In Star Trek IV: The Voyage Home, the crew of the Enterprise hijacks a Klingon battle cruiser. In case you’re not a die-hard trekkie, that was quite a feat for a Federation starship. Usually the Klingon ships are the ones ambushing with impunity, thanks to a secret “cloaking device” that renders them invisible to light or radar.
Is such a device really feasible? Invisibility has long been one of the marvels of science fiction and fantasy, from H.G. Wells’s The Invisible Man to the The Lord of the Rings to the Harry Potter series. Yet physicists have doggedly dismissed such vanishing acts as impossible, claiming that escaping detection violates the laws of optics and does not conform to the known properties of matter.
[ad:51 1121]But today the impossible may become possible. New advances in “metamaterials”—man-made materials that can, in a sense, control the movement of light—are forcing a major revision of optics textbooks. Working prototypes of such materials have actually been built in laboratories, sparking intense interest from the media, industry, and the military, eager to know how the visible could someday become invisible.
Modern optics truly began with work done by James Clerk Maxwell, a Scottish physicist, in the mid-nineteenth century. At Cambridge, where Isaac Newton had worked two centuries earlier, Maxwell excelled as a student of what would now be called mathematical physics. Calculus—an invention of Newton’s that uses equations to describe how objects move in space and time—armed Maxwell with the mathematical tools to explore the nature of electromagnetism.
Maxwell began with physicist Michael Faraday’s discoveries that electricity could generate magnetism, that magnetism could generate electricity, and that each could be thought of as a force field. Maxwell rewrote Faraday’s depictions of force fields in the precise language of calculus, producing eight fierce-looking differential equations—one of the most important series of equations in modern science. (Every physicist and engineer in the world has to sweat over them when mastering electromagnetism in graduate school.)
Next, Maxwell asked himself a fateful question: if changing magnetic fields create electric fields and vice versa, what happens if those fields are constantly generating each other in a never-ending pattern? Maxwell found that electric-magnetic fields behave much like ocean waves—for example, in the way they undulate through space. He calculated the speed of the waves and to his astonishment, found it to be the speed of light! Upon discovering this fact in 1864, he wrote prophetically: “This velocity is so nearly that of light that it seems we have strong reason to conclude that light itself . . . is an electromagnetic disturbance.”
It was perhaps one of the greatest discoveries in human history. For the first time the secret of light was revealed. Maxwell suddenly realized that the brilliance of the sunrise, the blaze of the setting sun, the dazzling colors of the rainbow, and the stars in the firmament could all be explained in terms of waves. Today we realize that the entire electromagnetic spectrum—from radio waves, including broadcast frequencies and radar, through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays—can all be described by Maxwell’s wave theory of light.
Maxwell’s theory of light, paired with the insight that all matter is made up of atoms, provides simple explanations for the phenomena of optics and lays the foundation for invisibility. Most solids, for example, are opaque because light rays, traveling as waves, cannot pass through the dense matrix of atoms. Many liquids and gases, by contrast, are transparent because the wavelengths of visible light can pass more readily through the larger spaces between their loosely arranged atoms. Diamonds and other crystals are something of an exception: they are both solid and transparent. That’s because the atoms of a crystal, while tightly packed, are arrayed in a precise lattice structure that offers many straight pathways for a light beam to take.
[pagebreak]Invisibility is a property that would clearly have to arise at the atomic level, via Maxwell’s equations, and hence would be exceedingly difficult, if not impossible, to duplicate using ordinary means. To make a solid boy like Harry Potter invisible, you would have to liquefy him by boiling, crystallize him, heat him again, and then cool him, all of which would be quite an accomplishment, even for a wizard.
[media:node/934 horizontal caption medium left] Of course, short of changing someone or something’s atomic structure, there are other optical options. At the heart of current invisibility research is the manipulation of something called the “index of refraction.” When you put your hand in water, or look through the lenses of your glasses, you’ll notice that water and glass distort and bend the path of ordinary light—that’s refraction. Light slows down when it enters a dense, transparent medium, but in a pure vacuum the speed of light always remains the same. The index of refraction of any given material is obtained by dividing the speed of light by the slower speed of light inside the medium. Since the speed of light divided by itself equals 1, a natural material’s index of refraction is always greater than 1. It is also ordinarily a constant: a beam of light bends at a particular angle when it enters a given substance, such as glass, and then keeps going in a straight line [see illustration left].
But what if you could control the index of refraction at will, so that, for instance, it changed continuously from point to point in the glass? If a beam of light could create its own path—slithering around an object’s atoms like a snake—and exit the material along the same line it entered, then the object could be invisible. To do this, however, the object would need to bend light in unorthodox ways, which would require using a medium with a negative index of refraction, and that’s exactly what every optics textbook for decades said was impossible. Yet in 2006, researchers at Duke University’s Pratt School of Engineering in Durham, North Carolina, and at Imperial College London successfully defied conventional wisdom and made objects that were “invisible” to microwave radiation—by manipulating refraction.
