Microwave seminar | 19 February 2024

The role of nonlinearity on topology has been investigated extensively in Hermitian systems, while nonlinearity has only been used as a tuning knob in a PT-symmetric non-Hermitian system. Here, this talk shows that nonlinearity plays a crucial role in the topological classification of a non-Hermitian system. We start with a simple and intuitive example by demonstrating with both theory and circuit experiments an exceptional nexus, a higher-order exceptional point with a hybrid topological invariant, within only two coupled resonators with the aid of nonlinear gain. The anisotropic critical behavior of the eigenspectra is verified with experiments. After that, we show that the eigenbasis of the Hamiltonian governing the system dynamics can still be complete at some nonlinear exceptional points. We present a circuit experiment to demonstrate one such NEP. Our findings lead to advances in understanding the peculiar features of nonlinear non-Hermitian systems, possibly opening new avenues for applications.