Enhanced Strong Coupling of TMDC Monolayers by Bound State in the Continuum

Transition-metal dichalcogenides (TMDCs) monolayers have been considered a perfect platform for realizing exciton-polariton at room temperature due to their direct bandgap and large binding energy of exciton. It is well established that strong coupling depends on the field enhancement induced by optical nanocavity with a high-quality factor (Q-factor). In this work, the enhanced strong coupling between the exciton of TMDC monolayer and the cavity resonance based on a symmetry protected magnetic dipole (MD) bound state in the continuum (BIC) and electric toroidal dipole (TD) BIC is demonstrated. It is found that strong coupling can be realized between the exciton in a TMDC monolayer and quasi-BIC (QBIC) by varying the incidence angle, period of the grating, the width of the slit, and the position of the slit for symmetry protected BIC. Besides, strong coupling between exciton and TD BIC is also demonstrated by integrating a WSe₂ monolayer onto a compound grating. It is found that Rabi-splitting strongly depends on the location of TMDC monolayer, Q-factor of the resonator, and the thickness of the structure. By carefully adjusting these three critical parameters, Rabi-splitting can be up to 38 (1L-WSe₂), 65 (1L-WS₂), 40 (1L-MoSe₂), and 60 meV(1L-MoS₂).