The unique electronic properties of the surface electrons in a topological insulator [1] are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state [2]. Using time- and angle-resolved photoemission spectroscopy [3] [4], we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands when the pump pulse is present [Fig. 1] [5]. These photon dressed surface bands exhibit polarization-dependent band gaps at avoided crossings [Fig. 2A and B]. Circularly polarized photons induce an additional gap at the Dirac point [Fig. 2C], which is a signature of broken time-reversal symmetry on the surface. The size of the gaps are consistent with the calculation based on Floquet-Bloch theory [6]. This results in a Chern insulator as originally proposed by Haldane [7]. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter.

© 2014 Optical Society of America

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