Long wavelength quantum cascade lasers for applications in the second atmospheric window at wavelength of 9-11 microns

In the last few years there has been significant progress made in the development of high power and high efficiency quantum cascade lasers in the wavelength range of 4 to 5 microns, while QC lasers in the second atmospheric window have been experiencing performance development at a slower pace. Now similar improvements in the QCL design and growth used for the mid-wave IR (MWIR) can be applied to the long-wave IR (LWIR) with some important differences and adaptations to the challenges presented by the operation at longer wavelengths. These include, among others, a smaller optical confinement, larger losses and inter-miniband leakage, stronger sensitivity to background doping, and the need for thicker waveguides. These factors generally result in the degradation of laser characteristics as the emission wavelength increases. Here we present three new designs in the wavelength range of 8.9 to 10.6 μm and compare their performance and design metrics along with two reference designs in the same spectral range. A selective strain design emitting at 10.3 μm achieved threshold currents and slope efficiencies very close to the reference design emitting at 9.9 μm - thus providing longer wavelength emission with no performance deterioration. From the comparison of the designs presented here, after taking into account the differences in performance metrics of devices designed to operate at longer wavelengths, we can point out the contribution to the laser characteristics of the carrier leakage from the upper lasing state to the upper miniband and to the continuum, and of the coupling strength between injector and upper lasing level. We find that designs with similar metrics but larger splitting between ground injector and upper lasing level exhibit superior performance than those with smaller coupling.