Tunable quasi-bound states in the continuum with intrinsic chirality in a phase-change metasurface.

The realization of switchable high- factors with intrinsic chirality is of paramount importance for various applications involving active and nonlinear metadevices that exhibit chiral responses. Here, we present a chiral metasurface that employs amorphous phase-change material, which enables the formation of quasi-bound states in the continuum characterized by inherent optical chirality. By simultaneously breaking both in-plane and out-of-plane symmetries, the metasurface supports multiple high- resonances that produce pronounced circular dichroism responses. Through eigenmode simulations and multipolar decomposition analysis, we demonstrate that the quasi-bound states in the continuum originate from various multipolar contributions. These modes are distinguished by sharp spectral features and circular dichroism values nearing unity, indicating near-perfect polarization selectivity. Furthermore, we illustrate that the intrinsic chirality and high- response can be actively deactivated by inducing the phase transition in the GeSbTe material to its crystalline state, which leads to significant optical losses and the suppression of the quasi-bound states in the continuum. Our findings establish a versatile and tunable photonic platform for dynamically reconfigurable chiroptical devices, offering promising prospects for applications in optical communication, biosensing, and quantum photonics.