Maximum chirality in planar metasurfaces induced by strong coupling of quasi-bound states in the continuum

TL;DR

This paper demonstrates that maximum optical chirality in planar dielectric metasurfaces can be achieved through in-plane asymmetries that induce strong coupling of quasi-bound states in the continuum, leading to opposite circular dichroism responses.

Contribution

It introduces a novel method to realize maximum chirality in planar metasurfaces via in-plane symmetry breaking and strong QBIC coupling, expanding design possibilities.

Findings

Strong coupling of QBICs causes mode splitting with opposite CD responses.

Identification of exceptional points at the transition between coupling regimes.

Planar structures can achieve maximum chirality without vertical symmetry breaking.

Abstract

Achieving intrinsic optical chirality requires breaking all mirror symmetries of an object, and maximum chirality, which allows interaction with only one helicity of light, is particularly promising for applications such as chiral sensing, emission, and lasing. Traditionally, designing maximum chirality in dielectric metasurfaces has relied on precise engineering of vertical symmetry breaking, which presents significant fabrication challenges. Motivated by recent efforts towards enhanced chiral responses in planar structures, we demonstrate that maximum chirality can be achieved in a planar dielectric metasurface through controlled in-plane asymmetries. Specifically, the introduced perturbation induces strong coupling between two accidentally degenerate quasi-bound states in the continuum (QBICs) with orthogonal polarization states, which results in mode splitting into symmetric and antisymmetric modes, each exhibiting opposite circular dichroism (CD) responses. This behavior is quantitatively confirmed using quasinormal mode perturbation theory, by which we also identify a pair of exceptional points (EPs) at the transition between weak and strong coupling regimes. This work expands the existing approaches to maximum chirality in planar structures and aims to inspire future innovations in metasurface design.