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).
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