Photoluminescence Saturation and Exciton Decay Dynamics in Transition Metal Dichalcogenide Monolayers

We study the photoluminescence saturation and exciton decay dynamics in monolayer transition transition-metal dichalcogenides (TMDs). Monolayer MoSe$_{\mathrm{2}}$ shows a PL saturation at a very low excitation intensity, more than two orders of magnitude lower than monolayer MoS$_{\mathrm{2}}$. Transient reflection spectroscopy shows that nonlinear exciton-exciton annihilation is the dominant exciton decay mechanism in monolayer MoSe$_{\mathrm{2}}$, in contrast to the exciton decay in monolayer MoS$_{\mathrm{2}}$ Furthermore, we measure exciton lifetime \textgreater 125 ps for monolayer MoSe$_{\mathrm{2}}$ much longer than the several-ps exciton lifetime in MoS$_{\mathrm{2}}$. We find that that the difference in their exciton lifetime can explain both the dramatically different exciton decay mechanism and PL saturation behavior of MoSe$_{\mathrm{2\thinspace }}$and MoS$_{\mathrm{2}}$ monolayers.