High Field Terahertz Spectroscopy of Optically Excited Transition Metal Dichalcogenides*

2D transition metal dichalcogenides (TMDs) with semiconducting bandgaps exhibit strong optical responses including exciton resonances and spin-dependent photocarrier dynamics due to their unique band structure. We investigate the ultrafast dynamics of optically excited carriers in CVD grown large-grain MoS₂ and WSe₂ monolayer and multilayers in the presence of strong terahertz (THz) fields, employing time-resolved THz-control/optical-probe and optical-pump/THz-probe spectroscopy. We generate free carriers via optical excitations above the A and B exciton transitions in MoS₂ and WSe₂, and observe the relaxation dynamics with strong single-cycle THz pulses. The photocarriers have a long recombination lifetime exceeding 1 ns, while increasing optical pump intensity opens additional relaxation pathways such as carrier-carrier and carrier-lattice scattering, leading to fast carrier relaxation (~100 ps) at the early stage. Unlike 3D semiconductors such as Si and GaAs, in which strong THz fields enhance transparency by driving hot electrons into sidebands via intervalley scattering, MoS₂ and WSe₂ exhibit no pronounced signatures of field-driven intervalley scattering. Analyzing the THz waveforms transmitted through the optically excited MoS₂ and WSe₂, we obtain the temporal evolution of the complex conductivity spectra as photocarriers decay. The THz conductivity of optically excited MoS₂ and WSe₂ exhibit subtle differences, indicating photocarriers undergo distinct relaxation processes. This optical and THz study lends to future applications of TMDs in photonic and optoelectronic devices.