Higher-order exceptional points unveiled by nilpotence and mathematical induction

Abstract: Non-Hermitian systems can have peculiar degeneracies of eigenstates called exceptional points (EPs). An EP of $n$ degenerate states is said to have order $n$, and higher-order EPs (HEPs) with $n \ge 3$ exhibit rich intrinsic features potential for applications. However, traditional eigenvalue-based searches for HEPs are facing fundamental limitations in terms of complexity and implementation. Here, we propose a design paradigm for HEPs based on a simple property for matrices termed nilpotence and concise inductive procedure. The nilpotence always guarantees a HEP with designated order and helps divide the problem. Our inductive routine can repeatedly double EP order starting from known designs, such as a $2 \times 2$ parity-time-symmetric Hamiltonian. By applying our framework, we readily design reciprocal photonic cavity systems operating at HEPs with up to $n=14$ and find their unconventionally chiral, transparent, and enhanced responses. Our work opens up extensive possibilities for investigations and applications of HEPs in various physical systems.