  Plasmonic nanopatch antennas that incorporate dielectric gaps hundreds of
picometers to several nanometers thick have drawn increasing attention over the
past decade because they confine electromagnetic fields to grossly
sub-diffraction limited volumes. Substantial control over the optical
properties of excitons and color centers confined within these plasmonic
cavities has already been demonstrated with far-field optical spectroscopies,
but near-field optical spectroscopies are essential to an improved
understanding of the plasmon-emitter interaction at the nanoscale. Here, we
characterize the intensity and phase-resolved plasmonic response of isolated
nanopatch antennas with cathodoluminescence microscopy. Further, we explore the
distinction between optical and electron-beam spectroscopies of coupled
plasmon-exciton heterostructures to identify constraints and opportunities for
future nanoscale characterization and control of hybrid nanophotonic
structures. While we observe substantial Purcell enhancement in time-resolved
photoluminescence spectroscopies, negligible Purcell enhancement is observed in
cathodoluminescence spectroscopies of hybrid nanophotonic structures. The
substantial differences in measured Purcell enhancement for electron-beam and
laser excitation can be understood as a result of the different selection rules
for these complementary experiments. These results provide a fundamentally new
understanding of near-field plasmon-exciton interactions in nanopatch antennas
that is essential to myriad emerging quantum photonic devices.
