Non-Hermitian topological phase transitions controlled by nonlinearity

Manipulating topological invariants is possible by modifying the global properties of optical devices to alter their band structures. This could be achieved by statically altering devices or dynamically reconfiguring devices with considerably different geometric parameters, even though it inhibits switching speed. Recently, optical nonlinearity has emerged as a tool for tailoring topological and non-Hermitian (NH) properties, promising fast manipulation of topological phases. In this work, we observe topologically protected NH phase transitions driven by optical nonlinearity in a silicon nanophotonic Floquet topological insulator. The phase transition occurs from forbidden bandgaps to NH conducting edge modes, which emerge at a nonlinearity-induced gain-loss junction along the boundaries of a topological insulator. We find static NH edge modes and dynamic phase transitions involving exceptional points at a speed of hundreds of picoseconds, which inherently retain topological protections against fabrication imperfections. This work shows an interplay between topology and non-Hermiticity by means of nonlinear optics, and it provides a way of manipulating multiple phase transitions at high speeds that is applicable to many other materials with strong nonlinearities, which could promote the development of unconventionally robust light-controlled devices for classical and quantum applications.