Browsing by Subject "Metamaterials"

Now showing 1 - 7 of 7
  • Thumbnail Image
    Item
    Development of High Efficiency Metamaterial Antenna Structures for Near-Field and Far-Field Applications
    (2022-08) Dave, Aditya
    With the advent of mmwave 5G, and future G technologies, there is a path paved for multitude of applications in the cellular, augmented and virtual reality (AR/VR), internet-of-things (IoT) etc. domains. There is a need for compact, highly directional and low-loss antennas for reduced size, greater coverage, low power consumption. Partially reflective surfaces as superstrates are well known for enhancing antenna radiation. However, in the past, electrically large surfaces were used with little regards to the size and aperture efficiency of the antennas. In this dissertation, compact source antennas are used with smaller 2D metamaterial superstrates acting as partially reflective surfaces (PRS) to form metamaterial antenna (MMA) block. The dissertation is divided into three segments. After going over the theoretical framework for infinite periodic surfaces and development of equivalent circuits in chapter 2, finite PRS surfaces with source antennas are analyzed in chapters 3 and 4. Different types of PRS surfaces and source antennas are changed one at a time to explain the design methodology and arrive at highly aperture efficient MMA blocks. In chapter 5, single MMA block is used to create virtual arrays using beam-splitting PRS designs and analyze its performance with conventional arrays. The single MMA blocks are also showcased in array element reduction applications to reduce feedline complexities associated with conventional arrays. Chapter 6 focuses on formation of passive phased arrays using near field phase manipulation properties of the PRS. This property is used to create dual beam antennas. Next, designs that focus on creating polarization splitters using yet another variation of PRS, called beam and polarization splitting PRS (BPS-PRS) are proposed. The dual beam antennas and polarization splitters can be applied to emerging multiple-input multiple-output (MIMO) communication applications. MMAs are also useful as GNSS positional sensors as seen by their low phase center variation properties which are also showcased. Finally, chapter 7 focuses on near-field applications of the MMA by proposing free space vertical interconnects and power dividers that are useful for high frequency printed circuit board (PCB) integrated chip-to-chip intra-connects and interconnects. Additional loss reduction technologies that are useful for on-chip silicon implementations are also demonstrated by using Copper nanowires on coplanar waveguide transmission lines for frequency ranges up to 180 GHz. Chapter 8 concludes the work and gives directions for the future work.
  • Thumbnail Image
    Item
    Dynamic Homogenization Of Linear Waves In Periodic Media And Origami-Inspired Structures
    (2022-03) Oudghiri-Idrissi, Othman
    This dissertation aims to establish a comprehensive analytical framework for dynamic homogenization of wave motion at arbitrary frequency in (i) “perforated” periodic con- tinua, and (ii) periodic origami-inspired structures described via “bar-and-hinge” com- putational paradigm. For a given spectral (i.e. frequency-wavenumber) content the body force acting on the structure, the “activated” Bloch eigenstates of the lattice are iden- tified and classified depending on the multiplicity of participating energy levels. In the vicinity of an isolated dispersion surface (single energy level), an effective field equation with homogenized source term is formulated (via projection onto the dominant Bloch eigenstate) to obtain the leading- and second-order approximations of both macroscopic, i.e. “mean”, and microscopic wave motion. When the activated spectral neighborhood features more than one dispersion surface, the zeroth- and first-order systems of effec- tive field equations with homogenized source terms are formulated, covering a variety of topological configurations such as Dirac points, avoided crossings, and near-Dirac points. On setting the source term to zero, the featured system of equations degenerates to a low-order algebraic eigenvalue problem that accurately captures the local geometry of (a cluster of) dispersion surfaces. The proposed homogenization framework is veri- fied by comparing the asymptotic approximation of the dispersion relationship with its numerically-evaluated counterpart and deployed to approximate the total and effective motion in: (i) two-dimensional (2D) Kagome lattice featuring nearly-hexagonal Neu- mann exclusions, (ii) 2D square lattice of circular Dirichlet obstacles, (iii) 2D Miura-ori origami structure, and (iv) 1D Miura tube. Specifically, the asymptotic model is shown to approximate the dispersion relationship in the neighborhood of isolated dispersion surfaces and tight clusters thereof with equal fidelity. It is also found that the homog- enized model is capable of accurately capturing the body-force induced waveforms in a lattice, both in terms of macroscopic i.e. effective motion and microstructural motion when higher-order models are considered.
