How to Build an Invisibility Cloak

Using strange new materials not found in nature, physicists can make an object disappear.

By Josie Glausiusz|Monday, November 20, 2006
RELATED TAGS: MATERIALS SCIENCE

Rare is the moment when Harry Potter fans, Star Trek aficionados, H. G. Wells enthusiasts, and theoretical physicists unite in a moment of ecstasy. But that instant came last May with a flurry of dramatic headlines. "Scientists may be able to make magic like Harry Potter," wrote the Associated Press. "Here's how to make an invisibility cloak," shouted MSNBC. "Cast no shadows," said The Economist. For Duke University physicist David Smith, though, the oddest moment was seeing his work flashed on the CNN crawl the same day it appeared in Science Express, the online edition of the journal Science. "It was surreal," he says. "The story was generating a huge splash before the scientific community had a chance to take a critical look."

Smith hardly fits the profile of a media celebrity: soft-spoken, patient, and bespectacled, he has the pale-skinned hue of a man who has perhaps spent too much time in a windowless lab fiddling with wires. All at once, however, he and his postdoc Dave Schurig became the targets of intense public interest. Reporters called from around the world, crackpots sent long letters hand-scrawled with dubious hypotheses, and a Korean television crew flew in to the leafy Duke campus, posing Schurig, graduate student Jonah Gollub, and technician Bryan Justice in lab coats in front of their intricate machinery. "They wouldn't film until we put the white coats on. We didn't even have any—we had to borrow them," recalls Schurig.

All this fuss over a theory not only unproved but so dense with equations it is all but incomprehensible to the average person. What sparked such fascination was the mind-bending notion itself: Smith, Schurig, and their coauthor John Pendry of Imperial College London proposed that by using a novel class of composite materials, they could manipulate light so as to render an object invisible to the eye. Suddenly, Harry Potter's invisibility cloak, Star Trek's Romulan ship-concealing devices, and H. G. Wells's bandaged Invisible Man seemed the stuff of testable science. In a more practical vein, the finding could have profound implications for military technology, wireless communication, and even interplanetary exploration.

In September, the journal Science accepted a paper from Smith and Schurig that proved that their method was more than just a thrilling hypothesis. They had succeeded in cloaking a small cylindrical object—shielding it not from visible light but from microwaves, a form of electromagnetic radiation with a substantially longer wavelength, which makes the cloaking effect considerably easier to achieve. The way they did it most closely recalls not Harry Potter but another fictional character: the Invisible Woman, a Marvel Comics superhero who can bend light waves at her command, rendering her body and clothing imperceptible. By tightly controlling the bending, or refraction, of microwaves as they pass through a custom-built material, the Duke researchers could force them to detour around an object so that the microwaves are neither absorbed nor reflected. If they performed the same feat with visible light, a viewer looking directly at the object would see only what lies behind it, as if the object were not even there.

To explain their technique, Smith and Schurig invoke the example of a mirage on a hot summer road. When light rays from the sky hit the hot, thin air just above the surface of the asphalt, they bend. Although light moves through a vacuum at a constant speed, it slows down when traveling through any transparent medium, like water or glass. Light travels faster in the hot, thin air close to the road than it does in the cold, dense air above, and that difference in speed is what causes it to shift direction as it crosses the boundary between the two. Rays once headed from the sky to the ground are redirected to your eye, making the road shimmer like water. In effect, the mirage is cloaking the (now invisible) road behind an image of the blue sky.

To similarly cloak something from electromagnetic radiation, Smith and Schurig must bend the incoming beam around the object in a tightly controlled manner. They managed to do so using a class of recently created "metamaterials" that possess an ability, not found in nature, to bend light at extreme angles (a property known as negative index of refraction). The team's metamaterials consist of thin, rigid sheets of fiberglass insulator stamped with neat rows of conducting metal shapes like loops, coils, or tiny rectangles. The metal circuitry is designed to direct incoming electromagnetic radiation—in this case, microwaves—so it moves in a specific way.

All electromagnetic radiation has two intertwined components: a magnetic field and an electric field. As Schurig explains, these can be redirected when they interact with a material. "Materials are made of atoms, and these atoms respond to electromagnetic waves by acting like a little tiny magnet," he says. Electrons begin moving in circles in response to the magnetic field, as well as back and forth in reaction to the electric field—and the moving charges produce fields of their own. The challenge for the Duke team was to find the right shapes and dimensions for the metal circuitry on the metamaterials so they could precisely dictate how the electrons move around, which in turn controls how the incoming radiation is bent.

To demonstrate their system in action, Smith and Schurig walk into their lab, a room lit with fluorescent bulbs and littered with wires, pliers, plugs, pulleys, flashlights, foam cladding, microscopes, computer terminals, and a lone bicycle. The object to be cloaked is just a small copper cylinder filled with black foam: 5 centimeters (2 inches) in diameter and 1 centimeter (0.4 inch) tall. For the experiment it is sandwiched between two horizontal aluminum plates, the bottom one 3 feet square and the top one 4 feet square. Leading in from the front of the apparatus is a wire that feeds microwaves toward the cylinder as it sits in the center of the bottom plate. Around it, Smith and Schurig have arranged concentric rings of metamaterials, with the empty spaces between the rings forming narrow channels. Having carefully varied the properties of the circuits on those surrounding rings, they can now bend microwaves to flow around the cylinder like water flowing around a pebble in a stream. This makes the object undetectable to an instrument downstream that measures microwaves.

