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.
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