Quantum Cascade lasers in the Mid-infrared

Cascade lasers includes quantum cascade lasers (QCLs) and interband cascade lasers (ICLs). QCL and ICL have similar structures and working principles. A common feature of these two kinds of cascade lasers is electron recycling, which means that one electron can produce multiple photons. This greatly improves the efficiency of the devices at these wavelengths, because it is difficult to find good narrow gap materials which would allow lasing at this wavelength. Therefore, cascade effect can mitigate this problem. In addition, Mid-infrared (MIR) light sources is in great demand because this region contains absorption features of many important molecules, and therefore, can provide a variety of applications. It is important to investigate and evaluate the performance of current state-of-art devices, and it is worth exploring the way to improve the performance of devices. This can make the applications more competitive in the market and can help to increase the efficiency. This thesis will talk about some simulations of QCLs and ICLs and will focus on the discussion on experimental results for fabricated structures. In this project, we investigated the use of nextnano to simulate previously designed QCLs as well as some initial attempt on ICLs. QCLs with different number of periods were then fabricated and tested experimentally to explore the effect of increasing the number of active regions in the structure. We found a reduced threshold current density in the device fabricated from MR 3881 (42 periods) compared with MR 3877 (35 periods), and the reduction in threshold current density matched up with the increase of gain. The calculation of internal loss was also carried out and compared with literature. Moreover, the testing of highly-reflective (HR) coated devices for MR3877 were also attempted by evaporating an amount of MgF2 and a reduction in threshold current was obtained.