Robust Chiral High-Q Response Driven by Quasi-Bound Flatband in the Continuum

Chiral bound states in the continuum (BICs) with infinite radiation quality factor (Q-factor) and topological robustness have provided a new paradigm for enhancing chiral light-matter interaction. However, the strong momentum-space dispersion leads to a rapid collapse of the Q-factor when the resonance deviates from the ideal BICs condition, severely limiting its practical applications. Here, by strategically engineering the interaction among four counterpropagating folded guided modes, we theoretically propose and numerically realize the chiral flatband quasi-BICs. The pronounced chiroptical response arises from the destructive interference of two antiphase oscillating intrinsic modes, whereas the flatband characteristic originates from the interactions among opposite curvature hybrid guided modes. Compared with conventional BICs featuring strongly dispersive bands, our dispersionless BICs provide stronger light field confinement, enabling highly efficient third-harmonic generation (conversion efficiency up to 2.3 × 10^-3 under a pump intensity of 1 MW/cm^-2). Our work establishes a new benchmark for miniaturized nonlinear chiral sources and may pave the way for the development of novel chiral optical device.