The aim of this work was to develop an envelope function method to calculate the electronic states and optical properties of complex quantum well heterostructures, and to demonstrate its effectiveness by application to some device structures of topical interest. In particular, structures have been considered which might form the basis of intensity modulators and polarization insensitive amplifier devices for light at a wavelength of 1.55 µm. The modulator structures considered all have the general form of two coupled quantum wells of different widths as the active region. The application of an electric field in the growth direction is intended to result in a shift in the energy and spatial localisation of the confined states and produce an increase in the absorption coefficient at longer wavelengths than the zero field absorption edge. The effectiveness of certain structures is examined in terms of field induced absorption increase at 1.55 µm. A system which shows a significant increase in absorption coefficient at this wavelength on application of a practical electric field has been identified as a possible candidate for an intensity modulator. In the case of the amplifier, the active region of the most promising structure considered consists of a stepped well which comprises two layers, one with tensile and one with compressive strain. It is known that the presence of the two oppositely strained layers can result in the TE and TM gain peaks appearing at similar photon energies. Our calculations show that a suitable choice of strain and layer widths can result in a small or zero difference between the TE and TM gains at 1.55 µm, which can be important for the polarization insensitive operation of devices in optical communications applications. In order to predict the optical properties of quantum well devices it is necessary to calculate the electron and hole states for a range of in-plane wavevectors. The calculations developed and carried out in this work are based on a multi-layer (eight band) k.p model including strain effects. The interfacial boundary conditions which result from approximations to Burt's exact envelope function theory are included in the model. The effect of an electric field is modelled by including a potential energy term in each layer Hamiltonian which is equal to the average energy shift across the layer in question due to the presence of the field. The model has been developed with flexibility in mind and has applications beyond the specific devices considered in this thesis.

Theoretical models for the electrokinetics of weakly permeable porous colloidal particles are absent from the literature. The understanding of this topic will be advanced through a systematic analysis of the standard electrokinetic equations, resulting in a theory for the electrophoretic mobility of weakly permeable porous colloidal particles. / The standard electrokinetic equations are employed to model the flux of solvent and ions outside the porous particle. To be consistent with this approach, the flux of solvent and ions in the pores must also be governed by the standard electrokinetic equations. However, in practice, only transport phenomena on the particle scale are observed and it is sufficient for information regarding pore-scale behaviour to be retained purely in the form of averaged quantities. To complete the theoretical description, the standard electrokinetic equations outside the particle must be coupled to particle-scale transport equations inside the particle via boundary conditions at the porous/free-fluid interface. / It has been shown experimentally and theoretically for coupled Stokes and Darcy flows, that the correct interfacial boundary condition for the tangential external flow is given by the Beavers-Joseph-Saffman (BJS) condition. The effect of the BJS boundary condition on the hydrodynamic drag on an oscillating porous particle is investigated. It is found that the particle may be regarded as impermeable with a slip length independent of frequency, and the resulting drag is significantly reduced in comparison with an equivalent impermeable particle that does not exhibit a slip length. / The transport of a general electrolyte solution through a rigid porous body subjected to a static (d.c.) electric field is studied. The pore-scale description is given by the standard electrokinetic equations, including the effects of ion diffusion, electromigration and convection. Homogenization theory is used to derive transport equations that capture the particle-scale behaviour. It is proven that the transport coefficient tensors obey Onsager’s reciprocal relations and the diagonal coefficient tensors are positive definite. / New interfacial boundary conditions are derived using conservation arguments supplemented by Stern-layer theory. When combined with the particle-scale transport equations, these boundary conditions incorporate four principal effects into the standard electrokinetic model: solvent slip and Stern-layer ionic conduction at the interface, and macroscopic ionic conduction together with the electroosmotic flow of solvent through the particle. / The method of matched asymptotic expansions is then used to construct an approximate solution to the aforementioned system, in the thin double-layer limit. An expression for the electrophoretic mobility of a weakly permeable colloidal sphere is produced that consists of a generalization of Smoluchowski’s formula to encompass porous particles, and a next order correction. For the first time, the effects of solvent slip and Stern-layer ionic conduction within the porous/free-fluid interface, in conjunction with macroscopic ionic conduction and electroosmosis through the particle, are exhibited. It is shown that solvent slip at the porous interface is overwhelmingly the dominant effect on the mobility of weakly permeable porous colloidal particles.

No description available.

Les écoulements turbulents à phases séparées sont présents dans de très nombreuses applications. Cependant, la simulation de tels écoulements avec une interface déformable constitue l'un des problèmes les plus complexes de la mécanique des fluides numérique. La prise en compte du bilan des contraintes normales est au cœur du problème de déformation de l'interface. Dans le travail présenté ici, nous développons un algorithme permettant de simuler des écoulements diphasiques incompressibles et turbulents en suivant le déplacement de l'interface par une approche lagrangienne. Les équations de Navier-Stokes instationnaires écrites en variables vitesse-pression sont résolues dans les deux phases en utilisant des maillages curvilignes orthogonaux. Dans un premier temps, nous introduisons un schéma de raccordement des vitesses tangentielles et des cisaillements. Ce schéma est appliqué afin de simuler l'interaction de deux écoulements turbulents séparés par une interface plane. La turbulence est traitée par une approche de simulation des grandes échelles utilisant un modèle dynamique. Un algorithme original est ensuite développé dans le but de satisfaire de façon non-itérative à la fois la continuité des vitesses normales et des contraintes normales sur l'interface et l'incompressibilité dans les deux phases. Différentes simulations d'écoulements diphasiques avec interface déformable sont réalisées afin de valider ces développements. / Turbulent incompressible two-phase separated flows are present in many applications. However, simulation of such flows with a moving interface is one of the most challenging problems in todays computational fluid dynamics. Taking properly into account the normal stress budget accross the interface is the main difficulty of moving interface problems. This work deals with the development of a boundary-fitted method for computing turbulent incompressible two-phase flows. The interface displacement is achieved through a Lagrangian approach. The unsteady Navier-Stokes equations written in a velocity- ressure formulation are solved within the two phases using an orthogonal curvilinear grid. In a first step, we introduce a scheme allowing tangential velocities and shear stresses to match across the interface. We apply this technique to compute the countercurrent flow generated by two streams separated by a plane interface. This scheme is then applied to compute various situations involving the interaction between two turbulent flows separated by a flat interface. The turbulence is treated by using the Large Eddy Simulation approach with a dynamic model. An original algorithm is then developed to satisfy without any internal iteration the continuity of normal velocities and stresses across the interface and the incompressibility condition within both phases. Several simulations of two-phase flows with a moving interface are carried out to validate these developments.