Tunable multi-frequency exceptional points in non-Hermitian terahertz metasurfaces

Abstract Exceptional points (EPs) in non-Hermitian metasurfaces have garnered considerable attention due to their unique advantages in cutting-edge applications such as ultra-sensitive sensing and unidirectional reflectionlessness. However, existing studies on metasurfaces employing both active and passive tuning mechanisms can only observe a single EP, which fails to meet the requirements for multi-frequency responses or multifunctional integration, thus limiting the enhancement of device performance. In this study, we design a terahertz (THz) non-Hermitian metasurface device that is actively tuned by the phase-change material VO 2 . By keeping the geometric dimensions of the device unchanged, we achieve the simultaneous induction and detection of multi-frequency EPs at multiple frequency points. Through the regulation of VO 2 conductivity, the gain-loss distribution of the system can be continuously controlled, leading to the degeneracy of eigenvalues and eigenstates across multiple discrete frequency bands, thereby forming multi-frequency EPs. Furthermore, the design of chiral structures demonstrates that, under identical conductivity conditions, the eigenstates of the original metasurface structure and its chiral counterpart can degenerate into circularly polarized states with opposite rotations, enabling the switching of polarization chirality. These results illustrate that the deep integration of phase-change materials, non-Hermitian photonics, and electromagnetic manipulation in metasurfaces provides a novel design paradigm for the dynamic regulation of multi-frequency EPs and circular polarization control, laying a foundation for the development of high-performance and multifunctional integrated photonic platforms in the THz regime.