Chirality-induced strong coupling of leaky guided modes in a dielectric metasurface waveguide governed by symmetry breaking

Coherent coupling between optical modes plays a vital role in manipulating the light field and its interaction with matter. Here, we investigate the chirality-induced strong coupling between leaky guided modes (GMs) with orthogonal polarizations in a chiral metasurface waveguide in the near-infrared frequencies, which consists of a square lattice of Ge dimers with broken symmetry and an underlying Si₃N₄ waveguide layer deposited on a silica substrate. We find that mode coupling between two orthogonal GMs can be induced by introducing chirality to the metasurface waveguide when illuminated with circularly polarized light. The coupling strength can be manipulated from the weak to strong coupling regime by simply changing the rotation angle of nanorods in the metasurface, enabling the generation of an exceptional point (EP) and large Rabi splitting (∼ 2 meV). Moreover, the polarization state of the hybrid mode can be continuously modified from linear to circular polarization via strong coupling. More intriguingly, we reveal that mode coupling is associated with the hybridization of circular dichroism (CD) of leaky GMs in the chiral coupled system, rendering a dual-band high CD response with a linewidth of 0.18 nm. Our findings indicate the possibility of manipulating the coupling of high-Q guided modes with chiral nanostructures by symmetry breaking and suggest potential applications in efficient chiral sensing, polarization detection, multimodal chiral lasing, and nonlinear optics.