Extreme Optical Field Confinement and Enhancement in a Plasmonic Picopatch within a Nanoparticle-on-Mirror Resonator
Plasmonic resonances in metallic nanogaps can confine light into nanometric regions, but reaching modes of volume $\approx 1$ nm$^3$ remains challenging. We present a detailed theoretical analysis of the optical modes of an experimentally-motivated nanoresonator configuration that contains a picopatch formed by the lifting of a few gold atoms in the gap of a Nanoparticle-on-Mirror (NPoM) structure. Classical simulations indicate that the plasmonic modes associated with the picopatch geometry can confine light to extremely small regions and are highly sensitive to the picopatch size and shape, enabling broad tunability. Furthermore, these modes can couple strongly with other nanocavity modes of the structure, as identified from a clear anti-crossing behavior of the polaritonic resonances. Remarkably, up to $\approx 2000$-fold electric field enhancement inside the picopatch and tiny effective mode volumes approaching $\approx 1$ nm$^3$ are obtained. Further, changing the morphology of the picopatch does not modify the qualitative findings, and 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, picopatches are an intriguing alternative to reach extreme optical field confinement and enhancement in plasmonic cavities.