High-efficiency, high-power mid-infrared quantum cascade lasers [Invited]

The step-taper active-region (STA) design concept is implemented for ~5.0 μm-emitting quantum cascade lasers (QCLs) grown by metal-organic chemical vapor deposition (MOCVD). Carrier-leakage suppression yields high characteristic temperatures for the threshold-current density Jth, T0, and for the slope efficiency ηsl, T1: 226 K and 653 K. Resonant-tunneling extraction from the lower level results in miniband-like extraction. In turn, the internal efficiency ηi is found, from a variable mirror-loss study, to be ~77%; thus approaching the ~90% upper limit, when employing only inelastic scattering. Considering interface-roughness and alloy-disorder scattering, the transition efficiency reaches values of ~95%. Then, the injection efficiency is ~81%, and, for λ = 4.6 μm, the wallplug-efficiency ηwp upper limit reaches 41.2%. Results include 4.2 W/A single-facet ηsl and 0.96 kA/cm2Jth values. Buried-heterostructure (BH) QCLs provide single-facet 2.6 W continuous-wave (CW) power and 12% CW ηwp. Optimized 8 μm-emitting, STA-design QCLs provide 2 W/A ηsl, and 1.1 kA/cm2Jth; and BH devices yield single-facet 1 W CW power and 6% CW ηwp.