Near-unity circular dichroism and superchiral enhancement in quasi-BIC dielectric metasurface

Abstract

The intrinsically weak chirality of natural materials severely limits the probing of chiral light–matter interactions, which motivates the development of nanophotonic platforms for enhancing and controlling optical chirality. This work proposes and investigates an asymmetric double-slot disk (ADSD) dielectric metasurface to achieve strong intrinsic optical chirality and super-chiral near-field enhancement. The symmetric DSD is designed to support a true bound state in the continuum (BIC), which is confirmed through eigenmode simulations. Breaking the diagonal inversion symmetry transforms the ideal BIC into a quasi-BIC that exhibits near-unity circular dichroism (CD) with a quality factor (Q-factor) of ∼1000. Moreover, near-field simulation reveals that the quasi-BIC strongly confines chiral electromagnetic fields, showing an enhancement of the optical chirality density up to a factor of ∼40. Furthermore, introducing a weak chiral medium into the near field transfers the enhanced optical chirality to the far field, providing a substantial CD enhancement of ∼3000 compared to the structure without the metasurface. The near-unity chirality and strong enhancement of optical chirality density in the proposed metasurface enables high sensitivity chiral sensing and spectroscopy of weak chiral media.