In the past decade, the main effort has been directed towards reducing the injection current threshold for laser operation. High thresholds imply large input electrical powers and result in significant heating of the active region, which is detrimental to the laser operation. While QCL threshold power densities have remained high, there has been dramatic progress in the development of mid-infrared emitters with much lower thresholds using the so-called interband cascade laser (ICL), which combines the interband nature of the active transition with the multi-stage cascading inherent in the QCL design. The ICL current threshold is typically limited by non-radiative Auger recombination, which in the mid-infrared turns out to be far weaker than the intersubband scattering that dominates the QCL threshold. The authors of this Optics Express paper have demonstrated that their recent reductions in the ICL threshold, achieved by rebalancing the electron/hole density ratio in the active region by heavily n-doping the electron injectors, can be extended to longer emission wavelengths of lambda = 4.7 and 5.6 µm. While previous ICLs in this spectral range were limited to cryogenic operation, the improved devices operate in continuous-wave mode at room temperature and lase up to 60 C. The power-density thresholds are approximately an order of magnitude lower than in state-of-the-art quantum cascade lasers in this spectral range. All these developments will contribute noticeably to the compactness, portability, and lifetime of laser-based sensing systems in the mid-infrared.
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