Ultrafast Terahertz Probe of Transient Evolution of Charged and Neutral   Phase of Photoexcited Electron-hole Gas in Monolayer Semiconductor
  We investigate the dynamical formation of excitons from photoexcited electron-hole plasma and its subsequent decay dynamics in monolayer MoS2 grown by chemical vapor deposition using ultrafast pump and terahertz probe spectroscopy. Different photoexcited electron-hole states are resolved based on their distinct responses to THz photon and decay lifetime. The observed transient THz transmission can be fit with two decay components: a fast component with decay lifetime of 20 ps, which is attributed to exciton life time including the exciton formation and subsequent intraexciton relaxation; a slow component with extremely long decay lifetime of several ns due to either localized exciton state or a long live dark exciton state which is uncovered for the first time. The relaxation dynamics is further verified by temperature and pump fluence dependent studies of the decay time constants. 
Xuefeng Liu, Qingqing Ji, Zhihan Gao, Shaofeng Ge, Jun Qiu, Zhongfan   Liu, Yanfeng Zhang, Dong Sun
cond-mat.mtrl-sci