Reconfigurable high-Q terahertz chiral sensing enabled by quasi-bound states in the continuum in a Ge2Sb2Te5 metasurface

Abstract

Achieving high-Q resonances in active metasurfaces is fundamentally limited by the intrinsic optical loss of phase-change materials (PCMs). In order to address this limitation, we propose a reconfigurable terahertz chiral metasurface based on Ge₂Sb₂Te₅ (GST225) that excites intrinsically chiral quasi-bound states in the continuum (quasi-BICs) through controlled in-plane symmetry breaking of an I-shaped unit cell. When GST225 is in the amorphous state, the metasurface supports a narrow, high-Q chiral resonance with a circular dichroism (CD) of 0.98 and a Q factor of 1150, effectively suppressing the total damping of the system. This quasi-BIC further enables ultrasensitive refractive-index sensing, achieving a sensitivity of 0.61 THz/RIU and a figure of merit (FOM) of 305. In contrast, the crystalline phase produces a broadband, spectrally flat chiral response (CD > 0.98), allowing reversible switching between broadband and high-Q narrowband chiral modes using only the GST225 phase transition. This work demonstrates the realization of intrinsically chiral quasi-BIC-driven broadband-to-narrowband switching with simultaneously high CD, high Q, and high FOM in the terahertz regime. These results offer a promising approach for developing multifunctional terahertz devices capable of both high-performance chiral sensing and dynamic spectral reconfiguration.