DISAPPEARING ACT: A new approach to cloaking does away with complex engineered structures. The concept even works for visible light, provided in this diagram by a horizontal laser. Image: COURTESY OF V. SHALAEV
In recent years, optics researchers have come up with numerous concepts for invisibility cloaks—camouflaging that would effectively reroute light so as to conceal an object within. Most of those approaches have relied on so-called metamaterials, carefully engineered structures designed to have bizarre optical properties. But a team of researchers has now designed a much simpler cloaking apparatus that does away with the need for metamaterials entirely. Even if a Harry Potter–style invisibility cloak never materializes, the simplification of such extreme light manipulation may have more immediate applications in communications and computing.
In a paper published online this week in Physical Review Letters, researchers from BAE Systems in Washington, D.C., Towson University and Purdue University describe an experimental setup using two gold-coated surfaces, one curved and one flat, to conceal an area in between. In addition to the simplicity of the design [pictured at left], the researchers say that the reflective surfaces, which form what is known as a tapered waveguide, circumvent one of the main pitfalls of metamaterials, which is the high absorption rate of light passing through them. The waveguide also appears to work for multiple wavelengths of visible light, something metamaterials have struggled to do.
Study co-author Vladimir Shalaev, a professor of electrical and computer engineering at Purdue, was part of a group that in 2007 designed a visible-light cloak using metamaterials. But that cloak worked only at a predefined wavelength of light and concealed a very small area, calling attention to the shortcomings of metamaterial approaches. "From the very beginning we realized the huge challenge of doing this, and although we tried and keep trying to do this, the challenge is enormously difficult," Shalaev says. "It's not fundamentally impossible, but it's really, really hard."
Looking for a simpler way, the researchers realized that they could custom-design waveguides to mimic the properties of metamaterials, specifically the gradient of the material's refractive index, a measure of the speed at which light passes through. The result, Shalaev says, allows light to bend around the central cloaked area "like water flows around a stone."
Other pioneers in the field say the research provides an elegant and novel approach to a thorny problem. John Pendry, a physicist at Imperial College London, says that the tapered waveguide is "a very clever idea." Physicist Ulf Leonhardt of the University of Saint Andrews in Scotland agrees, calling the paper "a brilliant piece of work, a wonderfully simple idea." One caveat that both researchers point out is that the new approach conceals a two-dimensional area rather than a three-dimensional one. "The things you might want to cloak are probably not confined to two dimensions," Pendry says. To achieve the far-fetched dream of cloaking a human being, for instance, a three-dimensional space would have to be concealed.
But Shalaev and Pendry note that the ultimate application of the technology may not involve invisibility at all but could instead facilitate more powerful optical communication links and circuits. "The cloaking concept is a demonstrator that shows the interested person how powerful these new technologies are that we're developing," Pendry says. He adds that Shalaev's team and others in the field "can cloak things because they have great power in controlling and guiding radiation, but the real applications may not be in cloaking at all. They may be in pushing signals around some circuit which uses light instead of electrons, for example."