Multioctave Terahertz Frequency Synthesis via Simultaneous Excitations of Intrinsic and Extrinsic Nonlinearities in Metasurfaces

The advancements of terahertz (THz) technologies for high-speed wireless communication and high-resolution radar systems are fundamentally constrained by the lack of efficient, compact, and broadband THz sources. Frequency synthesis via nonlinear conversion offers a compelling route for generating multiple THz frequencies from one or two fixed inputs, but its practical implementation is limited by the scarcity of nonlinear materials with strong responses at THz frequencies. Here, we demonstrated the process of THz frequency synthesis in metasurfaces, generating multiple discrete THz frequency lines with multioctave broadband coverage. The metasurface enables the simultaneous excitations of intrinsic third-order and extrinsic second-order nonlinearities, respectively arising from the inherent nonlinear responses of the materials and from magnetoelectric coupling induced by the resonant behavior of the metasurface. These dual nonlinear interactions facilitate the co-occurrence of five distinct processes: second- and third-harmonic generation, sum- and difference-frequency generation, and four-wave mixing. As a result, the metasurface generates 10 discrete THz frequencies spanning a 13-fold spectral range from 0.3 to 3.9 THz, corresponding to 3.7 octaves, with only two frequency inputs. This work provides a scalable and integrable approach to ultrabroadband, frequency-agile THz sources and nonlinear devices.