  Manipulating spin polarization of electrons in nonmagnetic semiconductors by
means of electric fields or optical fields is an essential theme of the
conceptual nonmagnetic semiconductor-based spintronics. Here we experimentally
demonstrate a method of generating spin polarization in monolayer transition
metal dichalcogenides (TMD) by the circularly polarized optical pumping. The
fully spin-polarized photocurrent is achieved through the valley dependent
optical selection rules and the spin-valley locking in monolayer WS2, and
electrically detected by a lateral spin-valve structure with ferromagnetic
contacts. The demonstrated long spin lifetime, the unique valley contrasted
physics and the spin-valley locking make monolayer WS2 an unprecedented
candidate for semiconductor based spintronics.
