Generation of continuous-wave 1-11 THz radiation with intersubband polaritonic metasurfaces

Tunable sources of narrowband continuous-wave (CW) electromagnetic radiation in the terahertz (THz) spectral range, spanning from 0.1 to 15 THz, are notoriously difficult to produce. Despite significant research efforts to close the “THz gap”, there are still no tunable CW sources with reasonable (>0.1 μW) power output in the spectral range 6-11 THz. In this work, we demonstrate a CW THz 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 (DFG) in a deeply subwavelength nonlinear metasurface with a second-order nonlinear response 3 orders of magnitude larger than that of the best nonlinear crystals used for THz generation. Due to its subwavelength thickness, the metasurface is mostly free from both phase-matching and absorption constraints. Tunable THz radiation is generated by pumping the metasurface with two milliwatt-level CW mid-infrared (mid-IR) lasers in a simple modular setup that superficially resembles that employed by photomixers. Our work opens the 6-11 THz spectral range for applications that require tunable narrowband CW radiation sources, including high-resolution spectroscopy and metrology, radio-astronomy, microscopy, and sensing.