  Monolayers of semiconducting van der Waals solids, such as transition metal
dichalcogenides (TMDs), acquire significant electric polarization normal to the
layers when placed on a substrate or in a heterogeneous stack. This causes
linear coupling of electrons to electric fields normal to the layers.
Irradiation at oblique incidence at frequencies above the gap causes interband
transitions due to coupling to both normal and in-plane ac electric fields. The
interference between the two processes leads to sizable in-plane photocurrents
and valley currents. The direction and magnitude of currents is controlled by
light polarization and is determined by its helical or non-helical components.
The helicity-dependent ballistic current arises due to asymmetric
photogeneration. The non-helical current has a ballistic contribution (dominant
in sufficiently clean samples) caused by asymmetric scattering of photoexcited
carriers, and a side-jump contribution. Magneto-induced photocurrent is due to
the Lorentz force or due to intrinsic magnetic moment related to Berry
curvature.
