Robust quasi-bound states in the continuum in bilayer terahertz metasurface with S 4 point-group symmetry

Metasurface-based bound states in the continuum (BICs) enable exceptional spatiotemporal control of light and substantially enhance light–matter interactions, making them highly attractive for applications in sensing, nonlinear optics, and integrated photonics. However, conventional BICs in single-layer metasurfaces rely on in-plane symmetry and are highly susceptible to structural perturbations, offering limited design flexibility and constraining both their robustness and functional diversity. To overcome these limitations, we theoretically propose and experimentally demonstrate a new class of symmetry-protected quasi-BICs in bilayer terahertz metasurfaces. These states are realized by coupling in-plane rotational and out-of-plane reflective symmetries through S4 point-group operations. Moreover, we show that such QBICs exhibit strong robustness against structural defects and fabrication imperfections. This work overcomes the robustness limitations of conventional single-layer metasurface BICs and establishes a general design paradigm for photonic devices that harness BICs with enhanced stability and scalability.