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

A quantum cascade (QC) laser with a heterogeneous cascade containing two substacks previously optimized to emit at 5.2 μm and 8.0 μm wavelengths, respectively, is presented. The low-temperature performance of the two-wavelength laser is comparable to the respective homogeneous stack lasers, indicating no penalty from the heterogeneity of the cascade. Each substack is apportioned the optimum fraction of the applied bias. This demonstrates the general applicability of this scheme. In addition, an etch-stop layer inserted between the two substacks allowed fabrication of a "tap" into the cascade. The latter was used to selectively manipulate the laser threshold of one substack, turning the 8.0 μm laser on and off while the adjacent 5.2 μm QC laser was operating undisturbed.