Control over topological insulator photocurrents with light polarization

Three-dimensional topological insulators 1 , 2 , 3 represent a new quantum phase of matter with spin-polarized surface states 4 , 5 that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission 4 , 5 , 6 , 7 , 8 and tunnelling 9 , 10 spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions 11 , 12 , 13 and spin-polarized electrical currents 14 , 15 . 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 Bi 2 Se 3 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 11 , 12 , 13 , 14 , 15 and lead to novel opto-spintronic devices 16 . A topological insulator illuminated with circularly or linearly polarized light produces a photocurrent that depends on the helicity or polarization of the light, respectively.