  Monolayer transition metal dichalcogenides have recently become a playground
for spin- and valleytronics research. Their low energy spectrum can be
described by Dirac cones on the corners of Brillouin zone, but the physical
properties are richer than those of graphene since the spin degeneracy is
lifted and the optical selection rules are valley dependent. This has been
exploited for the optical injection of spin and valley polarized currents by
the application of static electric fields. In this paper we consider an
all-optical method for the injection of charge, spin and valley polarized
currents. The presence of both a fundamental optical field and its second
harmonic can lead to the injection of currents due to a nonlinear effect
involving the quantum interference between one- and two-photon absorption
processes. We analyze how the injected quantities can be controlled through the
parameters of the incident light fields, allowing capabilities of control
beyond those achieved with static fields, and discuss the conditions for
experimental verification of our results.
