Room temperature terahertz nonlinear quantum cascade lasers and their applications

Recent progress of terahertz semiconductor sources based on intra-cavity nonlinear frequency mixing in mid-infrared quantum cascade lasers will be reviewed. Relying on the dual-upper-state active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase matching scheme, these devices demonstrated drastic improvements in performance in recent years and can now produce terahertz radiation from 0.6 to 6 THz with power output sufficient for imaging and spectroscopic applications. In fact, the high-quality terahertz imaging has been obtained by a THz imaging system using these terahertz quantum cascade laser sources.