Enhancing the light-matter interactions in two-dimensional materials via optical metasurfaces has attracted much attention due to its potential to enable breakthrough in advanced compact photonic and quantum information devices. Here, we theoretically investigate a strong coupling between excitons in monolayer WS₂ and quasi-bound states in the continuum (quasi-BIC). In the hybrid structure composed of WS₂ coupled with asymmetric titanium dioxide nanobars, a remarkable spectral splitting and typical anticrossing behavior of the Rabi splitting can be observed, and such strong coupling effect can be modulated by shaping the thickness and asymmetry parameter of the proposed metasurfaces, and the angle of incident light. It is found that the balance of line width of the quasi-BIC mode and local electric field enhancement should be considered since both of them affect the strong coupling, which is crucial to the design and optimization of metasurface devices. This work provides a promising way for controlling the light-matter interactions in strong coupling regime and opens the door for the future novel quantum, low-energy, distinctive nanodevices by advanced meta-optical engineering.

Strong coupling exhibits the unique ability to preserve quantum sates between light and matter, which is essential for the development of quantum information technology. To explore the physical mechanism behind this phenomenon, we employ the tight-binding method for expanding the temporal coupled-mode theory, with the absorption spectrum formula of coupled system directly obtained in an analytical way. It reveals all the physical meaning of parameters defined in our theory and shows how to tailor line shapes of the coupled systems. Here we set an example to manipulate the strong coupling in a hybrid structure composed of excitons in monolayer ${\mathrm{WS}}_2$ and quasibound states in the continuum supported by the ${\mathrm{TiO}}_2$ nanodisk metasurfaces. The simulated results show that a clear spectral splitting appeared in the absorption curve, which can be controlled by adjusting the asymmetric parameter of the nanodisk metasurfaces and well fitted through our theoretical predictions. Our work not only gives a more comprehensive understanding of such coupled systems but also offers a promising strategy in controlling strong light–matter coupling to meet diversified application requests.

We investigate the strong coupling between excitons and quasibound states in the continuum (BICs) resonance in a bulk ${\mathrm{WS}}_2$ metasurface. Here, we employ bulk ${\mathrm{WS}}_2$ to construct an ultrathin nanodisk metasurface, supporting the symmetry-protected magnetic dipole quasi-BIC resonance, which can self-hybridize with the excitons and lead to a strong light-matter interaction enhancement within the structure without the necessity for an external cavity. This strong coupling can be characterized by the considerable Rabi splitting of 159 meV and the clearly anticrossing behavior appearing in the absorption spectrum. Furthermore, we analyze such light-matter coupling by constructing a Hamiltonian model including the surplus excitons and tune the interaction from weak- to strong-coupling regimes via the tunability radiation loss of the quasi-BIC resonance. Our results have great potential for manipulating the exciton-polaritons at room temperature and provide a promising prospect for photonic devices that exploit strong coupling in applications.

The strong chiral light-matter interaction is crucial for various important fields such as chiral optics, quantum optics, and biomedical optics, driving a quest for the extreme intrinsic chirality assisted by ultra-high-quality ($Q$-) factor resonances. In this quest we propose a straightforward method to achieve extreme intrinsic chirality in lossless planar structures by manipulating the quasi-BIC through in-plane perturbation. The temporal coupled-mode theory is employed to derive the conditions necessary for achieving maximal intrinsic chirality. The quasi-BIC should be excited within the transparent spectral range of the structure and couple with $x$- and $y$-polarized waves with the same intensity but a phase difference of $\pi /2$. For an illustration, a planar chiral dielectric dimeric waveguide grating is designed that strongly interacts with left circularly polarized light while decoupling from right circularly polarized light through in-plane symmetry engineering. Furthermore, by adjusting the magnitude of the in-plane asymmetry, we can independently manipulate the $Q$ factors of the chiral quasi-BIC while maintaining near unity circular dichroism. Our results provide a simple yet powerful paradigm for achieving extreme intrinsic chirality on an easily manufacturable platform, which may have potential applications in chiral emission, chiral sensing, and enantiomer separation.