Terahertz quantum cascade lasers employing convex phonon well

The 3-level resonant-phonon scheme, a predominant approach for achieving high-temperature operation in terahertz quantum cascade lasers, has recently gained conspicuous progress by purifying the lasing of relevant quantum levels from nonrelevant ones. However, as this 3-level system typically relies on a module architecture with only two quantum wells, the engineering flexibility is limited; for instance, the transparency of injector barriers for the resonant tunneling process is sacrificed to balance the trade-offs between injection selectivity and parasitic leakages through nonrelevant levels. This study presents the convex phonon well concept to expand engineering possibilities for levels of purification. The effectiveness of this concept is initially confirmed by observing lasing at 201 K in experiments.