Supervised-learning-enabled EM-driven development of low scattering metasurfaces

The recent advances in the development of coding metasurfaces created new opportunities to elevate the stealthiness of combat aircrafts. Metasurfaces, composed of optimized geometries of meta-atoms arranged as periodic lattices, are devised to obtain desired electromagnetic (EM) scattering characteristics, and have been extensively exploited in stealth applications to reduce radar cross section (RCS). They rely on the manipulation of backward scattering of electromagnetic (EM) waves into various oblique angles. Despite potential benefits, a practical obstacle hindering widespread metasurface utilization is the lack of systematic design procedures. Conventional approaches are largely intuition-inspired and demand heavy designer’s interaction while exploring the parameter space and pursuing optimum unit cell geometries. Another practical obstacle that hampers efficient design of metasurfaces is implicit handling of RCS performance. To achieve essential RCS reduction, the design task is normally formulated in terms of phase reflection characteristics of the unit cells, whereas their reflection amplitudes—although contributing to the overall performance of the structure—is largely ignored. A further practical issue is insufficiency of the existing performance metrics, specifically, monostatic and bistatic evaluation of the reflectivity, especially at the design stage of metasurfaces. Both provide a limited insight into the RCS reduction properties, with the latter being dependent on the selection of the planes over which the evaluation takes place. As a consequence of raised concerns, the existing design methodologies are still insufficient, especially in the context of controlling the EM wavefront through parameter tuning of unit cells. Furthermore, they are unable to determine truly optimum solutions. Therefore, we have introduced a novel machine-learning-based framework for automated and computationally efficient design of metasurfaces realizing broadband RCS reduction. We have employed a three-stage design procedure involving global surrogate-assisted optimization of the unit cells, followed by their local refinement. In its final stage, a direct EM-driven maximization of the RCS reduction bandwidth has been performed, facilitated by appropriate formulation of the objective function involving regularization terms. Moreover, to handle the combinatorial explosion in the design closure of multi-bit coding metasurfaces, a sequential-search strategy has been developed that enabled global search capability at the concurrent unit cell optimization stage. Latterly, the metasurface design task with explicit handling of RCS reduction at the level of unit cells has been introduced that has accounted for both the phase and reflection amplitudes of the unit cells. The design objective has been defined so as to directly optimize the RCS reduction bandwidth at the specified level (e.g., 10 dB) w.r.t. the metallic surface. The appealing feature of the said framework has consisted in its ability to optimize the RCS reduction bandwidth directly at the level of the entire metasurface as opposed to merely optimizing unit cell geometries. Besides, the obtained design has required minimum amount of tuning at the level of the entire metasurface. Lastly, a new performance metric for evaluating scattering characteristics of a metasurface, referred to as Normalized Partial Scattering Cross Section (NPSCS), has been proposed. The metric involved integration of the scattered energy over a specific solid angle, which allows for a comprehensive assessment of the structure performance in a format largely independent of the particular arrangement of the scattering lobes. Our design methodologies have been utilized to design several instances of novel scattering metasurface structures with the focus on RCS reduction bandwidth enhancement and the level of RCS reduction. Experimental validations confirming the numerical findings have been also provided. To the best of the author’s knowledge, the presented study is the first systematic investigation of this kind in the literature and can be considered a step towards the development of efficient, low-cost, and more high performing scattering structures

Surrogate-Assisted Design of Checkerboard Metasurface for Broadband Radar Cross-Section Reduction

Publisher's version (útgefin grein)Multiple-input multiple-output (MIMO) antennas are considered to be the key components of fifth generation (5G) mobile communications. One of the challenges pertinent to the design of highly integrated MIMO structures is to minimize the mutual coupling among the antenna elements. The latter arises from two sources, the coupling in the free space and the coupling currents propagating on a ground plane. In this paper, an array of H-shaped parasitic patches is proposed as a decoupling structure for compact MIMO antennas to reduce propagation of the coupling currents on a shared ground plane. The proposed decoupling structure is generic, and it can be applied to different antenna configurations as demonstrated in the work. Furthermore, it is employed to develop a new high-performance compact dual-band MIMO structure featuring acceptable level of element coupling at both operating frequencies. The design is validated both numerically and experimentally. The mutual coupling levels are less than -17 dB and -20 dB, with the total efficiency of 89% and 90%, and the realized gain of 6.6 dB and 7 dB at the two resonant frequencies of 5 GHz and 6 GHz, respectively. Topological complexity of the compact MIMO systems featuring elaborated decoupling structures, a large number of geometry parameters, as well as the necessity of handling multiple performance figures, constitute the major challenges of antenna design, in particular, its re-design for various specifications. To alleviate these difficulties, the paper also provides a procedure for rapid geometry scaling of the dual-band MIMO antennas. Our approach is based on inverse surrogate modeling methods, and results in numerically-derived expressions that enable a precise control over the operating antenna bands within broad ranges thereof (from 4 GHz to 8 GHz for the lower band, and from 1.1 to 1.3 ratio of the upper to lower operating frequency). The aforementioned procedure is accompanied by an optimization-based design refinement scheme. A practical utility of the procedure is corroborated using multiple verification case studies as well as physical measurements of the antenna designed for the exemplary set of performance specifications.Peer reviewe

