Multi-layer metasurfaces for manipulating the propagation of microwaves along surfaces and edges

Abstract

This thesis comprises original experimental studies on surface waves propagating on metasurfaces at microwave frequencies. These studies are supported by matching simulation data, obtained by means of Finite Element Method modelling. The structures studied throughout this thesis are comprised of more than one layer of sub-wavelength elements arranged in different periodic lattices. However, these layers are very thin compared to the size of the elements comprising the arrays, resulting in structures that are extremely sub-wavelength in the (out-of-plane) dimension. The work carried out as part of this thesis is divided in three main blocks, each of them looking at different features of metasurfaces, aiming to maximise different properties. The first type of structure presented in this thesis are designed and engineered to maximise three main properties. These are the mode index of the modes bound to the structures, its in-plane isotropy and the bandwidth of operation of prospective devices based on such metasurfaces. Whereas previous work in this field has considered single layers, the novelty here is the introduction of additional layers in order to increase the effective mode index of the modes supported by the structures. These extra layers create a capacitive effect between the overlapping areas of metal, therefore increasing the confinement of the waves. This is increased even further by minimising the separation between such layers. The second main goal in the design of the metasurfaces was to create a frequency independent mode index, aiming for the prospective development of broadband devices. For this, higher symmetries between the layers comprising the structures were introduced. Following the studies of the infinitely periodic metasurfaces and its properties, the suitability for their implementation as graded index devices is proven. Such devices are based on a graded mode index or surface impedance profile across the structure, which modifies the propagation of the wave. In the case of metasurfaces, the grading of the mode index is achieved by gradually varying the size or shape of the elements comprising the structure. This technique is used to design and manufacture two working planar Luneburg lenses, which are characterised experimentally and their performance compared with simulation data. The Luneburg lenses designed and manufactured as part of the work contained in this thesis have the novelty of a higher fractional bandwidth of operation compared to similar metasurface devices, reaching $73\%$. Another piece of work contained in this thesis involves a structure that guides microwaves with very high phase and group indices compared to similar metasurfaces. Its design is a simple two-layer discontinuous crossed metal-strip array. However, the novelty of this structure resides in the length of the metal strips, which extend to several unit cells. This work focuses on the isotropic wave dispersion shown at the lower frequencies. However, in addition to this, two of the higher frequency bands give rise to very strong negative dispersion, and also strong beaming occurs, which can be tailored easily by modifying the relative orientation of the layers. The third piece of work included in this thesis focuses on the propagation of edge modes along the termination of a particular metasurface structure with hexagonal symmetry. Our metasurface is comprised of two layers of hexagonal arrays of circular metal patches. This structure, in addition to supporting a bound wave that propagates isotropically across the two-dimensional structure, also supports an edge mode that propagates only along its termination. Here, the propagation of the mentioned edge mode has been extensively studied. Firstly, its propagation along finite strips is considered, followed by its use to guide the electromagnetic field around different shapes. Finally, the coupling of two of these edge modes across small gaps between two terminated structures is explored, with different symmetries between them. In all cases, samples are designed, fabricated and experiments have been carried out and the original results obtained have been compared with simulation data calculated with a finite element method modelling software.