Determining Band Splitting and Spin-Flip Dynamics in Monolayer MoS 2 .

Femtosecond helicity-resolved pump-probe spectroscopy is performed to study the spin and valley dynamics in monolayer (ML) MoS 2 . Both the bright to dark intravalley exciton transition (∼50 fs) and the reverse transition process (<50 fs) are directly monitored. It suggests that the bright exciton state of ML MoS 2 is lower in energy than the dark one, which is also confirmed by observing the temperature-dependent co-polarized photobleaching dynamics of A and B excitons. Furthermore, the band splitting in the conduction band of ML MoS 2 with a value of 15 ± 0.3 meV is determined by fitting the temperature-dependent ratios of the population in bright and dark states using the Boltzmann distribution law. Such minor band splitting allows the phonon-mediated intravalley spin-flip to even occur from the lower to the upper conduction band within tens of femtoseconds, which will have non-negligible effects on the performance of these ML MoS 2 -based optoelectronic and photonic devices.