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TR10: Invisible Revolution

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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.

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