Slow-light and high-sensitivity terahertz sensing enabled by multifunctional resonant metasurfaces

Resonant metasurfaces provide a promising platform for manipulating light–matter interactions at terahertz frequency, enabling functionalities such as slow-light propagation and high-sensitivity sensing. Here, we demonstrate a symmetry-controlled metasurface that supports a tunable transition between quasi-bound states in the continuum (quasi-BICs) and an electromagnetically induced transparency (EIT)-like response. By varying the rotation angle of the meta-atoms, the system evolves from an isolated quasi-BIC state to a strongly coupled regime where the quasi-BIC coherently interacts with an electric dipole mode, giving rise to a pronounced EIT window. Multipole decomposition reveals the evolution of the radiative channels and clarifies the mechanism underlying the BIC–EIT transition. Owing to the hybrid spectral response, the metasurface simultaneously supports slow-light propagation with group delays up to 30.3 ps in simulations and 14.2 ps experimentally, and high-sensitivity terahertz sensing enabled by the quasi-BIC. As a proof-of-concept application, in situ detection of furfural in transformer insulating oil is demonstrated with concentrations down to 1 ppm. These results establish symmetry-controlled metasurfaces as a versatile platform for integrating coherent wave manipulation and ultrasensitive sensing in terahertz photonic devices.