Photonic Realization of a Transition Between Topological Phases Residing in the Weak and Strong Floquet Driving Regimes

A photonic Floquet topological insulator has previously been experimentally realized in an array of evanescently-coupled helical waveguides. In the topological regime probed by that experiment, the chirality of the single topological edge mode was the same as the chirality of the helices. Here we demonstrate the rich structure present in Floquet systems by fixing the helix chirality while moving into the strong driving regime to observe a topological transition in which the edge mode reverses its propagation direction, yielding the counterintuitive result that an increase of the driving amplitude can have the same effect as reversing the helix chirality. We experimentally observe this transition while overcoming the bending loss typically associated with the strong driving regime, despite the fact that the helix radius is on the same scale as the size of the entire array. The two topological phases observed in our experiment can be understood as originating from a Floquet realization of the Haldane model.