▎ 摘　　要

A transient topological response in graphene is driven by a short pulse of light. When the Fermi energy is in the predicted band gap the Hall conductance is around two conductance quanta. An ultrafast detection technique enables the measurement. Many non-equilibrium phenomena have been discovered or predicted in optically driven quantum solids(1). Examples include light-induced superconductivity(2,3) and Floquet-engineered topological phases(4-8). These are short-lived effects that should lead to measurable changes in electrical transport, which can be characterized using an ultrafast device architecture based on photoconductive switches(9). Here, we report the observation of a light-induced anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure(4,5), similar to the band structure originally proposed by Haldane(10). This includes an approximately 60 meV wide conductance plateau centred at the Dirac point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau the estimated anomalous Hall conductance saturates around 1.8 +/- 0.4 e(2)/h.