Nikolay Zheludev's voice in the U.K. traverses the Atlantic Ocean on wires, fiber-optic cables and microwaves and reaches me in New York City. The delay and noise make a conversation difficult. Speaking from the University of Southampton, he is describing man-made structures called metamaterials and how they will make almost any conceivable device or application faster, cheaper and more efficient. Our transoceanic conversation is a case in point, he notes: over a metamaterial-augmented all-optical network, the static, the awkward pauses and the cross-talk at the ends of our sentences would be eliminated. “We are no longer limited by what nature gives us or what we can cook with chemistry,” Zheludev says. “We can do better.”
Metamaterials are made up of tiny arrays of microscopic elements such as metal rings or rods that can bend, scatter or transmit electromagnetic radiation in ways that no natural material can. The elements must be smaller than the wavelength of the radiation they are intended to manipulate. Picture a net so small it can deflect a wave of light the way the tall netting behind home plate on a baseball field deflects foul balls. Now imagine that by tweaking the size and composition of the gaps in the net, you can not only deflect light or allow it to pass through but also alter its trajectory, change its color or even make it disappear. Metamaterials manipulating light in these ways could form the basis for more reliable wireless Internet connectivity, denser data storage and all-around more capable electronics, not to mention a smartphone as thin as a credit card.
Yet many of these improvements will become reality only if and when metamaterials are made that work with visible light. At present, metamaterials work best for longer-wavelength radiation such as radio waves and microwaves, which require elements that are on the order of tens of millimeters. Such elements can be easily fabricated by several common manufacturing techniques.
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In January a team led by David Smith of Duke University demonstrated a metamaterial-based microwave camera that requires minimal data storage and sensors, which could replace bulkier, costlier microwave imagers now used in some airport security booths. A company called Kymeta is also using Smith's work in a new, low-power, high-bandwidth reconfigurable antenna for airplanes, ships, trains and cars that could go on sale as soon as next year, bringing cheaper high-speed satellite Internet to passengers in those vehicles. And researchers are devising “invisibility cloaks”—metamaterial shells that can bend radio waves or microwaves around objects to conceal them from radar.
Making metamaterials for shorter wavelengths such as visible light is more difficult because it entails making elements much smaller than a micron, approaching the sizes of components on modern computing chips. Furthermore, many applications require a configuration of elements that can change to manipulate light in different ways, on the fly.
Zheludev calls these dynamic arrangements “metadevices,” and with collaborators he has already created a few in his laboratory. In March his team published a “proof of concept” for an optical metadevice made of nanoscale elements etched in gold films, which are then connected to microscopic strings. Each element's position can be electronically controlled through the strings, thereby allowing the device to be reconfigured in real time to alter how it transmits or reflects visible light. Zheludev says the technology could make an ideal switch for ultrafast optical communications and computers.
The best way to master metamaterials for visible and near-infrared light, however, may be to first perfect flat, two-dimensional “metasurfaces,” according to researchers such as Federico Capasso of Harvard University. Only then could scientists consider making more complicated, three-dimensional assemblies for applications such as true-color holographic displays or Harry Potter–style invisibility cloaks that make people and objects we can see disappear. His group's most notable success so far is arguably its “flat lens,” which focuses a beam of light into a pinpoint and could lead to wafer-thin smartphones and digital cameras; lenses and batteries are the limiting factors in further flattening these products.
Although much work remains before metamaterials become common in practical products, “I do think this stuff has the right flavor,” Capasso says. “Influential people are taking notice. They tell me, ‘Federico, what you are cooking smells good!’”
