| Quantum Cascade (QC) lasers are mid-infrared semiconductor light sources which attract considerable interest for mid-infrared sensor systems, especially for high sensitivity and selectivity detection of chemical vapors in security, medical, and environmental applications. For some of these practical sensing applications, high temperature (room temperature and above), high power, continuous-wave (CW) operation is desirable for the purposes of simplifying the system and increasing the sensitivity. However, due to the thermal effects associated with the high threshold and low wall-plug efficiency, the performance of QC lasers is severely limited at high temperature. To overcome this problem, this dissertation shows how to optimize the QC laser performance through high gain active region and low loss waveguide designs, and advanced device processing and packaging for better thermal management.; First, a self-consistent thermal model of QC lasers has been developed, and combined with band-structure design and waveguide design to provide a comprehensive design tool which includes the consideration of thermal management. Using this model, room temperature, CW QC lasers at lambda ≈ 8.2 mum have been designed, grown and demonstrated, which includes the first room temperature CW QC lasers grown by metal-organic chemical vapor deposition (MOCVD) without lateral regrowth. Next, by using similar design strategies, high performance, room temperature, CW operation has been extended to longer wavelengths of 9.6-10.3 mum within the second atmospheric window, which are useful for sensing important gases such as diisopropyl methylphosphonate (DIMP, an explosive stimulant), etc. Third, to better understand the laser performance and further improve the laser designs, a systematic study on temperature-dependent optical gain and waveguide loss was conducted in those high performance QC lasers using Hakki-Paoli technique. Besides confirming the expected magnitude and temperature dependence of the gain coefficient, the results indicate a 2-3 times higher waveguide loss than the one calculated from free carrier absorption, which indicates the presence of other loss mechanisms inside the laser active core. Finally, using strain-compensated InGaAs/AlInAs material, a high performance, short wavelength lambda ≈ 5.3 mum QC laser was demonstrated with very small heat dissipation of 1.2 - 1.7 W for room temperature, CW operation. These lasers may lead to the first uncooled, room temperature, CW laser packages. |
