Achieving Higher-Order Exceptional Points in a Terahertz Metasurface.

Exceptional points (EPs) are unique features of non-Hermitian systems, where eigenvalues and eigenvectors coalesce, enabling functionalities such as ultrasensitive sensing and topological energy transfer. While higher-order EPs have been studied in microdisk cavities and circuits, their realization in metasurfaces has been challenging. In this work, we demonstrate the first realization of a third-order EP (EP3) in terahertz metasurfaces by tuning the near-field interaction between three gold split-ring resonators. This transition from second-order EP (EP2) to EP3 is validated by the simultaneous collapse of three eigenvalues and eigenfunctions. Using a global optimization algorithm combined with neural networks, we optimized the condition for EP3 in a high-dimensional parameter space. Our terahertz metasurface is suitable for ultrasensitive biosensing applications, offering a novel mechanism for EP3 realization and paving the way for advanced optical communication and sensing systems.