Near-Unity Chiral Terahertz Generation Driven by Asymmetric Quasi-Bound States in the Continuum

Efficient generation of terahertz (THz) radiation with both high emission strength and near-unity chiral purity remains a central challenge in nonlinear chiroptics. Here, we numerically demonstrate a framework for chiral THz generation from asymmetric lithium niobate (LN) metasurfaces enabled by chiral quasi-bound states in the continuum (q-BICs). By simultaneously breaking in-plane and out-of-plane symmetries, structural chirality is directly transferred to the optical-rectification process, enabling deterministic control of nonlinear THz emission. The resulting metasurface achieves a full-spectrum averaged THz circular dichroism (CD) of 0.99, while maintaining THz emission amplitudes comparable to nonlinear metasurface emitters under 100 fs excitation. The same symmetry-engineered mechanism yields an SHG-CD of 0.99, confirming the robustness of the nonlinear chiral response. In addition, the platform supports dynamic spectral tuning through pump-pulse engineering without sacrificing strong chiral selectivity. These results establish chiral q-BICs as a general strategy for engineering nonlinear chiro-optical interactions and provide a scalable route toward chip-integrated chiral THz photonic devices based on the lithium-niobate-on-insulator (LNOI) platform.