3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

This work was funded in part by the Predoctoral Grant FPU18/01965 and in part by the financial support of BBVA Foundation through a project belonging to the 2021 Leonardo Grants for Researchers and Cultural Creators, BBVA Foundation. The BBVA Foundation accepts no responsibility for the opinions, statements, and contents included in the project and/or the results thereof, which are entirely the responsibility of the authors.Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques.Predoctoral Grant FPU18/01965BBVA Foundatio

A Measured Rasorber with Two Absorptive Bands

In this paper, a novel rasorber with both Frequency Selective Surface and absorptive periodical structure is developed. This rasorber works as a radome and an absorber. Firstly, the design procedure of the unit cell of this rasorber is explained. Then, the characteristics of the manufactured rasorber are measured. The characteristics include the transmission/reflection coefficients of the rasorber, the radiation properties of the horn antenna covered by the novel rasorber and the scattering features of this rasorber. In the passband, the gain of the antenna with our rasorber radome is only 1~2 dB lower than the one of the horn antenna without any radome. Furthermore, the radome has little effect on the radiation patterns of the horn antenna. In the absorptive bands of the rasorber, the electric level of the scattered electromagnetic wave from the rasorber can be 16.5 dB lower than the one from a metallic plane with the same size as the rasorber. The feature proves that this rasorber can be a good candidate in the stealth radar radome area

Investigation of Radar Signal Interaction with Crossflow Turbine for Aviation Application

The increased adoption of wind energy is an important part of the push towards a net zero-emission economy. One obstacle that stands in the way of a higher rate of wind energy adoption is the interference that wind turbines cause to nearby radar installations. Wind turbines negatively affect the performance of nearby radar sites in a variety of different ways. Almost all types of radar are affected in at least one of these ways.In order to understand the degree to which an object such as a wind turbine interacts with radar, it is important to have detailed radar cross section (RCS) data for the object. In this work, a novel, low-cost, scale model radar cross section characterization system is presented with various advantages over traditional designs. This system was used to characterize the RCS of the novel Crossflow wind turbine. Additionally, work has been carried out on the characterization of metamaterial absorber coatings that can be applied to new and existing turbines for the purposes of reducing their radar cross section and the degree to which they cause radar inter-ference. The works presented can be leveraged to reduce concerns around radar interference from wind turbines, as well as to iteratively generate ge-ometries with lower radar cross sections for the aviation and infrastructure sectors, ultimately accelerating the pace of wind energy adoption and the move towards a net zero-emission economy

Guest Editorial Special Cluster on Functionalized Metasurface-Based Covers and Unconventional Domes for Dynamic Antenna Systems

The papers in this special section focus on recent advancements in this field and provide an overview of the potential applications of this technology in the next generation antenna devices, with emphasis on metamaterial-based enhancements enabling real-time control of their radiation characteristics

