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Active Cloaking Device Widens Light Scattering Coverage Range

23 December 2013

Researchers at The University of Texas at Austin have proposed an active “invisibility cloak” that can drastically reduce the scattering over a wide frequency range in the microwave regime, orders of magnitude broader than any available passive cloaking technology.

“I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them,” said Andrea Alù, associate professor at the Cockrell School of Engineering.

Cloaks have so far been realized with passive technology typically based on metamaterials or metasurfaces that can suppress the scattering of light that bounces off an object, making an object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio wave detectors.

“Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation,” Alù said. “We prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks.”

In a recent research paper, Alù’s team was able to prove that all available cloaking techniques based on passive cloaks are constrained by Foster’s theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum.

In contrast, an active cloak based on active metasurfaces, such as the one designed by Alù’s team, can break Foster’s theorem limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth.

The active cloaks can suppress 100 times or more the detectability at specific design frequencies of passive cloaks. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.

Alù and his team are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed to use these cloaks to improve biomedical sensing, near-field imaging and energy harvesting devices.

The team’s paper, “Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces,” was published Dec. 3 in Physical Review Letters.

The Air Force Office of Scientific Research, the Defense Threat Reduction Agency and a National Science Foundation CAREER Grant provided support for the research.

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