A disappearing act

Making a material that could achieve this bending of light is not a simple task. But over the past 10 years or so, there has been some development of so called ‘metamaterials’ – materials that have a very specific geometry, which allows them to capture light and guide it around an object. The individual subunits of the metamaterial, however, need to be smaller than the wavelength of light, so they need to be made of nanoscale structures.

Metamaterials have already been used by scientists to make ‘super lenses’ that can focus light to a point smaller than its wavelength, allowing optical microscopes to view much smaller objects than is possible with normal lenses.1 However, if metamaterials are to be used to render large objects invisible, for example, in applications such as military camouflage, several practical and theoretical challenges must be overcome.

First, you would need to identify the perfect geometry of individual nanoparticles, ie the way they should be positioned with respect to each other so that the light can be ‘guided’ in the right direction. This ‘invisibility’ property would need to work for all the colours in the visible spectrum.

Finding the right nanoparticles is also very hard – scientists have tended to use gold, but have so far only been able to guide light along by a fraction of a millimetre. Scaling the metamaterial up so that it can be used to cover an object the size of the human body is another big challenge – you would need a lot of nanostructures with the right geometry.

In 2006, John Pendry and his team at Imperial College London, UK, and independently, Ulf Leonhardt, then at the University of St Andrew’s, UK, came up with the first design for a metamaterial that would, in theory, steer electromagnetic waves, specifically microwaves, around an object.^2,3 They worked with microwaves because they have a much longer wavelength than light, so larger metamaterials could in principle be made that would be easier to control.

Within months of Leonhardt’s and Pendry’s publications, a team led by David Smith of Duke University in Durham, North Carolina, US,⁴ had built a metamaterial that could guide microwaves around an object as a proof-of-concept. Smith’s metamaterials consisted of concentric rings, which housed simple electronic components. He used ‘split-ring resonators’ – tiny broken wire loops where the loop behaves as an inductor (a magnetic field component), and the break behaves as a capacitor (an electric field component). This set-up allowed the microwaves to interact with matter through both its electric and magnetic fields, which gives an additional degree of control, making it possible to guide the wave around an object.

The first metamaterials that worked on visible light were made in 2009 by physicists at the University of California, Berkeley, US⁵ and at Cornell University, New York, US.⁶ These ‘Carpet cloaks’ were made from silicon, and were able to disguise objects from light in the near infrared to the far red part of the electromagnetic spectrum. Rather than guiding light around the object, the cloaks – which are visible – are placed over an object. Normally this would produce a ‘bump’ in the fabric, however, the metamaterial cloak is effectively able to reflect light off its surface so that the bump remains hidden from specific directions.

The object, however, was very small, with a surface area of only a few square micrometres and a few hundred nanometres depth.

Since then, similar ‘cloaks’ have been made but again they can only conceal very small objects. An ‘invisibility cloak’ of any practical use would need to be scaled up significantly.

Valev, working with physicists at Cambridge University, UK, has gone some way to addressing this problem.⁷ The team developed a technique that uses light to thread strings of gold nanoparticles together, assembling a material a few billionths of a metre across. The material controls the way that light interacts through it, and can be produced in large quantities.

To make their metamaterial, the researchers used barrel-shaped macrocyclic molecules called cucurbiturils (CBs) to act as a scaffold and hold the gold nanoparticles together at just the right distance. Once the gold particles were in place, they then built a bridge between the particles so that they were connected electrically.

This was the real challenge as it is relatively easy to join a few nanoparticles together, but scaling that up into larger materials is hard to achieve because welding techniques would cause the particles to melt. So the researchers focused light from a laser onto the strings of particles in their CB scaffolds. This produced ripples of electrons at the surface of the gold nanoparticles, causing them to bond to one another and form a bridge. The use of ultrafast lasers meant that billions of these bridges could form in rapid succession, forming long strings, which could be stacked one on top of the other, similar to Lego bricks.

The method makes it possible to produce materials in much larger quantities than can be made through current techniques, and the fact that the Cambridge team was able to control the dimensions of the metamaterial so precisely, in a way that hasn’t been possible before, opens up a wide range of potential practical applications.

‘Our work has produced gold nanoparticles, where we can control an important dimension with a very high precision, in a reliable way, and the whole process is scalable, ie suitable to mass producing the nanoparticles,’ says Valev. ‘We threaded a continuous gold bridge between individual gold nanoparticles to form something that resembles the geometry of tiny pearl necklaces. It is quite ironic that we used light as a fabrication tool for a material that could one day become suitable for invisibility.’

