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Transport properties

Transport properties

Invisibility extends out of its shell

11 Nov 2008

Step inside and, at least at a certain wavelength, light would flow smoothly around you as though you were hardly there — that was the thinking behind the world’s first “invisibility cloak”, which was unveiled a couple of years ago. The catch was that, with no light entering the cloak, the inability of people to see you on the inside would be matched by your inability to see them on the outside.

Their research describes a perfectly reasonable idea Ulf Leonhardt, University of St Andrews

Now, a team led by Che Ting Chan at the Hong Kong University of Science and Technology claims to have solved this problem with a theoretical device that can cloak an object from a distance. The idea is that the cloak uses an in-built copy of the object to control how the cloak cancels the external scattering of electromagnetic waves. When light shines on the cloak and the remote object, both of them are invisible.

Light geometry

The invisibility cloaks built since 2006 have been based on a shell of material with a non-uniform, negative refractive index to guide light in curved lines. But it is not easy to figure out how to design the shell’s refractive-index profile so that the light can propagate neatly around a finite volume. The trick is to see the material as something that can perform a transform from cartesian co-ordinates, in which light travels in straight lines, to curved co-ordinates. Coupled with James Clerk Maxwell’s theory of electromagnetism, such transforms can provide a blueprint for a device that leaves a “hole” in space.

This is the basic method that Chan and colleagues have followed. However, the shell in their invisibility cloak would also contain a smaller, implanted copy of the object to be rendered invisible. The effect of this implanted copy is twofold (see above figure). Combined with the negative-refractive-index shell, it cancels the electromagnetic field in the space around the cloak and object. Meanwhile, combined with the core of the cloak, which is a dielectric, it restores the electromagnetic field — minus the scattering that the object would have created (arXiv:0811.0458).

“[Their research] describes a perfectly reasonable idea — the appearance of an object is cancelled by a ‘complimentary’ object placed at some distance,” says Ulf Leonhardt, one of the UK physicists who came up with the theory for invisibility cloaks. “However, it is also clear that this conjuring trick will only happen in steady state when the electromagnetic fields have settled to a stationary flux, and it will only work in a narrow band of the electromagnetic spectrum. Lack of absorption is critical as well. But I think this is an exciting new development.”

Selective cloaking

Chan told that he and his team are designing a simplified version of their device that they can pass on to an experimentalist colleague of theirs to implement. In the past, negative refractive indexes have been realized in synthetic “metamaterials”, which require careful engineering, and the Chinese team will have to employ these too.

Chan adds that they are also working on a cloak that can selectively cloak just part of an object. “This is much more difficult mathematically,” he says.

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