Frequency-Selective Surfaces for Microwave and Terahertz Spectra

FREQUENCYselective surfaces (FSSs) made of subwavelength periodic structures have been broadly applied in various electromagnetic applications. Their main function is to tailor the frequency response to incident waves, or to obtain electromagnetic (EM) properties that do not exist in homogeneous natural materials. When increasing the design complexity to enhance performance, however, the computation cost hikes dramatically in analysis and synthesis as additional design variables are introduced. In contrast to such complexity increase, this thesis aims at systematically developing effective and efficient design and optimization approaches for FSS-based structures adopting fundamental unit-cell patterns, such as rectangular patches, rings and grids. Additionally, impedance matching to free space is thoroughly investigated and adapted as a means towards performance improvement in both absorbers and filters. Hereby, multiple designs are demonstrated with realizations from the microwave to the terahertz (THz) frequency spectrum. In spite of their simplicity, the proposed designs outperform the state-of-the-art counterparts in the literature by fully exhausting the potentials of their spatial structures and material attributes. Specifically, Chapter 3 challenges a common belief that adding an impedance matching superstrate to an absorber will broaden its operation bandwidth at the cost of increased total thickness profile. This Chapter proves that it is possible to increase the bandwidth-to-thickness ratio. The concept is firstly demonstrated at the circuit level, and then verified by full-wave simulations. The optimization process can be illustrated with an admittance Smith chart. The distinctive performance of the proposed single- FSS-layer absorber is justified with a figure of merit (FoM) which comprehensively involves the relative bandwidth, the normalized thickness and the level of reflectivity. In Chapter 4, a semi-analytical approach for absorber design is developed by systematically combining analytical, empirical and numerical techniques. The optimization space can be simplified from millions of exhaustive search cases to merely a few hundreds of seed simulations, by exploiting insights into the linearity, scalability and independence regarding the major components of an absorber. For any specified level of absorption and operation bandwidth, the obtained semi-analytical algorithm enables fast synthesis of an absorber with a minimal thickness. Both absorbers proposed in the above chapters have been realized using patterned resistive layers and experimentally validated under oblique angles of incidence for transverse-electric (TE) and transversemagnetic (TM) modes. The design methods can be readily expanded for structures of multiple FSS layers. In the terahertz frequency range, common microfabrication technologies do not accommodate those resistive inks used for silk-printing lossy FSS patterns. As an alternative, a sub-skin-depth metal layer with nanoscale thickness is proposed in Chapter 5 to meet this requirement. The Drude model is adopted to simulate the ultra-thin metallic FSS, as it satisfactorily describes the frequency dependent properties of noble metals. The proposed absorber is robust to dimensional tolerance in fabrication and attains a stable absorption bandwidth under oblique impinging waves. In Chapter 6, a frequency reconfigurable terahertz bandpass filter is proposed and experimentally verified. It includes two identical double-layer FSSs separated by an air spacer which can be mechanically tuned. The filter allows a highly selective transmission sweeping across a wide spectrum. The underlying mechanism can be explained from two perspectives, namely through interpretation as Fabry-Perot resonant cavity and through consideration of the admittance Smith chart. The designed device is insensitive to fabrication tolerances and stable to oblique angle of incidence. The fabricated filter confirms a 40% tuning range and less than 1 dB insertion loss. This design is among the first few practical reconfigurable terahertz bandpass filters reported in the literature. Overall, the research outcomes suggest that developing complicated FSS patterns with a large number of degrees of freedom is unnecessary in many cases if the potential of fundamental geometries is fully exploited through rigorous algorithmic optimization methods. The design approaches illustrated in this thesis are generic to all FSS-based structures and can potentially be extended to multi-FSS layers and impedance surfaces, to satisfy performance requirements in specific applications.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 202

Stabilization of evanescent wave propagation operators

This paper presents a stabilized scheme that solves the wave propagation problem in a general bianisotropic, stratified medium. The method utilizes the concept of propagators, i.e., the wave propagation operators that map the total tangential electric and magnetic fields from one plane in the slab to another. The scheme transforms the propagator approach into a scattering matrix form, where a spectral decomposition of the propagator enables separation of the exponentially growing and decaying terms in order to obtain a well-conditioned formulation. Multilayer structures can be handled in a stable manner using the dissipative property of the Redheffer star product for cascading scattering matrices. The reflection and transmission dyadics for a general bianisotropic medium with an isotropic half space on both sides of the slab are presented in a coordinate-independent dyadic notation, as well as the reflection dyadic for a bianisotropic slab with perfect electric conductor backing (PEC). Several numerical examples that illustrate the performance of the stabilized algorithm are presented

Artificial materials for microwave applications

This thesis has focussed on the properties and manufacturing techniques of artificial RF materials. These artificial materials can be divided into two types depending on the whether their individual unit cell is resonant or non-resonant. Both these types have been discussed. It has been shown that the efficiency and bandwidth of a patch antenna using a flexible 3D printed substrate can be improved by using composite materials as heterogeneous substrates. Composite materials with a large range of relative permittivity values were manufactured by combing 3D printing with commercial laminates. An equation to design such composite materials has been presented. The engineering tolerance and repeatability of 3D printing as a manufacturing process to fabricate ‘on demand’ dielectrics has been presented. [Continues.

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance