Design of Ultra-compact Graphene-based Superscatterers

The energy-momentum dispersion relation is a fundamental property of plasmonic systems. In this paper, we show that the method of dispersion engineering can be used for the design of ultra-compact graphene-based superscatterers. Based on the Bohr model, the dispersion relation of the equivalent planar waveguide is engineered to enhance the scattering cross section of a dielectric cylinder. Bohr conditions with different orders are fulfilled in multiple dispersion curves at the same resonant frequency. Thus the resonance peaks from the first and second order scattering terms are overlapped in the deepsubwavelength scale by delicately tuning the gap thickness between two graphene layers. Using this ultra-compact graphene-based superscatterer, the scattering cross section of the dielectric cylinder can be enhanced by five orders of magnitude.Comment: This paper has been accepted by IEEE Journal of Selected topics in Quantum Electronic

Computer-aided design and simulation of current sensitive electromagnetic actuators.

The thesis presents the development of CAD techniques in the design and simulation of electromagnetic actuators. It also demonstrates the application of analytical and finite element CAD techniques on the behaviour, evaluation and optimal design of current sensitive electromagnetic actuators. Both clapper and stepper motor actuators are investigated based on the magnetic saturation and polarisation principles. An analytical method is outlined and applied in the design of the actuators and their evaluation. The two scalar potential approach is investigated in 3D nonlinear magneto-static finite element computation which is used in the simulation and prediction of the performance of the actuators. A 2D nonlinear transient electromagnetic finite element method, taking account of the 3rd dimension, is developed and the actuators for time-varying current control is evaluated. A decoupled 3D finite element integral method is postulated to predict the dynamic transient response characteristics of the actuators. Analysis is used to examine the relationships of the actuator sensitivity to manufacturing tolerances which cover a wide range of design and control variables. The actuator sensitivity is quantified in terms of critical design factors and a range of airgap settings. The actuators are optimised and quantitative comparisons between the predictions and the test results for the clapper actuators are discussed. The deficiencies of the clapper type actuators are examined. Anovel approach electromagnetic actuator is proposed to obviate these deficiencies. This actuator is based on the combination of the principles of magnetic saturation and the stepper motor. A disc magnet stepper motor is investigated and employed in the actuator. The dependency of the actuator performance on specific key factors is obtained and optimised. A technique is developed to reduce detent torque to a low level. The optimal design of the stepper motor actuator is presented, together with the evaluation of its static and dynamic characteristics. Finally, an actuator combining the stepper motor and clapper actuator is postulated and evaluated. The aims of the work presented here are to: explore the use of Computer Aided Design techniques in the design and analysis of high sensitivity current-operated electromagnetic actuators; to optimise actuators and improve the sensitivity; to develop design and evaluation approaches capable of solving the magnetic actuators transient dynamic performance and practical engineering design problems; to investigate the actuators dependence on the key design parameters in order to determine the range of manufacturing tolerances and to optimise the actuator; to develop a novel magnetic actuator capable of operating with the highest possible sensitivity to current, diminishing the need for very finely machined surfaces and obviating the separate use of the toroid transformer. This also involves developing the analytical techniques

Clamping Force Distribution within Press Pack IGBTs

Press pack insulated gated bipolar transistors (PP IGBTs) have been gradually used in the high-voltage and high-power-density applications, such as the power system and electric locomotive, with its advantages of double-sided cooling, higher power density, and easy to connect in series compared with traditional wire-bonded power IGBT modules. However, the clamping force is quite important for PP IGBTs because too much clamping fore will cause mechanical damage to the silicon chips and too little clamping force will increase the junction temperature of the silicon chips due to the increased thermal contact resistance. And eventually it leads to thermal damage. Furthermore, the clamping force distribution within PP IGBTs is affected by many factors, and they can be divided into the internal and external factors. The finite element analysis model of the PP IGBTs is established based on the theory of elastic mechanics to obtain the influence of the affect factors, including the external clamping modes, spring design, thermal stress, the machining accuracy, and so on. The contribution of those affect factors to the clamping force distribution is ranked, and this can be a guideline not only for users but also for the manufacturers

Light interaction with multilayer arbitrary anisotropic structure: an explicit analytical solution and application for subwavelength imaging

A systematic analytical approach to simulate the propagation of electromagnetic plane waves in multilayer anisotropic structures, where the layers can have arbitrary oriented optical axis, is presented. The explicit expressions for the vector polarizations of electric and magnetic fields inside a randomly oriented anisotropic medium are derived. The developed algorithm operates with analytic 4×4 matrices to calculate the transmission and reflection coefficients. This algorithm is suitable to investigate the near-field/far-field electromagnetic wave interaction at any angle of incidence for numerous intriguing applications. The procedure is applied to design anisotropic single and multilayer lenses for subwavelength imaging

Experimental observation of superscattering

Superscattering, induced by degenerate resonances, breaks the fundamental single-channel limit of scattering cross section of subwavelength structures; in principle, an arbitrarily large total cross section can be achieved via superscattering. It thus provides a unique way to strengthen the light-matter interaction at the subwavelength scale, and has many potential applications in sensing, energy harvesting, bio-imaging (such as magnetic resonance imaging), communication and optoelectronics. However, the experimental demonstration of superscattering remains an open challenge due to its vulnerability to structural imperfections and intrinsic material losses. Here we report the first experimental evidence for superscattering, by demonstrating the superscattering simultaneously in two different frequency regimes through both the far-field and near-field measurements. The underlying mechanism for the observed superscattering is the degenerate resonances of confined surface waves, by utilizing a subwavelength metasurface-based multilayer structure. Our work paves the way towards practical applications based on superscattering