An interplay between optical rectification and transient photocurrent effect on THz pulse generation from bulk MoS₂ layered crystal

Abstract

Development of novel schemes for efficient terahertz (THz) generation from transition metal dichalcogenides (TMDs) are useful for realizing integrated THz devices based on them and also, understanding of the related fundamental processes from such studies will guide to suitable designs. Here, we report the THz emission efficiency of bulk MoS2 layered crystal at varying femtosecond excitation wavelengths, from 550 nm to the telecommunication wavelength of 1550 nm. By using both the below bandgap excitation at longer wavelengths and the above bandgap excitation at shorter wavelengths, we resolve THz emission contributions from resonant and non-resonant optical rectification processes, and the surface field induced transient photocurrent effect (TPE). A relatively much larger contribution to THz emission from the TPE than the resonant optical rectification is measured for the above bandgap excitation. We have measured a clear difference between the resonant and nonresonant optical rectification processes. The pure optical rectification part is exclusively determined from detailed experiments using excitation intensity, polarization angle, and azimuthal angle dependent measurements. For the above bandgap excitation, the THz emission gets highly saturated with the increasing excitation intensity. Also, the value of the saturation intensity increases (decreases) with the excitation photon energy (wavelength). Interestingly, we find that the linear polarization angle and the azimuthal angle dependent THz signal due to resonant optical rectification is π/2 phase offset relative to that due to the nonresonant optical rectification.

Ultrafast Shift Current in SnS2 Single Crystals: Structure Considerations, Modeling, and THz Emission Spectroscopy

“…[9][10][11][12] Another class of materials where THz emission spectroscopy has been applied to probe nonlinear optical properties is transition-metal dichalcogenides (TMDs) of the formula MX 2 , layered 2D semiconductors with van der Waals (vdW) interactions between the layers. [13][14][15][16][17] In a prototypical TMD material, MoS 2 , trigonal prismatic coordination exists between the Mo and S atoms. It does not have inversion symmetry in a monolayer form and exhibits second-order nonlinearities.…”

“…[17][18][19] In a naturally found and most commonly observed phase of MoS 2 , known as 2H-MoS 2 , bulk inversion symmetry is restored by layer stacking and experimentally observed shift current response arises solely in the surface layer. [13][14][15][16][17] In other members of layered MX 2 TMDs, such as TiS 2 and TiSe 2 , ZrS 2 , VSe 2 , and SnS 2 , chalcogenide atoms in a single layer do not stack directly above one another but are staggered, resulting in octahedral rather than trigonal coordination. [18,[20][21][22] In this case, monolayers possess inversion symmetry, ruling out second-order nonlinear effects such as the shift current or optical rectification.…”