Extreme Polaritonic Interactions in a Room‐Temperature Designable Sub‐Nanocavity Quantum Electrodynamic Platform

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 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 ( meV, ) and ultrastrong lower‐branch polaritonic photoluminescence with up to 10‐fold enhancement. This architecture provides a controllable pathway to access picocavity‐like behavior and opens new opportunities for single‐molecule spectroscopy and the exploration of nano‐cQED.