  Three-dimensional topological insulators represent a new quantum phase of
matter with spin-polarized surface states that are protected from
backscattering. The static electronic properties of these surface states have
been comprehensively imaged by both photoemission and tunneling spectroscopies.
Theorists have proposed that topological surface states can also exhibit novel
electronic responses to light, such as topological quantum phase transitions
and spin-polarized electrical currents. However, the effects of optically
driving a topological insulator out of equilibrium have remained largely
unexplored experimentally, and no photocurrents have been measured. Here we
show that illuminating the topological insulator Bi2Se3 with circularly
polarized light generates a photocurrent that originates from topological
helical Dirac fermions, and that reversing the helicity of the light reverses
the direction of the photocurrent. We also observe a photocurrent that is
controlled by the linear polarization of light, and argue that it may also have
a topological surface state origin. This approach may allow the probing of
dynamic properties of topological insulators and lead to novel opto-spintronic
devices.
