Abstract-An ultrafast terahertz probe of the transient evolution of the charged and neutral phase of photo-excited electron-hole gas in a monolayer semiconductor

Xuefeng Liu1,2, Hongyi Yu3, Qingqing Ji4, Zhihan Gao1,2, Shaofeng Ge1,2, Jun Qiu1,2, Zhongfan Liu4,Yanfeng Zhang4,5 and Dong Sun1,2


http://iopscience.iop.org/article/10.1088/2053-1583/3/1/014001/meta

We investigate the dynamical formation of an exciton from photo-excited electron-hole plasma and its subsequent decay dynamics in monolayer MoS2 grown by chemical vapor deposition (CVD) using ultrafast pump and terahertz probe spectroscopy. Different photo-excited electron-hole states are resolved based on their distinct responses to THz photon and decay lifetimes. The observed transient THz transmission can be fitted with two decay components: a fast component with a decay lifetime of 20 ps, which is attributed to the exciton lifetime, including its formation and subsequent intra-exciton relaxation; a slow component with an extremely long decay lifetime of several ns, possibly due to a long-lived dark exciton state. The relaxation dynamics are further supported by temperature and pump-fluence-dependent studies of the decay time constants. The sign of the transient THz observed in this experiment is the opposite of that measured in a recent parallel transient THz work on MoS2 [1]. The observed decay dynamics are also different, and the possible reasons for these discrepancies are discussed.

Abstract-Ultrafast Terahertz Probe of Transient Evolution of Charged and Neutral Phase of Photoexcited Electron-hole Gas in Monolayer Semiconductor

Xuefeng Liu1,2,Qingqing Ji3
, Zhihan Gao1,2, Shaofeng Ge1,2,Jun Qiu1,2, Zhongfan Liu3
, Yanfeng Zhang3,4, Dong Sun1,2, *
1 International Center for Quantum Materials, School of Physics, Peking University, Beijing
100871, P. R. China
2 Collaborative Innovation Center of Quantum Matter, Beijing 100871,P. R. China
3 Center for Nanochemistry(CNC), Beijing National Laboratory for Molecular Sciences,
College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary
Studies, Peking University, Beijing 100871,P. R. China
4 Department of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, People’s Republic of China
*
Correspondance and request for materials should be addressed to D. S. (email:
sundong@pku.edu.cn)

http://arxiv.org/ftp/arxiv/papers/1410/1410.2939.pdf

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.