You’d never guess it from the title of their paper but researchers at the University of Texas at Austin have conceptualized and designed a battery-operated invisibility cloak, according to a Dec. 18, 2013 news item on Nanowerk,

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.

Andrea Alù, associate professor in the Department of Electrical and Computer Engineering at the Cockrell School of Engineering, and his team have proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.

The Dec. 18, 2013 University of Texas at Austin news release (also on EurekAlert), which originated the news item, describes the current state of cloaking technology,

Cloaks have so far been realized 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 (a flexible, ultrathin metamaterial) 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 colors of light or specific frequencies of operation,” said Alù, David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. In this paper, on the contrary, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. 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.”

The news release details the new battery-powered design,

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.

In a related paper, published in Physical Review X in October, Alù and his graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may indeed become transparent, but if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without. The October paper proves 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.

“In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the bandwidth of operation,” Alù said.

The researchers are continuing to work both on the theory and design behind their non-Foster active cloak, and they plan to build a prototype.

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.

Here’s a link to and a citation for the team’s paper about active cloaking,

Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces by Pai-Yen Chen, Christos Argyropoulos, and Andrea Alù. Phys. Rev. Lett. 111, 233001 (2013) [5 pages] DOI: 10.1103/PhysRevLett.111.233001

This paper is behind a paywall.

Here’s a link to and a citation for the related and previously published paper (authors: Alù and Monticone),

Do Cloaked Objects Really Scatter Less? by Francesco Monticone and Andrea Alù. Phys. Rev. X 3, 041005 (2013) [10 pages] DOI: 10.1103/PhysRevX.3.041005

The authors have included both an abstract and a popular summary. I’ve excerpted the popular summary,

From ancient times, humanity has been fascinated by the concept of invisibility, and recently, scientists have moved a step closer to bringing this idea to reality by exploiting engineered artificial materials, or metamaterials. Several recent studies have indeed shown that a properly tailored metamaterial cover can, in principle, render an object invisible when illuminated by an electromagnetic wave oscillating at the specific frequency of interest. Yet, experimental realizations and theoretical investigations have consistently shown that reducing the visibility of an object with a passive cloak in a specific window of the electromagnetic spectrum is generally accompanied by a drastic increase of its visibility in other frequency ranges. Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility.

In this paper, we quantitatively assess the potentials and limitations of passive cloaks in terms of overall visibility, integrated over the entire frequency spectrum. Quite surprisingly, our results show that any linear, causal, and passive invisibility cloak, without special superconducting features, is deemed to increase the scattering and visibility of the original uncloaked object, when integrated over all frequencies. This result confirms that the most popular cloaking devices actually scatter more, not less, when considered over a sufficiently broad frequency range, allowing easy detection using, e.g., pulsed excitation.

Our general theorem holds a relevant exception if specific covers with a strong static diamagnetism are considered, and, based on this principle, we propose a technique to reduce the global scattering, as well as the local response around a frequency of interest, using diamagnetic and superconducting thin cloaking layers. More generally, our results provide a quantitative measure to compare the overall performance of different cloaking devices and generally assess their detectability. These findings may open important research directions in the quest for invisibility, not only in the electromagnetic domain but also for acoustic, mechanical, and matter waves.

This is an open access paper.