Terahertz source frequency comb based on difference frequency generation from a mid-IR quantum cascade laser

Optical frequency combs in the mid-IR and THz ranges are in great need for spectroscopy and frequency metrology. However, there is no chip-based THz frequency comb source at room temperature. The objective of the proposed research is to demonstrate a room temperature, continuous wave (CW), monolithic THz frequency comb source with sub-milliwatt level output power. The approach is to use a mid-infrared (mid-IR) quantum cascade laser (QCL) engineered with flat-top gain and large nonlinearities for the intracavity difference frequency generation (DFG) of THz frequency comb. Dual-section Fabry Pérot-distributed feedback (FP-DFB) cavity design is proposed to render the pump source with unique single- and multi-modes operation. In CW operation, the dispersed multi-modes can be locked to the equally spaced comb modes via four wave mixing and near-zero group velocity dispersion. Such cavity scheme provides a unique design freedom for the acquiring of a stable DFB-comb operation, and thus a THz comb output via the DFG processing and broadband Čerenkov phase matching scheme. The proposed approach offers a solution of on-chip generation of THz frequency comb at room temperature.