Monolayer MoS₂ is a 2D material beyond graphene that is widely studied, mainly due to its interesting valley degree of freedom as discovered independently by two individual groups in 2012. One group of them is also member of ICQM: Professor Ji Feng and Professor Enge Wang's group (Nature Communication, 3, 887 (2012)). The excellent control of valley degree of freedom by light has been demonstrated experimentally shortly after the theoretical prediction, but without knowing the detail dynamics of valley carriers.

In the newly published paper “Valley Carrier Dynamics in Monolayer Molybdenum Disulfide from Helicity-Resolved Ultrafast Pump–Probe Spectroscopy”, by ICQM member Professor Dong Sun's group, in collaboration with Professor Ji Feng's theoretical group, has investigated the valley related carrier dynamics in monolayer molybdenum disulfide using helicity resolved non-degenerate ultrafast pump-probe spectroscopy. The measurement is performed at the vicinity of the high-symmetry K point under the temperature down to 78K. The result shows that the valley polarization is preserved for only several picoseconds before scattering process makes it undistinguishable. The work attributes the major valley polarization mechanism to be the exciton trapping by defects.  One of the referees praises the paper 'reports a pioneering study on valley related carrier dynamic in MoS₂ monolayers using the polarization resolved transient differential reflection spectroscopy'. These results not only resolve the confusion of large variance of degree of valley polarization reported in the literature, but also elucidate the important role of defects state in valley carrier dynamics of MoS₂.