Journal
SCIENCE
Volume 342, Issue 6157, Pages 453-457Publisher
AMER ASSOC ADVANCEMENT SCIENCE
DOI: 10.1126/science.1239834
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Funding
- U.S. Department of Energy (DOE) [DE-FG02-08ER46521, DE-SC0006423]
- Army Research Office (ARO-DURIP) [W911NF-09-1-0170]
- DOE, Basic Energy Sciences Office, Division of Materials Sciences and Engineering [DE-SC0006418]
- NSF [DMR-0819762]
- U.S. Department of Energy (DOE) [DE-SC0006418, DE-SC0006423, DE-FG02-08ER46521] Funding Source: U.S. Department of Energy (DOE)
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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 symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter.
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