The active regions of quantum-cascade lasers (QCLs) are made of a superlattice of quantum wells (QWs) and barrier layers. In conventional QCLs, the barriers all have the same alloy composition. For QCLs emitting continuous-wave (CW) in the 4.5 to 5.0 μm range, this results in thermally activated electron leakage from the upper laser level to the continuum, and low maximum wall-plug-efficiency values of around 12% at room temperature—short of the theoretical 28%. Researchers at the University of Wisconsin (Madison, WI), Lehigh University (Bethlehem, PA), and the Naval Research Laboratory (Washington, DC) say that this problem can be corrected by using metal-organic chemical vapor deposition to grow QWs and barriers that vary in composition.
The researchers fabricated 4.8-μm-emitting indium-phosphide-based QCLs using the deep (highly strained) well approach, using tall barriers in and around the active region for strain compensation. In addition, the conduction-band edges of the injector and extractor regions were tapered. The characteristic temperature coefficient T0 for the threshold current density Jth was boosted from 143 K for conventional QCLs to 253 K over the 20°–90°C temperature range. The design should allow single-facet room-temperature CW wall-plug efficiencies of up to 22%. Contact Dan Botez at [email protected].