Self-Aligned Nanoring Doublet as a Single Particle Nanocavity for Strong Coupling Interaction with Biomolecules

Achieving strong coupling in practical applications of cavity quantum electrodynamics 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 utilize a plasmonic structure based on a gold self-aligned nanoring doublet (Au SAND) that incorporates an intraparticle nanogap. Through dark-field scattering measurements on individual particles, we observe the emergence of exciton-plasmon polaritons, i.e., 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 SAND morphology and the cavity resonance wavelength relative to the exciton absorption, while temporal coupled-mode theory quantifies the strength of coupling. 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.