Chiral optical modulation of Si-Ge2Sb2Te5 composite metasurfaces driven by quasi-bound states in the continuum

Chiral metasurfaces with high quality factors (Q-factors) are a promising platform for achieving chiral optical responses. However, the optical properties of most metasurfaces are fixed once they are fabricated. Here, we study the effect of phase-change material Ge 2 Sb 2 Te 5 (GST) on the chiral optics of a planar chiral Si metasurface driven by bound states in the continuum (BICs), as the refractive index of the phase-change material changes before and after the phase change. The planar chiral silicon metasurface is capable of generating near-unity (0.99) circular dichroism and giant (0.996) linear dichroism in the infrared region. Notably, phase-change material GST is integrated into the Z-shaped Si metasurfaces. We actively tune the dissipative loss by causing the GST to undergo a phase transition, thereby modulating the optical chirality. In addition, we numerically simulate the effect of the thickness of the phase-change layer and the embedded position on the optical response. Compared with single-functional metasurfaces, this device exhibits better flexibility and more functionalities. It is demonstrated that the optical chirality of the metasurface can be well controlled using the phase-change material GST.