Long-wavelength (λ ≈ 12- 16 μm) and cascaded transition quantum cascade lasers

Long-wavelength (12 - 16 μm) Quantum Cascade (QC) lasers are crucial devices for improving the detection sensitivity of QC-laser based sensing for important gases including BTEX (benzene, toluene, ethylbenzene, and xylenes) or uranium hexafluoride. A high-performance QC laser emitting at ∼ 14 μm is reviewed, optimized by employing a diagonal optical transition and a two-phonon-continuum depletion scheme. It shows a low threshold current density of 2.0 kA/cm2, a peak power of 336 mW, all at 300 K, as well as a high characteristic temperature ∼ 310 K over a wide temperature range around room temperature (240- 390 K). Single-mode operation is demonstrated with short cavities, with a mode-hop-free continuous tuning range of ∼ 5.5 cm-1. The ridge-width dependence of threshold of ∼ 14 μm QC lasers by both wet etching and dry etching is studied. The main challenge for narrowing wet-etched ridges is the high loss caused by mode coupling to surface plasmon modes at the insulator/metal interface of sloped sidewalls. Conversely, dryetched ridges avoid surface plasmon mode coupling due to the absence of transverse magnetic polarization for the vertical insulator and metal layers. To further improve the efficiency of QC lasers, a same-wavelength cascaded transition approach is developed, with two sequential cascaded transitions at the same wavelength ∼ 14.2 μm in each stage. This same-wavelength cascaded-transition QC gain medium was inserted between two conventional QC stacks at the same wavelength. Slope efficiency is increased by 46% when laser operation changes from the single-transition region to the cascaded-transition region.