Polarization-Driven Reversible Switching between Weak and Strong Coupling in Plasmonic Nanocavities.

Achieving dynamic control of light-matter coupling regimes in plasmonic nanocavities at room temperature is pivotal for quantum technologies but remains challenging due to limitations in polarization-selective excitation efficiency. Here, we demonstrate a polarization-driven reversible switch between weak and strong coupling at the vertical incidence. Leveraging radial vector beam (RVB's) cylindrical symmetry, we generate a confined longitudinal electric field that directly couples to nanoparticle-on-mirror plasmonic modes without sample tilting. This strategy enhances the local electric field by 327-fold (71% higher than linearly polarized beam, LPB) and compresses the mode field volume, amplifying the coupling strength to g = 107 meV, surpassing the strong coupling criterion. Using Rhodamine 800 as a quantum emitter, we demonstrate reversible all-optical switching between a Purcell-enhanced weak coupling regime (under LPB) and a strong coupling regime with 32.8 meV Rabi splitting (under RVB) within the molecule-nanocavity coupling system characterized by highly resolved Rabi splitting in the fluorescence spectra. Further optimization via Au nanoparticle size (R = 40 nm) and collective molecular coupling (N ≥ 5) establishes a ternary synergy for robust quantum control. This noninvasive, polarization-mediated platform enables on-demand manipulation of quantum states for reconfigurable nanophotonic devices.