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

Long-wavelength (lambda approximate to 12 - 16 mu m) 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 similar to 14 mu m 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/cm(2), a peak power of 336 mW, all at 300 K, as well as a high characteristic temperature similar to 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 similar to 5.5 cm(-1). The ridge-width dependence of threshold of similar to 14 mu m 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, dry-etched 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 similar to 14.2 mu m 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.