Scientists Seek to Hide Combat Forces
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This illustration shows the structure of a new device created by Purdue University researchers to overcome a fundamental obstacle in using new metamaterials for radical advances in optical technologies, including a possible invisibility cloak. The material developed by the researchers is a perforated, fishnet-like film made of repeating layers of silver and aluminum oxide. (Birck Nanotechnology Center, Purdue University) |
When scientists at the U.S. Army Research Office set out in 2003 to build a light-controlling synthetic material, they had no idea what the result would be—but they knew it would be big. A few years later, the research led them to ask if an invisibility cloak would be possible, and with each passing year, they get a little closer to making that science fiction fantasy a reality.
Theoretically, a cloaking material will bend light in such a way that soldiers on the battlefield will be rendered invisible to enemy eyes. Not only that, but the same technology could lead to other, more near-term innovations, such as a superpowerful microscopic lens capable of identifying biological agents on the battlefield, a solar energy collector so effective it has been dubbed a “black hole” for light, and vastly improved information processing technologies for computers.
“In the Army Research Office, we try to fund the most fundamental research—when you don’t always know what you’re going to get. We may have a hope or a goal or a desire, but we just do the research and see what happens,” says Dr. Richard Hammond, a theoretical physicist who leads the invisibility effort for the Army Research Office.
Research into invisibility centers on the development of metamaterials—artificial, manmade materials designed to provide properties not found in nature. For example, scientists can use nano-sized metal rods implanted into silicon and arranged in such a way as to manipulate electromagnetic waves, including light waves in the visible spectrum, and microwave or infrared signals, which are not visible to the human eye. All natural materials—water, glass, air—have a refractive index greater than one. Metamaterials with a negative refractive index—known as negative-index materials—have opened up a whole new world of physics research.
“We realized that if we can make negative-index materials, we can make light behave in almost any way we want. Now, we can do all kinds of things we couldn’t do before,”
To cloak objects within the visible spectrum, a device must meet two requirements—it cannot allow light to reflect off of the object, and it must cause light to bend around the object to prevent the appearance of a dark, shadowlike shape. Imagine someone being surrounded by a cylindrical object made of a metamaterial,
Through two multidisciplinary university research initiatives, the Army Research Office is working with scientists on two teams, which are led by
Duke researchers made a major breakthrough in 2006 when they revealed a small-scale prototype of a cloaking device that shielded the object within from microwaves. The device deflects microwave beams so that they flow around the hidden object, similar to water flowing around a rock, making it appear as if the object does not exist at all. To accomplish this feat, the scientists used standard circuit board technologies—copper rings and wires patterned onto sheets of fiberglass composite. The device is less than five inches across. To make work easier, it is designed to provide invisibility in two dimensions rather than three. It includes strips of metamaterial fashioned into concentric, two-dimensional rings and is used with a narrow beam of microwave radiation. The precise variations in the shape of copper elements patterned onto their surfaces determine their electromagnetic properties.
To assess the cloak’s performance, researchers situated it between two metal plates inside a test chamber and fired a microwave beam at it. They used a specially designed detecting apparatus to measure the electromagnetic fields inside and outside the cloak, and using an animated representation of the data, they found that the wavefronts of the beam separate and flow around the center of the cloak.
Although hailed as a major breakthrough, the accomplishment by Duke’s researchers was easier than creating a cloaking device that works in the visible spectrum, partly because microwaves are longer than light waves and also because a visible-spectrum cloak will have to work simultaneously with every color of light wave—red, orange, yellow, green, blue, indigo and violet, a task that will be much harder to accomplish.
“In the lab, Duke researchers showed that the microwaves hit the cylinder, go around it like they’re supposed to, and go out the other side. That was another kick start in this field to show that it looks like you can really make things invisible,”
Experts agree that making an object invisible to light is the biggest challenge, while shielding something from radio waves or sound waves should be much easier.
“I think we may be just a few years away from lab demonstrations of materials that can hide objects decently well from radio waves. It is important to note that these invisibility materials are thick and rigid shells, so they are definitely not suitable for all applications,” says Steven Cummer, associate professor of electrical and computer engineering at Duke’s Pratt School of Engineering. “I think we might be even closer to demonstrations of making objects invisible to sound waves. This builds on the same basic mathematical idea and uses composite acoustic materials to manipulate sound waves instead of electromagnetic waves. And again, we are working towards making thick and rigid shells that are not suitable for all applications. For example, something not moving would be much easier to apply this to.”
Scientists at Purdue recently made progress in cloaking objects in the visible spectrum. One significant problem has been the loss of too much light, which is absorbed by the metamaterials. In August, Purdue researchers announced that they used a fishnet-like film made of repeating layers of silver and aluminum oxide, which not only prevented the absorption of light, but also amplified the light.
The accomplishment is important in part because the loss of light provided ammunition for skeptics, according to Vladimir M. Shalaev, professor of electrical computer engineering at Purdue. “The first objective was to demonstrate that you can have metamaterials with no losses, and that is what we just demonstrated,” Shalaev explains. “Cloaking or any other applications, such as an optical black hole, will require really advanced metamaterials with an absence of losses. It’s not like we completely solved it, but we proved it is feasible.”
The material works only with one color of light at a time. The fact that different colors of light travel different paths is known as the bandwidth—or the dispersion—problem. Scientists are not yet sure how to tackle it, but they are generating ideas.
“One of the big problems, at this point, is how to make the actual material. It’s not like we can grab a blanket and become invisible, but maybe decades from now we’ll have that technology. I think we have a lot of the theory worked out,”
Even if scientists are able to create a cloaking device in the decades to come, the technology is likely to have some weakness. “The thing about being invisible is that no one can see you, but you can’t see anybody else, either. No light is reaching you, so you’re blind as long as you’re invisible,”
But the research promises more than just invisibility. Scientists already have developed and demonstrated a lens far more powerful than conventional lenses.
In addition, researchers theorize they might one day be able to build a black hole for light—a solar energy collector on steroids. Conventional solar collectors use a mechanical design that allows them to adjust to the sun as it rises and sets, but a solar collector made of a metamaterial will effectively and efficiently absorb sunlight no matter the sun’s position. And it potentially could be made small enough so that soldiers can carry it along and collect enough solar energy for recharging batteries and powering the myriad electronic devices used on the modern battlefield.
The research also may lead to new and improved integrated circuitry for computers and other electronic devices, experts say. The size of circuitry is currently limited by the wavelength of light because anything smaller cannot be controlled adequately. But with metamaterials, electronic circuit boards might be reduced in size by a factor of 10,
Despite the progress, scientists find themselves in the same place they started. They still do not know what benefits will arise from the research, but they express confidence that it will be something big.
“Since the technology allows us to do things differently—better, quicker, smaller—lots of electronics and photonics people are looking into new kinds of components, new kinds of transistors, new kinds of lasers. So, it’s the myriad smaller things that will come out of the larger things we’re looking at. We know it’s going to open a lot of doors. We don’t know exactly what yet, but we know it’s going to be great,”
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