Toward Long-Endurance Flight- Tamkang’s Aspect of Micro Ornithopters

[[notice]]補正完

Nanophotonics for dark materials, filters, and optical magnetism

Research on nanophotonic structures for three application areas is described, a near perfect optical absorber based on a graphene/dielectric stack, an ultraviolet bandpass filter formed with an aluminum/dielectric stack, and structures exhibiting homogenizable magnetic properties at infrared frequencies. The graphene stack can be treated as a effective, homogenized medium that can be designed to reflect little light and absorb an astoundingly high amount per unit thickness, making it an ideal dark material and providing a new avenue for photonic devices based on two-dimensional materials. Another material stack arrangement with thin layers of metal and insulator forms a multi-cavity filter that can effectively act as an ultraviolet filter without the usual sensitivity of the incident angle of the light. This is important in sensing applications where the visible part of the spectrum is to be removed, allowing detection of ultraviolet signals. Finally, achieving a magnetic material that functions at optical frequencies would be of enormous scientific and technological impact, including for imaging, sensing and optical storage applications. The challenge has been to find a guiding principle and a suitable arrangement of constituent materials. A lattice of dielectric spheres is shown to provide a legitimately homogenized material with a magnetic response. This should pave the way for experimental studies. More specifically, a graphene stack is designed, fabricated and characterized. The structure shows strong absorption of light. Spectroscopic ellipsometry is used to obtain the complex sheet conductivity of graphene. Further modeling results establish the graphene stack as the darkest optical material, with lower reflectivity and higher per-unit-length absorption than alternative light-absorbing materials. An optical bandpass filter based on a metal/dielectric structure is modeled, showing performance that is largely independent of the angle of incidence. Parametric evaluations of the reflection phase shift at the metal-dielectric interface provide insight and design information. Filter passbands in the ultraviolet (UV) through visible or longer wavelengths can be achieved by engineering the dielectric thickness and selecting a metal with an appropriate plasma frequency, as demonstrated in simulations. A lattice of suitable dielectric particles is shown to fulfill the requirements for a magnetic optical material. Using Mie theory, the microscopic origin of the magnetic response is explicitly identified as being due to the magnetic dipole resonance of an isolated sphere. This provides a design basis, and dielectric and lattice requirements with candidate dielectrics that will allow magnetic materials to be designed and fabricated for optical applications are presented

Nanophotonics for dark materials, filters, and optical magnetism

Research on nanophotonic structures for three application areas is described, a near perfect optical absorber based on a graphene/dielectric stack, an ultraviolet bandpass filter formed with an aluminum/dielectric stack, and structures exhibiting homogenizable magnetic properties at infrared frequencies. The graphene stack can be treated as a effective, homogenized medium that can be designed to reflect little light and absorb an astoundingly high amount per unit thickness, making it an ideal dark material and providing a new avenue for photonic devices based on two-dimensional materials. Another material stack arrangement with thin layers of metal and insulator forms a multi-cavity filter that can effectively act as an ultraviolet filter without the usual sensitivity of the incident angle of the light. This is important in sensing applications where the visible part of the spectrum is to be removed, allowing detection of ultraviolet signals. Finally, achieving a magnetic material that functions at optical frequencies would be of enormous scientific and technological impact, including for imaging, sensing and optical storage applications. The challenge has been to find a guiding principle and a suitable arrangement of constituent materials. A lattice of dielectric spheres is shown to provide a legitimately homogenized material with a magnetic response. This should pave the way for experimental studies. More specifically, a graphene stack is designed, fabricated and characterized. The structure shows strong absorption of light. Spectroscopic ellipsometry is used to obtain the complex sheet conductivity of graphene. Further modeling results establish the graphene stack as the darkest optical material, with lower reflectivity and higher per-unit-length absorption than alternative light-absorbing materials. An optical bandpass filter based on a metal/dielectric structure is modeled, showing performance that is largely independent of the angle of incidence. Parametric evaluations of the reflection phase shift at the metal-dielectric interface provide insight and design information. Filter passbands in the ultraviolet (UV) through visible or longer wavelengths can be achieved by engineering the dielectric thickness and selecting a metal with an appropriate plasma frequency, as demonstrated in simulations. A lattice of suitable dielectric particles is shown to fulfill the requirements for a magnetic optical material. Using Mie theory, the microscopic origin of the magnetic response is explicitly identified as being due to the magnetic dipole resonance of an isolated sphere. This provides a design basis, and dielectric and lattice requirements with candidate dielectrics that will allow magnetic materials to be designed and fabricated for optical applications are presented

