High-Q flexible terahertz sensor based on quasi-bound states in the continuum

Metasurface devices applied in the terahertz (THz) band can effectively address the scarcity of natural materials for terahertz functional devices. They are capable of flexibly manipulating the amplitude, phase, polarization and other physical characteristics of terahertz waves, holding promising application prospects in the research and development of cutting-edge technologies such as next-generation wireless communication, imaging, sensing, and radar stealth. In this paper, a high-Q terahertz refractive index sensor based on quasi-bound states in the continuum (QBICs) is proposed. The structure consists of a metallic unit array deposited on a low-loss flexible polyimide substrate. When the designed array structure satisfies mirror symmetry and central symmetry, symmetry-protected bound states in the continuum (SP-BICs) can be excited in the transmission spectrum. By adjusting the geometric parameters of the metallic structure to break the structural symmetry, the transition from BICs to QBICs can be realized, thereby endowing the proposed terahertz metasurface sensor with a high-quality factor. The integrated application of polyimide and QBICs in terahertz sensing technology helps solve the key bottlenecks of insufficient Q-factor and flexible conformal measurement in terahertz sensor design and further broadens the application scope of terahertz technology.