Recent Progress in Terahertz Quantum Cascade Lasers

Terahertz quantum cascade lasers (QCLs) emit radiation due to intersubband optical transitions in semiconductor superlattices that could be engineered by design. Among a variety of possible design schemes, we have pursued designs that utilize strong electron-phonon interaction in the semiconductor as a means to establish population inversion for optical gain. This report describes the recent progress in phonon-depopulated terahertz QCLs. Operation above 160 K has been realized in GaAs/AlGaAsbased QCLs with metal-metal waveguides for frequencies ranging from 1.8-4.4 THz (λ ~ 170-70 μm). A record highest operating temperature of 186 K has been demonstrated for a 3.9-THz QCL based on a diagonal design scheme. Also, operation down to a frequency of 1.45 THz (λ ~ 205 μm) has been achieved. Whereas metal-metal waveguides provide strong mode confinement and low loss at terahertz frequencies, obtaining single-mode operation in a narrow beam-pattern-posed unconventional challenges due to the subwavelength dimensions of the emitting aperture. New techniques in waveguide engineering have been developed to overcome those challenges. Finally, a unique method to tune the resonant-cavity mode of metal-metal terahertz "wire lasers" has been demonstrated to realize continuous tuning over a range of 137 GHz for a 3.8-THz QCL.