【Link：https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.146102】

        Mode coupling is a cornerstone of modern photonics, giving rise to many important phenomena.However, it remains an open question whether weak chirality can induce strong coupling between resonance states due to the extremely weak κ of natural chiral substances (e.g. ,amino acids, proteins, sugars), resulting in very weak coupling strength.Recently, a joint team of Prof. Yang Chen, Dong Wu and Jiaru Chu from the School of Engineering, University of Science and Technology of China, and Prof. Cheng-Wei Qiu's team from the Department of Electrical and Computer Engineering, National University of Singapore, have realized the strong coupling between two optical modes induced by weak chiral materials for the first time. It is theoretically verified that the circular dichroism (CD) signal of chiral matter is enhanced by three orders of magnitude.

        Here, we realize strong coupling between quasi-bound states in the continuum of a high-Q metasurface, assisted with externally introduced enantiomers of weak chirality. We establish a chirality-involved Hamiltonian to quantitatively describe the correlation between the coupling strength and the chirality of such systems, which provides an insightful recipe for enhancing the coupling of resonant states further in the presence of quite weak chirality. Consequently, high-sensitivity chiral sensing is demonstrated, in which the circular dichroism signal is enhanced 3 orders higher than the case without strong coupling.

        This study provides a new way to realize the strong coupling of modes, and has important theoretical value in chiral optics and micro-nano optics. It also has important applications in chiral molecular detection, circularly polarized laser emission and quantum optics.

Fig.1. (a) Schematic of the paradigmatic system. The geometric parameters of the metasurface are period a = 550 nm,radius r = 200 nm,perturbation size Δ = 10 nm,and thickness t = 165.9 nm.(b)The characteristic frequencies of the upper and lower branches under different thicknesses t obtained by simulation and model calculation, as well as the eigenfrequencies of TM1 and TE4 modes before coupling (dashed line).

Fig.2. (a)CD spectra of metasurfaces of different thickness. (b)The absorption spectrum and CD spectrum of circularly polarized light at metasurface thickness t=165.9 nm.