Extreme Optical Field Confinement and Enhancement in a Plasmonic Picopatch within a Nanoparticle-on-Mirror Resonator

TL;DR

This paper provides a detailed theoretical analysis demonstrating that a nanogap with a picopatch in a nanoparticle-on-mirror structure can confine and enhance optical fields to extremely small volumes (~1 nm^3) with high tunability and strong mode coupling.

Contribution

It introduces the concept of picopatches in NPoM structures as a novel way to achieve extreme optical confinement and enhancement, supported by classical simulations and analysis.

Findings

Electric field enhancement up to 2000-fold in the picopatch

Mode volumes approaching 1 nm^3

Strong coupling and anti-crossing of polaritonic resonances

Abstract

Plasmonic resonances in metallic nanogaps can confine light into nanometric regions, but reaching modes of volume $\approx 1$ nm$^3$ remains challenging. Here we present a detailed theoretical analysis of the optical modes of a nanoresonator that contains a picopatch formed by the lifting of a few gold atoms in the gap of a Nanoparticle-on-Mirror (NPoM) structure. This configuration is motivated by recent experiments that suggest that local lifting of an atomic monolayer from metallic substrates can occur randomly due to optical or thermal forces. We show through classical simulations that the plasmonic modes associated with the picopatch geometry can confine light to extremely small regions and are highly sensitive to the size and shape of the picopatch, enabling broad tunability. Furthermore, these modes can couple strongly with other nanocavity modes of the structure, as identified by the presence of a clear anti-crossing of the resulting polaritonic resonances. Remarkably, up to $\approx 2000$-fold electric field enhancement in the middle of the picopatch and tiny effective mode volumes that approach $\approx 1$ nm$^3$ are obtained. We also confirm that changing the morphology of the picopatch does not modify the qualitative findings, and verify that increasing the absorption losses in the classical simulations, to mimic quantum (non-local) effects in the metal permittivity, decreases the electric field enhancement only moderately. Compared to the standard picocavities formed by single-atom protrusions, this work shows that picopatches are an intriguing alternative to reach extreme optical field confinement and enhancement in plasmonic cavities.