PORTLAND, Ore. — 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.

"According to our numerical computer simulation, it is possible to cloak relatively large regions of space, even from multi-frequency sources, using destructive interference," said professor Graeme Milton, a researcher at the University of Utah. He performed the work with Fernando Guevara Vasquez and Daniel Onofrei.

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.

Theoretically, the active cloaking mechanism should be able to render objects invisible to any wavelength down to those 10 times smaller than the size of the cloaked area. For instance, microwaves have wavelengths of about four inches, so an active cloak could hide objects as large as 40 inches from radar detection.

Metamaterials, on the other hand, can only render objects invisible for a single wavelength of the same size as the object, or less than four inches in the radar example above.

The researchers predict that engineers will be able to use their method to create active invisibility cloaks that could shield submarines from sonar, planes from radar and buildings from earthquakes.

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