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

Development of novel schemes for efficient terahertz (THz) generation from transition metal dichalcogenides 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 (OR) processes, and the surface field induced transient photocurrent effect (TPE). A relatively much larger contribution to THz emission from the TPE than the resonant OR is measured for the above bandgap excitation. We have measured a clear difference between the resonant and nonresonant OR processes. The pure OR 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 OR is & pi;/2 phase offset relative to that due to the nonresonant OR.