Realization of Unidirectional Soliton-like Edge States in Nonlinear Floquet Topological Insulators

Over the past decades, intriguing topological states have been extensively studied in electronic, photonic, ultracold atomic, and other systems. How topological edge states behave in the presence of inter-particle interactions and nonlinearity is an important and open question in this field. Here we demonstrate unidirectional soliton-like nonlinear states on the edge of photonic Floquet topological insulators formed by modulated optical waveguides. A well-controlled modulation of waveguide paths gives rise to a topologically nontrivial photonic band, characterized by an integer-valued topological invariant (winding number). Optical Kerr nonlinear interaction is introduced by using intense laser pulses. The observed non-diffracting, soliton-like wavepackets slowly radiate power because of the intrinsic gaplessness of the system. The rich and distinct localization characteristics measured as a function of nonlinearity confirms the existence of these soliton-like edge states. Our results are universal to other interacting bosonic systems, described by the focusing nonlinear Schrödinger equation or the attractive Gross Pitaevskii equation.


Ref. S. Mukherjee, M. C. Rechtsman, arXiv:2010.11359 [physics.optics] (2020).