Floquet topological photonic crystals with temporally modulated media.

We show that Floquet topological insulating states can exist in two-dimensional photonic crystals made of time-variant optical materials. By arranging the modulating phases, it facilitates effective gauge fields that give rise to topological effects. The band structures demonstrate the existence of topologically non-trivial bandgaps, thereby leading to back-scattering immune unidirectional edge states owing to bulk-edge correspondence. With these first-principle numerical results, we then verify the topological order for every Floquet band via Wilson loop approach. In the final paragraph, the possible experimental implementation for Floquet topological photonics is also discussed.