High Performance RF Metamaterials

Metamaterials can have electric or magnetic properties that cannot be achieved with conventional, naturally occurring materials. By introducing structures that are small compared to the wavelength of operation, materials can be designed with effective permittivity and permeability values that can be large or small, positive, zero or even negative at any selected frequency, and metamaterials can even have a negative refractive index.Materials with negative refractive index show many exotic characteristics. For example, the resolution of a lens made using negative refractive index material is not limited by the wavelength of radiation: the focus can be perfect. Losses in the material inevitably reduce the resolution achievable, but recent work has shown that sub-wavelength imaging is indeed possible. However, no free-space experiment on a metamaterial with negative refractive index has yet been performed to demonstrate this exotic effect. We propose to work at RF frequencies (20 - 200 MHz), using a metamaterial known as Swiss Rolls . Then length scales very much smaller than a wavelength (e.g. A/a >100) are readily obtained, which greatly reduces the demand on the metamaterial properties and provides a unique opportunity to test the theoretical predictions. We plan to build a slab of high performance metamaterial, and use it to test whether we can form cm-scale images with radio waves. .Although metamaterials with negative refractive index are often called Left Handed , they are not chiral or optically active. Very recently, it was shown that genuinely chiral metamaterials can be realised, and that these materials offer a new route to negative refractive index - the negative refractive index arises from the nature of the medium, whose ingredients are chirality and a resonance to produce a band-gap. Although there are naturally existing active chiral systems (e.g. sugar solution), the level of activity is too low (by typically five orders of magnitude) to display a negative refractive index. The Swiss Roll design, wound with a small helical offset, provides a practical realisation of a sufficiently active resonant chiral structure.. This is very compact, with a scale much less than 1/100th of the free space wavelength. Theory predicts that this material will exhibit massive circular dichroism and will also have a negative refractive index near the permeability resonance. We propose to exploit the Swiss Roll design to fabricate suitable chiral material, and to make a comprehensive study of these extraordinary electromagnetic characteristics.Artificial magnetic materials based on non-magnetic conducting elements, which can provide a designed magnetic response at RF frequencies, are a natural match for magnetic resonance (MR) applications. We have already demonstrated that Swiss Roll metamaterials can be applied in the MRI environment as a flux duct linking the imaged region to the detector and that improved material can act as a faceplate for MRI, transferring magnetic field image information faithfully from the input to the output face. We have also explored the construction of an RF yoke for directing and concentrating magnetic flux. This could provide enhanced sensitivity, permit the use of a remote detector (possibly cooled), or allow the applied RF field distributions to be tailored to particular requirements. Furthermore, the RF fields in MRI are circularly polarised, so the recently proposed chiral materials could be extremely powerful in MRI applications. We propose to develop these concepts to make practical devices, and in particular to address issues relating to material losses and associated noise.