Evolution of pairwise scattering exceptional points via acoustic metasurfaces

Degenerate states including exceptional points (EPs) and diabolic points (DPs) arise due to the underlying symmetry in physical systems. The interplay between different symmetry breakings opens a promising route for exceptional wave manipulation. Here, we conceptually demonstrate and experimentally prove that breaking parity symmetry and time-reversal symmetry through spatial perturbation and non-Hermitian perturbation, respectively, result in the evolution of EPs in pairs in a scattering system. These pairwise scattering EPs, which are orthogonal to each other and can be interconverted by mirror inversion, evolve continuously in the perturbation space and ultimately merge into a special non-Hermitian degenerate state—a non-Hermitian DP. The EPs and DP observed here exhibit distinct topological structures from different planes in the perturbation space, thus both carrying hybrid topological charges. Based on these findings, we show that metasurfaces at EPs can encode differences in scattering asymmetry, allowing for a complete yet arbitrary wave manipulation beyond previously reported non-Hermitian scattering metasurfaces. Our findings establish a general framework for exploring extreme wave scattering through combined-perturbation-driven degeneration evolution.