Fig. 1. Light-induced topological Floquet bands in graphene and device architecture used to detect ultrafast anomalous Hall currents.

a, A coherent interaction between graphene and circularly polarized light was predicted to open a topological band gap in the effective Floquet band dispersion⁴. b, The gap is characterized by the presence of Berry curvature (Ω), which is identical in both valleys. The experimental signature of the induced nontrivial topology is the emergence of anomalous Hall currents. c, Exfoliated graphene monolayer with four electrical contacts (right) and a photoconductive switch for current detection (left), connected by a microstrip transmission line. The graphene was optically driven using an ultrafast mid-infrared circularly polarized laser pulse (red beam). The generated helicity-dependent anomalous Hall currents I_x [⟳ - ⟲] were probed after a variable time delay at the photoconductive switch, which was activated by a second laser pulse (green beam). Anomalous Hall currents were measured as a function of source-drain voltage bias V_y and backgate voltage V_g, the latter of which controlled the graphene Fermi level (E_F).