vol 3 issue 8

Out of mind, out of sight

30 May 2008
By Sian Harris
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Person in 'invisible' coat

The theory of invisibility is becoming clearer

Materials like those used in Roman glassware may hold the key to making people and objects invisible. Their possible applications go far beyond superhero antics.

At 11 years of age, Harry Potter inherited an invisibility cloak from his father. Many young people would envy the truancy and rule-breaking opportunities that this garment gave its owner. But, according to scientists, devices to turn objects and people invisible are not necessarily confined to the realm of fiction.

Theoretical physicists have been battling with this challenge for some years and now believe that invisibility 'cloaks' are possible. What's more, their fabrication is relatively simple if we use so-called metamaterials known for their interesting optical properties.

The idea behind invisibility is to make light travel around an object in the same time that it would take it to go in a straight line if the object was not there. To achieve this, the object must be surrounded by a material that can propagate light at a speed faster than in a vacuum.

According to Ulf Leonhardt of the School of Physics and Astronomy at the University of St Andrews, this isn't impossible. "Having the phase velocity faster than light in a vacuum does not conflict with relativity theory," he says, and explains that changing the phase velocity is important because it controls refraction and that is what causes light to bend rather than go in a straight line.

Another theorist in this area, Sébastien Guenneau, of the Department of Mathematical Sciences at the University of Liverpool, sees the process as similar to creating a sort of black hole. "We do geometrical transforms that blow up a point to become a bowl so that it makes an invisible region," he says.

The physics behind invisibility research is complicated, and the majority of the work is theoretical. The theorists and their experimental collaborators have their sights on its practical applications and believe that the materials to do this already exist.

Metamaterials, where the structure as well as the chemistry influences the optical properties, have been known for a long time. The practice of adding compounds to glass to colour it is ancient. For example, the Romans used gold chloride to create the red glass known as ruby glass, which can appear in different colours depending on the light conditions. The optical properties (in this case, colour) of this type of 'metamaterial' changed depending on the size or chemistry of the colloidal particles.

No matter how attractive ancient glass specimens could appear, such experimentation would have been very unlikely to result in invisibility cloaks. Modern metamaterials are carefully designed to fit the predictions of theoretical calculations. Like their ancient predecessors, the metamaterials of today typically contain metals such as gold in a host, which nowadays might be glass or a low-loss plastic. Instead of colloidal particles, the metal is arranged in a tiny pattern of wires or rings within its host. Leonhardt, for example, described his design as 'a miniature hedgehog in glass'.

Bending light the wrong way

Playing around with the structure of these materials creates local resonances at certain wavelengths, and this can have dramatic effects on electromagnetic fields. Metamaterials with a negative refractive index have already been produced by bending the light in the opposite way from the expected one - the sort of approach required to make sure that something cannot be seen.

What would an invisibility cloak look like?

John Pendry, of Imperial College London Department of Physics, who has gained a knighthood for his theories about metamaterials, points out that the word 'cloak' is a bit of a misnomer. "It's not Harry Potter's invisibility cloak. It's more like Harry Potter's invisibility shed," he smiles. "Although the metamaterial shell can be any shape, it needs to be quite thick. The thickness depends on the size of the object to be concealed, and an object with a diameter of 10cm would need a cloak that is at least 1cm, ideally more like 3cm, in thickness. Thinner is harder to engineer."

In practice, the theorists and their experimental collaborators do not have invisibility cloaks as their immediate targets. Indeed, the first 'cloaks' being constructed are for radio waves, microwaves and radar. "We came up with the invisibility concept not because we wanted to make a cloak but because we wanted to show that we can do clever things with electromagnetic waves," says Pendry. He has a long-standing collaboration with David Smith and his team at Duke University in the US turning the metamaterials theories into practice. One of the most exciting developments from this partnership has been the construction of a real-life cloak able to hide a copper cylinder from microwaves.

The reason for starting with microwaves is that the applications for cloaking this region of the electromagnetic spectrum are more obvious. It is easy to imagine why the military might want to conceal an aircraft from a radar, say.

There are technical reasons too: to cause electromagnetic waves to bend around an object, the elementary cells of the metamaterial making up the cloak must be smaller than the wavelength of the waves. This is relatively straightforward for radio waves and microwaves, where the wavelengths are of the order of metres and centimetres respectively. In contrast, visible light has wavelengths of the order of hundreds of nanometres, meaning that the structure of the metamaterials would also have to be engineered at the nanoscale. Despite the current enthusiasm about nanotechnology, making such small structures is still a big challenge.

Although it is fairly commonplace to etch two-dimensional nanoscale details onto the surface of silicon chips, it would be much harder to come up with the complex three-dimensional structures required to make metamaterials that work at optical wavelengths.

