Now you see them: Smith, left, and Schurig have captured the imagination of scientists and daydreamers alike

Les Todd

A few months ago, Harry Potter was all over thenews again, and not because his creator, J.K. Rowling, had completed her final book about the boy wizard and his friends. Rather, a group of scientists at Duke had invented a device that reroutes light to create a “hole” in space and hide objects from prying eyes—a device that was repeatedly compared with Harry’s magical invisibility cloak.

Like many non-magical, “muggle” versions of things, the Duke device isn’t perfect. For now, it makes objects invisible only to microwaves; humans, whose eyes work in visible light, see the objects just fine.

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Still, the promise of attaining this elusive super power, however distant, captured the imagination of millions. The Duke team’s achievement, detailed in the November 2006 issue of the journal Science, was heralded as an astonishing success, praised by the scientific community, and reported in every major newspaper in the world: Invisibility, a favorite staple of fantasy and science fiction, was one tantalizing step closer to reality.

The cloaking device was created by a team led by David R. Smith, an associate professor of electrical and computer engineering at Duke’s Pratt School of Engineering, and David Schurig, a research associate in electrical and computer engineering at Pratt. For their achievement, Smith and Schurig were named two of the world’s top fifty scientists by Scientific American in 2006.

But while the concept of invisibility is in itself fascinating, the researchers say the larger impetus for their research was a desire to probe the possibilities of a recently developed class of engineered substances, called “metamaterials,” the full potential of which has yet to be realized. The invisibility device was a dazzling proof-of-concept experiment intended to demonstrate the power of this new class of manmade materials.

Metamaterials are allowing scientists to control light in ways unknown in nature and considered impossible only a few years ago. By breaking rules long considered inviolate in physics, metamaterials are changing how scientists think about light and are breathing new life into well-established fields such as optics and electromagnetism.

In the future, the Duke team’s metamaterials could be used to conceal military aircraft from radar better than current stealth technology, protect people and electronics against harmful electromagnetic radiation, create super-sensitive solar cells, or focus light rays into tight beams, enabling a satellite orbiting Mars, for example, to transmit power to a rover on the planet’s surface.

Two other classes of metamaterials, being developed by other researchers, have the potential to create “super lenses” that could be fitted onto microscopes and allow scientists to peer into the mysterious inner workings of living cells, or to shepherd electrons more precisely and efficiently for the construction of smaller electronics and faster computers.

In the very broadest sense, all of these classes of metamaterials function by controlling how light behaves when it comes in contact with the material. How they do this varies, but all of these metamaterials have one thing in common: Unlike natural materials, structure is more important for determining their optical properties than chemistry. In other words, how a metamaterial’s atoms and molecules are arranged is more important for controlling its interactions with light than what those atoms and molecules are actually made of.

Schurig compares the difference between conventional and meta- materials with the different ways that ink can be arranged on a sheet of paper. “If you had a sheet of paper with ink all over it in random patterns, that’s different from printed text,” he says. The arrangement of ink into precise patterns—letters—is what makes printed text readable. Similarly, Smith says, it is the arrangement of atoms and molecules into larger structures in metamaterials that lets them perform their seemingly magical tricks with light. By patterning things on the macroscopic level of multi-atom structures, scientists can create effects not possible in ordinary materials.

The Duke team’s cloaking device is surprisingly small—less than five inches across. It consists of ten concentric rings of fiberglass and looks like a loosely coiled roll of movie film. Etched in copper on each ring are numerous U-shaped patterns, repeated in three rows along the outside of the rings. In their experiments, the researchers placed a small, squat, copper cylinder about a half-inch tall and two inches in diameter into a hole in the center of the device. The entire setup was sandwiched between two horizontal aluminum plates, and microwave light was beamed in through the gap onto the device. Light rays striking the device get channeled around the rings and emerge on the opposite side.

“The microwaves come in and are swept around the cloak and avoid the interior region. So it looks as if they just pass through free space,” Smith explains. The researchers liken the effect to water flowing virtually undisturbed around a smooth rock in a stream—but with one important difference. “In the stream, the rock pushes water out of the way,” Schurig says. “The metamaterial doesn’t push the light outside of itself, it guides it through itself.”

Put another way, the device functions not unlike a beltway diverting traffic around a city: Cars traveling along a linear street enter the ring road and circle partway around the city before emerging on the other side, onto another linear street.

Light is an energy wave made up of intertwined electric and magnetic fields hurtling through space at a swift 186,000 miles per second. Humans are only sensitive to electromagnetic waves within a narrow range of frequencies, called the visible spectrum. Our eyes perceive the different frequencies as colors. We can’t see electromagnetic waves of longer frequencies, such as radio waves and microwaves, or of shorter frequencies, such as X-rays and gamma rays.

Electromagnetic waves interact with matter by influencing the motions of the electrons in the atoms making up the matter. Light’s magnetic field causes the electrons to move in circles, while its electric field makes the tiny charged particles bob up and down. If not interacting with light, the electrons in atoms “would move like free particles,” Schurig explains. “They wouldn’t move in any particular direction. If they were sitting, they’d stay sitting. If they were moving in a straight line, they would continue to do so.”

So light influences matter, but the reverse is also true: The electrons’ motions generate electric and magnetic fields of their own, and these fields in turn interact with the electric and magnetic fields of light to influence its direction and speed.