High-CD chiral metasurfaces with robustness and active tunability via bound states in the continuum

Chiral metasurfaces have attracted considerable interest for applications in photonics, optical communications, and stereoscopic displays, particularly in scenarios requiring high robustness and tunable chiral responses. However, most chiral metasurfaces exhibit strong sensitivity to geometric perturbations, resulting in significant variations in circular dichroism (CD) and limited fabrication tolerance. In addition, their inherently static and narrowband chiral responses limit their applicability in dynamic or multifunctional photonic systems. In this work, we propose a chiral metasurface that integrates quasi-bound states in the continuum (Q-BICs) with a phase-change material. Through the simultaneous breaking of in-plane $C_2$ rotational symmetry and mirror symmetry, the structure achieves high CD between $-0.75$ and $-0.98$ across a wide range of asymmetry parameters (16 to 60) in the near-infrared region. Furthermore, dynamic tuning of the chiral response is achieved by inducing thermally driven, reversible phase transitions between the amorphous and crystalline states of the material. When the phase-change material is in the amorphous state, the metasurface achieves a maximum CD value of $-0.98$. In contrast, the highest CD value in the crystalline state is $-0.5$. The flexible and reversible tuning of chiral response with high robustness can be readily achieved by in-plane architecture without resorting to traditional out-of-plane symmetry breaking. These findings provide a promising pathway for the development of robust and tunable chiral optical devices.