External excitation enabled chirality reversal of exceptional points in an effective anti-PT-symmetric non-Hermitian system

As a kind of phase singularity in physical systems, exceptional points (EPs) give rise to a plethora of counterintuitive phenomena, such as the asymmetric state transfer and unidirectional reflection. Especially, the eigenvectors of EP are self-orthogonal, and an inherent chirality can be determined because of the “missing dimension.” Beyond traditional wisdom, in this work, we proposed theoretically an effective anti-parity-time symmetry in a non-Hermitian system under the external excitation, and the chirality reversal from resonance EP to absorption EP is demonstrated experimentally. The essence of this phenomenon is that the external input is involved in the establishment of the eigenstates of the system. Moreover, the critical condition of chirality reversal between two types of EPs in the non-Hermitian system is analytically given. This external excitation-controlled EP not only paves an effective way to study the fundamental non-Hermitian physics in an open system but also holds great potential in the applied photonic devices such as the chiral antenna, polarization converter, and wireless communication.