Multidimensional conduction-band engineering for maximizing the continuous-wave (CW) wallplug efficiencies of mid-infrared quantum cascade lasers

By tailoring the active-region quantum wells and barriers of 4.5-5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature T₀ (253 K) and the slope-efficiency characteristic temperature T ₁ (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ_4g decreases, resulting in basically the same room-temperature (RT) threshold-current density J_th as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the _τ4g value while further suppressing carrier leakage. As a result, experimental RT J_th values from moderate-taper TA 4.8-μm emitting QCLs are ∼14% less than for conventional QCLs and T₁ reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the τ_4g value, due to Stark-effect reduction and strong asymmetry. Then, the RT J_th value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.