The optical properties of nanogap plasmonic cavities formed by a NanoParticle-on-Mirror (NPoM, or patch antenna) are determined here, across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. Full understanding of the confined optical modes allows these nanocavities to be utilized in a wide range of experiments across many fields. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter G = n/t 0.47 . Simple tuning of mode resonant frequencies and strength is found for each quasi-normal mode, revealing a spectroscopic "fingerprint" for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. This is applied to determine the most likely nanoscale morphology of facets hidden below each NPoM in experiment, as well as to optimize the constructs for different applications. Simple scaling relations are demonstrated, and an online tool for general use is provided.

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.

Plasmonic nanoantennas have the ability to confine and enhance incident electromagnetic fields into very sub-wavelength volumes, while at the same time efficiently radiating energy to the far-field. These properties have allowed plasmonic nanoantennas to be extensively used for exciting quantum emitters—such as molecules and quantum dots—and also for the extraction of photons from them for measurements in the far-field. Due to electromagnetic reciprocity, it is expected that plasmonic nanoantennas radiate energy as efficiently as an external source can couple energy to them. In this paper, we adopt a multipole expansion (Mie theory) and numerical simulations to show that although reciprocity holds, certain plasmonic antennas radiate energy much more efficiently than one can couple energy into them. This work paves the way towards designing plasmonic antennas with specific properties for applications where the near-to-far-field relationship is of high significance, such as: surface-enhanced Raman spectroscopy, strong coupling at room temperature, and the engineering of quantum states in nanoplasmonic devices.

Plasmonic nanoantennas are able to produce extreme enhancements by concentrating electromagnetic fields into sub-wavelength volumes. Recently, one of the most commonly used nanoantennas is the nanoparticle-on-mirror geometry, which allowed for the room temperature strong coupling of a single molecule. Very few studies offer analysis of near-field mode decompositions, and they mainly focus on spherical and/or cylindrically-faceted nanoparticle-on-mirror geometries. Perfectly spherical nanoparticles are not easy to fabricate, with recent publications revealing that a rhombicuboctahedron is a commonly occurring nanoparticle shape – due to the crystalline nature of metallic nanoparticles. In this paper, we perform a quasi-normal mode analysis for the rhombicuboctahedron-on-mirror nanoantenna and map the field distributions of each mode. We examine how the geometry of the cavity defines the near-field distribution and energies of the modes, and we show that in some cases the mode degeneracies break. This has a significant impact on the radiative emission and far-field profile of each mode, which are measured experimentally. Understanding how realistic nanoantenna geometries behave in the near-field and far-field helps us design antennas with specific properties for controlling and sensing quantum emitters in plasmonic systems.

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