Observation of Broadband Terahertz Chirality and Chiral Quasi-BICs in Silicon Membrane Metasurfaces

In the terahertz (THz) regime, broadband chirality, high-quality-factor chiral resonances, and active tunability have largely been realized in separate metasurface designs, which restricts their integration within a single platform. Here, we present a chiral silicon membrane metasurface platform that unifies broadband THz chirality, chiral quasi-bound states in the continuum (QBICs), and dynamic tunability in a compact, single-material architecture. We demonstrate a pronounced broadband chiral response spanning from 0.324 THz to 0.417 THz, with an average circular dichroism of 0.782 and a maximum value of 0.883, enabling spin-selective broadband THz imaging. The membrane geometry intrinsically supports multiple chiral bound states in the continuum, which are converted into chiral QBICs through controlled breaking of in-plane inversion symmetry. By optically tuning the conductivity of silicon using continuous-wave laser excitation, both the broadband chiral response and the QBIC resonances can be dynamically modulated. This work establishes a versatile and scalable electromagnetic platform for chiral wave manipulation, polarization control, and multifunctional THz imaging.