  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.

Optics