Observation of Floquet-Bloch States on the Surface of a Topological Insulator

The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort midinfrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon-dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal…

• NS
  National Science Foundation

  Awards: 0819762, DMR-0819762
• UD
  U.S. Department of Energy

  Awards: DE-SC0006423, DE-SC0006418, DE-FG02-, DE-FG02
• MR
  Materials Research Science and Engineering Center, Harvard University

  Award: DMR-0819762
• DO
  Division of Materials Research

  Awards: 0819762, DMR-0819762
• AR
  Army Research Office

  Awards: W911NF-09-1-0170, W911NF
• DO
  Division of Materials Sciences and Engineering