Effects of Elastic Scattering on High-Performance Step-Taper Active-Region Quantum Cascade Lasers

Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and ultrafast, miniband-like carrier extraction. In turn, that has led to very high internal-efficiency eta(i) values: similar to 77 % and 80-86 % from similar to 5.0 mu m- and 8-9 mu m-emitting QCLs, respectively. Based on extracted parameters that characterize the interface-roughness (IFR) scattering, a study has been performed of the effects of elastic scattering, both IFR and alloy-disorder (AD) scattering, on 5.0 mu m-emitting STA-QCLs. We find that the laser-transition efficiency eta(tr) is enhanced by similar to 15 % (i.e., from 83 % to similar to 95 %) due to the much stronger effect of elastic scattering on the lower-laser-level lifetime than on the effective upper-laser-level lifetime. In turn, the injection efficiency: eta(inj) = eta(i) / eta(tr), reaches similar to 81 %; that is, the highest injection-efficiency value obtained to date from QCLs. Furthermore, we find that the projected upper limit for the pulsed wall-plug efficiency can reach values as high as 44.4 % for 4.6 mu m-emitting devices; thus, raising the possibility of CW operation of 4.5-5.0 mu m-emitting QCLs with wallplug-efficiency values as high as 40 %.