Principle of the THz nonlinear metasurface and resonance characterization. (a) Schematic of the designed metasurface. (b) Schematic of dynamically controlling carrier dynamics during a terahertz pulse by tuning optical pump fluence and time delay. The optical excitation defines the transition from the static to the active state. (c) Illustration of orientations of the local electric field E_y(ω), magnetic field B_z(ω), the Lorentz force F_B(2ω), and the carrier drift in the pumped GaAs driven by the local electromagnetic field. (d) The current disturbance (white arrow), the local enhancement of magnetic field in the z-axis B_z(ω), the local enhancement of electric field in the y-axis (E_y(ω)), and the normalized distribution of the Lorentz force F_B(2ω). (e) Photomicrograph of the fabricated metasurface, where the insert shows the unit cell of the metasurface: W = 4 μm, G = 4 μm, L = 24 μm, and the period of the unit cell is 42 μm. (f) Simulated and measured transmission spectrum of the metasurface.