Robust optical emission polarization in MoS 2 monolayers through selective valley excitation

We report polarization resolved photoluminescence from monolayer ${mathrm{MoS}}_{2}$, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90$%$ polarized emission at 4 K is observed with 40$%$ polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T indicating robust, selective valley excitation.