Extreme polaritonic interactions in a room-temperature deterministic sub-nanocavity quantum electrodynamic platform

TL;DR

This paper demonstrates a stable, controllable nano-cavity quantum electrodynamics platform using van der Waals materials and gold clusters, achieving extreme light-matter interaction confinement at room temperature.

Contribution

It introduces a novel nano-cQED platform with ultrasmall gold clusters in a nanoparticle-on-mirror cavity, enabling picocavity-like behavior at room temperature.

Findings

Observation of multi-branch Rabi splittings

Ultrastrong polaritonic photoluminescence enhancement

Deep-subwavelength mode volumes achieved

Abstract

Pushing nanoscale optical confinement to its ultimate limits defines the regime of nano-cavity quantum electrodynamics (nano-cQED), where light--matter interactions approach the fundamental quantum limits of individual atoms, e.g., picocavities. However, realizing such extreme confinement in a stable and controllable manner remains a key challenge. Here, we introduce a van der Waals material-based nano-cQED platform by coupling monolayer MoS2 excitons to plasmonic sub-nanocavities formed via assembly of ultrasmall gold clusters (3-5 nm) in the nanogap of a nanoparticle-on-mirror nanocavity. These clusters emulate the field-confining role of atomic protrusions of the picocavities through a resonance-insensitive lightning-rod effect, achieving deep-subwavelength mode volumes. In this nano-cQED testbed, we observe pronounced multi-branch Rabi splittings and ultrastrong lower-branch polaritonic photoluminescence with up to 10^4-fold enhancement. This deterministic architecture provides a controllable pathway to access picocavity-like behavior and opens new opportunities for single-molecule spectroscopy and the exploration of nano-cQED.