Planar Chiral Metasurface Based on Coupling Quasi-Bound States in the Continuum

Chiral metasurface based on bound state in the continuum (BIC) possesses extremely high Q factor. However, intrinsic chiral BIC only can be realized by breaking out-of-plane mirror symmetry with complicated 3D structures. In this work, a design of planar intrinsic chiral metasurface supporting orthogonal BICs is proposed. The coupling strength, eigenfrequency, and radiation loss of the two quasi-BICs can be tuned by metasurface parameters. By adjusting the metasurface parameters, the two quasi-BICs merge to an exceptional point (EP). Around EP, one of the BICs transforms to chiral BIC with circular eigenstate polarization. In the parameter space, different elliptical eigenstate polarizations with the same helicity wind around the EP. The polarizations of the quasi-BICs nearly fully cover the Poincaré sphere by changing the geometry parameters. The superchiral fields of the quasi-BICs will also be greatly enhanced near the EP. The simulation result shows high circular dichroism at resonance frequency. The design provides a new strategy for polarization detection and chiral light sources.