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.