Tuesday, 11 October 2011
Underwater cloaking devices could be a step closer thanks to heated sheets of carbon nanotubes that deflect light from the surface of an object – just like a mirage.
Desert mirages occur when surfaces warmed by the sun bend light rays so that photons from the sky, rather than those reflected from the surface, reach an observer's eye – an effect known as "photothermal" deflection.
Existing invisibility cloaks use arrays of electromagnetic antennas to steer photons around an object, but these so-called metamaterials typically cover a narrow range of frequencies.
Ali Aliev and colleagues at the University of Texas in Dallas embedded a sheet of carbon nanotubes into aerogel, a foam-like material. When electrically heated, the nanotubes bent light waves to create a mirage, effectively cloaking the sheet and anything behind it.
Aliev says the mirage forms because the nanotubes transfer heat to the surrounding air more efficiently than regular metals, allowing a steeper temperature gradient to form near the device's surface. Because photothermal deflection depends on light's ability to propagate faster through hotter, less-dense material, the device works better when the temperature gradient is steeper. Plus, he adds, because carbon nanotubes do not store heat well, the mirage can be turned on and off quickly.
It works best when immersed in liquids, which tend to support steeper temperature gradients than gases. Aliev says it could also be used to create acoustical mirages using a similar effect, which might be used to hide future stealth submarines. The temperature required depends on the medium, but is less than the boiling point of most liquids.
Posted by sachi at Tuesday, October 11, 2011
Tags : New Technology