Great for peering into the girl's locker room..

Saw this on the news today and thought it was fascinating:

Working invisibility cloak created at last
16:17 19 October 2006
NewScientist.com Justin Mullins

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An invisibility cloak that works in the microwave region of the electromagnetic spectrum has been unveiled by researchers in the US. The device is the first practical version of a theoretical set-up first suggested in a paper published earlier in 2006.

The cloak works by steering microwave light around an object, making it appear to an observer as if it were not there at all. Materials that bend light in this way do not exist naturally, so have to be engineered with the necessary optical properties.

Earlier in 2006, John Pendry, a theoretical physicist at Imperial College London, UK, and colleagues showed how such an invisibility cloak could, in theory, be made (see Physicists draw up plans for real 'cloaking device'). Now David Smith and colleagues at Duke University in North Carolina, US, have proved the idea works.

In recent years, materials scientists have made rapid progress in making so-called "metamaterials", which can have exotic electromagnetic properties unseen in nature. These are made up of repeating structures of simple electronic components such as capacitors and inductors.

In 2001, Smith built a metamaterial with a negative refractive index, which bends microwaves in a way impossible for ordinary lenses. Now he has gone one step further.

Easy does it

"It's a real breakthrough," says Ulf Leonhardt, a physicist at the University of St Andrews in Scotland. "Paradoxically, it turns out to be easier to construct an invisibility cloak than it was to make the negative lens."

To simplify the problem, Smith's cloak works in only two dimensions. It is about the size of a movie reel canister and consists of a series of concentric rings, each housing a set of simple electronic components that distort an electromagnetic field as it passes through.

"Rather than the cloak's material properties being the same everywhere, its material properties vary from point to point and vary in a very specific way," says Smith. The combined effect of the rings is to steer microwaves around the central region of the device in which Smith hid a copper ring.

To study the effect of his cloak, Smith took images of microwaves flowing through the rings, like water waves moving across a pond. Without the cloak in place, the microwaves were reflected and diffracted by the copper ring. But with the cloak in place, the distortion was dramatically reduced.

Slight distortion
"It's not perfect," says Leonhardt. "If you could see in the microwave region of the spectrum, the copper ring would not quite disappear. You'd see perhaps a shadow and some slight distortion where the copper ring ought to be."

The device has another important limitation – it works only at a single specific frequency of microwave. "How it might be possible to make a device that works over a range of frequencies is an open problem," says Leonhardt. But Smith now hopes to build a 3D structure that could hide an object completely from view.

So far, the technology works only in the microwave region of the spectrum. The problem with visible light is that it has a much smaller wavelength, meaning an optical metamaterial would have to be built on the nanoscale, which is beyond the limits of current nanotechnology. It, too, would only work at a specific frequency.

"It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device," says Smith.

Journal reference: Science: (DOI: 10.1126/science.1133628)