Optical driving has been proposed as a means of engineering topological properties in topologically trivial systems. One proposal for such a “Floquet topological insulator” is based on breaking time-reversal symmetry in graphene through a coherent interaction with circularly polarized light . This was predicted to lift the degeneracy of the Dirac point, opening a topological band gap in the resulting photon-dressed band structure accompanied by the formation of dressed chiral edge states . While quantum simulation experiments have validated aspects of this proposal [3,4], and Floquet–Bloch bands have been observed in a topological insulator , the electrical transport properties of such a light-induced topological state have remained elusive in a real material.
In this talk, I will report on our recent observation of a light-induced anomalous Hall effect in monolayer graphene driven by an intense femtosecond pulse of circularly polarized light . We probed electrical transport using an ultrafast device architecture based on photoconductive switches. The dependence of the anomalous Hall effect on a gate potential used to tune the equilibrium Fermi level revealed multiple features that reflect a Floquet-engineered topological band structure, similar to the band structure originally proposed by Haldane . This included an approximately 60 meV wide conductance plateau centered at the Dirac point, where a gap of equal magnitude was predicted to open. We found that when the Fermi level was tuned within this plateau, the estimated anomalous Hall conductance saturated around 1.8 ± 0.4 e^2/h.
Speaker: James McIver.
Max Planck Institute for the Structure and Dynamics of Matter (MPSD).
James received his PhD in 2014 from Harvard University, where he investigated the nonlinear optical and optoelectronic properties of topological insulators. He then came to MPSD on a Humboldt postdoctoral fellowship, during which time he developed ultrafast circuitry to probe the topological transport properties of photon dressed states in graphene. He now leads the Ultrafast Quantum Transport group in the Condensed Matter Dynamics Department at MPSD.