Parity–time symmetry in a non-Hermitian toroidal terahertz metasurface

Abstract In this article, we investigate parity–time (PT) symmetry in a non-Hermitian terahertz (THz) metasurface comprising orthogonally placed metallic resonators exhibiting toroidal behavior. Metals with different conductivities are diagonally placed in proximity. PT symmetry is observed when toroidal resonators are displaced diagonally, owing to strong near-field coupling between resonators. This is observed through the appearance of an exceptional point (EP) when resonators are displaced to a certain distance, leading to optimal coupling. Beyond the EP, the proposed metasurface design undergoes a sudden change from the unbroken phase to the broken phase, resulting in a transformation from a PT-symmetric state to a PT-asymmetric state. These observations are also apparent from the measurements of eigenvalues, phase spectra, and eigenstates at different displacement values of one toroidal resonator with respect to the other. We propose a coupled mode theory to validate the numerically obtained transmission results. Furthermore, a Poincaré sphere and circular transmission components are reported to explain the emergence of the EP and hence PT symmetry in the toroidal metasurface. The study reported here enhances our fundamental understanding of PT symmetry and its emergence in THz metasurfaces via toroidal excitation, which can pave the way for innovative applications such as enhanced sensing, phase and polarization modulation, etc.