Tunable terahertz intrinsic chiroptical absorption empowered by accidental bound states in the continuum

Terahertz (THz) bound states in the continuum (BICs) present a significant opportunity to achieve ultra-high quality (Q) factors in chiroptical responses, which are essential for high-resolution spin-selective devices. However, most high-Q chiroptical resonances rely on symmetry-protected BICs, attained either via electromagnetic oblique incidence or structural anisotropy to obtain extrinsic chirality (false chirality). Recently, the intrinsic chirality (true chirality) has been achieved through the breaking of both in-plane (C2) and out-of-plane (σz) symmetries. In this Letter, we utilize two accidental BICs to demonstrate dual-frequency intrinsic chirality without breaking σz symmetry. By carefully tuning structural parameters, we can manipulate the circularly polarized states (C points) located at the Γ-point in k-space, leading to the achievement of intrinsic chiral quasi-BICs. Under left/right circularly polarized wave illumination, the maximum optical absorption is 99.56%/5.69% at 0.4469 THz and 3.37%/98.78% at 0.4584 THz, with circular dichroism (CD) maxima of 0.939 and −0.954, and Q-factors of 2917 and 1433, respectively. The dual-frequency intrinsic chirality can be dynamically controlled through changing the Fermi level EF of the graphene substrate. The CD peak values for quasi-BIC 2 and quasi-BIC 3 can be continuously tuned within the ranges of 0.059–0.792 and −0.218 to −0.889, respectively, as EF increases from 0.05 to 1.00 eV. Our work provides a unique design path for achieving optical intrinsic chirality, with potential applications in THz biomedical detection, radar stealth, and other spin-selective devices.