Carrier and polarization dynamics in monolayer MoS$_2$: temperature and power dependence

In monolayer (ML) MoS$_2$ optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical k-valley initialization [1,2,3]. Here we present the first time resolved photoluminescence (PL) polarization measurements in MoS$_2$ MLs [4], providing vital information on the electron valley dynamics. Using quasi-resonant excitation of the A-exciton transitions, we can infer that the PL decays within $\tau \simeq$4ps. The PL polarization of P$_c \approx 60\%$ remains nearly constant in time for experiments from 4K - 300K, a necessary condition for the success of future Valley Hall experiments [1]. $\tau$ does not vary significantly over this temperature range. This is surprising when considering the decrease of P$_c$ in continuous wave experiments when going from 4K to 300K reported in the literature [2,3]. By tuning the laser following the shift of the A-exciton resonance with temperature we are able to recover at 300K $\sim80\%$ of the polarization observed at 4K. For pulsed laser excitation, we observe a decrease of P$_c$ with increasing laser power at all temperatures.\\[4pt] [1] Xiao et al, PRL 108, 196802 (2012).\\[0pt] [2] Mak, et al Nat. Nanotech. 7, 494 (2012).\\[0pt] [3] Sallen et al, PRB 86, 081301 (2012).\\[0pt] [4] Lagarde et al, arXiv:1308.0696.