Observation of modes reversion by encircling exceptional points in high-order non-hermitian system

The special properties of non-trivial topological structures usually come from a so-called edge mode which emerges in the band gap after the band inversion. The topological winding number of exceptional points (EPs) in non-Hermitian physical systems can be regarded as another type of topology. In photonics, the topological properties of EPs have been observed widely both in adiabatic and non-adiabatic second-order non-Hermitian systems. Here, we construct a higher-order non-Hermitian system with meta-atoms, including two independent second-order EPs. By circling EPs in different paths in the Riemann parameter surface, we observe extraordinary modes of reversion and repulsion, which is quite different from the conventional second-order EP systems. We propose a theoretical model of three-resonance non-Hermitian system and verify experimentally the mode reversion by encircling exceptional points in the microwave band, which may pave a new way for studying the topological physical mechanism of non-Hermitian systems, and provide a practical experimental system for its application.