Observation of Dual-Band Intrinsic Chirality in Underetched Silicon Metasurfaces via Quasi-BICs

Intrinsic chirality via bound states in the continuum (BICs) offers a promising approach to spin-selective light-matter interactions but often relies on complex three-dimensional (3D) or 2.5D geometries that hinder CMOS compatibility. Here we demonstrate a fabrication-friendly strategy for intrinsic chiral quasi-BICs (QBICs) in silicon (Si) metasurfaces by exploiting a single-step underetch process to introduce controllable vertical asymmetry. Combined with mild in-plane perturbation, this enables independent control over two symmetry-breaking pathways, yielding dual-band intrinsic circular dichroism (CD) with a tunable sign and magnitude. We develop a chiral coupled-mode theory (CCMT) for universal design rules linking the sign of CD to the spinning-dependent coupling phase for maximizing intrinsic chirality. Guided by this framework, we experimentally demonstrate near-unity intrinsic chirality (CD = 0.90) and an ultrahigh quality factor exceeding 3000 in the telecom band. Our findings provide a CMOS-compatible route to robust intrinsic chirality and offer a versatile platform for advanced chiral metadevices.