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Optical physics

Optical physics

How to walk through walls

13 Dec 2010
Illusion optics create an invisible gateway

Imagine being able to walk through a solid wall. That sort of trick might sound far-fetched, but it’s a little closer to reality now that researchers in China have created what they call an “invisible gateway”.

Huanyang Chen at Soochow University, Jiangsu, says that the effect is a bit like “platform nine and three-quarters” – that is, the fictional area of King’s Cross railway station in the Harry Potter books that is only accessible through a secret, illusionary wall. Although the researchers’ current demonstration is based on an electrical circuit for radio waves, Chen claims that it could also work for visible light.

The idea for the invisible gateway stems from so-called transformation optics, which gave us the first invisibility cloak back in 2006. Yet the invisible gateway is almost the opposite of a cloak: rather than bend light round an object to make the object invisible, the device makes an object – a wall – appear that isn’t really there. It is, according to Chen’s group, the first demonstration of illusion optics.

Network of capacitors and inductors

Chen, whose colleagues are based at the Chinese Academy of Sciences, Beijing, and the Hong Kong University of Science, created the invisible gateway using a network of capacitors and inductors. The network forms a channel that separates two electric conductors – the walls – one of which contains a slab of material with a negative index of permittivity and refraction. The combination of these two materials allows collective waves of electron, called plasmons, to form on the surface. The plasmons prevent electromagnetic waves from passing through the channel. To an observer, the channel looks like a continuation of the walls – so long as they are looking at electromagnetic radiation between 45 and 60 MHz.

This does demonstrate that the principle works Tom Driscoll University of California, San Diego

Tom Driscoll, a researcher who studies novel electromagnetic devices at the University of California, San Diego, calls the demonstration a “good step”, although he notes that the progression to devices that work with visible light and at human scales are “decades or more away”. “The total sample size is quite small compared with the wavelength used, so I would like to have seen a bigger example,” he says. “However, this does demonstrate that the principle works.”

Martin McCall, a theoretical physicist at Imperial College, London, also thinks that the invisible gateway is an interesting development. “It’s a viable addition to the pile of interesting electromagnetic structures being produced,” he says.

Chen and colleagues’ invisibility gateway is one of many ideas to have been realised used transformation optics in recent years. Last year, groups at Cornell University and the University of California at Berkeley independently created 2D cloaks that operated at optical wavelengths. Earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

In 2008, Chen’s group proposed what might be the next step on these lines – a device that can cloak objects at a distance.

The research is available in Phys. Rev. Lett. 105, 233906

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