James Bond may have to up his game. Cloaking materials can now hide tiny microphones placed on a wall – and they will do the job at all visible wavelengths.
Optical cloaks can hide a free-floating object by bending light all the way around it, but only at specific wavelengths. They are usually made of synthetic metamaterials, which have a structure on a scale smaller than the wavelength of light they are meant to deflect.
Bumps or objects on a floor or wall are relatively easy to hide, since merely changing the angle at which light bounces off them can make the surface look flat. Previously, such "carpet cloaking" had only been achieved at infrared and microwave wavelengths.
Now two different cloak designs have managed to conceal bumps over the full visible spectrum. Chris Gladden and Majid Gharghi of the University of California, Berkeley, etched holes into a thin layer of silicon nitride deposited on porous glass. Varying the diameter of the holes between 20 and 65 nanometres – smaller than the wavelengths of visible light – changed the way the layer refracted light, allowing its interaction with the porous glass substrate to cloak a small bump (Nano Letters, DOI: 10.1021/nl201189z).
Earlier this year, Baile Zhang and colleagues at the Singapore-MIT Alliance for Research and Technology achieved a similar effect with polarised light, whose electric field is lined up in one direction. The team aligned calcite crystals, which have refractive properties that depend on the electric field's direction, to hide a 2-millimetre-high bump (Physical Review Letters, DOI: 10.1103/PhysRevLett.106.033901).
Gladden and Gharghi also used polarised light in their test, "but it's not required" for the design, says Gladden. He says the ability to use normal unpolarised visible light would allow their setup to be used for a wider variety of applications beyond cloaking. "We could use the same approach in solar energy devices to control sunlight and potentially increase efficiency," says Gharghi. This could be done by focusing light to higher intensity or diverting light around obstructions such as current-collecting wires.
The Berkeley demonstration "is an excellent and significant advance over their previous work in the infrared", says George Barbastathis of the Singapore-MIT group. But he says that using a natural material like calcite offers a big advantage over nano-fabrication. "Our cloak is about 10,000 times bigger than the Berkeley cloak, and also, I estimate, more than 10,000 times cheaper."
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