Towards efficient three-dimensional wide-angle beam propagation methods and theoretical study of nanostructures for enhanced performance of photonic devices

In this dissertation, we have proposed a novel class of approximants, the so-called modified Padé approximant operators for the wide-angle beam propagation method (WA-BPM). Such new operators not only allow a more accurate approximation to the true Helmholtz equation than the conventional operators, but also give evanescent modes the desired damping. We have also demonstrated the usefulness of these new operators for the solution of time-domain beam propagation problems. We have shown this both for a wideband method, which can take reflections into account, and for a split-step method for the modeling of ultrashort unidirectional pulses. The resulting approaches achieve high-order accuracy not only in space but also in time. In addition, we have proposed an adaptation of the recently introduced complex Jacobi iterative (CJI) method for the solution of wide-angle beam propagation problems. The resulting CJI-WA-BPM is very competitive for demanding problems. For large 3D waveguide problems with refractive index profiles varying in the propagation direction, the CJI method can speed-up beam propagation up to 4 times compared to other state-of-the-art methods. For practical problems, the CJI-WA-BPM is found to be very useful to simulate a big component such as an arrayed waveguide grating (AWG) in the silicon-on-insulator platform, which our group is looking at. Apart from WA beam propagation problems for uniform waveguide structures, we have developed novel Padé approximate solutions for wave propagation in graded-index metamaterials. The resulting method offers a very promising tool for such demanding problems. On the other hand, we have carried out the study of improved performance of optical devices such as label-free optical biosensors, light-emitting diodes and solar cells by means of numerical and analytical methods. We have proposed a solution for enhanced sensitivity of a silicon-on-insulator surface plasmon interference biosensor which had been previously proposed in our group. The resulting sensitivity has been enhanced up to 5 times. Furthermore, we have developed an improved model to investigate the influence of isolated metallic nanoparticles on light emission properties of light-emitting diodes. The resulting model compares very well to experimental results. Finally, we have proposed the usefulness of core-shell nanostructures as nanoantennas to enhance light absorption of thin-film amorphous silicon solar cells. An increased absorption up to 33 % has theoretically been demonstrated

Model Identification and Robust Nonlinear Model Predictive Control of a Twin Rotor MIMO System

PhDThis thesis presents an investigation into a number of model predictive control (MPC) paradigms for a nonlinear aerodynamics test rig, a twin rotor multi-input multi-output system (TRMS). To this end, the nonlinear dynamic model of the system is developed using various modelling techniques. A comprehensive study is made to compare these models and to select the best one to be used for control design purpose. On the basis of the selected model, a state-feedback multistep Newton-type MPC is developed and its stability is addressed using a terminal equality constraint approach. Moreover, the state-feedback control approach is combined with a nonlinear state observer to form an output-feedback MPC. Finally, a robust MPC technique is employed to address the uncertainties of the system. In the modelling stage, analytical models are developed by extracting the physical equations of the system using the Newtonian and Lagrangian approaches. In the case of the black-box modelling, artificial neural networks (ANNs) are utilised to model the TRMS. Finally, the grey-box model is used to enhance the performance of the white-box model developed earlier through the optimisation of parameters using a genetic algorithm (GA) based approach. Stability analysis of the autonomous TRMS is carried out before designing any control paradigms for the system. In the control design stage, an MPC method is proposed for constrained nonlinear systems, which is the improvement of the multistep Newton-type control strategy. The stability of the proposed state-feedback MPC is guaranteed using terminal equality constraints. Moreover, the formerly proposed MPC algorithm is combined with an unscented Kalman filter (UKF) to formulate an output-feedback MPC. An extended Kalman filter (EKF) based on a state-dependent model is also introduced, whose performance is found to be better compared to that of the UKF. Finally, a robust MPC is introduced and implemented on the TRMS based on a polytopic uncertainty that is cast into linear matrix inequalities (LMI)

Data-driven model reduction-based nonlinear MPC for large-scale distributed parameter systems

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordModel predictive control (MPC) has been effectively applied in process industries since the 1990s. Models in the form of closed equation sets are normally needed for MPC, but it is often difficult to obtain such formulations for large nonlinear systems. To extend nonlinear MPC (NMPC) application to nonlinear distributed parameter systems (DPS) with unknown dynamics, a data-driven model reduction-based approach is followed. The proper orthogonal decomposition (POD) method is first applied off-line to compute a set of basis functions. Then a series of artificial neural networks (ANNs) are trained to effectively compute POD time coefficients. NMPC, using sequential quadratic programming is then applied. The novelty of our methodology lies in the application of POD's highly efficient linear decomposition for the consequent conversion of any distributed multi-dimensional space-state model to a reduced 1-dimensional model, dependent only on time, which can be handled effectively as a black-box through ANNs. Hence we construct a paradigm, which allows the application of NMPC to complex nonlinear high-dimensional systems, even input/output systems, handled by black-box solvers, with significant computational efficiency. This paradigm combines elements of gain scheduling, NMPC, model reduction and ANN for effective control of nonlinear DPS. The stabilization/destabilization of a tubular reactor with recycle is used as an illustrative example to demonstrate the efficiency of our methodology. Case studies with inequality constraints are also presented.The authors would like to acknowledge the financial support of the EC FP6 Project: CONNECT [COOP-2006-31638] and the EC FP7 project CAFE [KBBE-212754]

Polymer-Based Micromachining for Scalable and Cost-Effective Fabrication of Gap Waveguide Devices Beyond 100 GHz

