Giant Circular Dichroism in Chiral Broad Flat-Band Quasi-BIC Metasurface.

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.