Quantum Cascade Lasers (QCLs) are semiconductor lasers that, in contrast to common diode lasers, operate on intersubband transitions in the conduction band of the materials in the heterostructure. This means that device characteristics are defined by quantum well structure rather than material bandgaps. A superlattice of quantum wells is engineered to give off a photon of light per stage. The stages are then repeated multiple times, allowing an electron to cascade from one stage to the next, potentially releasing a photon through stimulated emission at each stage.

As electrons cascade from one stage to the next, there are chances for the electron to transition into states that do not allow for stimulated emission. Such states are referred to as parasitic states, and are a source of inefficiency in a QCL. The percent chance that a cascading electron will transition into the upper laser level of a stage is the injection efficiency.

Wall-plug efficiency is the percent of input electrical energy converted into useful laser light in a QCL. It is the net description of the effects of injection efficiency, slope efficiency (the amount of optical power generated per unit current injected into a QCL) and threshold current (the minimum current required to generate laser light.) High injection and slope efficiencies, along with a low threshold current, allow for high wall-plug efficiency.

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