Also, the metals typically used in metamaterials are more absorbent in the optical range, making it harder to avoid the appearance of a dark shadow where some of the light has been absorbed.

When it comes to making people invisible, there appears another problem: directing light around the person hidden within the 'cloak' would leave him/ her in complete darkness.

"To enable the guy inside the cloak to see the outside you would need to make two holes for light to come in, and then the person would only be able to see in that direction," points out Guenneau. "If you were observing the landscape, you would see two small anomalies, but it would be hard to tell whether there was anything there."

Invisible - but only for one colour

Visible light brings a further challenge: the visible part of the spectrum goes from wavelengths of about 300nm to around 800nm.

Metamaterials work by resonance at a given frequency (related to the wavelength) and will only resonate perfectly at that very frequency. Because the range for visible light is so broad, people predict that current approaches would only make an object invisible at a particular colour. "At the moment, it works only in one frequency range, so if you wore special glasses, the object in question could appear invisible," says Leonhardt.

This could be useful in seeing specific details in the absence of a particular colour: measurements at a particular wavelength could be made with an endoscope, for example, without disturbing what is
being measured. But having to ask everybody else to wear special glasses to appear invisible would be an obvious drawback for an aspiring superhero.

Pendry is more optimistic though. "It is true that the cloak would only be perfect at one frequency," he agrees, before pointing out that windows are very transparent at a wide range of useful frequencies and that it may be possible to create visible-region metamaterials with similar ranges. "At other frequencies, you might get a bit of scattering so that it might look like a coloured soap bubble," he predicts.

Guenneau, in his turn, suggests that the metamaterials might be made more broadband by including several different structures in them. "I believe that by the end of this year we will have a cloak for a given visible wavelength. We have to wait for a decade for invisibility for the whole 300-800nm range though."

The ideas for invisibility could have dramatic implications in other parts of the electromagnetic spectrum.

Leonhardt believes that the first applications for these theories will be in wireless technology. For example, guiding waves around objects and focusing them in a particular place could be very important for antenna design, both in helping antennas receive very weak signals and in shielding other areas from microwaves.

"It's clear that the secret services and US military are funding research in this area," he assumes.

Pendry agrees: "The military are generally prepared to pay top money for the best know-how, and they already have an interest in stealth technology.

"They'd obviously like to make their soldiers invisible but are more likely to be interested initially in making their equipment undetectable to radar."

But the military is not the only group that has noticed the potential for this technology. David Smith's group is said to be working with a car manufacturer that has an in-vehicle radar collision system based on terahertz radiation. Many materials that might be used for radar are not active at terahertz frequencies, but metamaterials are.

Tsunamis and earthquakes

Possible metamaterials applications go beyond the electromagnetic spectrum.

Pendry and colleagues at Imperial College's Faculty of Medicine, for example, are looking at whether they can push static magnetic fields in magnetic resonance imaging (MRI) systems. This would enable operators to take sensitive instruments into the magnetic field. Making an instrument 'invisible' to the field would allow screening without affecting the sensitive field outside.

In addition, this technology could help screen MRI operators themselves. Although there is believed not to be any risk from MRI systems, there is a precautionary desire to protect operators who use it every day.

Guenneau in Liverpool is taking the concept still further. He and collaborators in L'Institut Fresnel in Marseille, France are applying the same invisibility equations to seismic waves, water waves and sound waves.

The potential applications of this research are exciting: buildings could be made 'invisible' to earthquakes and islands could be protected from tsunamis. On a smaller scale, recording studios could be made completely soundproof.

There is a challenge though: while the theoretical equations are the same, the materials to redirect such different wave types are very different.

Nonetheless, the team in Marseille is already running experiments with flexible plates for anti-earthquake systems.

The would-be teenage wizards are likely to be adults before practical applications of this research appear but, when they do, avoiding lessons and sneaking off to sweetshops will be at the very bottom of the list of invisibility cloaks' amazing capabilities.

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Invisibility gimmick

Camouflage and clever camera work

Using metamaterials is not the only way of creating 'invisibility cloaks'. Back in 2003, scientists at the University of Tokyo in Japan demonstrated a different approach, which they called optical camouflage. Instead of bending waves around an object, this approach used virtual reality.

A jacket is coated in a 'retro-reflective' material, which has a surface structure consisting of very small beads. This means that the surface only reflects light back in the direction it came from and that images are reflected clearly even in bright sunlight.

A camera records what is going on behind the jacket's wearer and these images are then projected onto the reflective surface. The result is that the jacket and the person inside it appear transparent and the scene behind can be seen 'through' them.

Although this technique relies on clever camera tricks, its developers believe that it has plenty of uses. For example, it could enable surgeons to see through their own fingers and surgical tools during operations. It could also help pilots to land planes by making the floors of their cockpits transparent or help drivers to reverse-park their cars by enabling them to 'see through' their vehicles.

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