Silicon 'carpet' masks objects at infrared wavelengths.
Invisibility 'carpets' that conceal objects by making bumps look flat can work under near-infrared light, two teams of physicists have shown. And making a similar device that shields objects in visible light should be relatively straightforward, they say.
Xiang Zhang and his colleagues at the University of California, Berkeley1, and Michal Lipson's team at Cornell University, Ithaca, New York2, have independently created slightly different versions of the silicon carpet.
In both designs, a mirrored edge that contains a bump appears flat, allowing an object to be tucked behind the bump without being seen. Infra-red light rays shone on the bump are bent by the surrounding material, making it appear that the radiation that bounces back has been reflected by a flat mirror (see video).
The disguised bumps are only around 3 micrometres across, says Thomas Zentgraf, a member of the Berkeley team — and the effect only works for light rays confined to run on a flat two-dimensional plane through the cloak, hitting the bump head-on. Given this proof of concept, however, larger bumps and even three-dimensional cloaking could be within reach. "It should be possible to make this work at visible light — it is just a question of device fabrication," Zhang says.
“This is a huge step — moving from microwave and radiofrequency cloaks right up to the threshold of the visible. John Pendry , Imperial College London”
True invisibility cloaks — which would create the appearance of empty space, rather than of flat mirrors — rely on metamaterials: materials whose complex internal structure can steer light around objects. But so far, such cloaking metamaterials have worked only at very specific wavelengths, and in the microwave region of the spectrum.
Last year, John Pendry of Imperial College London, together with Jensen Li — now on Zhang's Berkeley team — put forward the idea of a cloaking carpet, which would, in theory, be easier to make3. By January 2009, David Smith of Duke University, North Carolina, had published a version of the carpet that worked at a broad band of microwave frequencies4.
"This is a huge step — moving from microwave and radiofrequency cloaks right up to the threshold of the visible," says Pendry.
The cloak made by Zhang's team consists of a 250-nm-thick silicon sheet, divided by a silica slab from a silicon wafer below. The sheet is drilled through with holes 110 nanometres across; Lipson's team uses a similar set-up, but the silicon is patterned with 50-nanometre-wide posts. Whether holes or posts, these features are arranged to gradually alter the refraction of light as it passes through the silicon.
The Berkeley researchers say their cloak will work with light across wavelengths of 1,400–1,800 nanometres, whereas Lipson says her scheme will work at wavelengths as low as 1,000 nanometres. Visible light, however, has even lower wavelengths of 400-700 nanometres.
Because silicon absorbs visible light, a different material — such as titanium oxide — would need to be used to create a carpet that could fool the human eye, Pendry notes. And then there would be the complications of fabricating a cloak in three dimensions.
Nonetheless, the silicon carpet might prove useful. Zhang's team think that its main applications will lie in the silicon wafers of the semiconductor industry — particularly in circuits that couple light with electronics for optical computing.
"We can use this carpet to manipulate light on the nanoscale in silicon chips, avoiding obstructions that would normally scatter it," Zentgraf explains. Zhang also envisages the carpets being used to hide defects in the expensive, finely patterned lithography masks used to etch circuits into a silicon chip. But a cloak that would actually render objects invisible "is still a scientific fantasy at this point," he adds.
Valentine, J., Li, J., Zentgraf, T., Bartal, G. & Zhang, X. Nature Materials, doi: 10.1038/NMAT2461 (2009).
Gabrielli, L. H., Cardenas, J., Poitras, C. B. & Lipson, M. arXiv:0904.3508v1.
Li, J. & Pendry, J. B. Phys. Rev. Lett. 101, 203901 (2008).
Liu, R., Ji, C., Mock, J. J., Chin, J. Y., Cui, T. J. & Smith, D. R. Science 323, 366–369 (2009).