New software has enabled metamaterials to work with a broad band of frequencies.
Metamaterials interact with light in ways that appear to violate the laws of physics. They can bend light around an object as if it weren’t there, or narrow the resolution of light microscopes down to a few nanometers. But metamaterials must be painstakingly structured at the nano- and microscales in order to achieve these exotic effects. Now the Duke University researcher who built the first invisibility cloak in 2006 has created software that speeds up the design of metamaterials. He and his colleagues have used the program to build a complex light cloak that’s invisible to a broad band of microwave light–and they did it in only about 10 days.
David R. Smith of Duke and Tai Jun Cui of Southeast University, in Nanjing, China, led the work, which is a landmark in the field of metamaterials. The cloak that the researchers built works with wavelengths of light ranging from about 1 to 18 gigahertz–a swath as broad as the visible spectrum. No one has yet made a cloaking device that works in the visible spectrum, and those metamaterials that have been fabricated tend to work only with narrow bands of light. But a cloak that made an object invisible to light of only one color would not be of much use. Similarly, a cloaking device can’t afford to be lossy: if it lets just a little bit of light reflect off the object it’s supposed to cloak, it’s no longer effective. The cloak that Smith built is very low loss, successfully rerouting almost all the light that hits it.
“Their cloak … answers the naysayers who predicted that cloaks would always be narrowband and lossy,” says John Pendry, chair in theoretical solid-state physics at Imperial College London. Pendry did the theoretical work upon which both the first invisibility cloak and its new successor are based. “Needless to say, I am delighted with this development,” says Pendry. He and his Imperial College colleague Jensen Li proposed a theoretical version of a broadband cloak just last year, and at that time, he says, he “did not expect such rapid experimental progress.”
The broadband cloak is a rectangular structure measuring about 50 by 10 centimeters, with a height of about 1 centimeter. It’s made up of roughly 600 I-shaped copper structures. Making each structure is a simple matter, says Smith. “They’re copper patterns on a circuit board, cut up and arranged. It’s a well-known, inexpensive technology.” The hard part is determining the dimensions of each of these 600 structures and how to arrange them. With the first light cloak, which had only 10 such pieces, “we had to design each element by numerical simulations,” Smith says. Applying the same approach to the more complicated cloak would have eaten up months.
Even for physicists and engineers, the math involved in the theoretical design of cloaking devices is very difficult, says Nicholas Fang, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. The way that a material interacts with light’s magnetic and electric components is taken into account in determining the size, shape, and orientation of each structure in a metamaterial. Pendry and Li’s theoretical work described how to make a broadband cloak by using materials structured so that they have an electrical response to light, but not a magnetic one. But it wasn’t clear how to put this idea into practice. The Southeast University researchers developed new algorithms to greatly speed up the process, says Smith. These algorithms make it possible to quickly predict how a structure with a particular shape will interact with light.
The cloak itself, described this week in Science, is indeed impressive, says Fang, who’s working on metamaterials for super-resolution biological imaging. But what’s more exciting is that the new approach to design will accelerate the development of other metamaterials. Smith says that he and his group have already moved beyond the cloak reported in Science, but because their latest work is unpublished, he can’t specify what they’ve made. “Now [that] this is becoming a more feasible technology,” he says, “we will start to see a lot more of it.”
Other applications of metamaterials, says Smith, include optical devices that take light energy and concentrate it, instead of turning it away–conceptually, the opposite of a cloak. “You could improve solar cells by making structures to increase the field strength of the light,” he says. The new work suggests that this could be done over the whole spectrum of wavelengths found in sunlight. Similarly, broadband “hyperlenses” that gather up light missed by normal lenses could revolutionize biological imaging. Fang and others have developed narrowband hyperlenses with resolutions of only a few nanometers, which make the molecular workings of cells visible. A broadband hyperlens could work with all colors of visible and infrared light.
The ultimate goal, says Pendry, is cloaking in the visible-light spectrum, and Smith’s latest work points the way forward. “There are no insuperable obstacles to making a cloak work at optical frequencies,” Pendry says. “The Duke paper brings this goal a step closer.”