Full Control of Plasmonic Nanocavities Using Gold Decahedra‐on‐Mirror Constructs with Monodisperse Facets

Bottom‐up assembly of nanoparticle‐on‐mirror (NPoM) nanocavities enables precise inter‐metal gap control down to ≈ 0.4 nm for confining light to sub‐nanometer scales, thereby opening opportunities for developing innovative nanophotonic devices. However limited understanding, prediction, and optimization of light coupling and the difficulty of controlling nanoparticle facet shapes restricts the use of such building blocks. Here, an ultraprecise symmetry‐breaking plasmonic nanocavity based on gold nanodecahedra is presented, to form the nanodecahedron‐on‐mirror (NDoM) which shows highly consistent cavity modes and fields. By characterizing > 20 000 individual NDoMs, the variability of light in/output coupling is thoroughly explored and a set of robust higher‐order plasmonic whispering gallery modes uniquely localized at the edges of the triangular facet in contact with the metallic substrate is found. Assisted by quasinormal mode simulations, systematic elaboration of NDoMs is proposed to give nanocavities with near hundred‐fold enhanced radiative efficiencies. Such systematically designed and precisely‐assembled metallic nanocavities will find broad application in nanophotonic devices, optomechanics, and surface science.