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Materials science

Unexpected tricks of the light

The buzz over invisibility cloaks is fun — while it lasts. But metamaterials are likely to transform optics through more mundane applications. Katharine Sanderson reports.

There is a world where objects can be made invisible, where light can be bent the wrong way, and where images of incredibly small objects can be brought into sharp focus by a superlens. That magical world doesn't sound very real, and it isn't. It exists mostly in the minds of theoretical physicists. But under the guise of metamaterials, some of these weird properties are making it into the real world, and beginning to attract commercial interest that goes beyond boyish excitement about 'invisibility cloaks'.

David Smith was the first to demonstrate how a metamaterial could have a negative refractive index. Credit: J. WALLACE/DUKE PHOTOGRAPHY

In reality, metamaterials applications are likely to be much more mundane, says Michael Wiltshire, a researcher at Imperial College London. Wiltshire predicts that metamaterials will find their first uses in antennas for telecommunication or in biological imaging. It's a far cry from last year's headlines about Harry Potter and magic cloaks but a first step to a world of new optical tricks.

Metamaterials are engineered to have features that are about the same size as, and usually smaller than, the wavelength of electromagnetic radiation they are being used to manipulate. The features are often small metallic wires and coils, which together manipulate the electrical and magnetic components of electromagnetic waves in seemingly unnatural ways. It is these tiny features of their design, rather than the atomic structure of the material itself, that determine its properties.

The field got going in 2000 when John Pendry at Imperial College suggested that metamaterials could be used to make a 'superlens'1 that could image objects smaller than the wavelength of the incoming light. Making a lens that could bring such tiny features into focus would require a material with a negative refractive index, one that would actually bend incoming light the wrong way. This was achieved in 2001, although not for visible light, when David Smith at Duke University in Durham, North Carolina, made a two-dimensional metamaterial from interlocking units of fibreglass board patterned on one side with a copper strip and on the other with split-ring resonators2.

Smith's cloaking device currently works only for microwaves. Credit: D. SMITH

But the public's imagination was really caught in May 2006 when Pendry suggested that, theoretically, metamaterials could be used to make an invisibility cloak. Again, Smith's team followed just five months later with a practical demonstration of invisibility. Smith's cloak used concentric rings of fibreglass imprinted with copper wires (see picture). This contraption, when placed around an object, directed microwaves around that object, much as a stick dipped in a stream interrupts the flow of water. When the microwaves met up behind the object they behaved as if the object hadn't been there3.

This cloaking prototype only works in two dimensions, and for a narrow band of wavelengths, but it was a proof of principle. With this demonstration came promises of the ultimate camouflage — in short, applications that would have military agencies salivating.

According to Pendry, the first metamaterial was developed by the US military in the 1950s, and its interest in the topic has never died. “The primary interest for sure is from the military,” says Smith, who received most of his research funding from the military. In 2001, the US Defense Advanced Research Projects Agency (DARPA) set up a metamaterials research programme. In total, it has invested US$40 million over six years — enough for a young field to get going but small change for an agency whose annual budget is several billion dollars. “Metamaterial developments so far are exciting and promising,” a DARPA spokesperson says, “but they still require significant development to qualify for 'real world' applications.” Not even Smith thinks that a fully functional invisibility cloak will become a reality. “We won't be able to cloak a fighter jet,” says Smith. “I don't know how large a structure we could eventually cloak. Right now, we are trying to play around with the parameters to implement a full cloak, but it is much more difficult.” Mind your Ps Pendry was working on radar-absorbing materials for a military contractor when he came across the work of Victor Veselago, a physicist at the Russian Academy of Sciences in Moscow. Metamaterials take advantage of two fundamental properties of materials — their electric permittivity and magnetic permeability, which are the amounts by which the electric and magnetic parts of a wave interact with a material. Most, but not all, materials have positive values for these two parameters. And in the 1960s, Veselago expanded previous work to study what happens when both the permittivity and the permeability are negative. This leads to a negative refractive index — the amount by which light is bent as it passes from one medium into another — and as Veselago showed, to new optical properties. It was Pendry who realized that with metamaterials Veselago's ideas could become reality. DARPA's initial interest was in applications beyond optics, such as magnets for more powerful motors, radar applications, and textured surfaces that could control a material's heat-handling capabilities. DARPA now thinks, and Wiltshire agrees, that the most feasible advances are likely to be ultra-high-frequency antennas for possible radar applications. DARPA won't reveal much about its radar work, but others are pursuing commercial applications of metamaterials for radio-frequency antennas. Conventional antennas need to be half as high as the wavelength of the radio waves, which can be many metres for low radio frequencies. More compact antennas are needed for the next generation of hand-held devices such as mobile phones and laptops. Fast and small To reduce the wavelength (and the size of antennas), one option is to increase the frequency; at 30 gigahertz the wavelength is one centimetre. But these higher-frequency waves are blocked more easily by obstacles, making them impractical for networks in urban areas. In 'normal' materials the wavelength is inversely proportional to the frequency of the wavelength, but in a negative-index antenna, something strange happens — the apparent wavelength decreases as the frequency decreases. “This is quite the opposite of what happens in conventional materials,” says George Eleftheriades, an electrical engineer at the University of Toronto in Canada. According to Eleftheriades, by exploiting this property, the height of the antenna could be just one-thirtieth of the wavelength, making compact millimetre-sized devices a real possibility (see above). “Now, the wavelength reduces as the frequency is reduced,” says Eleftheriades, “hence short antennas can be constructed even at the low frequencies (1–5 gigahertz) at which most wireless telecom systems work.” Negatively refracting materials (left) are being used in devices such as the compact antenna (right). Credit: J. B. PENDRY; G. ELEFTHERIADES Developing metamaterials for antennas has allowed Eleftheriades to bag a commercial contract with Nortel Networks in Toronto, and a project funded jointly by Nortel and the Canadian government to the tune of$400,000. He also has a patent on his antenna design.

