Nat. Commun. 10, 2403 (2019)

A frequency comb in the terahertz region and that operates at room temperature could prove useful for applications in spectroscopy. The device, designed and built by scientists from Northwestern University in the US, relies on difference-frequency generation from a mid-infrared (mid-IR) quantum cascade laser (QCL). A distributed feedback grating is integrated into the QCL’s cavity in order to simultaneously generate a single mode at one distinct wavelength in the mid-IR as well as a mid-IR comb. The two are then mixed together to perform down-conversion into the terahertz region. The approach is attractive as it offers a compact semiconductor chip-based approach to comb generation and operates at room temperature. When the laser was driven at a current of 1.55 A, a total of 5 comb lines with a spacing of 245 GHz were generated between 2.2 and 3.3 THz.