The scientists designed a kind of metamaterial: a substance that has optical properties not found in nature. Nathan Myhrvold, former chief technology officer at Microsoft, says metamaterials “will completely change the way we approach optics and nearly every aspect of electronics. . . . [They] can perform feats that would have seemed miraculous a few decades ago.”
The metamaterial, made by the Duke University engineers, relied upon tiny implants that forced electromagnetic waves to bend in unorthodox ways. Tiny electric circuits were embedded in a concentric series of bands arranged rather like the coils of an electric oven. Those implants—a mixture of ceramic, Teflon, fiber composites, and metal—made it possible to channel the path of microwave radiation in a specific way around the device. A small copper cylinder inside the device was undetectable to microwave radiation. But it did cast a minuscule shadow. True invisibility requires eliminating all reflections and shadows.
To visualize how the radiation bends around the device, think about the way a river flows around a boulder. Because the water quickly wraps around the boulder, the rock is undetectable downstream. Similarly, metamaterials can continuously alter and bend the path of microwaves so that they flow around a cylinder, for example, essentially making everything inside the cylinder invisible to microwaves [see illustration below].
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In general, the internal structures implanted inside such a metamaterial must be smaller than the wavelength of the radiation to be redirected. For example, for a metamaterial to bend the path of microwaves with a wavelength of three centimeters, it must have tiny implants embedded in it that are smaller than three centimeters. But to make an object invisible to green light, with a wavelength of 500 nanometers (nm), the metamaterial must have structures embedded within it that are only about 50 nm long. (One nanometer is a billionth of a meter in length. Approximately five atoms fit within a single nanometer.) Building such atomic-scale structures requires advanced nanotechnology at the very limits of modern engineering. Pushing those limits is perhaps the key problem engineers face in their attempts to create a true invisibility cloak.
The rac is on—just as it is in the computer-chip industry—to use nanotechnology to achieve better performance at smaller scales. Ever since the announcement that metamaterials have been fabricated in the laboratory, there has been a stampede of activity in this area, with new insights and startling breakthroughs coming every few months. And the goal is to create metamaterials that can bend visible light, not just microwaves.
Advances made in the semiconductor industry have spurred the creation of new metamaterials. A technique called “optical lithography” enables engineers to place hundreds of millions of tiny transistors onto a silicon wafer no bigger than your thumbnail. Scientists use ultraviolet radiation to “etch” tinier and tinier components onto a silicon chip. At present, the smallest components that one can create with this etching process are about 30 nm (or about 150 atoms) across. With the etching technology, a group of scientists created the first metamaterial that operates in the visible range of light: scientists in Germany and at the U.S. Department of Energy (DOE) announced in late 2006 that, for the first time in history, they had fabricated a metamaterial that worked for red light. How did they achieve the “impossible”? Physicist Costas M. Soukoulis of the DOE’s Ames Laboratory in Iowa joined forces with Gunnar Dolling and Martin Wegener of the University of Karlsruhe and Stefan Linden of the Karlsruhe Research Institute in Germany. They started with a glass sheet, and then deposited a thin coating of silver, followed by magnesium fluoride and another layer of silver, forming a fluoride “sandwich” that was only 100 nm thick. Next, using standard etching techniques, they made a large array of microscopic square holes, 100 nm wide, in the sandwich to create a grid pattern resembling a fishnet. Then they passed a red light beam with a wavelength of 780 nm through the material and measured its refractive index, which was -0.6. (Previously, the smallest wavelength of radiation bent by a metamaterial was 1,400 nm, which put it outside the visible light spectrum, in the range of infrared.)
Physicists foresee many applications of this invisibility technology. Metamaterials “may one day lead to the development of a type of flat superlens that operates in the visible spectrum,” says Soukoulis. “Such a lens would offer superior resolution over conventional technology, capturing details much smaller than one wavelength of light.” Breakthrough applications of such a “superlens” would include the photographing of microscopic objects, such as the inside of a living human cell, with unparalleled clarity; it could make possible the early diagnosis of diseases in a developing baby inside the womb. Ideally, one would be able to take photographs of a DNA molecule without having to use clumsy X-ray crystallography.
So far, Soukoulis’s group has only been able to manipulate red light. The next step would be to create a metamaterial that would bend red light entirely around an object, rendering it invisible to that light.
In the running for future developments, another area of invisibility research shows promise: light transistors. The goal of “photonic crystal” technology is to create a chip that uses light, rather than electricity, to process information. That entails using nanotechnology to etch and mold tiny components on a wafer, such that the index of refraction changes with each component.
Light transistors have several advantages over those using electricity. For example, there is much less heat loss in photonic crystals. (Advanced silicon chips generate enough heat to fry an egg. Thus chips must be continually cooled down or they will fail, and keeping them cool is very costly.) Not surprisingly, the science of photonic crystals is ideally suited for metamaterials, since both technologies involve manipulating the index of refraction of light at the nanoscale.
