No Harry Potter stuff: 'Invisibility cloak' for real


Forget Harry Potter, wizards and witches - this one's for real. Scientists claim to have developed materials that can now perfectly reflect the background, in effect acting as an invisibility cloak.  

Since time immemorial, the elusive power of invisibility has been linked to the even-more-elusive status of invincibility.

While invincibility has remained constrained to the realm of fairytales and, of course, popular video games, the idea of invisibility, however surrealistic it might sound, actually became a practical matter for the scientific community in 2006, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm, now termed transformation optics.

In plain English, that means materials that would, on their own surface, reflect what lies behind them, much like a reverse camera reflecting the image on its face, thus rendering the camera ‘invisible’. Only, these materials (artificially structured metamaterials, if you insist) are not electronic in nature, and that makes the whole idea really, really interesting.

Click here to watch a video of how the ‘perfect invisibility cloak’ works.

The idea gained currency when, in a study published in May 2006 and conducted by Sir John Brian Pendry, David Schurig, and David R. Smith, the authors showed how, by using the freedom of design that metamaterials provide, electromagnetic fields can be redirected at will and proposed a design strategy.

In the study, a simple illustration was given of the cloaking of a proscribed volume of space to exclude completely all electromagnetic fields. Their work, the authors then said, had relevance to exotic lens design and to the cloaking of objects from electromagnetic fields.

Since then and until now, efforts to manufacture the perfect cloak came close to it but could not really surpass the stumbling block of the ‘cloak’ leading to a shadow on the imagery behind it, rendering the illusion darker than its surroundings.

“The complexity of the transformation optics material prescription has continually forced researchers to make simplifying approximations to achieve even a subset of the desired functionality,” say Nathan Landy and David R. Smith in a new study (published in Nature Materials), that claims to have cleared that hurdle.

“These approximations place profound limitations on the performance of transformation optics devices in general, and cloaks especially,” they say.

“Here, we design and experimentally characterise a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics.”

In plain English, Prof Smith and his Duke colleague Nathan Landy have now reworked how the edges of a microwave cloak line up, ensuring that the light passes around the cloak completely with no reflections.

The trick they employed was to use a diamond-shaped cloak, with properties carefully matched at the diamond’s corners, to shuttle light perfectly around a cylinder 7.5cm in diameter and 1cm tall.

“This to our knowledge is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility,” Prof Smith told BBC News.

However, as with the previous cloaks, even this one has the limitation that the illusion is complete in just 2D – i.e., it only works in one direction and if you tilt your head to look from the side, the ‘invisibility’ illusion will be shattered.

“It’s like the card people in Alice in Wonderland,” Prof Smith has been quoted as saying. “If they turn on their sides, you can’t see them but they’re obviously visible if you look from the other direction.”

Click here to watch a video of how the ‘perfect invisibility cloak’ works



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