Recent Progress in Terahertz Quantum Cascade Lasers

摘要

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 $hbox{160 K}$ has been realized in GaAs/AlGaAs-based QCLs with metal--metal waveguides for frequencies ranging from $1.8$-- $4.4 ;hbox{THz}$ ($lambdasim 170hbox{rm --}70, mu{rm m}$). A record highest operating temperature of $hbox{186 K}$ has been demonstrated for a 3.9-THz QCL based on a diagonal design scheme. Also, operation down to a frequency of $1.45 ;hbox{THz}$ ($lambdasim 205 ,mu{rm m}$) 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.