driven by intense terahertz fields*

We investigate high-field photocarrier dynamics in a 2D semiconductor, WSe₂, employing time-resolved nonlinear THz spectroscopy. Non-equilibrium photocarriers in multilayer WSe₂ injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. THz absorption in optically excited WSe₂ rises rapidly in the low THz field regime and more gradually ramps up at high intensities. The nonlinear THz absorption in WSe₂ is extraordinary, since intense THz pulses enhance transparency in conventional 3D semiconductors such as Si, Ge and GaAs as well as another 2D material, graphene. The spectrally analyzed nonlinear THz absorption provides insights into the underlying microscopic processes of the high-field photocarrier dynamics in WSe₂. The permittivity spectrum of the optically excited WSe₂ is predominantly imaginary and inversely proportional to the frequency at low frequencies, which is consistent with the THz properties of conducting media. The conductivity spectrum, however, shows prominent spectral features, implying that the photocarriers undergo resonant interactions such as carrier-phonon scattering. The high-speed control of the electronic structure of 2D semiconductors via THz fields can provide promising platforms not only for nanoelectronic and spintronic applications, but also for exploring fundamental physical processes such as phase transitions and many-body interactions.