Multiple Exceptional Rings in One-Dimensional Perovskite Photonic Crystals

Exceptional rings (ERs) are high-dimensional non-Hermitian topologies formed by exceptional points, significantly enriching the topological properties of non-Hermitian systems. Because of the intricate topology and symmetry requirements, the realization of ERs generally demands complex structures and precise parameter tuning, resulting in relatively few experimental observations in high-dimensional periodic systems. Here, we show that even the simplest 1D non-Hermitian periodic systems can support multiple ERs, enabled by the system's multiple degrees of freedom which naturally accommodate diverse non-Hermitian perturbations. The proposed system can host one to four ERs as well as hybrid ER by simply tuning the non-Hermitian loss, with a bulk Fermi torus in momentum space serving as the fundamental origin of this tunability. Through experiments using 1D perovskite photonic crystals, we observe the ERs associated bulk Fermi torus at visible frequencies. Our work establishes a simple platform for realizing complex non-Hermitian topologies, enabling optical-frequency applications harnessing non-Hermitian physics.