Monday, March 12, 2007

TR10: Invisible Revolution

Artificially structured metamaterials could transform telecommunications, data storage, and even solar energy, says David R. Smith.

By Philip Ball

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David R. Smith led the team that built the world’s first “invisibility shield” (above). The shield consists of concentric circles of fiberglass circuit boards, printed with C-shaped split rings. Microwaves of a particular frequency behave as if objects inside the cylinder aren’t there--but everything remains in plain view.
Credit: David Deal

This article is one in a series of 10 stories we're running this week covering today's most significant emerging technologies. It's part of our annual "10 Emerging Technologies" report, which appears in the March/April print issue of Technology Review.

The announcement last November of an "invisibility shield," created by David R. Smith of Duke University and colleagues, inevitably set the media buzzing with talk of H. G. Wells's invisible man and Star Trek's Romulans. Using rings of printed circuit boards, the researchers managed to divert microwaves around a kind of "hole in space"; even when a metal cylinder was placed at the center of the hole, the microwaves behaved as though nothing were there.

It was arguably the most dramatic demonstration so far of what can be achieved with metamaterials, composites made up of precisely arranged patterns of two or more distinct materials. These structures can manipulate electromagnetic radiation, including light, in ways not readily observed in nature. For example, photonic crystals--arrays of identical microscopic blocks separated by voids--can reflect or even inhibit the propagation of certain wavelengths of light; assemblies of small wire circuits, like those Smith used in his invisibility shield, can bend light in strange ways.

But can we really use such materials to make objects seem to vanish? Philip Ball spoke with Smith, who explains why metamaterials are literally changing the way we view the world.

Technology Review: How do metamaterials let you make things invisible?

David R. Smith: It's a somewhat complicated procedure but can be very simple to visualize. Picture a fabric formed from interwoven threads, in which light is constrained to travel along the threads. Well, if you now take a pin and push it through the fabric, the threads are distorted, making a hole in the fabric. Light, forced to follow the threads, is routed around the hole. John Pendry at Imperial College in London calculated what would be required of a metamaterial that would accomplish exactly this. The waves are transmitted around the hole and combined on the other side. So you can put an object in the hole, and the waves won't "see" it--it's as if they'd crossed a region of empty space.

TR: And then you made it?

DRS: Yes--once we had the prescription, we set about using the techniques we'd developed over the past few years to make the material. We did the experiment at microwave frequencies because the techniques are very well established there and we knew we would be able to produce a demonstration quickly. We printed millimeter-scale metal wires and split rings, shaped like the letter C, onto fiberglass circuit boards. The shield consisted of about 10 concentric cylinders made up of these split-ring building blocks, each with a slightly different pattern.

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TR10: Invisible Revolution

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