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Team achieves breakthrough in invisibility

The towers of the Gothic Wonderland aren't its only resemblance to Hogwarts-thanks to improvements to an invisibility cloak made by researchers in the Pratt School of Engineering.

A team of researchers led by David Smith, William Bevan professor of electrical and computer engineering, made a new breakthrough in cloaking technology this month through a new design algorithm.

"The phenomenon we've achieved is to cloak something on the ground," said Ruopeng Liu, a third-year graduate student in electrical and computer engineering. "But the real important thing is the underlying technology that we've improved.... It's just like day and night."

Originally developed in 2006, the invisibility cloak is made of metamaterials that are able to guide waves around the cloak, Smith said. The previous version did not cloak in the visible spectrum and only worked for a single microwave frequency, he added.

Although the current iteration still does not work in the visible spectrum, the new cloak can work at a broad spectrum of frequencies from 10 megahertz to 18 gigahertz, and can be produced in significantly less time.

Through the use of a new algorithm, the new cloak can be designed in a few seconds, said Liu, who is also the first listed author of the study published in the Jan. 16 issue of "Science" journal. The cloak is 20 inches by 4 inches and is less than an inch thick.

Another important feature of the new design is its low absorbance of the surrounding waves, a problem that has plagued previous cloaks, said Tie Jun Cui, Chang-Kang chair professor at China's Southeast University and one of the collaborators on the study. The current cloak does not absorb the waves as they pass through the metamaterial.

"It is very loss-less, hence the invisible effect is very good," Cui said. "The performance is much better than the first one."

Both Smith and Cui noted, however, that an invisibility cloak that works in the visible spectrum, which is at a much higher frequency-400 terahertz-is still far off.

"At this point, I think it's not very easy to make it in the visible frequency," Cui said. "Right now, the structure was realized using some structure using PCB technology. When the frequency goes higher, it is very difficult to realize it using that way."

Liu added that the current cloak only works with polarized electromagnetic waves in two dimensions-meaning that invisibility is only achieved with synchronized light waves coming from a single direction. Cui noted, however, that work on a three-dimensional cloak is in progress and should be complete by the end of the year.

Despite all the hype, the groups' goals are not to create the fabled cloak seen in the Harry Potter series.

"We're not really trying to make an invisibility shield and hide them from people looking," Smith said. "We're just trying to think about interesting things to do that are matched for the materials that we make."


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