Aishwarya Sharma
Second Year
Mechanical
Invisibility has long been one of the marvels of science fiction and fantasy, from the pages of The Invisible Man, to the magic invisibility cloak of the Harry Potter books, or the ring in The Lord of the Rings. Yet for at least a century, physicists have dismissed the possibility of invisibility cloaks, stating flatly that they are impossible: They violate the laws of optics and do not conform to any of the known properties of matter.But today the impossible may become possible. New advances in "metamaterials" are forcing a major revision of optics textbooks.Working prototypes of such materials have actually been built in the laboratory, sparking intense interest by the media, industry, and the military in making the visible become invisible. It was not until the work of Scottish physicist James Clerk Maxwell,one of the giants of nineteenth-century physics, that physicists had a firm understanding of the laws of optics. Maxwell set out with a clear goal, to express the revolutionary findings of Faraday and his force fields through precise differential equations.
Maxwell began with Faraday's discovery that electric fields could turn into magnetic fields and vice versa. He took Faraday's depictions of force fields and rewrote them in the precise language of differential equations, producing one of the most important series of equations in modern science. They are a series of eight fierce-looking differential
equations. And all of us had to sweat over them during the past few years. During the course of his studies, Maxwell discovered that light waves are nothing but electromagnetic waves. It was a profound discovery. Maxwell's theory of light and the atomic theory give simple explanations for optics and invisibility. In a solid, the atoms are tightly packed, while in a liquid or gas the molecules are spaced much farther apart Most solids are opaque because light rays cannot pass through the dense matrix of atoms in a solid, which act like a brick wall. Many liquids and gases, by contrast, are transparent because light can pass more readily between the large spaces between their atoms, a space that is larger than the wavelength of visible light.
There are some important exceptions to this rule. Many crystals are both solid and transparent But the atoms of a crystal are arranged in a precise lattice structure, stacked in regular rows, with regular spacing between them. Hence there are many pathways that a light beam may take through a crystalline lattice. Therefore, although a crystal is as tightly packed as any solid, light can still work its waythrough the crystal.
Clearly, invisibility is a property that arises at the atomic level, via Maxwell's equations, and hence would be exceedingly difficult, if not impossible, to duplicate using ordinary means. To make Harry Potter invisible, one would have to liquefy him, boil him to create steam, crystallize him, heat him again, and then cool him, all of which would bequite difficult to accomplish, even for a wizard. But perhaps the most promising new development involving invisibility is an exotic new material called a "metamaterial," which may one day render objects truly invisible. Ironically, the creation of metamaterials was once thought to be impossible because they violated the laws of optics. But in 2006 researchers at Duke University in Durham,North Carolina, and Imperial College in London successfully defied conventional wisdom and used metamaterials to make an object invisible to microwave radiation. Although there are still many hurdles to overcome, for the first time in history we now have a blueprint to render ordinary objects invisible.
What are these metamaterials? They are substances that have optical properties not found in nature. Metamaterials are created by embedding tiny implants within a substance that force electromagnetic waves to bend in unorthodox ways. 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. If the metamaterial can eliminate all reflection and shadows, then it can render an object totally invisible to that form of radiation.
Scientists successfully demonstrated this principle with a device made of ten fiberglass rings covered with copper elements. A copper ring inside the device was rendered nearly invisible to microwave radiation, casting only a minuscule shadow. At the heart of metamaterials is their ability to manipulate the refractive index. Usually, the index of refraction is a constant A narrow beam of light is bent when it enters glass and then keeps going in a straight line. But assume for the moment that you could control the index of refraction at will, so that it could change continuously at every point in the glass. As light moved in this new material, light could bend and meander in new directions, creating a path that would wander
If one could control the index of refraction inside a metamaterial so that light passed around an object, then the object would become invisible. To do this, this metamaterial must have a negative index of refraction, which every optics textbook says is impossible.
But Fans of Harry Potter or Star Trek may have to wait. While a true invisibility cloak is possible within the laws of physics, as most physicists will agree, formidable technical hurdles remain before this technology can be extended to work with visible light rather than just microwave radiation.
In general, the internal structures implanted inside the metamaterial must be smaller than the wavelength of the radiation. For example, microwaves can have a wavelength of about 3 centimeters, so for a metamaterial to bend the path of microwaves, it must have tiny implants embedded inside it that are smaller than 3 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 nanometers long-and nanometers are atomic-length scales requiring nanotechnology. This is perhaps the key problem we face in our attempts to create a true invisibility cloak. The individual atoms inside a metamaterial would have to be modified to bend a light beam like a snake.
A milestone in the quest for invisibility came when the silicon wafer etching technology was used by a group of scientists to create the first metamaterial that operates in the visible range of light. Scientists in Germany and at the U.S. Department of Energy announced in early 2007 that, for the first time in history, they had fabricated a metamaterial that worked for red light. The "impossible" had been achieved
in a remarkably short time. Physicist Costas Soukoulis of the Ames Laboratory in Iowa, with Stefan Linden, Martin Wegener, and Gunnar Dolling of the University of Karlsruhe, Germany, were able to create a metamaterial that had an index of -.6 for red light, at a wavelength of 780 nm. The scientists first started with a glass sheet, and then deposited a thin coating of silver, magnesium fluoride, and then another layer of silver, forming a "sandwich" of fluoride that was only 100 nm thick.
