Trion valley dynamics in monolayer WSez

We present results of polarization-resolved differential transmission experiments, in which pump and probe pulses are quasi-resonant with charged and neutral excitons in monolayer WSe 2 . By selecting co- and cross-circular polarization configurations for pump and probe pulses, the excitations in the K and K' valleys can be individually addressed [1]. From far above the band edge excited pump-probe experiments, it is known that in these materials a rapid depolarization mechanism exists, leading to a fast (few ps) population transfer between the K and K' valleys [2]. In our experiments [3], we demonstrate that for quasi-resonant excitation, depolarization only occurs if the exciton or the blue wing of the trion transition is pumped. In case of pumping in the red wing of the trion peak, we observe a remarkably stable valley polarization, lasting for >25 ps.