  Electronic and spintronic devices rely on the fact that free charge carriers
in solids carry electric charge and spin, respectively. There are, however,
other properties of charge carriers that might be exploited in new families of
devices. In particular, if there are two or more conduction (or valence) band
extrema in momentum space, then confining charge carriers in one of these
valleys allows the possibility of valleytronic devices. Such valley
polarization has been demonstrated by using strain and magnetic fields, but
neither of these approaches allow for dynamic control. Recently, optical
control of valley occupancy in graphene with broken inversion symmetry has been
proposed but remains experimentally difficult to realize. Here we demonstrate
that optical pumping with circularly-polarized light can achieve complete
dynamic valley polarization in monolayer MoS2, a two dimensional (2D)
non-centrosymmetric crystal with direct energy gaps at two valleys. Moreover,
this polarization is retained for longer than 1 ns. Our results demonstrate the
viability of optical valley control and valley-based electronic and
optoelectronic applications in MoS2 monolayers.
