Observation of Terahertz Planar Chirality Enabled by C‐Points in Accidental Bound States in the Continuum

Abstract C points, circularly polarized states in momentum space, play crucial roles in chiral emission, chiral lasers, and polarization‐resolved photodetection. Recently, symmetry‐protected bound states in the continuum (SPBICs) have been proposed in chiral metasurfaces with ultra‐high quality ( Q ) factors. However, the chirality enabled by C points typically requires breaking both in‐plane and out‐of‐plane symmetries, which poses significant fabrication challenges. Here, planar chirality enabled by C points based on two accidental BICs (ACBICs) is theoretically and experimentally demonstrated. By tailoring the unit lengths and of the rectangular lattice, the two ACBICs can transition to quasi‐ACBICs without breaking structural out‐of‐plane symmetry. Under this strategy, arbitrary eigenpolarization states at the point can be achieved, including C points, thus achieving planar chiral quasi‐ACBICs. Theoretically, by tailoring and , the C points can be shifted from north/south to south/north on the Poincaré sphere, enabling modulation of circular dichroism (CD) from 0.99/−0.99 to −0.99/0.99. Such a design of silicon metasurfaces with terahertz chiral resonants are experimentally confirmed. This work provides a universal yet simple design strategy for high‐ Q chiral metasurfaces, offering potential applications in chiral drug development, biosensing, and light detection.