The optical and thermal properties of quantum cascade lasers

The optical and thermal properties of quantum cascade lasers (QCLs) are investigated through the development of comprehensive theoretical models. The optical properties of various multilayer quantum cascade laser waveguides are investigated by solving Maxwell’s equations using a transfer-matrix method. The complex material refractive indices are calculated using a Drude-Lorentz model which takes into account both phonon and plasma contributions to the material properties. A Caughey-Thomas-like mobility model is used to estimate the temperature dependence of the electron mobility which is found to have a significant effect on the optical waveguide properties. The incorporation of this effect leads to better agreement with experimentally measured threshold current densities.
In order to investigate the thermal properties of QCLs, a multi-dimensional anisotropic heat diffusion model is developed which includes temperature-dependent material parameters. The model is developed using finite-difference methods in such a way that is can be solved in both the time-domain and in the steady-state. Various heat management techniques were compared in the time-domain in order to extract the heat dissipation time constants. In the steady-state, the model is used to extract the temperature dependence of the cross-plane thermal conductivity of a GaAs-based THz QCL and compare the thermal properties of THz and InP-based mid-infrared QCL optical waveguides.
In addition, fully self-consistent scattering rate equation modelling of carrier transport in short-wavelength QCLs is carried out in order to understand the internal carrier dynamics. This knowledge is then used to optimise the device design and the model predicts significant improvements in the performance of the optimised device.