Outflanking Bandwidth Limitations for Terahertz Generation in Silicon Metasurfaces

Terahertz (THz) radiation is of utmost interest for applications ranging from communications to safety and biological spectroscopy. However, the generation of THz frequencies above 5 THz is challenging due to absorption in crystals and pump bandwidth requirements. Here, a silicon-based metasurface, featuring resonant modes in the near-infrared (NIR), is employed as nonlinear platform for the generation of THz radiation via degenerate four wave mixing process. We show that it is feasible to achieve on demand frequency generation by sweeping the central wavelength of a visible narrowband pulse and keeping the NIR pump beam unaltered, thus avoiding the need of broadband pump pulses. The usage of a metasurface allows to drastically reduce the absorption in the THz region. Our results pave the way to broadband spectroscopy in the THz region.