| The quantum cascade laser (QCL) is an intersubband semiconductor laser. Since its invention in 1994, the world had witnessed continuous output power improvement for this kind of device. But the wall plug efficiency (WPE), i.e., the energy conversion efficiency, remained only a few percent prior to this work. The objective of this work is to explore and overcome the limiting factors and experimentally demonstrate the highest WPE.;The fundamental working principles of a QCL are understood by a simplified rate equation model and a more complicated quasi-equilibrium thermal model with all relevant scattering mechanisms included. On the device level, the WPE in pulsed mode is theoretically decomposed into four sub-efficiencies, the internal quantum efficiency, voltage efficiency, electrical efficiency and optical efficiency, with experimental methods developed to measure all of them. The strategy of improving the WPE takes the route of understanding each sub-efficiency, finding its relation to the device structure, and then modifying the device structure to achieve a higher pulsed WPE. For cw operation, advanced thermal packaging is explored to bring the cw WPE as close to the pulsed WPE as possible. At this time, record WPE of 27% and 21% have been achieved in room temperature pulsed and cw mode operation, respectively. |
