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Traditional Limits on Invisibility Cloak Disappear in U Texas Research

• By Dian Schaffhauser
• 12/20/13

The creation of a real-life invisibility cloak just took another stride forward. Engineers at the University of Texas at Austin have come up with a design for a cloaking device that makes it "invisible" or undetectable to parts of the electromagnetic spectrum over a greater number of frequencies.

The new design uses "active" technology, which calls for drawing energy from a battery. The work is being led by Andrea Alù, an associate professor at the Cockrell School of Engineering. Alù and his team recently published a paper on the work in a December issue of Physical Review Letters.

Traditionally, the researchers have stated, cloaks have been designed using "passive" technology. Passively cloaked objects may become transparent, but if lit up with white light, which is composed of many colors, they become more visible with the cloak than without. This, the researchers explained, is a constraint imposed by Foster's theorem, which relates to the behavior of an electrical network when the electric current or voltage changes.

The U Texas research puts forth a design in which the proposed active cloak uses specialized materials, a battery, circuits and amplifiers to boost signals. According to the researchers, that approach allows for the cloak to be less discoverable across a broader set of frequencies.

"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," noted Alù.

The team started with a passive "metasurface" made from an array of metal square patches and laden with amplifiers that use energy from a battery to broaden the bandwidth. Metamaterials were introduced to the creation of cloaking devices by a team of Duke University engineers in 2009.

"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.

Eventually, the research team will go beyond theory to attempt to build a prototype cloak. They anticipate that the discoveries they make will have practical application in several areas, including sensing applications and wireless communications, in which they'll enable the suppression of interference from neighboring antennas.