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Non-equilibrium photocarriers in multilayer WSe₂injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. The THz absorption in optically excited WSe₂rises rapidly in the low THz field regime and gradually ramps up at high intensities. The strong THz pulses drive the photocarriers into sidebands of higher mobility and release trapped charge carriers, which consequently enhance the transient conductivity of WSe₂. The spectrally analyzed conductivity reveals distinctive features, indicating that the photocarriers undergo resonant interactions such as carrier-photon scattering.

We demonstrate that free-standing multi-walled carbon nanotubes exhibit extraordinary nonlinear terahertz responses upon optical excitation. Terahertz transmission of the photoexcited nanotubes rises in a narrow range of intermediate intensity with increasing intensity, while falling in the regime of low and high intensities. A theoretical analysis shows that the nanotube conductivity drops sharply in the region of intermediate intensity and soars elsewhere. Field-effect mobility and field-induced carrier multiplications are considered to be competing processes governing the rise and fall of the conductivity.

In WSe2 intense THz fields are found to enhance transmission at 400 nm, while reducing it at 800 nm. The differential transmission is proportional to the field amplitude. The nonlinear responses are fast, yet non-adiabatic.

CNTs exhibit extraordinary nonlinear THz responses upon optical excitation. Its conductivity reduces at intermediate intensities, while soaring elsewhere. Field-effect mobility and carrier multiplications govern the rise and fall of the conductivity.

A terahertz (THz) metamaterial consisting of radiative slot antennas and subradiant complementary split-ring resonators exhibits plasmon induced opacity in a narrow spectral range due to the destructive interference between the bright and dark modes of the coupled oscillators. Femtosecond optical excitations instantly quench the mode coupling and plasmon oscillations, injecting photocarriers into the metamaterial. The plasmon resonances in the coupled metamaterial are restored by intense THz pulses in a subpicoseond time scale. The strong THz fields induce intervalley scattering and interband tunneling of the photocarriers and achieve significant reduction of the photocarrier mobility. The ultrafast dynamics of the nonlinear THz interactions reveals intricate interplay between photocarriers and plasmon oscillations. The high-field THz control of the plasmon oscillations implies potential applications to ultrahigh-speed plasmonics.