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, limiting 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 0.324–0.417 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 intrinsi-cally supports multiple chiral bound states in the continuum, which are converted into chiral QBICs through con-trolled breaking of in-plane inversion symmetry. By optically tuning the conductivity of silicon using continu-ous-wave laser excitation, both the broadband chiral response and the QBIC resonances can be dynamically modu-lated. This work establishes a versatile and scalable electromagnetic platform for chiral wave manipulation, polariza-tion control, and multifunctional terahertz imaging.