Polymer Composites for Electrical and Electronic Engineering Application

Polymer composite materials have attracted great interest for the development of electrical and electronic engineering and technology, and have been widely applied in electrical power systems, electrical insulation equipment, electrical and electronic devices, etc. Due to the significant expansion in the use of newly developed polymer composite materials, it is necessary to understand and accurately describe the relationship between composite structure and material properties, as only based on thorough laboratory characterization is it possible to estimate the properties for their future commercial applications. This book focuses on polymer composites applied in the field of electrical and electronic equipment, including but not limited to synthesis and preparation of new polymeric materials, structure–properties relationship of polymer composites, evaluation of materials application, simulation and modelling of material performance

Microstructure design of magneto-dielectric materials via topology optimization

Engineered materials, such as new composites, electromagnetic bandgap and periodic structures have attracted considerable interest in recent years due to their remarkable and unique electromagnetic behavior. As a result, an extensive literature on the theory and application of artificially modified materials exists. Examples include photonic crystals (regular, degenerate or magnetic) illustrating that extraordinary gain and high transmittance can be achieved at specific frequencies. Of importance is that recent investigations of material loading demonstrate that substantial improvements in antenna performance (smaller size, larger bandwidth, higher gain etc.) can be attained by loading bulk materials such as ferrites or by simply grading the material subject to specific design objectives. Multi-tone ceramic materials have also been used for miniaturization and pliable polymers offer new possibilities in three dimensional antenna design and multilayer printed structures, including 3D electronics. However, as the variety of examples in the literature shows, the perfect combination of materials is unique and extremely difficult to determine without optimization. In addition, existing artificial dielectrics are mostly based on intuitive studies, i.e. a formal design framework to predict the exact spatial combination of dielectrics, magnetics and conductors does not exist. In the first part of this thesis, an inverse design framework integrating FE based analysis tool (COMSOL MULTIPHYSICS-PDE Coefficient Module) with an optimization technique (MATLAB-Genetic Algorithm and Direct Search toolbox) suitable for designing the microstructure of artificial magneto-dielectrics from isotropic material phases is proposed. Homogenizing Maxwell's Equations (MEQ) in order to estimate the effective material parameters of the desired composite made of periodic microstructures is the initial task of the framework. The FE analysis tool is used to evaluate intermediate fields at the "micro-scale" level of a unit cell that is integrated with the homogenized MEQ's in order to estimate the "macro-scale" effective constitutive parameters of the overall bulk periodic structure. Simulation of the periodic structure is an extremely challenging task due to the mesh at micro-level (inclusions much smaller than the periodic cell dimension) that spans over the entire bulk structure turning the computational problem into a very intensive one. Therefore, the proposed framework based on the solution of homogenized MEQ's via the micro-macro approach, allows topology design capabilities of microstructures with desired properties. The goal is to achieve predefined material constitutive parameters via artificial electromagnetic substrates. Physical material bounds on the attainable properties are studied to avoid infeasible effective parameter requirements via available multi-constituents. The proposed framework is applied on examples such as microstructure layers of non-reciprocal magnetic photonic crystals. Results show that the homogenization technique along with topology optimization is able to design non-intuitive material compositions with desired electromagnetic properties. In the second part of the thesis, approximation techniques to speed-up large scale topology optimization studies of devices with complex frequency responses are investigated. Miniaturization of microstrip antennas via topology optimization of both the conductor and material substrate via multi-tone ceramic shades is a typical example treated here. Long computational times required for both the electromagnetic analysis over a frequency range and the need for a heuristic based optimization tool to locate the global minima for complex devices present themselves as two important bottlenecks for practical design studies. In this thesis, two new techniques for speeding up the optimization process by reducing the number of frequency calls needed to accurately predict a multi-resonance type response of a candidate design are proposed. The proposed techniques employ adaptive sampling methods along with novel rational function interpolations. The first technique relies on a heuristic based rational interpolation using Bayes' theory and rational functions. Second, a rational function interpolation employing a new adaptive path based on Stoer-Bulirsch algorithm is used. Both techniques prove to efficiently predict resonances and significantly reduce the computational time by at least three folds

