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.
TR: So an object inside the shield is actually invisible?
DRS: More or less, but when we talk about invisibility in these structures, it’s not about making things vanish before our eyes–at least, not yet. We can hide them from microwaves, but the shield is plain enough to see. This isn’t like stealth shielding on military aircraft, where you just try to eliminate reflection–the microwaves seem literally to pass through the object inside the shield. If this could work with visible light, then you really would see the object vanish.
TR: Could you hide a large object, like an airplane, from radar by covering its surface with the right metamaterial?
DRS: I’m not sure we can do that. If you look at stealth technology today, it’s generally interested in hiding objects from detection over a large radar bandwidth. But the invisibility bandwidth is inherently limited in our approach. The same is true for hiding objects from all wavelengths of visible light–that would certainly be a stretch.
TR: How else might we use metamaterials?
DRS: Well, this is really an entirely new approach to optics. There’s a huge amount of freedom for design, and as is usual with new technology, the best uses probably haven’t been thought of yet.
One of the most provocative and controversial predictions came from John Pendry, who predicted that a material with a negative refractive index could focus light more finely than any conventional lens material. The refractive index measures how much light bends when it passes through a material–that’s what makes a pole dipped in water look as though it bends. A negative refractive index means the material bends light the “wrong” way. So far, we and others have been working not with visible light but with microwaves, which are also electromagnetic radiation, but with a longer wavelength. This means the components of the metamaterial must be correspondingly bigger, and so they’re much easier to make. Pendry’s suggestion was confirmed in 2005 by a group from the University of California, Berkeley, who made a negative-refractive-index metamaterial for microwaves.
Making a negative-index material that works for visible light is more difficult, because the building blocks have to be much smaller–no bigger than 10 to 20 nanometers. That’s now very possible to achieve, however, and several groups are working on it. If it can be done, these metamaterials could be used to increase the amount of information stored on CDs and DVDs or to speed up transmission and reduce power consumption in fiber-optic telecommunications.
We can also concentrate electromagnetic fields–the exact opposite of what the cloak does–which might be valuable in energy-harvesting applications. With a suitable metamaterial, we could concentrate light coming from any direction–you wouldn’t need direct sunlight. Right now we’re trying to design structures like this. If we could achieve that for visible light, it could make solar power more efficient.