In plane focus

A joint UK–US initiative predicts that in the future it will be possible to simply produce a flat, near-perfect lens using cost-effective materials.

Imaging systems, electronics and digital computers could benefit from a new generation of ‘near-perfect’ lenses, predicted by engineers at the University of Edinburgh and PennsylvaniaStateUniversity.

Such lenses have been the subject of much research over the past decade, but have depended on expensive and complex devices. But Tom Mackay of Edinburgh and Akhlesh Lakhtakia of PennState say the same properties could be obtainable with cheap materials and simple production processes.

Lenses and their properties, plus their drawbacks, have been known for centuries — the effectiveness of the lens depends on how well you can grind it. The theory of perfect lenses goes back to 1904, said Lakhtakia, and depends on how materials transmit the electrical and magnetic components of electrical and magnetic energy. This depends on properties called the dielectric and magnetic permittivity and, in theory, a material showing a certain combination of these properties will allow materials to refract light in the opposite direction from that seen with normal materials.

In nature, such materials do not exist, but in 2000, researchers in San Diego produced an assembly of intricately-shaped conducting components on an insulating platform which produced negative refraction of microwaves.

‘Basically, the material — or “metamaterial” as it’s known — affects the magnetic properties of the radiation in unexpected ways,’ said Mackay.

But, added Lakhtakia, there isn’t a niche application which anyone can find for negative refraction of microwaves.

‘Where there is a need for them is in optics. With a negatively-refracting material, you could make a completely flat, near-perfect lens. That means that it wouldn’t distort, giving it innumerable uses in TVs, DVD systems, and optical storage. And if it were flat, it would mean no more lens grinding; and it would be easier to integrate with electronic systems.’ The problem is, nobody has come close to a meta-material that works at optical wavelengths; and nobody has devised a material which doesn’t depend on expensive, complex components.

‘So far, the most promising results have been with very thin layers of nano-imprinted silver, but even that wasn’t spectacular,’ said Lakhtakia. Mackay and Lakhtakia’s theory is that reverse refraction should be possible much more simply: by mixing together granules of naturally occurring materials.

‘What you need for negative refraction is a material with a negative dielectric permittivity and a negative magnetic permittivity,’ said Mackay.

‘We know that they don’t exist, but we decided to calculate whether a mixture of two materials with negative dielectric permittivity might acquire both properties.’ Although he stressed that the research is still theoretical, Mackay said that there are ‘relatively simple, magnetic dielectric materials which exist in nature’ which should work.

‘It’s almost a matter of reverse engineering,’ he said. ‘We know what properties we need, and there are materials which have them. They’ll have different electromagnetic properties, but bring them together in close proximity and they’ll interact.’ Mackay and Lakhtakia are unwilling to reveal any more details about these materials.

The model proposed is of an intimate mixture of spherical or nearspherical granules. The smaller the granules, the smaller the wavelength of electromagnetic radiation they will be able to affect; so for visible wavelengths, some nanotechnology would be involved to make granules small enough. These could then be sandwiched between flat slabs of transparent material to make the lenses, said Mackay. Changing the proportions of the components would alter the power of the lens, he added.

Lakhtakia is confident the system will work, but said he is depending on granular materials and composites experts to make it a reality.