The terahertz (THz) frequency bands have gained attention over the past few years due to the growing number of applications in fields like communication, healthcare, imaging, and spectroscopy. Above 100 GHz transmission line losses become dominating, and waveguides are typically used for transmission. As the operating frequency approaches higher frequencies, the dimensions of the waveguide-based components continue to decrease. This makes the traditional machine-based (computer numerical control, CNC) fabrication method increasingly challenging in terms of time, cost, and volume production. Micromachining has the potential of addressing the manufacturing issues of THz waveguide components. However, the current microfabrication techniques either suffer from technological immaturity, are time-consuming, or lack sufficient cost-efficiency. A straightforward, fast, and low-cost fabrication method that can offer batch fabrication of waveguide components operating at THz frequency range is needed to address the requirements.A gap waveguide is a planar waveguide technology which does not suffer from the dielectric loss of planar waveguides, and which does not require any electrical connections between the metal walls. It therefore offers competitive loss performance together with providing several benefits in terms of assembly and integration of active components. This thesis demonstrates the realization of gap waveguide components operating above 100 GHz, in a low-cost and time-efficient way employing the development of new polymer-based fabrication methods.A template-based injection molding process has been designed to realize a high gain antenna operating at D band (110 - 170 GHz). The injection molding of OSTEMER is an uncomplicated and fast device fabrication method. In the proposed method, the time-consuming and complicated parts need to be fabricated only once and can later be reused.A dry film photoresist-based method is also presented for the fabrication of waveguide components operating above 100 GHz. Dry film photoresist offers rapid fabrication of waveguide components without using complex and advanced machinery. For the integration of active circuits and passive waveguides section a straightforward solution has been demonstrated. By utilizing dry film photoresist, a periodic metal pin array has been fabricated and incorporated in a waveguide to microstrip transition that can be an effective and low-cost way of integrating MMIC of arbitrary size to waveguide blocks

Modeling of Self-Assembled Quantum Dot Lasers

The study of active region structure for semiconductor lasers began in the 1960s. Most recently, quantum dot (QD) based lasers have attracted increasing attention. Modeling is crucial for the design of semiconductor QD-based lasers. Many attempts have been made to the macroscopic and, particularly, the microscopic modeling of III-V semiconductor QD as well as its applications during these decades. However, these proposed approaches use a very similar but outdated way to calculate the elastic strain field, referred to as one-step model, without rigorous consideration of the influence of the growth interruption in double-capping procedure, as the latter is currently used in epitaxial self-assembly for the control over the size of QDs. This thesis aims to contribute to the design improvements of QD-based laser applications through more accurate modeling. In this thesis, we have focused on improving the modeling accuracy by elaborately analyzing the elastic strain and quantum confinement potential. By applying this accurate modeling methodology, not only the general semiconductor QD-based lasers but also the structures with an interlayer/sublayer or tightly coupled QD ensemble can be numerically modeled, giving rise to the possibility for predicting the behavior and even structural design of lasers, paving the way to potentially novel applications. The following work has been done in this thesis. Firstly, we propose an accurate method of modeling a single QD, including a thorough so-called two-step elastic strain analysis, by considering the influence of growth interruption. A series of settings in terms of the three-dimensional (3D) geometry of QD and surrounding matrix are considered. The 3D confinement potential profile is found significantly different compared with the counterpart using the conventional one-step model. The electronic band structure is then calculated by using the strain-dependent eight-band k ∙ p method. The simulation results by using the two-step model are found in better agreement than one-step model in comparison with measurements. Moreover, the impact of the quaternary compositions of barrier material is, for the first time, systematically studied. Secondly, the two-step model is further extended to three- and multi-step analysis to model the structures with additional GaP ultrathin layer above or beneath the QDs. It is found that, instead of preventing the As/P exchange, the main impact of GaP interlayer/sublayers is enhancing the quantum confinement and thereby blue-shifting the emission peak. Based on the ability to efficiently shifting the spectrum, a new vertically chirped multi-layer structure is proposed. By simultaneously optimizing the interlayer/sublayer thickness and double-capping settings, a total gain spectral bandwidth of 245.7 nm (i.e. 30% increase) is predicted, and peak wavelength is shortened to 1510 nm (i.e. 70 nm blueshift, in comparison to the case without interlayer/sublayer). Thirdly, laterally and vertically coupled QDs are modeled to investigate a variety of coupling effects in the active region of lasers. In particular, multi-step strain analysis is applied to the modeling of closely stacked QDs to reproduce a more realistic unidirectional compressive strain accumulation, evidenced by the morphological observation of cross-section images obtained from measurements. A “quasi continuum band” formed by the mixing of bonding and antibonding states is found, giving rise to the possibility of emission at excited state (ES) instead of the ground state (GS). Using this feature, a new laser structure allowing two-state lasing under continuous wave (CW) electrical pumping is proposed for the first time and characterized through the simulation of spectral linewidth and relative intensity noise (RIN). The new structure exhibits lower (i.e. −130 versus −110 dBc/Hz) integrated RIN compared with the conventional counterpart under relatively high CW current injection. Overall, this thesis sheds light on new device physics and provide guidelines to realize QD-based lasers with new features, and would be interesting to the scientific community

Modelling of Floods in Urban Areas

This Special Issue publishes the latest advances and developments concerning the modelling of flooding in urban areas and contributes to our scientific understanding of the flooding processes and the appropriate evaluation of flood impacts. This issue contains contributions of novel methodologies including flood forecasting methods, data acquisition techniques, experimental research in urban drainage systems and/or sustainable drainage systems, and new numerical and simulation approaches in nine papers with contributions from over forty authors