Exceptional point driven by asymmetry in meta-resonators in a non-Hermitian complementary terahertz metasurface

In this paper, we investigate a non-Hermitian open terahertz metasurface comprising complementary structures capable of exhibiting parity-time symmetry. The metasurface consists of two asymmetric resonators of different sizes, representing effective gain and loss elements, placed orthogonally in a strongly coupled near-field configuration. When one resonator is displaced diagonally with respect to the other, the exceptional point appears where the system undergoes a phase transition from a PT-symmetric to a PT-asymmetric state. We fabricate the samples in a clean room ambience to experimentally validate the exceptional points. Terahertz time-domain spectroscopy is performed on the fabricated samples to experimentally corroborate the transmission properties observed in numerical simulations. We further employ coupled mode theory to analyze and distinguish between the PT-symmetric state, exceptional point, and PT-asymmetric state. Theoretical framework enables the calculation of eigenvalues, phase spectra, and eigenmodes associated with the metamaterial design, thereby again corroborating the simulation results. Furthermore, we construct the Poincaré sphere to visualize the orientation of the polarization states of the eigenmodes, which again suggest the presence of an exceptional point. The study holds potential to develop highly sensitive terahertz devices, addressing limitations of conventional PT-symmetric systems that rely on traditional gain and loss media.