Recent Progress in Widely Tunable Single-Mode Room Temperature Terahertz Quantum Cascade Laser Sources

We present the operating principle, design, and performance of external-cavity (EC) and monolithic terahertz (THz) tuners based on intracavity difference-frequency generation (DFG) in midinfrared (mid-IR) quantum cascade lasers (QCLs). A DFG-QCL gain chip employed in a Littrow-type THz EC system was optimized for wide tunability in 1-6 THz range using broad mid-IR gain bandwidth active region with integrated optical nonlinearity and Cherenkov THz phase-matching scheme. The EC system demonstrated ultrabroadband single-mode tuning from 1.2 to 5.9 THz. Beam steering in THz far field due to refractive index dispersion of InP substrate has been successfully suppressed by bonding the QCL chip on a high-resistivity silicon substrate. We also discuss the design of compact widely tunable monolithic THz DFG-QCL sources. Devices use two electrically isolated grating sections for independent electronic tuning of the two mid-IR pump frequencies. THz DFG frequency tuning from 3.44 to 4.02 THz, corresponding to 580 GHz or 15% of center frequency, is obtained.