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

The Magic of Metamaterials

New manmade substances hold out tantalizing possibilities, from better microscopes and military-stealth technology to the Holy Grail of sci-fi fans—invisibility.
Writer: 
April 1, 2007
 

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.

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.

Deceptively simple: The cloaking device consists of ten concentric rings of fiberglass, with copper etched in repeating U-shaped patterns

Deceptively simple: The cloaking device consists of ten concentric rings of fiberglass, with copper etched in repeating U-shaped patterns. Les Todd

The cloaking device created by the Duke team takes advantage of this principle to bend light in precise ways. When light passes from air into a denser medium such as glass or water, it slows down and shifts direction or “refracts.”

“Light bends in glass [or water] because you’ve got moving electrons in [the] atoms whose motions create their own electromagnetic field,” explains Steven Cummer, an associate professor of electrical and computer engineering at the Pratt School who also worked on the invisibility device. Think of the classic high-school lab experiment, in which a straw is dipped into a glass cup half full of water. The submerged part of the straw looks as if it is no longer continuous with the portion above the water. The refraction of light as it passes from air into water creates the illusion that the straw is broken.

The direction and degree that light rays bend when entering a new medium is determined by an optical property called the index of refraction. For glass, water, and most other natural materials, the index of refraction is uniform throughout the entire material. Not so with the Duke team’s metamaterial. “In our case, the material properties vary from point to point in the cloak,” Schurig says.

This means light can bend in many different directions at once within the metamaterial. “The index of refraction varies throughout our material, and it’s that variation that causes the light rays to bend and go around the object,” Schurig explains.

While scientific interest in metamaterials has skyrocketed in recent years, their antecedents can be traced back to at least the time of the Romans, says Ulf Leonhardt, a professor of theoretical physics at the University of St. Andrews in Scotland. Roman craftsmen made bright, blood-red glass, called “ruby” glass, by mixing molten glass with microscopic spheres of gold. The diameter of each gold sphere was thousands of times smaller than the width of a human hair. At such a small scale, gold is no longer golden. Electrons on the surface of the tiny particles absorb blue and yellow light but reflect longer-wavelength red light. The result is red, not gold, tinted glass.

Like modern metamaterials, ruby glass was made by combining two or more natural materials to create a novel electromagnetic effect. The difference between the Romans and modern scientists is that “we now have more control,” Leonhardt says. “We can design structures more clearly and compute them in advance, instead of trying things out by trial and error. Our manufacturing processes are also much better than what the Romans were able to do.”

The story of modern metamaterials is much younger and is still being written, but it began with a group of scientists who figured out how to bend light the wrong way. Until just a few years ago, every material ever examined had a positive index of refraction, meaning it always caused light passing through it to bend to the right of the incoming beam. But in 1967 a Soviet physicist named Victor Veselago showed that it was theoretically possible to create a material with a negative index of refraction that could bend light to the left.

Veselago further demonstrated that a negative-index material would have startling and counterintuitive properties unlike anything found in nature. Children swimming in a pool of negative-index liquid, for example, would look as if they were doing backstrokes in air because their reflections would appear above the pool’s surface. And the aforementioned straw, if dipped into a glass of negative-index water, would appear to bend all the way out of the water in a V-shape.

For years, Veselago worked in vain to find or create a material with the remarkable electromagnetic properties his formulas predicted. But his efforts ended in failure and his idea eventually came to be regarded by the physics community as a fascinating but far-fetched possibility, the “unicorn” of electromagnetic research.

Indeed, negative-index research didn’t start to become serious science again for some thirty years. In 2000, a team that included Smith, then an associate adjunct professor of physics at the University of California at San Diego (UCSD), created one of the world’s first metamaterials, capable of doing exactly what Veselago had predicted.

Smith’s team built upon the work of another scientist, Sir John Pendry, a physicist now at Imperial College London. A few years before, Pendry realized that a material could be thought of as more than just a collection of identical atoms or molecules; it could also be composed of larger multi-atom structures precisely arranged in repeating patterns. Think of a valet-parked garage where each vehicle lines up perfectly with the next to form a tidy row-by-row pattern of cars. According to Cummer, Pendry’s main insight was that scientists could fabricate whole structures made up of multiple atoms that behaved like individual atoms in a normal material.

