Abstract Tunable sources of narrowband continuous-wave electromagnetic radiation in the terahertz spectral range, spanning from 0.1 THz to 15 THz, are notoriously difficult to produce. Despite substantial research efforts to close the ‘terahertz gap’, there are still no tunable room-temperature continuous-wave sources with powers larger than 0.1 μW in the spectral range 6–11 THz. Here we demonstrate a continuous-wave terahertz source with up to 14 μW of power output in the 6–11 THz band and a total tuning range of 1–11 THz. Our source is based on difference-frequency generation in deeply subwavelength nonlinear metasurfaces with a second-order nonlinear response three orders of magnitude larger than that of the best nonlinear crystals used for terahertz generation. Owing to their subwavelength thickness, the metasurfaces are mostly free from both phase matching and absorption constraints. Tunable terahertz radiation is generated by pumping the metasurfaces with two milliwatt-level continuous-wave mid-infrared lasers in a simple modular set-up that superficially resembles that employed by terahertz photomixers.
Krakofsky et al. (2026) studied this question.