Quantum cascade lasers with a heterogeneous cascade: Two- and multiple-wavelength operation

Unipolar Quantum Cascade (QC) lasers are easily recognized by the cascading scheme, in which electrons traverse a stack of many, typically 30 but sometimes up to 100, active regions alternated with injector regions, rather than only a single active region, as in conventional semiconductor lasers. So far, QC-lasers shared the characteristic, that all stages of the cascade were essentially identical. This makes perfect sense for lasers with optimized performance, with a low threshold current density and high optical output power. The possibility of heterogeneous cascades was sometimes discussed. However, it was uncertain if optimal operating conditions could be achieved for all components of the cascade. Here, we experimentally discuss three types of QC-lasers with heterogeneous cascades. The first type contains two sub-stacks, each using a previously optimized QC structure, connected by a thin InGaAs layer. This results in a QC-laser emitting simultaneously at 5.2 and 8.0 μm wavelength, with performance levels similar to those of the respective homogeneous stack lasers. It was not necessary to adjust the design electric field of the two stacks to match each other. Each sub-stack is apportioned the appropriate fraction of the applied bias. In addition, an etch-stop layer inserted between the two sub-stacks allowed fabrication of a "tap" into the cascade. The latter was used to selectively manipulate the laser threshold of one sub-stack, turning the 8.0 μm laser on and off while the adjacent 5.2 μm QC-laser was operating undisturbed. We also fabricated a doubly-single mode QC-distributed feedback laser with single-mode emission at 5.0 and 7.5 μm with simultaneous single-mode tunability. The second type of QC-laser contains a waveguide core with an interdigitated cascade of two different active regions with matching injectors and emitting at 8.0 and 9.5 μm wavelength simultaneously. Finally, the third type of QC-laser with heterogeneous cascade was designed to generate a broadband continuum. We observe gain from 5 to 8 μm and laser action continuously from 6 to 8 μm.