Resonant-Phonon Terahertz Quantum-Cascade Lasers and Video-Rate Terahertz Imaging

摘要

We review the development of terahertz quantum-cascade lasers (QCLs) that can be uniquely qualified based on a resonant-phonon depopulation scheme. Record performances in terms of operating temperature and optical power output are reported. The best temperature performance is achieved in the metal--metal (MM) waveguides, which provide near-unity mode confinement and low waveguiding loss at terahertz (THz) frequencies even for cavities with subwavelength dimensions. A pulsed operation up to a heat-sink temperature of $hbox{169 K}$ at $nusimhbox{2.7 THz}$ and a continuous-wave (CW) operation up to $hbox{117 K}$ at $nusimhbox{3 THz}$ are demonstrated with a five-level design that has a two-well injector region. Some of the key temperature degradation mechanisms for this design are discussed. For operation at lower frequencies ($nu≪hbox{2 THz}$), a one-well injector design is developed that reduces intersubband absorption losses in the injector region. A QCL operating at $nu=hbox{1.59 THz}$ ( $lambda=hbox{188.5} muhbox{m}$) up to a heat-sink temperature of $hbox{71 K}$ in cw mode is demonstrated with that design. To obtain high-power output and low beam divergence from the MM waveguides, a lens-coupled scheme is demonstrated. A peak power output of $hbox{145 mW}$ , a beamwidth of $hbox{4.8}^circ$ , and a maximum lasing temperature of $hbox{160 K}$ are obtained from a $hbox{4.1 THz}$ QCL in this configuration. In the latter part of the paper, we report on the demonstration of video-rate (20 frames/s) terahertz imaging with QCLs as the source for illumination and a $hbox{320}times hbox{240}$ element room-temperature microbolometer focal plane array as the detector. The QCLs for the imaging system are processed into semiinsulating surface-plasmon waveguides, and are operated in a cryogen-free thermomechanical cooler in quasi-CW mode at a heat-sink temperature of ${sim}hbox{30 K}$. Real-time imaging in transmission mode is demonstrated at a standoff distance of 25 m with a $nusimhbox{4.9 THz}$ QCL in this setup.