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Abstract:
Many striking non-equilibrium phenomena have been discovered or predicted in optically-driven quantum solids, ranging from light-induced superconductivity to Floquet-engineered topological phases. These effects are expected to lead to dramatic changes in electrical transport, but can only be comprehensively characterized or functionalized with a direct interface to electrical devices that operate at ultrafast speeds. Here, we make use of laser-triggered photoconductive switches to measure the ultrafast transport properties of monolayer graphene, driven by a mid-infrared femtosecond pulse of circularly polarized light. The immediate goal of this experiment is to probe the transport signatures of a predicted photon-dressed topological band structure in graphene, similar to the one originally proposed by Haldane. We report the observation of an anomalous Hall effect in the absence of an applied magnetic field. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that directly reflect the dressed band structure expected from Floquet theory, including a ~60 meV wide plateau centered at the Dirac point. We find that when the Fermi level lies within this plateau, the anomalous Hall conductance approaches 2e^2/h.
