Nonlinear second-order photonic topological insulators

Higher-order topological insulators are a novel topological phase beyond the framework of conventional bulk-boundary correspondence(1,2). In these peculiar systems, the topologically non-trivial boundary modes are characterized by a co-dimension of at least two(3,4). Despite several promising preliminary considerations regarding the impact of nonlinearity in such systems(5,6), the flourishing field of experimental higher-order topological insulator research has thus far been confined to the linear evolution of topological states. As such, the observation of the interplay between nonlinearity and the dynamics of higher-order topological phases in conservative systems remains elusive. Here we experimentally demonstrate nonlinear higher-order topological corner states. Our photonic platform enables us to observe nonlinear topological corner states as well as the formation of solitons in such topological structures. Our work paves the way towards the exploration of topological properties of matter in the nonlinear regime, and may herald a new class of compact devices that harnesses the intriguing features of topology in an on-demand fashion.