  In the past decade, advances in nanotechnology have led to the development of
plasmonic nanocavities which facilitate light-matter strong coupling in ambient
conditions. The most robust example is the nanoparticle-on-mirror (NPoM)
structure whose geometry is controlled with subnanometer precision. The excited
plasmons in such nanocavities are extremely sensitive to the exact morphology
of the nanocavity, giving rise to unexpected optical behaviors. So far, most
theoretical and experimental studies on such nanocavities have been based
solely on their scattering and absorption properties. However, these methods do
not provide a complete optical description of a NPoM. Here, the NPoM is treated
as an open non-conservative system supporting a set of photonic quasinormal
modes (QNMs). By investigating the morphology-dependent optical properties of
nanocavities, we propose a simple yet comprehensive nomenclature based on
spherical harmonics and report spectrally overlapping bright and dark nanogap
eigenmodes. The near-field and far-field optical properties of NPoMs are
explored and reveal intricate multi-modal interactions.
