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Despite the Harry Potter headlines they've inspired, efforts over the last few years to create a real-life invisibility cloak have been simple proofs of concept. They're lab experiments, and not the kind of thing that could mask a troublesome young wizard lurking in a library, much less a soldier on the battlefield. But in those early successes, some scientists see the potential for active cloaking technology that could hide objects in plain sight.

According to University of Texas researcher Andrea Alù, a battery-powered (or active) cloak composed of metamaterials might prevent a broadband radio signal from scattering off a target, an important step towards making objects undetectable to the human eye.

Metamaterials are man-made materials that can manipulate the electromagnetic spectrum in weird ways. The science of building them and tapping their emergent properties is still in its infancy. Alù and his team, for example, are working only with nonvisible wavelengths. But even at this early stage, metamaterials lend themselves to interesting military applications, such as better anti-radar stealth and antennas that don't interfere with each other.

"Our group was the first to theoretically show that metamaterials could provide a realistic route towards invisibility and cloaking," he says. "We are now working to experimentally prove broadband cloaks"—those with coverage for a wide spread of frequencies—"for radio waves and cloaked antennas. These possibilities are in reach within the coming months."

Negative Refraction

The idea behind metamaterials began with Russian physicist Victor Veselago. In 1967, Veselago noted that natural objects bend light in a particular direction. You see this positive refractive index when looking at a "crooked" pen through a glass of water. He then imagined a lens with a negative refractive index. In such a case, the light would bend in the opposite direction—the semi-submerged pen would look V-shaped.

That's the key to the invisibility potential of metamaterials. "We could use a negative refractive index metamaterial in combination with a normal material to bend light all the way around an object," says Mark Swihart, a University of Buffalo professor working with the nanoparticles that comprise metamaterials.

The reality in the lab, however, has been far more modest. In 2006, Duke University physicist David Smith managed to bend microwaves around arrays of specially designed copper rings. A year later, a University of Maryland team used concentric gold rings coated in transparent polymethyl methacrylate to bend light. But this was polarized cyan light injected onto the surface by an optical fiber—it wasn't the full visible spectrum. And both experiments were conducted in 2D. (Smith says he is now working on a more challenging 3D cloak.) So far, Swihart says, the metamaterials that work for visible wavelengths of light do so "in extremely thin films over small areas." The University of Maryland's cloak, for example, was thinner than a human hair.

For Alù, that's never going to be good enough. His team just published a paper in Physical Review X this past fall arguing that "true invisibility cannot be obtained with a passive cloak, whatever technique one uses, if not over a narrow range of wavelengths."

The problem with passive, or nonpowered, metamaterials is that they scatter light when trying to cover a broad spectrum. As a result, the cloak backfires. "When viewed at certain frequencies, passively cloaked objects may indeed become transparent. But if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without," Alù says.

Another issue, Smith says, is that materials used for passive cloaks attenuate, or reduce, a wave as it passes through them. "It's a pretty severe problem. If you tried to scale up and build a big cloak, and especially if that were tried in the optical wavelength it would look like something was absorbing your energy, so you would just see a big shadow."

Engage Cloak

The way forward is active-cloak, Alù says. His own approach combines a thin, flexible, conducting-metamaterial coating with a battery, circuitry, and an amplifier. When excited by an electromagnetic wave, the electrical current in the cloak radiates a wave opposite to the one scattering off the object, canceling it out. This extends the cloak's frequency coverage. Indeed, active cloaks are now a hot topic of research for this reason, Smith says, noting the parallel work of another scientist, George Eleftheriades at the University of Toronto.

As promising as his research is, even Alù is not working with visible light. But the University of Texas, whose research is partially funded by the U.S. Air Force Office of Scientific Research and the Defense Threat Reduction Agency, sees other military applications for metamaterials.

Consider stealth aircraft, which are hard to detect by radar largely because of their shape, which scatters the signal away from the radar receiver. However, this sort of evasion works only if the radar transmitter and receiver are monostatic, or located in the same spot. By contrast, receive-only passive radar is bistatic and uses other techniques, such as exploiting signals of opportunity, such as radio and TV, to beat stealth.

That's where Alù's cloaking technology comes into the picture. He says, "[O]ur cloaking technology is able to beat bistatic radars, since it cancels the scattering in all directions." Simply, he says, there are no reflected or scattered signals, not even "shadows," which can be used to locate the position of the stealth aircraft.

He now plans to tackle another problem for the military: "signal fratricide," or when friendly forces jam their own communications. This can happen when too many antennas are packed together on one vehicle. "Cloaked antennas can be made invisible to each other," he says, "even when they are placed very close to each other, eliminating unwanted interference and improving radio communications."

This may not be as mind-blowing as creating an invisible tank, but it does signify the gradual conquest of the electromagnetic spectrum.

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