Independent Control of Quality Factor and Circular Dichroism via Intrinsic Chiral Plasmonic Bound States in the Continuum

Chiral plasmonic metasurfaces face a fundamental trade‐off between high circular dichroism (CD) and large quality (Q) factors due to radiative losses from asymmetric geometries. Although photonic bound states in the continuum (BICs) can suppress radiative losses to enhance Q‐factors, current plasmonic chiral quasi‐BIC designs remain limited to 2D configurations or infrared regimes, which limits the control over optical chirality and resonance linewidth. Here, 3D symmetry‐broken plasmonic metasurfaces operating in the visible spectrum are introduced, enabled by a nanofabrication breakthrough integrating thermal scanning probe lithography (t‐SPL) and anisotropic etching. This methodology achieves nm‐scale height control in out‐of‐plane architectures, enabling chiral quasi‐BIC resonances with independent tuning of CD (0–0.6) and Q‐factor (10–55)—establishing unprecedented performance benchmarks in chiroptical plasmonic metasurfaces. Crucially, the 3D height asymmetry parameter independently governs CD intensity, while BIC‐engineered symmetry breaking enables precise Q‐factor tuning via radiative loss modulation. Hyperspectral CD mapping reveals that the decoupled control mechanism originates from orthogonalized multipolar interactions between in‐plane lattice modes and out‐of‐plane plasmonic couplings. By resolving long‐standing fabrication challenges in 3D metasurfaces, a universal framework is established for applications demanding concurrent chiral selectivity and ultraconfined fields, including chiral nanolasers, enantioselective nonlinear systems, and quantum emitter interfaces with spin‐photon interactions.