不同于传统的厄米系统, 非厄米系统存在奇异点。当某一参数演化到奇异点时, 非厄米系统发生相变,即两个本征态合并成一个。利用耦合的超材料人造原子, 研究了二态和四态的非厄米系统。通过记录电磁波开放系统相干吸收峰频率随损耗的变化, 研究了非厄米系统本征值的各种演化轨迹。理论和电磁场仿真结果表明, 四态非厄米系统的本征态通过不同的方式合并, 可以形成五种奇异点;其中, 由两个以上本征态合并产生的奇异点, 称为高阶奇异点。

Unlike the traditional Hermitian system, a non-Hermitian system has exceptional points. When one parameter evolves to an exceptional point, a phase transition occurs in the non-Hermitian system, i.e., two eigenstates collapse and their corresponding eigenvalues merge into one. The two-state and four-state non-Hermitian systems are studied by using the coupled metamaterial artificial atoms. Various evolution traces of the eigenvalues of non-Hermitian systems are studied by recording the variation of peak frequency of coherent absorption with the loss parameter in an open system for electromagnetic waves. Theoretical and electromagnetic-field simulated results show that, five kinds of exceptional points can form by different combination ways of eigenstates in the four-state non-Hermitian system. Among them, the exceptional points at which more than two eigenstates merge is called as high-order exceptional points.