Abstract-Terahertz surface emission from layered semiconductor WSe2

Keyu Si,  Yuanyuan, Huang, Qiyi Zhao, Lipeng Zhu, Longhui Zhang, Zehan Yao, Xinlong Xu,

https://www.sciencedirect.com/science/article/pii/S0169433218310651

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggests that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this semiconductor for THz devices.

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 from layered semiconductor WSe2

Keyu Si, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Longhui Zhang, Zehan Yao, Xinlong Xu, 

https://www.sciencedirect.com/science/article/pii/S0169433218310651

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe₂ due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is due to the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggested that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe₂ crystal is 3.11 mJ/cm² confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe₂ and promise the potential applications of this semiconductor for THz devices.

Abstract-Terahertz Surface Emission from Layered MoS2 Crystal: Competition Between Surface Optical Rectification and Surface Photocurrent Surge

Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Longhui Zhang, Chuan He, Qiyi Zhao, Jintao Bai, and Xin Long Xu


http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b09723?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry–breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary condition. The maximum of surface depletion field is estimated to be 1.45×104 V/cm with 130 nm in depth under -40oincidence. Interestingly, when the incident angle is tuned from -40o to 0o, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs (100). The results afford not only comprehensive understanding of THz radiation from layered materials like MoS2, but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.

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.