Intersubband optical processes in semiconductor quantum wells

In this thesis, several aspects of the optical properties of intersubband semiconductor \nlasers are studied theoretically, including the waveguiding properties of \nquantum cascade lasers (QCLs), the anticipated modulation bandwidth, gain and \nthreshold current of intersubband lasers, and the engineering of nonlinear susceptibilities \nin intersubband quantum well structures. Using two computational solvers, \nfor the Helmholtz and Schrödinger Equations respectively, optical waveguide structures \nand multi quantum well (MQW) structures are designed for subsequent research. \nThe waveguide design of a QCL reported by the Bell Labs reseachers is \nanalysed and improved upon. A four level rate equation model was used to obtain \nthe population inversion condition and modulation response for a triple quantum \nwell structure (TQW) designed for intersubband lasing. An analytical expression \nfor the modulation response is first obtained, followed by a numerical computation \nto verify the results. It is demonstrated that there is a unique dependence of the \nmodulation bandwidth upon the output power of the laser, and that the maximum \nmodulation frequency does not increase monotonically with optical output power \nas is the case with conventional semiconductor lasers. An expression describing \nthe optical gain of intersubband lasers is also derived. Using this, investigations \ninto the predicted achievable gain in mid-infrared (MIR) and near-infrared (NIR) \nintersubband lasers are conducted. It is found that the NIR gain is at least an \norder of magnitude higher than that of the MIR case. Self-consistent calculations \nof the optical gain are also undertaken, where the rate equations and the optical \ngain equations are solved alternately. An intersubband structure is designed for \nboth triple harmonic generation (THG) and four-wave mixing (FWM). The third \norder nonlinear susceptibilities of these respective processes in the structure were \ncalculated and found to be comparable to those of structures designed for just one \nprocess.