  • Thumbnail Image
    Item
    Investigation of Metamaterial Transmission line resonators for Ultra-High Field Magnetic Resonance Imaging RF Coils
    (2018-10) Panda, Vijayaraghavan
    The objective is to develop a highly efficient RF head coil on a thin substrate for the Ultra-high magnetic field (7 T and above) MRI systems. The artificial Metamaterial resonator is investigated for this purpose. Simulation and experimental results are provided for an 8-channel Metamaterial based RF coil in comparison with a standard high performance 8-channel dipole based RF coil for the 10.5 T MRI system. Each element is 180 mm (approximately a quarter of a wavelength λ0) long, identical, evenly spaced along the circumference of the cylindrical phantom, loaded with dielectric material, and referred to as inverted Metamaterial Zeroth Order Resonator. The resonator elements are open circuited, matched, and tuned to 447.06 MHz with the phantom. An unloaded to loaded quality factor ratio of 2.97 is obtained from the scattering matrix of the proposed design. The length independent nature of the proposed design and the flexibility of the lumped elements have provided an optimized element with a substrate thickness of roughly 3 mm (λ0/200). With the proposed design, a RF magnetic field strength (B1+) to √SAR ratio of 1.38 (compared to 1.46 for dipole) is obtained. Optimization of the physical design parameters, especially the distance between the element and the phantom, is performed to improve the transmission efficiency of the metamaterial based RF coil element. The amount of radiated power reaching 45 mm inside the phantom is used for the comparison. The optimal design for a 16 cm long, 7 T metamaterial resonator shows an increase of 1.1 dB and 3.2 dB in transmit power when compared to a dipole and microstrip element of same length. Similarly, the optimal design for an 18 cm long, 10.5 T metamaterial resonator shows an increase of 1.6 dB in power when compared to a 12 cm long microstrip and a 0.2 dB decrease in power when compared to an 18 cm long dipole. An electron band-gap (EBG) periodic structure is designed as the ground plane for the proposed metamaterial resonator. The simulation results show increased B1+ field magnitude when compared with the metamaterial resonator with a solid ground plane. Similarly, a technique of improving the surface current density by creating slots along the line is implemented in the 7 T microstrip resonator. The experimental results show an improved coil efficiency after including of the slots. The extendibility of the coil conductor length with the Metamaterial resonators is also shown by designing a 48 cm long, 10.5 T metamaterial loop body element (longer than a wavelength). Simulations and experimental results confirm the functionality of the loop element and show the comparison with a traditional loop element.
  • Thumbnail Image
    Item
    Nonlinear mechanisms of wave propagation in periodic structures: harmonic generation, dispersion correction, and their interplay
    (2020-12) Jiao, Weijian
    Periodic structures have been extensively investigated due to their unique dynamical properties, which have enabled a broad range of practical applications, especially in the context of wave control. Considering nonlinearity in periodic structures not only leads to a more complete description, but also opens new doors for the design of functional and tunable metamaterials. In this thesis work, we are interested in the dynamical behavior of periodic structures in the weakly nonlinear regime. In the case of quadratic nonlinearity, a well-known effect on wave propagation is second harmonic generation (SHG), which gives rise to a secondary harmonic in addition to the fundamental harmonic that is nearly identical to the linear response. This effect provides an opportunity to nonlinearly activate a second harmonic that exhibits complementary modal characteristics to those of the fundamental harmonic, thereby enriching the modal characteristics involved in the total response. Then, this modal enrichment functionality is explored in 1D periodic structures featuring internal resonators via numerical and experimental analysis, in which we use SHG as a mechanism to achieve energy trapping and localization in the resonators. Moreover, we extend our focus to experimentally demonstrate all the key components induced by SHG in 2D lattices of repulsive magnets supported by pillars. As for cubic nonlinearity, the effect on wave propagation is an amplitude-dependent correction of the dispersion relation, which can manifest either as a frequency shift or as a wavenumber shift depending on how the excitation is prescribed. Compared to the vast study on frequency shift, the scenario of wavenumber shift has only been marginally explored. To fill this gap, we first present a multiple scales framework to analytically capture the wavenumber shift on the dispersion relation of monatomic chains, showing that wavenumber shift is associated with harmonic boundary excitation. We then extend the framework to periodic structures with internal resonators to achieve tunability of locally resonant bandgaps. Last, we investigate the effects of the interplay between quadratic and cubic nonlinearities in periodic waveguides. Through two conceptual applications, we demonstrate that these effects can be leveraged to unveil an array of wave control strategies for the design of tunable metamaterials with self-switching functionalities.
