  Recently discovered materials called three-dimensional topological insulators
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 the protected metallic electronic
states confined to the material's surfaces. Electrons in these surface states
are spin polarized with their spins governed by their direction of travel
(linear momentum), resulting in a helical spin texture in momentum space. Spin-
and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only
tool capable of directly observing this central feature with simultaneous
energy, momentum, and spin sensitivity. By using an innovative photoelectron
spectrometer with a high-flux laser-based light source, we discovered another
surprising property of these surface electrons which behave like Dirac
fermions. We found that the spin polarization of the resulting photoelectrons
can be fully manipulated in all three dimensions through selection of the light
polarization. These surprising effects are due to the spin-dependent
interaction of the helical Dirac fermions with light, which originates from the
strong spin-orbit coupling in the material. Our results illustrate unusual
scenarios in which the spin polarization of photoelectrons is completely
different from the spin state of electrons in the originating initial states.
The results also provide the basis for a novel source of highly spin-polarized
electrons with tunable polarization in three dimensions.
