Multiband opposite-sign chirality and sensing empowered by multiple quasi-BICs in all-dielectric metasurfaces.

Chiral metasurfaces have emerged as powerful platforms for manipulating light polarization, with broad applications in sensing and optical communication. However, achieving high-performance, multiband chiral responses remains challenging because it requires simultaneously managing multiple resonances. In this work, we propose a planar all-dielectric chiral metasurface based on an array of bowtie-shaped nanostructures that enables versatile multiband intrinsic and extrinsic chiral responses. By strategically introducing perturbations to break structural symmetry, we successfully excite three distinct quasi-bound states in the continuum (quasi-BICs). At a geometric perturbation of δ = 40 nm, Mode 1 achieves near-perfect left-handed circularly polarized (LCP) transmission (TLCP = 0.994, CD = -0.93). Under oblique incidence (θ = 4°), Modes 1 and 2 exhibit strong LCP (CD = -0.84) and RCP (CD = 0.63) responses. Multipole decomposition confirms that magnetic dipole (MD) and toroidal dipole (TD) moments, respectively, dominate these resonances. Furthermore, the metasurface achieves a high FOM of 4830.4 RIU-1 for intrinsic chirality. This work provides an efficient strategy for multiband chiral photonics and integrated devices.