  • Thumbnail Image
    Item
    Reconfigurable wave manipulation in smart cellular solids
    (2017-07) Celli, Paolo
    Metamaterials are man-made materials designed to display properties which are not attainable by conventional materials. They owe their behavior to their mesoscale architecture, which often revolves around the periodic arrangement of a repetitive volume element, or unit cell. A particularly prominent application of metamaterials is in the context of wave control, where they have been used as mechanical filters, energy steering devices, and optical and acoustic cloaks. This thesis work tackles a few open problems within this fertile and fast-growing field. Specifically, one of our main aims is to unveil the relationship between the symmetry of the unit cell of a given periodic medium and the symmetry of its wave response, and to provide a mechanistic rationale for the generation of anisotropic wave patterns in specific frequency ranges. We then propose two strategies to modify these patterns in the context of periodic cellular solids (lattice structures). The first strategy, based on the concept of cell symmetry relaxation, relies on a symmetry-driven microstructural design of the unit cell, in which the geometric and material characteristics of certain microstructural features are modulated to modify the symmetry landscape of the cell. The second one, that we named anisotropy overriding, is based on the interplay between the intrinsically anisotropic wave patterns of the medium and the corrective action of a small number of strategically-placed resonators. We also propose tunable implementations of these strategies, which are achieved by incorporating into the periodic architectures smart material inserts (e.g., shunted piezoelectric patches and curlable dielectric elastomers) which are activated using external non-mechanical stimuli. The resulting wave manipulation effects are illustrated through a series of numerical simulations and experimental tests.
  • Thumbnail Image
    Item
    A Study of Bianisotropy in Split Ring Structures made with Graphene and Gold
    (2016-12) Ghosh, Amartya
    An electromagnetic study of sub-wavelength structures made with graphene and gold is done with a concentration on the electro-magnetic coupling of these structures. The aim of the thesis is to analyze the reflection and transmission coefficients from the numerical simulations done with the help of COMSOL. Then homogenize these periodic array of structures for varying thicknesses so that it behaves as a continuous medium in the long wavelength limit. The next goal is to retrieve the effective electromagnetic parameters like the permittivity, permeability and refractive index from this homogenized structure. This will lead to tuning the electromagnetic properties according to the requirements-the property which is not available in naturally occurring materials, because the electrical or magnetic properties in naturally occurring materials are fixed. This new kind of material is defined as the metamaterial. The effective parameters of these materials are dependent on the properties of the basic materials with which the periodic array of structures are made - for example, it will be seen later how the effective properties is different when graphene is used instead of gold. The approach here is to use a normally incident wave on these periodic arrangement of graphene or gold structures and extract the scattering coefficients. Then invert these reflection and transmission data using basic Maxwell's equations to determine the refractive index and the impedance of the multilayered slab. From here the self consistent equations, the permittivity and permeability is determined. When the metamaterial is made with graphene it is found that the continuous slab behaves as an optical non magnetic material while with gold it behaves as a magnetic material. Some studies are also done on the dispersion of graphene nanoribbons and the electromagnetic modes associated with it.
  • Thumbnail Image
    Item
    Topological Metamaterials: Beyond the Kagome Lattice
    (2024-02) Charara, Mohammad
    Metamaterials are architected solids composed of networks of individual building blocks, whose ensemble yields emergent properties that transcends those of the individual components. Within the broad family of metamaterials, Maxwell lattices are a type of lattice material composed of a network of interconnected bonds such that the number of degrees of freedom and constraints are equal in the bulk. Topological Maxwell lattices are a special subclass of this family that display the ability to localize floppy modes (zero-energy deformation modes) and stress on opposing edges of a finitedomain, a property referred to as topological polarization. This behavior, analogous to topological insulators in electronic and quantum systems, is protected by the topology of the lattice’s band structure (referred to as k-space topology) and formally captured by a topological invariant. When interpreted as structural systems, where the bonds are replaced by structural elements that can deform flexurally, and the ideal hinges are replaced by internal clamps of finite-thickness ligaments that can store bending energy, the polarization, and resulting asymmetry between the edges, is maintained, such that soft modes are still localized on one edge, but the zero mode mechanisms rise in the spectrum and morph into finite frequency phonons. While topological polarization has been well documented in the literature, most investigations are limited to configurations that can be regarded as variations of the kagome and square lattices. In this dissertation, we aim to expand the design space for topologically polarized systems through a series of strategies that increase the geometric complexity of the lattice unit cells. In the first presented strategy, we introduce complexity by extending the problem of polarization, native to 2D elastodynamics, to out-of-plane flexural mechanics, resulting in plate-like 2D-periodic lattices embedded in 3D space. We design bilayer systems which couple in-plane and flexural mechanics with the goal of exporting the topological polarization of kagome lattices to the flexural response. The second strategy introduces a framework for unit cell augmentation which increases the geometric complexity of a Maxwell lattice via a series of mirror operations on a kagome cell, resulting in macrocells with higher kinematic complexity, dubbed “superkagome” cells, and we study the circumstances under which these augmented configurations display polarization. For both strategies, we show that, in the limit of ideal lattice conditions, the obtained configurations enjoy topological protection. For their structural lattice counterparts, we document the signature of polarization via finite-element simulations and laser vibrometry experiments.