Strong coupling systems enable coherent energy exchange between a light field and material electrons in nanoscale space. Active manipulation of this phenomenon by external stimuli is crucial for the design of advanced optoelectronic devices. Two neglected points severely hinder the improvement of tuning accuracy: irreversible variation in cavity morphology and lack of control over the dielectric environment which may change during the coupling process. Here we present a chemical fine-tuning of the strong plasmon–exciton coupling process in tailored Au@Ag nanocavities. The silver shell thickness was carefully controlled to tune the plasmon resonance wavelength with an accuracy of ∼8 nm and facilitate hot spots at the edges to boost the plasmon–exciton coupling strength. Hybrid polariton states were further regulated across the zero-detuning point with a spectral accuracy of less than 1 nm via tuning the solvent refractive index, and a Rabi splitting as large as 194 meV was observed at room temperature. The fine-tuning of strong plasmon–exciton coupling by an adjacent dielectric environment provides a novel route to manipulate excitons in molecules and possesses great potential for chemical or biological sensing.

A tunable plexcitonic material that sustains multimode hybridization is highly desirable, which is vital for advanced quantum devices. However, the research about regulations of biexcitons‐plasmon coherent states has rarely been reported. Here we apply single‐nanoparticle scattering spectroscopy correlative with SEM imaging to identify biexcitons‐plasmon interaction in a metal‐semiconductor hybrid structure composed of a single Au@Ag nanoparticle, J‐aggregates molecules and tungsten disulfide (WS ₂ ) monolayer. The mode competition within the localized plasmonic hotspots (∼240 nm ³ ) is revealed by continuously regulating the J‐aggregates spacer. Two distinct anticrossings are observed at both excitons resonances, and large double Rabi splittings (137 meV and 124 meV) are obtained successfully. We establish experimentally that J‐aggregates and WS ₂ monolayer are responsible for the middle polariton states, while plasmon rarely contributes. Further calculations show that plasmonic nanocavity enables coherent energy exchange with different excitons by providing a highly enhanced localized E‐field. In addition, we find that the multimode coupling strengths can be efficiently tuned by changing the cavity morphology and environment temperature, where the tuning spectral accuracy can reach up to 1 nm. Our findings uncover the distinctive properties of biexcitons‐plasmon polaritons, suggest an easily obtainable multiqubit states platform, and open up a new way to construct nanoscale photonic devices.

Hybrid quasiparticles produced by the strong interaction between nanostructures and excitons will exhibit optical chirality when one of the coupled components is chiral. Due to the tunability of hybrid states, the coupled system has potential applications in chiral devices and chiral sensing. However, reported chiral materials including chiral molecules and three-dimensional chiral structures in the coupled system limit the application due to the weak chiroptical responses and difficult fabrication, respectively. In this paper, we design chiral quasibound states in the continuum (q-BIC) metasurface by introducing planar symmetry-breaking and z-axis perturbation into an array structure whose unit cell is a C4 rotational symmetric disk. By tuning the polarization state of the eigenmode, a significant chiroptical response is obtained in our q-BIC metasurface. Furthermore, mode splitting is observed not only in the reflection spectrum but also in the circular dichroism (CD) spectrum in the chiral q-BIC and monolayer WS₂ strong coupling system, which indicates the realization of the exciton-polariton optical chirality. More importantly, one order of magnitude difference in the reflection to left and right circularly polarized light is achieved resulting in significant CD signals. Our work provides a new strategy to realize the exciton polaritons with significant chiroptical responses, which exhibits promising applications in on-chip chiral devices.