On the symmetry and topology of plasmonic eigenmodes in heptamer and hexamer nanocavities

Plasmonics is expected to play a key role in nanotechnology, leading to intriguing routes in many engineering and biological applications. Recently, it has been realized that toroidal resonances could be an alternative to electric and magnetic resonances, which have governed the innovation of plasmonic applications so far. In a previous contribution, we proved the existence of toroidal moments in an oligomeric void-plasmonic structure 1]. In this article, we investigate the role of topology and symmetry in decomposing the various dipolar, quadrupolar, and toroidal moments, using energy-filtering transmission electron microscopy supported by three-dimensional finite-difference time-domain method simulations. The consequences of changing the topology on the toroidal character are discussed by comparing results obtained from nanoholes forming heptamer and hexamer nanocavity systems that were drilled into a thin silver film.