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16 September 2016

The Illusion of Invisibility Cloaks

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Becoming invisible is a wish that has fascinated scientists and writers of all ages. The image of Harry Potter disappearing with ease under his cloak of invisibility has intrigued us since the beginning of this century. Nowadays, researchers trying to obtain a similar effect have to do so in a world where there are no magical creatures from whose hair invisible fabrics are obtained, but instead they must deal with light waves, governed by the basic laws of physics.

Harry Potter, the magician created by the writer J.K. Rowling, and his invisibility cloak in a scene of the movie “The Philosopher’s Stone”. Credit: Warner Bros

Although it still seems a question of magic, the reality is that large European and American universities, such as Imperial College London, the University of California in Berkeley or the University of Texas in Austin, have departments that are dedicated almost exclusively to trying to use science to make us invisible. The approaches vary, but most researchers show caution with the current results. “Our theoretical analyses show that it is currently impossible to hide (from visible light waves) large objects like a person, a military tank or a spaceship,” researcher Francesco Monticone from the University of Texas tells OpenMind, recalling examples from science fiction cinema.

A ‘cloak’ for tiny objects

Expectations improve when it comes to hiding smaller objects. A study by the University of Berkeley in the journal Science in September 2015 garnered headlines like “Scientists are getting closer to an invisibility cloak“. The researchers, Xingjie Ni and Zhang Xiang, showed in their study how they had managed to make tiny objects (0.036 mm) invisible thanks to the use of metamaterials. These artificial materials, which are designed to have unusual electromagnetic properties, can redirect light around the object to be hidden.

Simulation results for the metasurface skin invisibility cloak developed by the University of California, Berkeley. Credit: Xingjie Ni et al.

It is similar to what happens if we put one foot in the river. The water stream separates when it reaches the foot, flows around it and then rejoins. That’s what researchers intend to do with the layers of metamaterials: get light rays to reach the object, go around it and then continue on their way without changes once they have passed it. When an object does not absorb or reflect the light it receives, it is not possible to see it from a distance, so that in practice it would become invisible. If we could place ourselves close enough, we could observe a slight deformation of the background behind the object because of the curve that the light rays make.

So they created and tested the effectiveness of an ultrathin layer –0.00008 mm thick– made of microscopic blocks, like a skin, that adapts itself to the shape of the object and makes it undetectable. This optical illusion is due to the layer that redirects the light waves away from the object, tricking the human eye into seeing a flat mirror instead of the object. “If we managed to make a curved surface appear flat, that means we can do a lot more”, claims Xingie Ni.

Although not as impressive, making very small objects invisible has some very useful applications. The team of Andrea Alu and Francesco Monticone is studying how covering concealed antennas could reduce interference in the current dense communication systems or how to use invisible nano-sensors to optimize optical images, such as those used in x-rays. “In this sense, invisibility cloaks are already real and practical, but they only work for small or single-colour objects. To have a human scale invisibility cloak like that of Harry Potter, we would need other types of structures that are powered,” muses Monticone.

Next challenge: Powered invisibility cloaks

So far, the options that have been explored are based on a passive invisibility cloak, in other words, that the only energy that influences it is that which comes from the light wave. These layers get their invisibility thanks to the optical illusion that is created when light strikes the metamaterials and is redirected to another point. However, metamaterials do not work with large objects or those with different colours, because they have many different wavelengths. For example, the colour blue has a longer wavelength than the colour red, so using one metamaterial for an object that had these two colours would not work: it would cover one of them, but the other colour would be seen.

For this reason, Monticone reveals that one of the possible ways to go beyond these limits is to power the devices that have to hide objects: “If we drew power from an external source such as a battery, perhaps the invisibility layer would be stronger and we could cloak larger objects and cover a wider range of colours.” The possibility of using this type of powered device comes to reality from fiction. In 1966, in an episode of Star Trek, Mr. Spock foreshadowed to the captain of his spaceship, James T. Kirk, that to hide such a large object as his vehicle would need a device with a power reserve: “Invisibility is theoretically possible, Captain, with a certain refraction. But the cost of the energy to achieve that might be huge,” warned the character in the science fiction series.

Thus, despite the caution, researchers have not lost hope: “We still have many challenges to overcome, and it might take us years or even decades, but we haven’t ruled out achieving a large-size invisibility cloak that works for all colours,” Monticone says.

By Beatriz Guillen for Ventana al Conocimiento


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