Then, using standard etching techniques, they created a large array of microscopic square holes in the sandwich, creating a grid pattern resembling a fishnet (The holes are only 100 nm wide, much smaller than the wavelength of red light) Then they passed a red light beam through the material and measured its index, which was -.6. Their next step would be to use this technology to create a metamaterial that would bend red light entirely around an object, rendering it invisible to that light. yet another group announced in mid-2007 that they have created a metamaterial that bends visible light using an entirely different technology, called "plasmonics." Physicists Henri Lezec, Jennifer Dionne, and Harry Atwater at the California Institute of Technology announced that they had created a metamaterial that had a negative index for the more difficult blue-green region of the visible spectrum of light.
The goal of plasmonics is to "squeeze" light so that one can manipulate objects at the nanoscale, especially on the surface of metals. under certain conditions, when a light beam collides with the metal surface, the electrons can vibrate in unison with the original light
beam, creating wavelike motions of the electrons on the metal surface (called plasmons), and these wavelike motions beat in unison with the original light beam. More important, one can "squeeze" these plasmons so that they have the same frequency as the original beam (and hence carry the same information) but have a much smaller wavelength.
In principle, one might then cram these squeezed waves onto Nanowires.With breakthroughs occurring in this field every few months, it's not surprising that some physicists see some sort of practical invisibility shield emerging out of the laboratory perhaps within a few decades. In the next few years, for example, scientists are confident that they will be able to create metamaterials that can render an object totally invisible for one frequency of visible light, at least in two dimensions.
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 .Next, scientists will have to create metamaterials that can bend light in three dimensions, not just for 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 have to solve the problem of creating metamaterials that can bend not just one frequency but many. This 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 based on layers, with each layerbending a specific frequency. The solution to this problem is not clear.Nevertheless, once an invisibility shield is finally made, it might be a clunky device. Harry Potter's cloak was made of thin, flexible cloth and rendered anyone draped inside invisible. But for this to be possible the index of refraction inside the cloth would have to be constantly changing in complex ways as it fluttered, which is impractical. More than likely a true invisibility "cloak" would have to be made of a solid cylinder of metamaterials, at least initially. That way the index of refraction could be fixed inside the cylinder. (More advanced versions could eventually incorporate metamaterials that are flexible and can twist and still make light flow within the metamaterials on the correct path. In this way, anyone inside the cloak would have some flexibility of movement).
But there is a flaw in this invisibility shield: anyone inside would not be able to look outside without becoming visible. Imagine Harry Potter being totally invisible except for his eyes, which appear to be floating in midair. Any eye holes on the invisibility cloak would be clearly visible from the outside. If Harry Potter were totally invisible, then he would be sitting blindly beneath his invisibility cloak.
Another way to render a person partially invisible is to photograph the scenery behind a person and then project that background image directly onto the person's clothes or onto a screen in front of him. As seen from the front, it appears as if the person has become transparent, that light has somehow passed right through the person's body. Naoki Kawakami, of the Tachi Laboratory at the University of Tokyo, has been hard at work on this process, which is called "optical camouflage." He says, "It would be used to help pilots see through the floor of the cockpit at a runway below, or for drivers trying to see
through a fender to park a car." Kawakami's "cloak" is covered with tiny light-reflective beads that act like a movie screen. A video camera photographs what is behind the cloak. Then this image is fed into a video projector that lights up the front of the cloak, so it appears as if light has passed through the person.
Prototypes of the optical camouflage cloak actually exist in the lab.If you look directly at a person wearing this screenlike cloak, it appears as if the person has disappeared, because all you see is the image behind the person. But if you move your eyes a bit, the image on the cloak does not change, which tells you that it is a fake. A more realistic optical camouflage would need to create the illusion of a 3-D image. For this, one would need holograms. A person could be rendered invisible if thebackground scenery was photographed with a special holographic camera and the holographic image was then projected out through a special holographic screen placed in front of the person. A viewer
standing in front of that person would see the holographic screen, containing the 3-D image of the background scenery, minus the person. It would appear as if the person had disappeared. In that person's place would be a precise 3-D image of the background scenery. Even if you moved your eyes, you would not be able to tell that what you were seeing was fake.
We should also mention that an even more sophisticated way of becoming invisible was mentioned by H. G. Wells in The Invisible Man, and it involved using the power of the fourth dimension. Could we perhaps leave our three-dimensional universe and hover over it from the vantage point of a fourth dimension? Like a three-dimensional butterfly hovering over a two-dimensional sheet of paper, we would be invisible to anyone living in the universe below us. One problem with this idea is that higher dimensions have not yet been proven to exist. Moreover, a hypothetical journey to a higher dimension would require energies far beyond anything attainable with our current technology. As a viable way to achieve invisibility, this method is clearly beyond our knowledge and ability today.
With all the technologies available today, it is clear that invisibility is not a pipedream. And experts in the field estimate that Within the next few decades, or at least within this century, a form of invisibility may become commonplace.
