Plasmonic nanocavity modes: From near-field to far-field radiation

TL;DR

This paper models plasmonic nanocavities, specifically nanoparticle-on-mirror structures, as open systems supporting quasinormal modes to provide a comprehensive understanding of their optical behaviors from near-field to far-field.

Contribution

It introduces a quasinormal mode framework for NPoMs, offering a complete optical description beyond traditional scattering and absorption analyses.

Findings

Identification of morphology-dependent nanogap eigenmodes

Spectral overlap of bright and dark modes

Insight into multi-modal optical interactions

Abstract

In the past decade, advances in nanotechnology have led to the development of plasmonic nanocavities which facilitate light-matter strong coupling in ambient conditions. The most robust example is the nanoparticle-on-mirror (NPoM) structure whose geometry is controlled with subnanometer precision. The excited plasmons in such nanocavities are extremely sensitive to the exact morphology of the nanocavity, giving rise to unexpected optical behaviors. So far, most theoretical and experimental studies on such nanocavities have been based solely on their scattering and absorption properties. However, these methods do not provide a complete optical description of a NPoM. Here, the NPoM is treated as an open non-conservative system supporting a set of photonic quasinormal modes (QNMs). By investigating the morphology-dependent optical properties of nanocavities, we propose a simple yet comprehensive nomenclature based on spherical harmonics and report spectrally overlapping bright and dark nanogap eigenmodes. The near-field and far-field optical properties of NPoMs are explored and reveal intricate multi-modal interactions.