Giant Circular Dichroism in Chiral Broad Flat‐Band Quasi‐BIC Metasurface

ABSTRACT Compact, high‐efficiency chiral light sources are essential for on‐chip integrated photonics, yet simultaneously achieving giant circular dichroism (CD), ultrahigh quality factor (Q), and angular robustness remains unresolved. Here, we report a chiral flat‐band metasurface that unifies giant CD, ultrahigh Q, and full angular resilience by merging symmetry‐broken bound states in the continuum (BICs) with a dispersion‐engineered photonic band. Platform‐tilted double rectangular nanopillars break both in‐plane and out‐of‐plane mirror symmetries, generating an intrinsical chirality and an ultraflat photonic band. The resulting quasi‐BIC resonance features a high Q‐value ≈ 1969 and near‐unity transmission CD (> 0.99). Crucially, the dispersionless ultraflat band ensures stable resonance and maintains high CD across a moderate angular range of ± 4°. Furthermore, strong slow light field confinement effectively quenches lateral leakage, enabling high ellipticity ( ρ > 0.99) and Q ≈ 1108 even in finite‐sized structures of merely eight periods. By synergistic integrating giant chiral response with flat‐band photonics, this strategy establishes a scalable route to ultralow threshold–integrated chiral photonic‐crystal surface‐emitting lasers (PCSELs) and angle robust chiral sensors. Compared with existing chiral flat‐band structures, our design achieves substantial, concurrent enhancement of circular dichroism and a quality factor, establishing a compelling platform for integrated chiral photonic devices.