Leonhardt, now professor of physics at the Weizmann Institute of Science, Israel, comments: ‘It is very impressive what the researchers have done – making materials that are held together by the forces of light. However, the question is: can both their texture and their refractive-index profile be controlled independently? Without that control, they will not be able to function as materials of cloaking devices.’

Another option is to use an ‘active’ metamaterial, which would rely on electrical power to make objects appear invisible. ‘So far the cloaks that have been made in the laboratory have only been able to make light travel a fraction of a millimetre’, says Valev. ‘People are trying to solve the issue by providing the material with “gain”. This means that, as light travels from one nanoparticle to the other, and as it gets progressively absorbed, the gain could help it to recover. Using external energy to keep the light travelling around the cloak in the way we want it to means that an actual “invisibility cloak” would likely not be a passive material, but would instead be powered.’

In 2013, Andrea Alu, University of Texas, US, designed an ‘active’ cloak using an ultrathin, electronic system, based on microwaves.⁸ Although active cloaks may make the prospect of invisibility more likely, according to Leonhardt, perfect invisibility may not be possible. ‘Perfect invisibility is impossible, but invisibility that is good enough is probably possible. I don’t think the main challenges come down to finding new materials, but rather clever ways of working with existing materials. Some scientists have achieved invisibility with quite ordinary materials. Creativity and imagination are the way forward.’

In September 2014, physicists from Rochester University, US, announced that they had made a cloaking device⁹ – comprising an array of four ordinary and inexpensive lenses – that could make three dimensional objects placed between the first and fourth lenses appear invisible. The researchers used a mathematical technique called ABCD matrices to calculate how light would bend when going through a series of lenses. The formula revealed that if they used four lenses with specific focal lengths, and set them apart at specific distances from each other, then the lenses would continue to show an observer a grid background, while an object placed behind the lenses would be unseen. As long as the object did not interrupt the path of light in the very centre of the lenses, a three dimensional object could remain invisible from multiple angles. The cloaked region of the lenses is therefore shaped like a long cylindrical donut, with objects being invisible as long as they remain out of the central ‘hole’.

‘The field of metamaterials is doing some amazing things, but it has been difficult to scale up the systems and to work in visible wavelengths,’ says John Howell, professor of physics at Rochester University, who developed the Rochester Cloak with graduate student Joseph Choi. ‘As for applications, it is hard to say. I’d say right now, it has just been a fun exercise.’

Although ‘invisibility cloaks’ are unlikely to appear in retail shops any time soon, if the technology can be scaled up it could potentially be used by the military in several ways. Stealth bombers, which currently rely on a curved shape and radar absorbent paintwork to avoid detection, would be much ‘stealthier’ if they were invisible to the human eye.

The chances are that the military is already investing in metamaterial research as a way of achieving invisibility, although even if they had been successful, we wouldn’t know about it.

Meanwhile, other researchers are looking to develop other types of ‘invisibility cloaks’. For example, ‘acoustical cloaking’, where sound rather than light is guided around an object, could be used by the military to protect submarines from sonar detection. As the metamaterials needed for such as application would need to be smaller than the wavelength of sound, which is about 1m, this should be much easier to achieve. Alternatively, there is the possibility of ’active camouflage’. Soldiers could wear textiles equipped with tiny photosensors that change the material’s colour according to its surroundings. A variety of flexible metamaterials have been developed which fit that brief,10 some of which take their cue from nature.

For now, however, it seems that invisibility cloaks will remain available only to the likes of Potter and his friends.

1 D. Melville et al, Optics Express, 2005, 13, 2127.
2 J. B. Pendry, D. Schurig and D. R. Smith, Science, 2006, 312(5781), 1780.
3 U. Leonhardt, Science, 2006, 312(5781), 1777.
4 D. Smith et al, Science, 2006, 324(5801), 977.
5 X. Zhang et al, Nature Materials, 2009, 8, 568.
6 M. Lipson et al, Nature Photonics, doi:10.1038/nphoton.2009.117.
7 L. O. Herrmann et al, Nature Communications, 2014, 5, 4568.
8 P. Chen, C. Argyropoulos and A. Alù, Phys. Rev. Lett., 2013, 111, 233001.
9 J. S. Choi and J. C. Howell, Optics Express, in press.
10 A Di Falco et al, New J. Phys., 2010, 12, 113006.