#7: Invisibility Becomes More than Just a Fantasy

Researchers are cloaking materials from light, sound, and even matter itself.

By Stephen Ornes|Sunday, December 21, 2008

Two years ago a team of engineers amazed the world (Harry Potter fans in particular) by developing the technology needed to make an invisibility cloak. Now researchers are creating laboratory-engineered wonder materials that can conceal objects from almost anything that travels as a wave. That includes light and sound and—at the subatomic level—matter itself. And lest you think that cloaking applies only to the intangible world, 2008 even brought a plan for using cloaking techniques to protect shorelines from giant incoming waves.

Engineer Xiang Zhang, whose University of California at Berkeley lab is behind much of this work, says, “We can design materials that have properties that never exist in nature.”

These engineered substances, known as metamaterials, get their unusual properties from their size and shape, not their chemistry. Because of the way they are composed, they can shuffle waves—be they of light, sound, or water—away from an object. To cloak something, concentric rings of the metamaterial are placed around the object to be concealed. Tiny structures—like loops or cylinders—within the rings divert the incoming waves around the object, preventing both reflection and absorption. The waves meet up again on the other side, appearing just as they would if nothing were there.

The first invisibility cloak [subscription required], designed by engineers at Duke University and Imperial College London, worked for only a narrow band of microwaves. Xiang and his colleagues created metamaterials that can bend visible light backward—a much greater challenge because visible light waves are so small, under 700 nanometers wide. That meant the engineers had to devise cloaking components only tens of nanometers apart.

Xiang’s group also cleared another design hurdle. A competing team had devised a metamaterial to cloak visible light, but it was just one atom thick, too flimsy to deflect anything more than a single sheet of incoming light. Xiang’s new metamaterials have heft.

Last March José Sánchez-Dehesa and Daniel Torrent, physicists at the Polytechnic University of Valencia in Spain, presented a design that would allow a cloaked submarine to hide from sonar. This technology could also allow an orchestra patron sitting behind a cloaked column to hear music as clearly as one in an unobstructed spot.

In September French and British physicists presented a plan for using metamaterials to shield shorelines from the impact of massive waves. Their proposed device [subscription required] would look like a scaled-up acoustic cloak: concentric circles of posts surrounding a hidden object. When a wave hits them, the posts would redirect it around the object without ever breaking the wave. The researchers say that such a device could be used to protect isolated spots in the ocean—like drilling platforms or low-lying islands—or coastal regions vulnerable to tsunamis.

But the weirdest extension of the cloaking concept is undoubtedly the “matter” cloak described this past year by Shuang Zhang, a postdoctoral associate in Xiang’s lab. Subatomic particles like electrons travel as waves, and Shuang showed how metamaterials could be used to divert an atomic wave the same way the invisibility cloak re­directs a light wave. If such a device could be scaled up to the human-size world (far from certain, alas), it might be able to steer a bullet around a bulletproof cloak.

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