Transition-induced Q-BIC-exciton polaritons in a permittivity-asymmetric system

Recent research on exciton polaritons (EPs) in two-dimensional semiconductors has garnered significant attention. Transition metal dichalcogenides, due to their exotic physical properties, hold great promise for applications in various fields, including Bose-Einstein condensation, nanophotonics, and quantum optics. In this work, using finite-element method calculations, we demonstrate a coupling regime transition induced multiple quasibound state in the continuum (Q-BIC) strong-coupling model in a permittivity-asymmetric tetramer metasurface system with ${C}_{4\mathrm{v}}$ symmetry that supports three high Q-factor resonance modes. A tungsten disulfide ($\mathrm{W}{\mathrm{S}}_{2}$) monolayer is positioned on the permittivity-asymmetric metasurface, enabling strong coupling between multiple strongly coupled Q-BICs and the $\mathrm{W}{\mathrm{S}}_{2}$ exciton, which can be modulated by adjusting the height and gaps within the tetramer. Interestingly, the resonances of the Q-BICs can form coupling regime transition induced hybridized EPs. Additionally, the coupling strength can be effectively controlled by manipulating the tetramer's gaps. This work represents the realization of EPs in a multiple Q-BIC permittivity-asymmetric system, and this coupling regime transition induced Q-BIC-EP permittivity-asymmetric system significantly reduces the mode volume, while simultaneously enhancing the Purcell effect and the photoluminescence efficiency. It offers promising pathways for practical device concepts in polaritonic applications and other fields of physical investigation, such as nanolasers, superfluidity, and quantum information technologies.