Observation of giant chiral response driven by quasi-bound states in the continuum in bilayer microwave metasurfaces

High-Q chiral responses can be achieved in symmetry-breaking metasurfaces based on quasi-bound states in the continuum (q-BICs). However, deep-subwavelength single-layer metallic metasurfaces in the microwave regime face inherent limitations in breaking out-of-plane symmetry to achieve chiral response. To overcome this challenge, we theoretically propose and experimentally implement a bilayer helical chiral metasurface in microwave frequencies, which is systematically verified to exhibit a high-Q chiral response with large circular dichroism (CD). The structure supports two bound states in the continuum (BIC) modes exhibiting distinct chiral characteristics, with one achieving strong extrinsic chirality under oblique incidence, while the other attaining strong intrinsic chirality with a CD value exceeding 0.9 over a wide range of asymmetry parameters through out-of-plane symmetry breaking. The evolution of the far-field polarization states of the two BIC modes reveals the generation mechanism of the strong chirality. At last, we perform microwave experiments to validate the intrinsic chiral response of the designed metasurface with out-of-plane symmetry breaking, and the microwave experimental measurements agree well with the simulation results. This work opens new avenues for high-performance microwave chiral metasurfaces, with promising applications in polarization control, chiral sensing, and next-generation wireless communication systems.