Bright Plasmons with Cubic Nanometer Mode Volumes through Mode Hybridization

We propose a new interpretation for light confinement in picocavities formed by ultrasmall metallic protuberances inside the gap of metal-insulator-metal nanoresonators. We demonstrate that the protuberances support dark resonances with mode volumes comparable to their geometric volumes and that their brightness can be enhanced by several orders of magnitude when they are strongly coupled with the modes of nanoresonators with nanometric dielectric spacers. With a simple and accurate closed-form expression, we clarify the role of gap plasmons in this coupling. Based on this understanding, we propose a general strategy, exploiting strong coupling to design extremely localized modes with cubic nanometer volumes and so-far unreached brightness.