Physicist Che Ting Chan, of the Hong Kong University of Science and Technology, told New Scientist:

Invisibility is just an illusion of free space, of air. We are extending that concept. We can make it look like not just air but anything we want.

With his equations, he can make visibility an illusion as well.

The device — which exists only as a design so far — would use existing metamaterial technology . It would create a series of filters that first render one object invisible and then another one visible. Continues New Scientist:

To make a cup look like a spoon, for example, light first strikes the cup and is distorted. It then passes through a complementary metamaterial which cancels out the distortions to make the cup seem invisible. The light then moves into a region of the metamaterial that creates a distortion as if a spoon were present. The result is that an observer looking at the cup through the metamaterial would see a spoon.

Although the technology as envisioned is active — requiring a knowing observer to watch the cup through the metamaterial to see the spoon, it could undoubtedly be made passive. The only current technological roadblock is the need for the metamaterial components to be smaller than a micrometer, as that's the wavelength of visible light. Scientists like John Pendry, who first came up with the theory behind invisibility cloaking, thinks that building it is well within the realm of human capacity.

Modified Invisibility Cloak Could Make The Ultimate Illusion [New Scientist]

[Image via NASA]

These photos come from Palmen’s series, “Camouflage” and “Surveillance Camera Project,” depicting people who have gone to extremes to hide themselves from sight. Her subjects, who are visible in the photos, are more like chameleons than completely invisible, but it is easy to imagine them as prototypical users of cloaking technology.

[Desiree Palmen via Environmental Graffiti]

You may remember the metamaterials that will make you invisible or the metamaterials that can act as cloaking devices; both of those findings are part of a great body of research on creating substances that transform the behavior of light. Citing Arthur C. Clarke, who said that "any sufficiently advanced technology is indistinguishable from magic," the New Journal of Physics explains the theory and the practice behind what they call "pure and applied magic":

Transformation optics gathers an unusual mix of scientists, ranging from practically-minded engineers to imaginative theoretical physicists and mathematicians or hybrids of all three. The engineers have been developing new materials with extraordinary electromagnetic properties, from materials for microwaves, to be used in radar or wireless technology, to materials for terahertz radiation and visible light. These materials typically are composites—they consist of artificial structures much smaller than the wavelength that act like man-made atoms, apart being much larger in size. The properties of these artificial atoms depend on their shapes and sizes and so they are tunable, in contrast to most real atoms or molecules. This degree of control is what makes these materials—called metamaterials—so interesting. Such new-won freedom invites the other side of the spectrum of scientists, the theorists, to dream. Just imagine there are no practical limits on electromagnetic materials—what could we do with them? One exciting application of metamaterials has been Veselago's idea of negative refraction, dating back to the 1960s. Metamaterials have breathed life into Veselago's idea, culminating in recent optical demonstrations. Another application is cloaking, developing ideas and first experimental demonstrations for invisibility devices. It turns out that both negative refraction and cloaking are examples where materials seem to transform the geometry of space.

Scientists Tomáš Tyc and Ulf Leonhardt — of Masaryk University, the University of St. Andrews, and the National University of Singapore — recognized the difficulty of constructing an actual device that contained an optical singularity. Achieving a refractive index of zero or infinity is possible in theoretical thought experiments, but understanding the visual reality of such a singularity is far more difficult. They found a way to mathematically transmute the singularity equations to make them more practical for actual optical devices; soon, it might be possible to understand and use the effects of these unconventional refractive indices.

In the same issue, Sergei A. Tretyakov, Igor S. Nefedov, and Pekka Alitalo of the Helsinki University of Technology introduce their field-transforming metamaterials — substances that can transform any electromagnetic fields surrounding them in a specified way. This has applications not only for invisibility cloaks, they wrote, but also for the creation of perfect lenses or artificial black holes. Yup, you read that right. This issue is full of research on the engineered direction of electromagnetic waves (i.e. not just light); when it comes to future possibilities for applying this research, all of the authors seem to be saying "you name it!"

We really aren't limited to invisibility capes, or even Create-Your-Own-Black-Hole (just add water). J.B. Pendry and Jensen Li, working at Imperial College of London and the University of California in Berkeley, applied metamaterial research to acoustics. Their contribution to the New Journal of Physics focuses on fashioning what they call a "broadband acoustic cloak" — they claim that with the right materials, sound waves "can be controlled and directed almost at will." And so visual invisibility meets acoustic invisibility: Once these scientists get it all right, they might just throw in the towel and become superheroes before they ever get around to explaining their research to us.

Cat's eye image from Wikimedia Commons.

