Anti-invisibility cloak would render invisible objects visible again

We have written several stories on how science was inching toward a true invisibility cloak (see, for example, this 12 August 2008 HS Daily Wire story and this 11 March 2008 HS Daily Wire story about invisibility cloaks, and this 30 June 2008 HS Daily Wire stoy about acoustic cloak), and the security and commercial applications of such cloaks. The action-reaction process never ends, though, and now theoretical physicists think they may have the makings of the world’s first “anti-invisibility cloak,” a device which, when layered beneath an invisibility cloak, renders any object within visible again. The anti-cloak could overcome one major limitation of the invisibility cloaks being designed and built in physics laboratories across the world: when you are invisible from the outside, the outside is invisible to you. An anti-cloak could allow a spy, whether man or machine, hidden inside a cloak periodically to peep out.

New Scientist’s Collin Barras writes that invisibility cloaks first came to the world’s attention in 2006 when John Pendry at Imperial College London and colleagues including David Smith’s team at Duke University in North Carolina designed a cloak that could make an object invisible to microwaves. Waves which would usually hit the cloaked object are carefully guided through the cloak and released at a point on the far side exactly equivalent to their point of entry. Any external observer sees through the cloak - and any objects hidden inside it — as if it were transparent.

Versions of this idea which worked in two dimensions for microwaves and later for visible light were built later in 2006, and this year physicists progressed towards making them in three dimensions. A perfect invisibility cloak, however, guides rays so effectively that none reaches the object within, keeping it in total darkness — a disadvantage if invisibility cloaks are ever to be used to shield tanks, steer microwaves in space, or hide humans.

Now Huanyang Chen’s team at Shanghai Jiao Tong University in China and colleagues at the Hong Kong University of Science and Technology have found a theoretical way to allow some light to penetrate through an invisibility cloak, so that anyone within can see out. Their work shows that an anti-cloak can reverse the light-bending effects of an invisibility cloak if it has exactly the opposite refractive properties. When pressed against the inner boundary of the invisibility cloak, the anti-cloak acts as a mirror. The path the light rays take through the anti-cloak is an exact reflection of their path through the invisibility cloak. In other words, the two cloaks cancel each other out, rendering the invisible visible.

Sebastien Guenneau at the University of Liverpool thinks the idea is an interesting one. “It’s a nice twist on Pendry’s work,” he says. He is quick to point out that the practicalities of making an anti-cloak mean it is unlikely to ever make it off the drawing board. “The cloak needs to be anisotropic, heterogeneous and have a negative index of refraction,” he says. In other words, it must treat light differently depending on the direction it is travelling in and which part of the anti-cloak it hits, as well as bending it the “wrong” way compared to all natural materials. “That’s an absolute killer combination in terms of metamaterial technology. It’s very hard to make an invisibility cloak - it’s going to be twice as hard to make an anti-cloak.”

-read more in Huanyang Chen, Xudong Luo, Hongru Ma, and C. T. Chan, “The Anti-Cloak,” Optics Express  6 (2008): 14603-608 (forthcoming)