Magnetoinductive waves in metamaterial-based structures

Metamaterials are artificial structures that exhibit unusual properties not seen in natural materials achieved by designing the composite unit cells on the subwavelength scale. Magnetic metamaterials consist of magnetically coupled resonators. Strong magnetic interactions between the unit cells in magnetic metamaterials give rise to magnetoinductive (MI) waves, which are slow waves with wavelengths shorter than the free-space electromagnetic wavelengths.
The aim of this thesis is to present our studies on three novel applications of metamaterialbased structures for manipulating MI waves. The flexible designs of metamaterial-based structures allow one to tailor the dispersion characteristics of MI waves. Analytical models are established based on the MI wave properties, which are verified by numerical and experimental results.
We first propose an approach for signal-guiding to specified directions and realising nearfield directionality via multiple channels in a MI waveguide, which is potentially useful for wireless power or data transfer. Employing two sources with a phase difference to a MI waveguide can suppress the waves travelling to one end. Analytical calculations and experimental validations are presented for both axial and planar structures operating in the microwave regime. Selective unidirectional signal propagation is also demonstrated for nanostructured metamaterial arrays, where there is purely electric coupling and electroinductive (EI) waves are supported, with analytical predictions verified by numerical calculations. Separating the sources by two unit cells leads to a split in the passband, and MI waves travel in opposite directions within the two branches. Such a system can be treated as a special case of diatomic structures with dual dispersion branches featuring both forward and backward waves. We thus propose and prove a fundamental rule that the direction of guided signals is controllable by the forward or backward nature of the supported MI or EI waves in a structure.
The sensitivity of resonant meta-atoms to the variations in local electromagnetic fields makes MI waveguides attractive for near-field remote sensing applications. We devise a 1D position sensor that supports MI waves. With single-port probing, a low-complexity algorithm (we named it as the Odd-Even algorithm) is developed for localising a metal object in close proximity. The localisation procedure relies on the unique reflection patterns of MI waves due to the standing waves formed between the signal injection point and the defect in the array created by the presence of the metal object. Weaker interactions between the object and the defect lead to increased difficulties for localisation. It is shown that terminating the array end with a matching load can suppress the interfering reflections, and improve localisation accuracy. The accuracy of the Odd-Even algorithm tested on the measured and analytically calculated reflection patterns are presented. In this proof-of-principle study, the possibility for remote discrete localisation of a conducting object on the MI sensing waveguide using a low-complexity algorithm has been demonstrated.
We then proceed to analyse far-field properties of structures supporting MI waves. Superdirectivity refers to antenna structures with directivity higher than that of a phased array of the same size, and it can be realised by a dimer of two magnetically coupled split ring resonators (SRRs), by exciting only one of the elements and imposing MI waves. The slow-wave nature of MI waves enables rapid spatial variation in the current distribution among elements, which is essential for achieving superdirectivity. In order to enable quick characterisation of superdirective structures that can be approximated as dipoles, we introduce new 2D equivalents to the traditional 3D directivity. The theoretical maximum achievable directivity values as well as the optimum conditions, for both the 3D and planar directivities are derived. Analytical results are verified and supported by the observations based on numerical calculations. It is shown that by optimising the planar directivity in the azimuthal plane, the 3D directivity is optimised simultaneously. We also demonstrate superdirectivity experimentally by measuring the in-plane field distribution produced by a dimer antenna. It is the first time that superdirectivity is realised using a PCB dimer structure.
Three areas of potential applications of MI waves are developed in this thesis, expanding the possibilities of near- and far-field manipulations using metamaterial-based structures by imposing MI waves.

Advisor: Shamonina, E
Stevens, C