Terahertz quantum cascade laser sources based on Čerenkov difference-frequency generation

Room-temperature terahertz (THz) quantum cascade laser (QCL) sources based on intra-cavity difference-frequency generation (DFG) with record THz conversion efficiencies is reported. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Čerenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-infrared quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. A lateral current extraction scheme is used. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency and directional THz output. Experimentally, proof-of-principle devices demonstrate a conversion efficiency up to 70 μW/W2 and provide output across a 1.2 - 4.5 THz spectral range.