Single Particle Plasmonic Nanocativy-Based Strong Coupling with Intrananogap Nanostructure

Achieving strong coupling in practical applications of cavity quantum electrodynamics (CQED) often poses significant challenges due to the delicate conditions required. In particular, the realization of strong coupling with single nanoparticles has been predominantly demonstrated using J-aggregate dye molecules. In this work, we present a novel CQED probe capable of transforming a nanocavity within a single nanoparticle. Our approach utilizes a plasmonic CQED structure based on a gold double nanoring (Au DNR) that incorporates an intraparticle nanogap. Through dark-field measurements on individual particles, we observe the emergence of exciton-plasmon polaritons, known as plexcitons, evidenced by the bifurcation of the scattering peak into two distinct resonances. Finite-element modeling is employed to elucidate the correlation between the Au DNR morphology and the cavity resonance wavelength relative to the exciton absorption, while temporal coupled-mode theory quantifies the strength of coupling. Verification of plexciton states is accomplished through photoluminescence measurements, which demonstrate light emission from the lower plexciton state. The proposed CQED nanocavity serves as a promising nanoprobe for the detection of small molecules within cells and offers a platform for groundbreaking biomolecular studies.