According to the Duke team, this experiment shows it should be possible to make an object invisible to the human eye as well, but there are major technical hurdles. For cloaking to work, the metal shapes stamped on the metamaterial must be smaller than the wavelength of the electromagnetic radiation that is aimed at them. The wavelength of the microwaves is a little over 3 centimeters (just over an inch), and the shapes on the surface of the metamaterials are closer to 3 millimeters long. Green light, by contrast, has a wavelength of 500 nanometers—60,000 times smaller—so the shapes that could cloak it would have to be around 50 nanometers long. Theoretically, you could pattern metamaterials at that tiny scale using specialized methods like focused beams of charged atoms, but such materials would be difficult to mass-produce.

At this point, then, cloaking objects from visible light is still pie in the sky. In the meantime, the far more accessible applications of microwave cloaking have already garnered intense interest—mainly from the military. Smith is up-front as he rattles off their funding sources: DARPA (the Defense Advanced Research Projects Agency); the Air Force, the Army, the Navy, the intelligence community. One of the technique's most practical and immediate uses would be to hide obstructions that block wireless communication. But since Smith and Schurig's technique bends electromagnetic radiation in a controlled manner, it could someday also be co-opted to focus or concentrate energy in highly efficient ways. For example, it could be used to create supersensitive solar cells or even to power a Mars rover that would gather energy from a microwave beam sent by a satellite orbiting the Red Planet.

The Duke researchers are not the only ones scrambling to create cloaking devices. When their theory first appeared in the May 26 edition of Science Express, it was published alongside an independent article that outlined a similar proposal. The author of that paper, theoretical physicist Ulf Leonhardt of the University of St. Andrews in Scotland, proposed using slightly different types of engineered materials to accomplish the trick. A few weeks before that, a pair of math-loving physicists, Graeme Milton of the University of Utah and Nicolae Nicorovici of the University of Sydney in Australia, came up with yet another, drastically different scheme for making objects the size of dust specks invisible.

The Milton-Nicorovici hypothesis, which is based on rigorously proved mathematical calculations, relies on the use of a superlens, a thin transparent film that can resolve light finer than its wavelength (long considered a theoretical impossibility), producing extremely sharp images. A superlens made from a thin film of silver could have a negative index of refraction, bending light outside of its normal path. "What we found was that if you put a speck of dust near the superlens and shine light on the dust, then part of the scattered light gets trapped at the front surface of the superlens," Milton explains. "That trapped light builds up in intensity until it almost exactly cancels the incoming light," in the same way that two colliding sound waves can zero each other out. It is as if there is no light there at all, and the dust particle becomes invisible. (For an action-packed movie of this phenomenon, see Milton's Web site at www.physics.usyd.edu.au/cudos/research/plasmon.html.)

So how far off is a real invisibility device? Could such a contraption ever be used to cloak an airplane, a tank, or a ship? Smith doesn't want to be snared by such hypothetical questions. "Reporters, they call up and they just want you to say a number," he says. "Number of months, number of years. They push and push and push and then you finally say, well, maybe 15 years. Then you've got your headline, right? 'Fifteen years till Harry Potter's cloak!' So I have to resist giving you a number."

One major problem with masking objects from visible light, says Schurig, is that light is composed of a range of colors, each with its own wavelength. "We don't know how much of that spectrum we could cloak all at once," he explains. "If you could get past these fabrication issues, you could cloak one color of light, and maybe you could cloak some range of visible light. We might be able to make the cloak work for a brief time, for a microsecond at red, a microsecond at green, a microsecond at blue, and you could make it look translucent. But we don't know that you could make something 100 percent invisible to the whole spectrum simultaneously."

Milton sounds a further note of caution. Of the Duke research, he says: "I think it's a brilliant idea. But there's a certain amount of skepticism in the scientific community in so far as the time line. I remember reading claims that you could cloak some factory that would be an eyesore. I think that's a bit far-fetched. You can make some small things invisible, but making larger things invisible will be a longer time in coming."

There are other factors that neither Harry Potter fans nor the series' fabulously wealthy author, J. K. Rowling, seem to have considered. Ulf Leonhardt—the only one of the researchers who admits to enjoying the books—explains that Harry can see through his cloak, which is made of a thin material in which he can walk and climb. "The present scheme assumes you have something very rigid" surrounding the object, Leonhardt says. "It's not a cloak, it's like a suit of armor. If you want to have something flexible, then the material also has to change its properties, like a chameleon. That is also possible in principle, but with present technology we're a long way away from that."

The other, bigger problem is that to see, the eyes must absorb light—which, of course, makes them visible. "If Harry Potter wants to see through his cloak, then his eyes would be visible, because they have to see. And if they have to see, they have to be seen," Leonhardt says. "For example, a fish that camouflages itself by being transparent has eyes that are not transparent, because they have to see. Yet Harry Potter can see through the invisibility cloak. That, I think, is not possible. He would be blind behind it."

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