Metamaterials and Transformation Optics

Our metamaterials research started in 2001, focused on the microwave regime and with a strong emphasis on industrial applications. Our work has covered a very broad range of research topics in metamaterials, including electromagnetic bandgap structures, high impedance surfaces and partially reflective surfaces. Our work on left-handed metamaterials is based mainly on numerical modelling. Due to the dispersive nature of metamaterials, the conventional FDTD method requires modification to tackle both frequency and spatial dispersions. Once the limitations of left-handed metamaterials were uncovered (for instance, limited bandwidth and high losses), we investigated many alternative materials and structures to enable applications, such as directive and compact antennas, subwavelength imaging and broadband microwave systems. We also developed techniques to model invisibility cloaks. Meanwhile, it has offered us an opportunity to work on the exciting concept of transformation optics/electromagnetics.

We recently successfully completed an EPSRC Programme Grant “QUEST”, with the aim to apply both transformation optics and metamaterials to practical applications at microwave frequencies. This included theoretical work, fundamental work on materials fabrication and investigation of numerical design tools, as well as numerous applications. We are currently working with our partners on two related projects that build, in various ways, on the QUEST results, namely AOTOMAT and SYMETA, as well as various industry-funded projects.

Flat Lens Modelling and Layered LHM Structures

Left-handed materials (LHMs) are those engineering composites that the electric permittivity and the magnetic permeability of a material are both negative. This was noted theoretically some time ago: in 1968, Victor Veselago of the Lebedev Physics Institute in Moscow described LHMs exhibiting an anti-parallel nature in its wave and Poynting vectors. This is opposite to conventional materials, within which electromagnetic waves carry energy in the same direction as they propagate, following what's called a right-hand rule. Exciting possible electrodynamic properties, such as a reverse Doppler shift, Cherenkov radiation and an inverse Snell effect, were identified. However, because of the unavailability of LHMs at that time, his idea was forgotten until recently, when Prof. Pendry (UK) demonstrated that materials with an array of split ring resonators (SRRs) produce negative permeability over certain frequency bands. Soon afterwards, by combining a two-dimensional (2D) array of SRRs interspersed with a 2D array of wires, Prof. Smith’s Group (USA) demonstrated, for the first time, the existence of LHMs.

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Wire Medium Imaging

We have investigated, through numerical simulation, whether or not the finite transverse dimensions of LHM slabs influence the quality of their sub-wavelength imaging. However, in practice, the fabrication of left-handed media remains problematic, since it requires the creation of negative permeability, which does not exist in nature. Furthermore, currently available designs of the left-handed media are very lossy at both microwave and optical frequencies, which restricts and even prevents their use in sub-wavelength imaging applications.

There is an alternative approach to sub-wavelength imaging, in the sense of mapping the source distribution in one plane onto another (the imaging plane). This approach involves neither the use of left-handed media nor does it capitalize on negative refraction or amplification of evanescent waves, which has been referred to as canalization. It is based on transporting both the propagating and evanescent spectra of a source by transforming them into propagating waves inside a slab of specially designed materials. Then, these propagating modes are capable of transporting sub-wavelength images from one interface of the slab to the other. The source must be placed very close to the front interface of the slab in order to minimize the degradation of its spectrum, which occurs when the fields propagate in free space. It is also necessary to minimize the reflection from the slab via an appropriate choice of its thickness. This is done by tuning the slab thickness for Fabry-Perot resonance, to reduce reflections from its interface for a wide range of angles of incidence, and minimize the interfering interactions between the source and the slab that can distort the image.

The material operating in the canalisation regime should have a flat iso-frequency contour, implying that it should support waves travelling in a certain direction with fixed phase velocity for any transverse wave vectors. The materials that fulfil this requirement are available at both microwave and optical frequency ranges. One such artificial material is the wire medium, comprised a regular array of parallel metallic wires.

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95GHz Woodpile EBG Antennas

Millimetre wave systems are becoming increasingly important in many applications because they can provide wider bandwidth for transmitting large amount of data and better resolution in radar systems. Electromagnetic Bandgap (EBG) structures, a class of metamaterial also known as photonic bandgap structures (PBG) in optics, are now finding numerous applications at microwave and millimetre wave frequencies . The full potential of EBG structures can be utilised with a full three dimensional (3D) bandgap. Thus, rapid and cost-effective fabrication techniques for 3D EBG structures are of significant importance. A woodpile structure, shown in the figure, exhibits a full 3D bandgap and can be easily fabricated for applications at microwave frequencies using columns of individually machined dielectric rods . However, at millimetre wave frequencies, conventional machining would not be convenient because of small dimensions (50–500 mm). Various sophisticated techniques such as silicon lithography are available for microstructures, but those are more appropriate for terahertz and photonic wavelength applications, and would be costly to fabricate 3D structures with large number of layers for applications at W-band (75-110 GHz). In this work, we present a direct rapid prototyping method for constructing 3D EBG materials for millimetre-wave applications, with a possible extension to higher frequencies based on extrusion free-forming of ceramic materials. The proposed fabrication method can also be versatile for constructing curved geometries and creating defects in layered structures.

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Highlights and Research Outcomes

Selected Research Grants and Projects

Selected Recent Publications

 

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