Using invisibility to increase visibility [via EurekAlert]
Focus on cloaking and transformation optics [New Journal of Physics]

Controlling the way light rays bounce off of and move through objects is no easy feat, but that's exactly what Berkeley's metamaterials do. All naturally-occurring materials have a positive refractive index. As light waves travel from one medium to another, the difference in the refractive index between the two will cause the light wave to bend at a certain angle. Consider what happens when you stick a straw into a glass of water — the straw appears to bend or break as it enters the water. What you're seeing is the way light bends as it moves from the air (which has a refractive index of about 1) and water (which has a refractive index of about 1.33). The light is still propagating forward, but it's made a slight turn, and so your eyes see a bendy straw.

In the case of negative refraction, the light waves behave much more oddly, as you can see in the above image by UC Berkeley's Jason Valentine and Robert Lee. Valentine explained to me that in negative refraction, a light ray no longer appears to be propagating forward — when it bends, it bends backward. The energy flow of the wave still moves in its forward direction, but the electric and magnetic components of the light ray seem to be traveling in reverse. They've turned far more drastically than they would in the natural phenomenon of positive refraction. So instead of seeing a bendy straw, once the metamaterial is combined with other light-bending tech, you'd see a straw that seemed to disappear.

In order to manipulate light at this level, you have to manipulate the structure of the material it's hitting at an extremely small scale. That's where metamaterials come in. Metamaterials negatively refract waves of visible light because they're woven out of materials smaller than the wavelengths of that light. If you think of a metamaterial as a piece of cloth, the "threads" in that make it up are somewhere between 400 and 700 nanometers in size. As fabrication techniques for such metamaterials have grown more and more advanced, this nanoscale structural manipulation has become possible, and UC Berkeley's team has used it to full advantage.

According to a release about Valentine's study:

"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," said Xiang Zhang, professor at UC Berkeley's Nanoscale Science and Engineering Center, funded by the National Science Foundation (NSF), and head of the research teams that developed the two new metamaterials. "Both bring us a major step closer to the development of practical applications for metamaterials."

A paper in the August 13 issue of Nature, co-authored by Valentine, Shuang Zhang, and Thomas Zentgraf (all members of Xiang Zhang's lab), explores one of these approaches. Valentine, Zhang, and Zentgraf layered conducting silver and non-conducting magnesium fluoride. Then, they cut tiny "fishnet" patterns into the material. The result is a metamaterial, pictured at the top of this page, that is capable of achieving a negative index of refraction at wavelengths as small as 1500 nanometers.

The second approach, detailed in the August 15 issue of Science, appears in a paper co-authored by Jie Yao, Zhaowei Liu, and Yongmin Liu (also all members of Zhang's lab). What these researchers did was grow silver nanowires inside aluminum oxide, to create a bulk metamaterial that is more than 10 times larger than the wavelength of visible light. The structure of this metamaterial, however, is still on a nanoscale. Though the Science metamaterial doesn't technically have a negative index of refraction, the geometry of its structural components interacts with light in a way that still achieves the backward-bending phenomenon of negative refraction. And it does this with light rays that have wavelengths as short as 660 nanometers.

The media is aflutter with ideas for possible applications of these new metamaterials, and they run the gamut from the visualization of individual molecules of DNA to the production of Harry Potter's invisibility cloak. Valentine cautions that cloaking devices are still in the future — in order to make things truly invisible, one would need to cover them with a large sheet of a metamaterial like these, and that's a fabrication challenge. In addition, though researchers have made a breakthrough in the way manufactured materials can control the bending of light rays, actual invisibility demands that each of the light waves around a given object are deflected in a certain way, creating a specific pattern of refraction that will hide that object.

Still, these metamaterials are making my heart beat faster. It's hard to deny the excitement that comes with knowing we can build substances that move light in ways no existing material can — that as far as refraction goes, we've got a one-up on nature.

Invisibility shields one step closer with new metamaterials that bend light backwards [UC Berkeley]

According to an article in the most recent issue of Physics World:

The "stealthiest" ship that currently exists is Sweden's Visby Corvette. Apart from being painted in grey dazzle camouflage and made of low-radar reflectivity materials, it also does not use propellers, which are the noisiest part of a ship. The vessel also has the lowest "magnetic signature" of any current warship.

But the next generation of warships could be truly invisible by exploiting "metamaterials" - artificially engineered structures first dreamt up by physicist John Pendry at Imperial College, London. Metamaterials are tailored to have specific electromagnetic properties not found in nature. In particular, they can bend light around an object, making it appear to an observer as though the waves have passed through empty space.

About the research, Chris Lavers writes, "If optical and radar metamaterials could be developed, they might provide a way to make a ship invisible to both human observers and radar systems, although the challenges of building a cloak big enough to hide an entire ship are huge."