Extrinsic and intrinsic chiral dual bound states in the continuum with opposite circular dichroism supported by an all-dielectric metasurface.

The precise modulation of circular dichroism (CD) is essential for applications in polarization control, sensing, and imaging. Here, we report a novel, to the best of our knowledge, dual-CD response mechanism based on bound states in the continuum (BICs) in an all-dielectric silicon metasurface. By leveraging two distinct topologically protected BICs and converting them into circularly polarized eigenstates, we first demonstrate a reversal of maximum chirality at oblique incidence without requiring any structural alteration. The CD signs across two frequency bands are opposite and can be dynamically tuned by introducing controlled perturbations to the metasurface geometry. Remarkably, the quasi-BIC modes exhibit near-ideal CD values of ±1, enabling complete spin-selective transmission. Subsequently, we introduced the inclination angle of the structure to move the circular polarization state near the center of the Brillouin zone, achieving a giant intrinsic chirality inversion. These controllable chirality reversals offer an unprecedented degree of freedom in multiband optical response design. Finally, we further demonstrate its application in enhancement of multiband chiral nonlinearity, showcasing spin-polarization-dependent nonlinear effects across different spectral bands. This work provides a generalizable approach for extrinsic and intrinsic chirality control in achiral metasurfaces and opens new avenues for developing multifunctional devices in areas such as biodetection, chemical sensing, and integrated chiral photonic systems.