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.
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.