The optical range is perhaps the most practically important for applications. John Pendry

Beyond antennas, “the optical range is perhaps the most practically important for applications,” says Pendry. Optical communication devices, such as fibre-optic cables, and biological imaging would be enhanced if light at optical wavelengths could be manipulated in new ways. But shrinking a metamaterial's features to match these wavelengths is more challenging: the material would need features that control multiple wavelengths of light simultaneously — and that would involve some very small and complicated engineering.

By making optical devices, physicists hope to beat the 'diffraction limit' and so achieve a superlens. For features smaller than roughly half the wavelength of light, diffraction in a normal lens distorts the image. A negative-index material that could focus visible light from subwavelength objects should produce distortion-free images. This would revolutionize biological imaging, Pendry argues, by bringing the details of cells and cellular processes into sharp focus.

In 2005, two teams from the United States and New Zealand showed that a simple superlens could be created for ultraviolet wavelengths from very thin layers of silver4,5. Silver naturally has a negative electric permittivity (but not permeability) at these wavelengths and is able to function as a superlens as long as the dis-tance between the object and lens is much less than the wavelength used. These experiments achieved only modest gains over the diffraction limit, but such superlenses are already being used to improve near-field infrared microscopy6. Smith admits that losses and imperfections in the metamaterial will limit the resolution that can ultimately be obtained, but insists, “The superlens as a concept is sound.”

Metamaterials are an answer searching for a question. Nathan Myhrvold

“It is a separate question as to whether any of these metamaterials technologies will be useful or commercial — that will depend on the competition,” Smith says. He argues that metamaterials ideas are already influencing the next generation of engineers: “So these concepts may seep into the techniques and bags of tricks that engineers employ without necessarily being identified as 'metamaterials'. It could be a very quiet revolution.”

For example, the aerospace company Boeing in Seattle, Washington, has explored the use of metamaterials with negative indices as lenses, again funded mainly by DARPA. Lenses are used in satellites to help focus microwave signals onto antennas, enhancing their detection from the ground. Using a negative-index lens in a communications satellite would enhance performance while reducing the weight of the lenses. This is a simple, albeit niche, application of a metamaterial, says Wiltshire. “What we're exploiting is nothing sexy in the metamaterial — it's mundane — the lens is lighter,” he explains.

Nathan Myhrvold, chief executive of Intellectual Ventures in Bellevue, Washington, and formerly Microsoft's first chief technology officer, claims to have more patents for metamaterials than anyone. “Dozens of them,” he says. The issue for metamaterial applications is not so much when, but rather where they will be used, he says: “At the moment metamaterials are an answer searching for a question — certainly from a commercial standpoint.” Like many in the field, he has faith that the technological breakthroughs will come.

References

1. Pendry, J. B. Phys. Rev. Lett. 85, 3966–3969 (2000).

2. Shelby, R. A., Smith, D. R. & Schult, S. Science 292, 77–79 (2001).

3. Schurig, D. et al. Science 314, 977–980 (2006).

4. Fang, N., Lee, H., Sun, C. & Zhang, X. Science 308, 534–537 (2005).

5. Melville, D. et al. Opt. Express 13, 2127 (2005).

6. Taubner, T., Korobkin, D., Urzhumov, Y., Shvets, G. & Hillenbrand, R. Science 313, 1595 (2006).

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Sanderson, K. Unexpected tricks of the light. Nature 446, 364–365 (2007). https://doi.org/10.1038/446364a

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