Nonperturbative Nonlinear Metasurfaces for Efficient Terahertz Harmonic Generation

The nonlinearities at terahertz (THz) frequencies have been recognized as key and promising approaches to advance the development of high-frequency THz sources via the harmonic generation process, which lies at the core of THz technology. However, the high-performance nonlinear THz materials possessing extreme conversion efficiency, high-volume on-chip integration, and room-temperature operation are still seemingly missing. Here, we propose a nonlinear metasurface for efficient THz third-harmonic generation (THG) based on the nonperturbative nonlinearity of doped silicon. By integrating aluminum grating, the significantly enhanced local field drives the intrinsic nonlinearity of silicon to the nonperturbation regime, where the conversion efficiency of THz THG approaching maximum can be achieved under moderate pump from table-top sources at ambient temperature. The measurements demonstrate that the nonlinear responses of metasurfaces are ∼1000 times stronger than the plain silicon film. Such a platform also allows us to experimentally obtain the nonlinear susceptibilities of silicon at perturbation and nonperturbation regimes, which are crucial for understanding the essences of nonlinear THz–matter interactions but scarcely available for most materials in this part of the spectrum. Our findings may open an avenue toward CMOS-compatible, room-temperature, and efficient THz sources and nonlinear electronics.