  A photoinduced current of a layered MoS2-based transistor is studied from
first-principles. Under the illumination of circular polarized light, a
valley-polarized current is generated, which can be tuned by the gate voltage.
For monolayer MoS2, the valley-polarized spin-up (down) electron current at K
(K') points is induced by the right (left) circular polarized light. The valley
polarization is found to reach +1.0 (-1.0) for the valley current that carried
such a K (K') index. For bilayer MoS2, the spin-up (down) current can be
induced at both K and K' valleys by the right (left) circular light. In
contrast to monolayer MoS2, the photoinduced valley polarization shows
asymmetric behavior upon reversal of the gate voltage. Our results show that
the valley polarization of the photoinduced current can be modulated by the
circular polarized light and the gate voltage. All the results can be well
understood using a simple kp model.
