Metamaterials and Their Applications

1. 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 a while ago; in 1968, Victor

Veselago [1] of the Lebedev Physics Institute in Moscow concluded LHMs exhibiting an anti-parallel nature in

its wave and Poynting vectors which is opposite to conventional materials that normally 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 reverse Doppler shift, Cherenkov radiation and inverse Snell effect,

were identified. But his idea was forgotten because of the unavailability of LHMs at that time and until recently,

Pendry et al [2] 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, Smith et al [3] demonstrated for the first time the existence of

LHMs.

Figure 1. Dispersive FDTD Simulation at QMUL verifying possible construction of Perfect Lens

from LHMs predicted by Pendry [5]

Figure 2. A snapshot of the near field (Ez) image through a multi-layer LHM slab

2 . 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 are very lossy, 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 the 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 traveling 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

comprising of, a regular array of parallel metallic wires.

3. 95GHz Woodpile EBG Antennas

Millimetr e 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 and also known as photonic bandgap structures (PBG) in

optics, are now finding numerous applications at microwave and millimetr e wave frequencies . The full potential

of EBG structures can be utili s ed 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 Fig. 1

exhibits a full 3D bandgap and can be easily fabricated for applications at microwave frequencies using columns

of individually machined dielectric rods . However, at millimetr e wave frequencies, conventional machining would

not be convenient because of small dimensions (50–500 m m). 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

millimetrewave applications, with a possible extension to higher frequencies based on extrusion freeforming of

ceramic materials. The proposed fabrication method can also be versatile for constructing curved geometries

and creating defects in layered structures.

References

For more information on our research, please contact Prof. Y. Hao .