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Opt Express. 2016 Oct 17;24(21):24483-24494. doi: 10.1364/OE.24.024483.

86% internal differential efficiency from 8 to 9 µm-emitting, step-taper active-region quantum cascade lasers.


8.4 μm-emitting quantum cascade lasers (QCLs) have been designed to have, right from threshold, both carrier-leakage suppression and miniband-like carrier extraction. The slope-efficiency characteristic temperature T<sub>1</sub>, the signature of carrier-leakage suppression, is found to be 665 K. Resonant-tunneling carrier extraction from both the lower laser level (ll) and the level below it, coupled with highly effective ll-depopulation provide a very short ll lifetime (~0.12 ps). As a result the laser-transition differential efficiency reaches 89%, and the internal differential efficiency η<sub>id</sub>, derived from a variable mirror-loss study, is found to be 86%, in good agreement with theory. A study of 8.8 μm-emitting QCLs also provides an η<sub>id</sub> value of 86%. A corrected equation for the external differential efficiency is derived which leads to a fundamental limit of ~90% for the η<sub>id</sub> values of mid-infrared QCLs. In turn, the fundamental wallplug-efficiency limits become ~34% higher than previously predicted.


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