Threshold Reduction in Quantum Cascade Lasers with Partially Undoped, Dual-Wavelength Interdigitated Cascades

A dual-wavelength quantum cascade (QC) laser with an interdigitated cascade of alternating active regions designed to emit at 8.0 and 9.5 m mm, respectively, is presented. Aside from providing two-wavelength operation, this laser design was used to test the role of extrinsic carriers in the injectors, which customarily bridge successive active regions together. An interdigitated cascade was grown with undoped injectors bridging 9.5 and 8.0 m mm active regions, but doped injectors bridging 8.0 and 9.5 m mm active regions. Clear laser action on both wavelengths demonstrates that doping of all injector regions is not a firm requirement for QC-lasers, as was so far believed. Furthermore, comparison with a conventionally doped interdigitated cascade QC-laser shows a threshold reduction by a factor of approximately 2 for the laser based on the active regions preceded by the undoped injector. This can be understood from the absence or strong reduction of impurity scattering related to the dopant ions.