A material made this way would be easier to manufacture because, as Leonhardt puts it, “scientists can more easily make structures than chemists can make new materials.” A metamaterial can also be designed to respond to the electric and magnetic fields that make up light in ways unknown in nature. (The prefix in “metamaterial” comes from the Greek word “meta,” meaning “beyond.”)

Soon after Pendry had his epiphany, his team created two modern-day metamaterials: One was an array of metal coils that manipulated only the magnetic field component of light; the other was a lattice of wires that affected only light’s electric field. “There was some interest in that, but it didn’t become this huge growing field until David Smith had the insight to put these two things together,” Schurig says.

Smith’s team combined Pendry’s metal coils and wires into a single metamaterial capable of manipulating both the electric and magnetic field components of light simultaneously. In this way, they were able to make Veselago’s fabled negative-index material a reality. The metamaterial created by Smith’s team “showed clearly that you could engineer something that was totally unlike anything you could get out of nature,” Cummer says.

Schurig, a graduate student at UCSD at the time, remembers the excitement—and skepticism—surrounding Smith and Schultz’s success. “About half the scientists who had heard about it didn’t believe it,” Schurig recalls. “But it was interesting enough that a few more people started working on it, and they were able to confirm those early results. More and more people have been pouring into the field ever since.”

Deceptively simple: The cloaking device consists of ten concentric rings of fiberglass, with copper etched in repeating U-shaped patterns

Deceptively simple: The cloaking device consists of ten concentric rings of fiberglass, with copper etched in repeating U-shaped patterns. Les Todd

One of those people was Schurig himself, who joined Smith’s team in 2000. “I was working on other things, but after a while I couldn’t resist,” Schurig says. “I could see that what [Smith] was doing was more interesting, so I dropped what I was doing and started working on that.” When Smith was offered a position as an associate professor at Duke in 2004, Schurig made the cross-country trip with him.

At Duke, Smith and Schurig began a long-distance collaboration with Pendry. In May 2006, the scientists published a paper in Science detailing the theoretical blueprint for another type of metamaterial, one that didn’t so much bend light in the wrong direction as bend it in many different directions at once. A scant five months later, the team announced the working prototype of its theory: the invisibility device.

“There are several possible goals one may have for cloaking an object,” says Schurig. “One goal would be to conceal an object from discovery by agents using probing or environmental radiation. Another would be to allow electromagnetic fields to essentially pass through a potentially obstructing object. For example, you may wish to put a cloak over the refinery that is blocking your view of the bay.”

While widely praised as a success, the Duke team’s device is still limited. Besides rendering the “cloaked” object invisible to microwaves, but not humans, it works only in two dimensions, so it is invisible only from the side. In addition, not all of the light is redirected; some of it gets absorbed or scattered, creating small reflections and shadows that give away its presence.

“Visible light would probably be the final frontier of metamaterials,” Schurig says. “It would be very difficult.” For a metamaterial to work, its composite structures must be smaller than the wavelength of light it is designed to manipulate. For example, microwaves have wavelengths of about 1.2 inches. The structures in the Duke team’s metamaterial are 0.13 inches wide—smaller by a factor of nine. To manipulate visible light using the same design, scientists would need to make metamaterials with parts tens of thousands of times smaller.

“It’s a real challenge to make optical metamaterials work because you have to fabricate very tiny structures,” Cummer says. “Nobody has figured out an especially good way of doing that. It’s being done in baby steps.”

The problem is more than one of scale, however. Even if the Duke team managed to shrink the structures in the invisibility device, it would not work as well in visible light as it now does with microwaves. That’s because the metals used in the Duke device’s construction would behave differently under optical wavelengths. They would become what scientists call “lossy,” absorbing the light instead of redirecting it. The object that a researcher wanted to render invisible would “just become very opaque, rather than transparent,” Smith says. “We need to make non-lossy materials,” adds Schurig, “and we might need to get away from metals to do it.”

But if these hurdles can be overcome, metamaterials have the potential to revolutionize everything from optics and electronics to biology. As a recent article in New Scientist put it, “Metamaterials will completely change the way we approach optics and nearly every aspect of electronics. Just as solid-state devices replaced vacuum tubes, metamaterial optics will make glass lenses a quaint artifact of an obsolete era.”

Invisibility might be just one of the seemingly magical technologies possible in a future where metamaterials are ubiquitous. The most remarkable uses for metamaterials likely haven’t even been dreamed up yet.