Resonant exceptional points sensing in terahertz metasurfaces

The phenomenon of simultaneous degeneracy of eigenvalues and eigenstates in non-Hermitian systems, known as exceptional points (EPs), renders systems in the vicinity of EP highly sensitive to perturbations in the environment. Recently, research works on terahertz metasurfaces primarily focused on finding the degenerate eigenvalue using the transmission matrix, known as the scattering EP, while neglecting exploration of another form of EP—the resonant EP. In this study, we numerically, theoretically, and experimentally investigate the resonant EP in terahertz metasurfaces, whose unit cell is composed of two unidentical length metal rods in a two-energy level non-Hermitian system. By altering two system parameters, we can manipulate the coupling between the two resonators, thereby directly realizing the singularity of complex frequency through only a single measurement. Furthermore, our simulations and measurements indicate a sensitivity of up to 9046 GHz/(RIU·mm), which underscores the high sensitivity characteristics of EP. Our approach not only contributes a fresh perspective on understanding EP in non-Hermitian terahertz metasurfaces but also proposes alternative methods for high-sensitivity terahertz sensing.