Second harmonic generation property of monolayer TMDCs and its potential application in producing terahertz radiation

Second harmonic generation (SHG) properties in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have aroused great interest. However, until now SHG for TMDC monolayer alloys is seldom investigated. Meanwhile, there is considerable controversy over the static SHG coefficients of monolayer MoS₂. The feasibility to produce terahertz (THz) radiation via SHG in pure and alloyed TMDCs has never been reported. We first calculate the SHG coefficients of monolayer MoS₂, MoSe₂, and MoS_2(1-x)Se_2x using the independent particle approximation plus scissors correction. We then simulate their THz absorption by applying density function perturbation theory plus the Lorentzian line and try to calculate their zero-frequency THz refractive index and birefringence. The physical property of MoS_2(1-x)Se_2x alloys is simulated by considering various combinations. Results indicate that monolayer MoS₂, MoSe₂, and MoS_2(1-x)Se_2x possess large static SHG coefficients and THz birefringence and display low absorption over broadband THz frequencies. Therefore, they have applications in producing THz radiation via SHG. This study demonstrates that THz radiation can be attained in a large number of monolayers and few-layers and will extend applications of 2D materials. Moreover, it is possible to identify the magnitude of static coefficients of single-layer MoS₂ by measuring THz intensities.