Abstract-Terahertz Probe of Photoexcited Carrier Dynamics in Dirac Semimetal Cd3As2

Wei Lu, Jiwei Ling, Faxian Xiu, Dong Sun

https://arxiv.org/abs/1807.03049

The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

Abstract-Dynamic Terahertz Response in the Dirac Semimetal Cd3As2 Induced by Ultrafast Optical Excitation

Wei Lu, Jiwei Ling, Faxian Xiu, Dong Sun,

https://www.osapublishing.org/abstract.cfm?uri=CLEO_QELS-2018-JW2A.133

Hot electron relaxation and coupling in Cd3As2 is studied by employing ultrafast time-domain THz spectroscopy. The excited carriers enhance the absorption of THz and relax in ps-time scales, the cooling rules consistent with mid-IR spectroscopy.

© 2018 The Author(s)

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.