Robust chirality via merging accidental BICs with net zero topological charge

Abstract

Chiral metasurfaces leveraging bound states in the continuum (BICs) offer a powerful route for enhancing light–matter interactions. However, existing quasi-BIC architectures typically face a fundamental trade-off between high quality (Q) factors and wide-angle chiral operation. Most designs confine strong circular dichroism (CD) to isolated points in momentum space (k-space) and often rely on intricate three-dimensional (3D) meta-atoms, rendering them highly sensitive to fabrication imperfections and angular misalignment. Here, we experimentally realize a planar dielectric metasurface that supports a wide k-domain chirality arising from accidental BICs with a net zero-topological-charge (ZTC). By lifting a Dirac-type degeneracy through controlled in-plane and out-of-plane symmetry breaking, we induce a deterministic topological evolution in which the same-handed circularly polarized (C) points migrate toward and accumulate near the Γ point, while the opposite-handed singularities annihilate or shift to higher k-space. This mechanism delivers record-level performance—an ultrahigh Q-factor (~ 104), near-unity linear and nonlinear CD (0.99/0.999), and robust angular coverage (|kxP/2π, kyP/2π|< 0.06)—all within a fabrication-friendly, single-layer dielectric platform. These results establish a new regime for chiral photonics, unifying high chiral purity, angular robustness, and topological stability in a scalable planar architecture.