Metasurface Based Mid-infrared Devices

The development of compact, efficient, and powerful mid-infrared devices is mainly restrained by the limited choice of materials due to the high loss of conventional optical materials in the mid-infrared range. The aim of this work was to find alternative novel materials which would enable the realization of devices with smaller size while maintaining its functionality. Metasurface and graphene have emerged as promising materials which can help us to manipulate the infrared light within nano-meter scale thickness. In this thesis, three different mid-infrared devices, thermal emitter, wave trapping sensor and phase modulator were designed based on either metasurface or both metasurface and graphene. Devices were all fabricated with modern semiconductor fabrication processes and their performances were also fully investigated, both experimentally and through simulations. A metasurface was first designed as a frequency selective layer on a graphene thermal emitter to tailor the graybody emission spectrum from a graphene filament into two discrete narrow bands for applications such as gas sensing or molecule detection. The emission and reflectance spectra of the devices were characterised using (FTIR) Fourier transform infrared spectroscopy and showed good agreement with simulations based on the Finite-difference time-domain (FDTD). method. The use of a metasurface to enhance the interaction between molecular vibrations and the evanescent waves, in a total attenuated reflectance system, was also explored. A complementary ring-resonator structure was patterned onto both silicon and SiO2/Si substrates, and the spectral properties of both devices were characterised using an FTIR-ATR system. Experiments were undertaken using 5µL mixtures containing trace amounts of butyl acetate diluted with oleic acid. Without the use of a metasurface, the minimum concentration of butyl acetate that could be clearly detected was 10%, whereas the use of the metasurface on the SiO2/Si substrate allowed the detection of 1% butyl acetate. Finally, graphene was integrated into a metasurface structure to achieve tunability of the design. The third device investigated was a phase modulator which shows the capability to change the amplitude and phase of the reflected wave by electrostatically gating the graphene from -90V to 90V. A dynamic beam steering lens model which is made up of a unit cell consisting of four phase modulator with different phase shift was also proposed to control the angle for the reflected wave from specular to 30°.Engineering and Physical Sciences Research Council (EPSRC

Optimal Grading for Strength and Functionality of Parts Made of Interpenetrating Polymer Networks: Load Capacity Enhancement

Uniform parts with stress concentrations or singularities are prone to failure under relatively small loads, which motivates researchers to seek methods to enhance the strength of these parts. This dissertation studies the optimization of material grading to design parts made of functionally graded interpenetrating polymer networks (FG-IPNs) to improve their load capacity. An acrylate/epoxy IPN with variations of elastic Young’s modulus, Poisson’s ratio, and ultimate stress at failure is used for optimization of a plate with stress concentration. The grading is optimized by attaching the finite element method (FEM) solver to a general purpose bound-constrained optimizer. Two examples, a plate with a hole and a bent bracket, show more than 100% improvement in the part’s load capacity when compared to the uniform IPNs. Parts with stress singularities are studied using a PMMA/PU IPN system. For this system, we have the elastic modulus and the critical stress intensity factor KIC as a function of the concentration of the components. A material mesh is utilized to control the grading near the crack tip and uniform material is assumed outside the tip area. The displacement correlation technique (DCT) is used to calculate stress intensity factors and the maximum hoop stress criterion is selected as the fracture criterion. Parts with edge cracks, interior cracks and interacting cracks under tension are considered. For the PMMA/PU IPN system, improvements in load capacity in the order of one hundred percent were commonly obtained through grading the region around the crack tip, compared to both optimal uniform plates, and plates with simple toughening of the region around the crack. In addition, in FEM modelling of FGM part with graded elements, the polynomial interpolations used in such elements can be prone to oscillations that can result in regions of negative elastic modulus, even with only positive nodal values of elastic moduli. The result of these negative modulus regions, even if the region is small, can be unexpected singularities in the solution. To avoid this potential problem, conditions for robust higher order materially graded elements were developed. Advisor: Mehrdad Negahba

New Trends and Applications in Femtosecond Laser Micromachining

This book contains the scientific contributions to the Special Issue entitled: "New Trends and Applications in Femtosecond Laser Micromachining". It covers an array of subjects, from the basics of femtosecond laser micromachining to specific applications in a broad spectra of fields such biology, photonics and medicine