"MATERIALS: Active invisibility cloaks could work at many wavelengths"

Invisibility cloaks usually harness exotic metamaterials to passively route electromagnetic waves around objects, effectively masking them from view. Unfortunately, passive cloaks only work for specific wavelengths enabling snoops to just switch scanning frequencies to defeat cloaking. A new active cloak, however, could mask objects from view at any wavelength by using a countering signal exactly matched to that of the scanning probe regardless of its frequency. Look for active cloaks to begin masking military projectiles within five years. R.C.J.



Active cloaking devices can use destructive interference, similar to noise-cancelling headphones, to render invisible areas up to 10 times larger than the wavelength of light being disguised. Unlike passive invisibility cloaks that use exotic metamaterials, active cloaks require as few as three antennas surrounding the cloaked area to render it invisible. Metamaterial cloaks work with free-air resonators that bend incoming waves of a single frequency around the object to be cloaked. Active cloaks, on the other hand, monitor the incoming waveforms composed of multiple frequencies, then produce a canceling signal that prevents the electromagnetic radiation from reaching the cloaked object. Outgoing radiation on the far side of the object is then reconstructed to make it appear as if the waves traveled unobstructed through the cloaked area.

Text: http://www.eetimes.com/showArticle.jhtml?articleID=219400397