Not to be outdone, yet another group announced in mid-2007 that they created a metamaterial that bends visible light using an entirely different technology, called “plasmonics.” Physicists Henri J. Lezec, Jennifer A. Dionne, and Harry A. Atwater at the California Institute of Technology had developed a metamaterial that had a negative index for the more difficult blue-green region of the visible spectrum of light. Unlike photonic crystals, which use light beams trapped inside crystals, plasmonics uses electrons synchronized with a light beam. (Electrons also behave like waves, as photons do.)
The goal of plasmonics is to “squeeze” light’s rapid information-carrying capacity into nanoscale spaces, especially on the surface of metals. The reason metals conduct electricity is that electrons are loosely bound to metal atoms, so they can move freely along the surface of the metal. The electricity flowing in the wires in your home is nothing more than loosely bound electrons flowing smoothly on the wires’ metal surface. But under certain conditions, a light beam colliding with the metal surface can create wavelike motions of the electrons on that surface, called plasmons. The wavelike motions of the plasmons vibrate in unison with the original light beam. More important, one can “squeeze” the plasmons so that while they still have the same frequency as the original beam (and hence carry the same amount of information), they have a much smaller wavelength. In principle, one might then cram the squeezed waves onto nanowires. As with photonic crystals, the ultimate goal of plasmonics is to create computer chips that compute using light and electricity, rather than electricity alone.
The Caltech group built their metamaterial out of two layers of silver, with a 500 nm silicon-nitrogen insulator in between, interrupted by a nanoscale prism of gold and silver layers. The prism was separated by only 50 nm of silicon nitride, which acted as a “waveguide” that could redirect the plasmonic waves. Laser light enters and exits the team’s cloaking apparatus via two slits carved into the metamaterial. By analyzing the angles at which the laser light is bent as it passes through the metamaterial, one can verify that the light is being bent with a negative index.
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Thanks to the intense interest in creating light transistors, progress in metamaterials will only continue to accelerate. Research in invisibility can therefore “piggyback” on the ongoing research in photonic crystals and plasmonics. Already hundreds of millions of dollars are being invested in the two fields with the goal of creating smaller, faster, and cooler replacements for silicon chip technology, and newer and better metamaterials will inevitably be spun off.
With breakthroughs occurring in this field every few months, it’s not surprising that some physicists see a practical invisibility shield of some kind emerging from the laboratory in the not so distant future. Scientists are confident, for example, that in the next few years they will be able to create metamaterials that can render an object totally invisible for one frequency of visible light, at least along a two dimensional plane. To do this would require embedding tiny nano implants not in regular arrays, but in sophisticated patterns, so that light would bend smoothly around an object.
The next challenge will be to create metamaterials that can bend light in three dimensions, not just on flat two-dimensional surfaces. Photolithography has been perfected for making flat silicon wafers, but creating three-dimensional metamaterials will require stacking wafers in a complex fashion.
After that, scientists will face the problem of creating metamaterials that can bend not just one frequency of light, but many. That will be perhaps the most difficult task, since the tiny implants that have been devised so far bend light of only one precise frequency. Scientists may have to create metamaterials in layers, with each layer bending a specific frequency. The solution to this problem is not clear.
When will an invisibility cloak be ready for the market? Sci-fi fans will have to wait. And even then, the first one may be a clunky device. Harry Potter’s cloak was made of thin, flexible cloth that rendered anyone draped in it invisible. But for that to be possible, the index of refraction of the cloth would have to be constantly changing in complex ways as it fluttered, which is impractical. More than likely, the first true invisibility cloak would be made of a solid cylinder of metamaterials. That way the index of refraction could be fixed. More advanced versions could eventually incorporate metamaterials that are flexible and can twist and still make light flow within them on the correct path. In this way, anyone inside the cloak would have some freedom of movement.
And, as some die-hards have pointed out, there is an inherent flaw in the invisibility shield: anyone inside would not be able to look out without becoming visible. Imagine Harry Potter being totally invisible except for his eyes, which appear to be floating in midair. Any eyeholes in the cloak would be clearly visible from the outside. To be totally invisible, Harry Potter would have to be sitting blindly beneath his cloak. (One possible solution to the problem might be to insert two tiny angled glass plates near the location of the eyeholes. The glass plates would split off a tiny portion of the light hitting the plates, and direct that light into the eyes. So most of the light hitting the cloak would still flow around it, rendering the person invisible; only a tiny amount would be diverted to allow the person to see.) As daunting as those difficulties are, scientists and engineers are optimistic that a real-life invisibility shield of some sort, worthy or not of the Klingons, can be built within two or three decades. In the meantime, there is still a lot of imagining that can be done, from the pages of a book to the intricate, ever-smaller designs of exotic materials.
This article was adapted from Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel, by Michio Kaku, © 2008. Reprinted with permission of Doubleday, a division of Random House, Inc. All rights reserved. Click here for ordering information.