Metamaterials Breakthrough Brings Invisibility Closer
Negative refraction of visible light is a step toward invisibility
PHOTO: Raymond Patrick/Getty Images
13 August 2008—New optical materials that bend light in unusual ways could lead to much tinier transistors, microscopes that are able to peer at smaller cellular structures, and—with a good deal of engineering—even invisibility cloaks.
Two new types of metamaterials, as the light-bending stuff is known, were developed by researchers at the Nano-Scale Science and Engineering Center at the University of California, Berkeley, and described in separate papers in Nature and Science . One is the first three-dimensional material to have a negative index of refraction, which allows it to bend light in the opposite direction of what you would expect from any other material. The other, also 3-D, doesn’t have a negative index but still provides some negative refraction, and unlike previous metamaterials, it does so in the visible part of the spectrum.
Natural materials have a positive index of refraction, which is a measure of how much they can bend a beam of light passing through them. Stick a pole into a swimming pool, and the portion below the surface appears to jut off at an angle, because of the difference between the indices of refraction of air and water. If the water had a negative index of refraction, the part of the pole below the surface would appear to be above the water.
An index of refraction has an electrical component (permittivity) and a magnetic component (permeability). Building a metamaterial with a structure having features substantially smaller than the wavelengths of light it’s meant to refract causes resonance between the atoms in the material and the photons. This reverses the permittivity and the permeability, making the refractive index negative.
This property of metamaterials opens the door to whole new ways of manipulating light. For instance, in cases when a normal lens cannot resolve anything smaller than half a wavelength of light, metamaterials could make a superlens that could resolve below this so-called diffraction limit. That could open up new possibilities in biomedical imaging, allowing scientists to look at the proteins inside cells. It would also allow the photolithography equipment used to make computer chips to build even smaller features than are currently possible without having to find new sources of smaller wavelengths.
Illustration: UC Berkeley
Fish Out of Water
A fish swimming in water appears slightly closer to the surface than it really is, because of the difference between the refractive index of air and that of water. If the fish were swimming in water with a negative index of refraction it would appear to be swimming above the water.
One of the new metamaterials, described in Nature , consists of 21 alternating 30-nanometer-thick layers of silver and 50-nm-thick layers of magnesium fluoride, a dielectric. Each layer is laid out like a fishnet, with lines 565 nm wide crosshatched by lines 265 nm wide. The material produced a negative index of refraction for infrared light with wavelengths between 1.5 and 1.8 micrometers. That broadband response is more useful than the single frequencies other metamaterials can handle, says Xiang Zhang, the professor at the Berkeley center who directs the research teams and is coauthor of the two papers. More important, the loss of light passing through the material was very low. That’s because the light essentially hopped from one dielectric to the next, with little of the absorbing resonance taking place in the metal. ”It’s like crossing a river,” says Zhang. ”You keep your feet on the stones and you don’t get wet.” The figure of merit used to measure light loss in a structure was 3.5 in this case, a great improvement over previous materials, most of which have been less than 1.0, Zhang says.
The second metamaterial consisted of a thin slice of alumina with pores etched into it and silver nanowires deposited into the pores. The nanowires were 60 nm in diameter and spaced 50 nm apart. Although the permeability didn’t differ from a normal material’s, the permittivity was reversed if the light entered the material at just the right angle. The researchers used the nanowire structure to negatively refract red light. Getting further into the visible or even the ultraviolet part of the spectrum, where wavelengths are even shorter, is an engineering challenge the group is working on, Zhang says.
David Smith, director of the Center for Metamaterials and Integrated Plasmonics at Duke University, in Durham, N.C., called the work ”a very significant milestone.” The fishnet structure, he says, is not yet practical because it has the negative index in only one direction. ”Still, I’m just really knocked out by how clean a result they were able to achieve at optical wavelengths using this structure,” he says. ”The Zhang group experiment should really prove to the skeptics that negative refraction at optical wavelengths is a reality.”
David Schurig, an assistant professor of electrical and computer engineering at North Carolina State University, in Raleigh, says the fishnet material could quickly lead to smaller waveguides and other devices for telecommunications because it’s already operating at telecom wavelengths.
The prospect that really captures the imagination—using these materials to cloak a person or an aircraft in a shield of invisibility—is much farther off, though Schurig says he’s optimistic about it. There’s no physical reason why you shouldn’t be able to direct light around an object and send it on in its original direction, rendering the object invisible. But one big challenge would be overcoming the loss of light as it travels through several centimeters or more of material. ”If you want to cloak something the size of a person, you need a figure of merit of something like 1 million instead of 3.5,” Schurig says.
About the Author
Neil Savage writes from Lowell, Mass., about lasers, LEDs, optoelectronics, and other technology. In the June issue of IEEE Spectrum he summarized the recent breakthroughs in graphene transistors.