  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 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. 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.
