Photoelectron spin-flipping and texture manipulation in a topological insulator

In a topological insulator, the surface-state electron spins are ‘locked’ to their direction of travel. But when an electron is kicked out by a photon through the photoelectric effect, the spin polarization is not necessarily conserved. In fact, the ejected spins can be completely manipulated in three dimensions by the incident photons. Recently discovered materials called three-dimensional topological insulators 1 , 2 , 3 , 4 , 5 constitute examples of symmetry-protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is their protected metallic Dirac fermion-like surface states. Electrons in these surface states are spin polarized with their spins governed by their momentum, resulting in a helical spin texture in momentum space 6 . Spin- and angle-resolved photoemission spectroscopy has been the only tool capable of directly observing this central feature with simultaneous energy, momentum and spin sensitivity 6 , 7 , 8 , 9 , 10 , 11 , 12 . By using an innovative photoelectron spectrometer 13 with a high-flux laser-based light source, we discovered a surprising property of these surface electrons. We found that the spin polarization of the resulting photoelectrons can be manipulated in three dimensions through selection of the light polarization. These effects are due to the spin-dependent interaction of the helical surface electrons with light, which originates from strong spin–orbit coupling. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from that of the originating initial states. The results also provide the basis for a source of highly spin-polarized electrons with tunable polarization direction.