The precise structural details of metallic nanogaps within optical antennae are found to dramatically modify the plasmonic response, producing a complex pattern of electromagnetic modes that can be directly observed in scattering experiments. We analyze this situation theoretically in the nanoparticle-on-mirror construct, which forms a plasmonic nanogap sensitive to even atomic-scale restructuring of nanoparticle morphology. We focus on the effect of nanoparticle faceting, which allows the formation of ultrathin cavities between the particle and the underlying metallic film in the nanoparticle-on-mirror geometry. Two different sets of modes are identified: longitudinal antenna modes, which are strongly radiative and excited for all facet width ranges, and transverse cavity modes produced at large facets and exhibiting extreme confinement. The interaction and hybridization of antenna and cavity modes is determined by their symmetry and the precise morphology of the nanogap edges. Understanding such complex optics from nanoparticle-on-mirror structures is important to elucidate a wide variety of emerging photochemical and optoelectronic processes.

C.T., R.E., and J.A. acknowledge financial support from Project No. FIS2013-41184-P of MINECO, and Project No. Etortek 2014-15 of the Department of Industry of the Basque Government. R.E. also acknowledges support from the program Fellows Gipuzkoa from the Gipuzkoako Foru Aldundia through FEDER funds of the European union “Una manera de hacer Europa.” J.J.B. acknowledges financial support from EPSRC Grants No. EP/G060649/1, No. EP/L027151/1, and No. EP/K028510/1, ERC Grant No. LINASS 320503, and Ikerbasque. J.M. acknowledges support from the Winton Programme for the Physics of Sustainability.

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