Large terahertz nonlinearity in two-dimensional electron gas metasurface

Nonlinear responses in the terahertz (THz) frequency range are essential for advancing THz sources and modulators. However, the development of THz nonlinear materials with efficient second- and third-order nonlinear susceptibilities at room temperature remains challenging. Here, we introduce a THz nonlinear metasurface based on gallium nitride two-dimensional electron gas (2DEG), capable of both second harmonic generation (SHG) and third harmonic generation (THG). By leveraging the magneto-electric coupling mechanism built in the metasurface, we induce anharmonic oscillations of electrons to achieve THz SHG with the effective second-order nonlinear susceptibility reaching 14.3 μm V^-1. Meanwhile, the localized electric field confinements in the same metasurface structure substantially improve the intrinsic third-order nonlinearity of the 2DEG as well, enhancing the THz THG by over two orders of magnitude. By simply scaling the structure of the metasurface, the working frequency of the intense nonlinear responses can be engineered at will. Our results provide a promising route to efficient THz second- and third-order nonlinearities within a single metasurface, which may open new pathways for developing highly integrated, room-temperature THz sources, as well as further advancements in high-speed electronics.