Ultrafast control of terahertz harmonic generation by optically modulating carrier dynamics in nonlinear metasurfaces

The development of terahertz (THz) sources with wide spectral coverage and dynamic control is critical for advancing high-speed communication, high-resolution imaging, and reconfigurable photonics. However, conventional nonlinear materials exhibit weak harmonic responses and limited tunability at room temperature. Here, we propose a nonlinear THz metasurface that simultaneously enables the generation and optical modulation of both second and third harmonics with ultrafast switching speeds. The metasurface locally enhances THz magnetic and electric fields, which drive photoinduced carriers into anharmonic oscillations and produce harmonic emissions. Owing to the rapid excitations of free carriers, we achieve the modulation of SHG and THG with switching times of 4.31 and 3.98 ps, respectively, and their extinction ratios exceed 1000. Moreover, tuning the optical pump fluence enables amplitude modulation of harmonic outputs by dynamically manipulating carrier density and mobility. This single-device approach demonstrates a unified platform for generating, enhancing, and dynamically controlling multiple THz harmonics, offering new opportunities for reconfigurable THz sources and adaptive nonlinear photonic devices.