Abstract-Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe2 Crystal with Time‐Resolved Terahertz Spectroscopy

Chuan He, Lipeng Zhu,  Qiyi Zhao,   Yuanyuan Huang,  Zehan Yao,   Wanyi Du,  Yuhang He,  Sujuan Zhang,  Xinlong Xu,


https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800290

The dynamics of photoexcited species is quite important for the development of next‐generation ultrafast optoelectronic devices based on transition metal dichalcogenides (TMDs). Herein, time‐resolved optical pump terahertz (THz) probe spectroscopy, which is sensitive to both bounded excitons and free electrons/holes, is employed to study the dynamics of photo‐induced carriers in the typical layered TMDs crystal tungsten diselenide (WSe2). Initial photoexcitation could generate both free carriers and excitons. The free carriers decay followed by phonon‐assistance (≈30 ps) and defect‐assistance (≈200–280 ps). The excitons decay followed by the phonon‐assisted recombination (≈100 ps) and the defect‐induced exciton separation (≈40–200 ps). With the increasing of pump fluence, more free electrons and holes will bind to form excitons by many‐body effect, while with the decay of time, more excitons will dissociate into free carriers by defects. The frequency‐dependent transient complex THz photoconductivity of layered WSe2crystal can be well described by Drude–Smith–Lorentz model, which suggests preferential free carriers backscattering due to the defects. The ratio of free carriers to excitons suggests that free carriers dominate after the photoexcitation, which is important for the optoelectronic devices such as solar cells and photodetectors.

Abstract-Terahertz surface emission of d-band electrons from a layered tungsten disulfide crystal by surface field

Longhui Zhang, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Zehan Yao, Yixuan Zhou, Wanyi Du, and Xinlong Xu

https://journals.aps.org/prb/accepted/ad078Od4Tf91333d54c29f0414038d0387a0cecfe

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide (WS2). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of MoS2\thinspace based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in WS2. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12{\%} approximately. All these suggest the main THz mechanism from WS2 is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of WS2 with the maximum surface depletion field of approximate 1.2 \texttimes 105 V/cm. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.