First battery-powered 'invisibility cloak' designed

Scientists have designed an 'invisibility cloak' that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.
Researchers at The University of Texas at Austin have proposed the first design of a cloaking device that uses an external source of energy to significantly broaden its bandwidth of operation.
The proposed active cloak will have a number of applications beyond camouflaging, such as improving cellular and radio communications, and biomedical sensing.
Cloaks have so far been realised with so-called passive technology, which means that they are not designed to draw energy from an external source.

They are typically based on metamaterials (advanced artificial materials) or metasurfaces (flexible, ultrathin metamaterials) 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.

They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.
"Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colours of light or specific frequencies of operation," said Andrea Alu, associate professor in the Department of Electrical and Computer Engineering at the Cockrell School of Engineering.

In this paper, on the contrary, we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks, Alu said.
"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," Alu said.
The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes possible the reduction of scattering over a greater range of frequencies.
This design, which covers a very broad frequency range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks.

The study was published in the journal Physical Review Letters.