Tuning of parity-time symmetry effect through exceptional points in non-Hermitian terahertz metasurface

In this article, we investigate parity-time (PT) symmetry in a non-Hermitian terahertz metasurface composed of two orthogonally oriented split ring resonators. Initially, one resonator is diagonally displaced relative to the other to study PT-symmetry in the system. The eigenvalues and eigenstates of the metasurface are examined at various displacement values to analyze the PT-symmetric phase transition. At a specific displacement, the metasurface design exhibits degenerate eigenvalues, marking the occurrence of an exceptional point. Beyond this point, the system transitions into a broken PT-symmetry phase. PT-symmetry is further explored by displacing the second resonator vertically and horizontally. This investigation reveals the presence of exceptional points during the transition from PT-asymmetry to PT-symmetry states. Additionally, the role of the split gap in the second resonator relative to the first is analyzed in this non-Hermitian system. It is observed that near-field coupling between the resonators plays a significant role during displacement. This is supported by the application of coupled mode theory to the metasurface. This comprehensive study on the impact of resonator displacement leading to exceptional points in a strongly coupled system holds potential for highly sensitive sensing applications and other advancements in photonics.