Can Weak Chirality Induce Strong Coupling between Resonant States?

ABSTRACT

Strong coupling between resonant states is usually achieved by modulating intrinsic parameters of optical systems, e.g., the refractive index of constituent materials or structural geometries. Externally introduced chiral enantiomers may couple resonances, but the extremely weak chirality of natural enantiomers largely prevents the system from reaching strong coupling regimes. Whether weak chirality could induce strong coupling between resonant states remains an open question. Here, we realize strong coupling between quasibound 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. Our findings present a distinct strategy for manipulating optical coupling between resonances, revealing opportunities in chiral sensing, topological photonics, and quantum optics.