Chirality reversal of exceptional points in a dual-channel graphene-tunable terahertz metasurface

In non-Hermitian systems, a special degeneracy known as an exceptional point (EP) arises from the coalescence of both eigenvalues and eigenvectors of the Hamiltonian. Chiral EPs, which exhibit polarization selectivity, have enabled numerous unique phenomena and potential applications, such as polarization selection, asymmetric energy transport, chiral enhancement effects, and topological manipulation. However, previous studies have typically realized chiral EPs by coupling two resonant structures with intrinsic chirality. This approach necessitates the separate fabrication of two metasurfaces and results in single-functionality devices, making flexible and controllable chiral reversal on a single platform challenging. To overcome this limitation, this study proposes a metasurface based on a triple-coupled resonator system. By tuning the Fermi energy of the graphene integrated at the gaps of two split-ring resonators, we modulate their dissipative loss, thereby achieving tunable chiral reversal on a single metasurface. Furthermore, by leveraging the phase-transition property of VO2, we realize chiral reversal in both transmission and reflection channels. This design not only overcomes the limitations of conventional methods but also paves the way for applications in reconfigurable chiral devices, polarization-selective photonic components, optical information processing with metasurfaces, and novel topological photonic platforms.