HIDING objects inside a cloak that channels light around them to make it look as if they aren鈥檛 there may soon be possible thanks to a breakthrough idea by materials scientists. It raises the prospect of invisibility shields that could hide objects sitting right under your nose.
Objects are visible simply because light scatters off their surfaces and into your eyes. So in theory, a cloaking device could work by steering light around an object so that you see only the light from behind it, and not the object itself. Now John Pendry, a theoretical physicist at Imperial College London, and his colleagues have worked out how this could be done with a spherical cloak that channels light around an object hidden at its centre (see Diagram).
The stuff that makes this plausible is a new generation of 鈥渕etamaterials鈥�, which can be tailored to have exotic electrical and magnetic properties not found in nature. The metamaterials developed so far consist of complex arrays of metal washer-like shapes and wires. The metal shapes are smaller than the wavelength of light and so interact with it, explains Pendry. 鈥淥n these scales, it is not the chemical properties of the metal that determine how it interacts with light, it鈥檚 the metal鈥檚 structure.鈥�
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The new idea is to build a sphere of metamaterial whose components are arranged in such a way that they bend radiation around the central cavity before sending it on its way, like a ring road diverting traffic around a town.
Team member David Smith at Duke University in North Carolina has already created a metamaterial that bends microwaves, and is now putting the cloaking idea into practice to make a microwave invisibility shield. 鈥淭he theory tells us the material properties we need at each point,鈥� says Smith. 鈥淭he challenge is to match those theoretical requirements in the actual material, point-by-point.鈥� The team hopes to complete it within a year.
The principle is exactly the same for visible light, but you may have to wait a little while for your invisibility bubble: nobody has yet succeeded in making metamaterials that work at optical wavelengths. However, 鈥渕any teams are already involved in shrinking metamaterials down to these scales,鈥� says Smith.
Another hurdle is that the materials can only steer light in a narrow band of wavelengths, which is fine for microwave radar, for example, but not the entire spectrum of visible light (Science, DOI: 10.1126/science.1125907).
