Jiangpeng Dong, Kevin-P. Gradwohl, Yadong Xu, Tao Wang, Binbin Zhang, Bao Xiao, Christian Teichert, and Wanqi Jie
Fig. 1. Characterization of the GaTe crystal by (a) an XRD spectrum; (b) a Raman spectrum with a laser wavelength of 785 nm; (c) optical absorption spectra, with the excitonic absorption observed around 1.62 eV. Inset in (c) shows square root of the absorption as a function of energy, where the linear extrapolation reveals an optical bandgap of 0.76 eV, associated with an indirect bandgap; (d) PL spectrum of GaTe at 10 K under 488 nm laser excitation.
https://www.osapublishing.org/prj/abstract.cfm?uri=prj-7-5-518
In this work, a model based on the optical rectification effect and the photocurrent surge effect is proposed to describe the terahertz emission mechanism of the layered GaTe crystal. As a centrosymmetric crystal, the optical rectification effect arises from the breaking of the inversion symmetry due to lattice reorganization of the crystal’s surface layer. In addition, the photocurrent surge originating from the unidirectional charge carrier diffusion—due to the noncubic mobility anisotropy within the layers—produces terahertz radiation. This is confirmed by both terahertz emission spectroscopy and electric property characterization. The current surge perpendicular to the layers also makes an important contribution to the terahertz radiation, which is consistent with its incident angle dependence. Based on our results, we infer that the contribution of optical rectification changes from 90% under normal incidence to 23% under a 40° incidence angle. The results not only demonstrate the terahertz radiation properties of layered GaTe bulk crystals, but also promise the potential application of terahertz emission spectroscopy for characterizing the surface properties of layered materials.
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