Nature Nanotechnol. (2011)

James McIver, David Hsieh and co-workers from the Massachusetts Institute of Technology and Harvard University in the USA have experimentally observed optically driven currents in a topological insulator. Theory predicts that circularly polarized light should be able to drive spin-polarized net electrical current because the light would induce interband transitions whose probability depends on the surface-state spin orientation, which in turn depends on the momentum (and hence the polarization). In their experiment, the researchers used 795 nm light and Bi2Se3, which is a topological insulator because the currents are allowed only near the surface owing to spin-degeneracy of the bulk bands. One of the main challenges in their work was separating and eliminating surface and bulk currents generated by other effects, such as laser-induced thermoelectric currents. The researchers therefore experimented with heat gradients between the device contacts with swept laser spots, and also used low laser intensities to minimize sample heating. After separating out any interfering effects, the team confirmed not only that the circularly polarized light generated a photocurrent, but also that the direction of the photocurrent can be controlled by the direction of the light momentum, as predicted.