Manipulation of Microwave Surface Waves Supported on Metamaterials

Abstract

The primary focus of the work presented in this thesis is experimental investigations into microwave surface waves that are supported by various metamaterial geometries. If the patterning of the metamaterial is either resonant or periodic, surface waves that are somewhat analogous to surface-plasmon-polaritons at optical frequencies can be supported at microwave frequencies. The precise form of the texturing strongly influences the surface eigenmodes that can be supported.

Two approaches to varying the surface-mode index, the ratio of the phase velocity of the surface wave to the speed of light in vacuum, are employed in order to design two different graded mode index surface wave devices: (i) an omnidirectional absorber that utilises the `fatal attraction' phenomenon and (ii) a Luneburg lens. The mode index is graded in the former by spatially varying the dielectric environment in the proximity of a metasurface, whereas in the latter, this is achieved by varying the patterning of the metasurface. Both devices are characterised through experiments, with the results compared to the predictions from full wave numerical simulations.

In a further study, investigations into surface waves with opposing group and phase velocities (analogous to bulk waves in negative refractive index media) is completed using Sievenpiper `mushroom' geometries with different symmetries. Measured instantaneous electric field maps allow the negative-index phenomenon to be directly visualised through phase sensitive measurements demonstrating that the wave-fronts propagate \it towards \rm a near-field point source that is exciting the surface waves. Measurements of the surface mode iso-frequency contours also reflect the negative index character of the surface mode.

Patterned metafilms that support coupled microwave surface waves are investigated experimentally. Both a single-layer and a bi-layer of metallic `dumbbell' arrays are shown to be able to support variations of such surface modes. Ultra-thin metafilms that are patterned with variations of the Pendry hole array are also shown to support such surface modes, although the very small thickness means that only the symmetric-in-charge surface mode disperses at microwave frequencies. This surface mode is used to demonstrate the self-collimation of microwave surface waves, with beams of surface waves excited from a point source, the number of which is dependent on the symmetry associated with the metafilm. A graded mode index device is also designed and characterised through experiments by varying the patterning of a metafilm.