Strong Terahertz Field Induced Nonlinear Optical Effects in WSe₂ and MoS₂

We study the effect of intense THz fields on the optical properties in WSe₂ and MoS₂, observing optical transmission through CVD grown large-grain samples. The transition metal dichalcogenides (TMDs) are 2D semiconductors exhibiting strong optical responses of excitonic nature. We employ time-resolved optical-pump/THz-probe and THz-control/optical-probe spectroscopy to investigate the ultrafast dynamics of photocarriers and nonlinear optical effects in TMD monolayer and multilayers induced by strong THz fields. We optically excite the sample with 400 or 800-nm, 100-fs pulses, where the photocarriers are injected at different side valleys. The optical excitation above the bandgap induce large absorption of THz radiation in the sample, while the unexcited sample, an insulator, is transparent in the THz range. The photocarrier relaxation shows distinctive dynamics depending on the sidevalleys where they are injected. The relaxation dynamics depends not only on the optical pump intensity and the photon energy, but also on the THz field strength. Furthermore, the strong THz fields produce large modulation of optical transmission, where the sign of change depends on the excitation photon energy. The differential transmission increases linearly with the THz field strength. The nonlinear optical effects exhibit sub-picosecond dynamics, comparable to the THz pulse duration. The optical responses are not completely transient, indicating that the THz induced modulation of the photocarrier relaxation occurs in several hundreds of femtosecond. The measurements on the high-field photocarrier dynamics provide insight into the light-matter interaction with many-bodies in the 2D semiconductors.