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The latest fashion in invisibility

Science / AAAS
This graphic shows a 3-D nanostructure, consisting of a bumpy gold surface
layer with the tailored "invisibility cloak" underneath. The cloak, made from
laser-sculpted layers of polymer, hides the bump from optical detection.

Scientists have designed a more stylish cloak of invisibility that can hide a bumpy feature from view, even if you're looking right at it from a wide range of perspectives. But don't expect boy wizard Harry Potter to be modeling this cloak anytime soon.

It's actually more like an ultra-thin carpet of invisibility, created from layers of laser-sculpted polymer and topped off with a bumpy coating of gold.

"In our carpet, you could put any object beneath the bump, and it would be hidden because the bump itself is hidden," Tolga Ergin, a physicist at Germany's Karlsruhe Institute of Technology, told me. The research conducted by Ergin and his colleagues was published online today by the journal Science.

Invisibility cloaks have long been a feature of fictional sagas including the Harry Potter book series and the "Star Trek" TV shows and films. As usual, fabricating such cloaks is harder to do in reality than it is in fiction. The challenge involves creating metamaterials that guide light waves on a detour around an object, instead of letting them hit the object and bounce back to the observer.

The first cloaks worked only in a two-dimensional plane, but last year researchers at Rice University unveiled a metamaterial that could produce a 3-D cloaking effect. The newly published German experiment could be considered the first 3-D demonstration of an invisibility carpet.

To create the carpet, the researchers laid down layers of polymer on a glass foundation, using a laser to sculpt each layer into a pattern of rods. This process is known as direct laser writing or 3-D laser lithography. The layers of rods were stacked on top of each other to produce a "woodpile" with a dent running down the middle. Then a thin reflective surface of gold was layered on top.

The resulting material, shown in the graphic above, is thinner than the width of a human hair with a depression that measured just one micron (a millionth of a meter) deep. The woodpile of rods is carefully structured so that light is refracted along complex paths that dodge the dented area. When scientists shone infrared light up through the woodpile, they could see the reflective gold surface. But the dent, which was a bump when seen from below, could not be detected. Instead, it looked as if the gold surface was perfectly smooth.

The invisibility effect wasn't restricted to just one viewing angle. "We saw that it worked quite well even to large angles," Ergin said.

The carpet had several limitations, however. "This is a great step they've made," said David Schurig, a physicist at North Carolina State University who helped create the first working 2-D invisibility cloak. "But there are a lot more steps to get to what you and most people imagine when you think of cloaking devices."

For example, the woodpile material works only down to the near-infrared part of the electromagnetic spectrum, around wavelengths of 1.2 microns. As the light came closer to the visible spectrum, scientists started seeing the pattern of rods instead of the gold surface beyond. In Harry Potter terms, the cloak itself became visible. "You start to see that there is something on top of this bump," Ergin said.

The size of the hidden region is also incredibly tiny - far too small for Harry to fit inside, even if he used a spell to shrink himself to the size of an ant. Making an invisibility carpet to cover larger objects would take much more time and effort.

Schurig also would like to see a device that routes light waves all the way around a hidden object, so that it truly looks as if there is nothing to see at all. "It'd be nice to hide things in free space, as opposed to beneath a surface, and it'd be nice to do it broadband in the visible spectrum," Schuring said.

Ergin acknowledged that his research group's invisibility carpet wouldn't do Harry much good. "Of course it is not a 'good' invisibility device, but it is one that is experimentally feasible, and it is a beautiful benchmark for showing these concepts for transformation optics.," he said.

The U.S. military has funded some research into cloaking technologies, including acoustic cloaks that could make submarines "invisible" to sonar, but Ergin emphasized that his work is going in a different direction. "This has absolutely nothing to do with going to military research and hiding tanks and soldiers," he told me.

Instead, he and his colleagues are focusing on exotic ways to twist and turn light waves using metamaterials. Such research could open the way for the development of super-fast optical circuitry. Another oft-mentioned application is the invention of super-antennas that could collect light from a range of directions and direct it to a single point. Metamaterials could also be used to simulate relativistic effects - for example, in optical black holes.

Wizards may not be modeling invisibility cloaks anytime soon, but you can expect research into cloaking effects to produce some real-life wizardry in the years ahead. Just don't ask Ergin to predict precisely what form those scientific spells will take.

"I can't really tell you what people might bring up in the next 10 years," he told me. "If I had 10 good ideas, I would have published them already."

Authors of the Science research on the three-dimensional invisibility cloak include Tolga Ergin, Nicolas Stenger, Patrice Brenner and Martin Wegener of the Karlsruhe Institute of Technology, as well as John B. Pendry of Imperial College London.

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