Theoretical investigation of active dual-band tuning of dielectric chiral metasurfaces via Q-BICs

While chiral metasurfaces based on quasi-bound states in the continuum (Q-BICs) enhance light-matter interactions, they are typically passive and single-band. This work introduces an actively tunable all-dielectric chiral metasurface that supports dual intrinsic Q-BICs resonances in the near-infrared. Through in-plane and out-of-plane symmetry breaking in Si, two high-quality (high- Q ) chiral modes with a near-unity circular dichroism of 0.93 are excited. Integrating the phase-change material Ge 2 Sb 2 Te 5 (GST) enables dynamic, mode-dependent chiroptical tuning: one mode provides a switchable on-off response for optical switching, while the other offers continuous, sensitive modulation for precision sensing. Eigenmode and multipole analyses trace these distinct behaviors to different dominant multipolar interferences. Consequently, this strategy delivers a reconfigurable, multifunctional platform that advances both the fundamental understanding of chiral Q-BICs and the design of integrated photonic devices.