersion of the plasmonic vortex in the spiral structure and examine the
effects of increasing topological charge on the plasmon phase and amplitude in
the detected CL signal. The vortex is mapped in CL over a broad spectral range,
and deviations between the predicted and detected positions of near-field
optical signatures of as much as 5 per cent are observed. Finally, enhanced
luminescence is observed from concentric spirals of like handedness compared to
that from concentric spirals of opposite handedness, indicating the potential
to couple plasmonic vortices to chiral nanostructures for sensitive detection
and manipulation of optical OAM.
