Symmetry and shape on the plasmonic behaviour of nanocavities

Plasmonic nanocavities confine light in deep subwavelength volumes and in recent years have enabled unprecedented control on light-matter interactions. A characteristic example is the nanoparticle-on-mirror geometry, which allows for the fabrication of very robust plasmonic gaps with sub-nanometre accuracy. Due to the extreme field confinement, the size and shape of plasmonic nanocavities dominate their optical response. But so far, the community has mainly focused on idealized spherical nanoparticles, ignoring that during their synthesis nanoparticles actually acquire polyhedral shapes, and that many different geometries can be synthesised these days. Here, we provide a complete description of the plasmonic modes in nanocavities made of three commonly occurring polyhedral nanoparticles (cuboctahedron, rhombicuboctahedron, decahedron). We show that the shape and symmetry of these plasmonic nanocavities dominate both their near- and far-field response, with intricate and rich optical behaviour. Through a recombination technique, the total far-field emission profile is obtained for an emitter placed at various nanocavity positions, which is crucial for understanding how energy couples in and out of the nanocavity. This work paves the way towards controlling light-matter interactions in extreme plasmonic environments for various applications, such as photochemical reactions and non-linear vibrational pumping.