Theoretical physicists think they might have the makings of the world鈥檚 first 鈥榓nti-invisibility cloak鈥�, a device which, when layered beneath an invisibility cloak, renders any object within visible again.
The 鈥榓nti-cloak鈥� could overcome one major limitation of the invisibility cloaks being designed and built in physics laboratories across the world: when you鈥檙e 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 to periodically peep out.
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Invisible beginnings
Invisibility cloaks first came to the world鈥檚 attention in 2006 when at Imperial College London and colleagues including 鈥榮 team at Duke University in North Carolina, US, designed a cloak that could make an object invisible to microwaves.
Waves that 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 鈥� and any objects hidden inside it 鈥� as if it were transparent.
Versions of that idea that worked in 2 dimensions for microwaves and later for visible light were built later in 2006, and this year physicists progressed towards making them in 3 dimensions.
But a perfect invisibility cloak guides rays so effectively that none reaches the object within, keeping it in total darkness 鈥� a disadvantage if invisibility cloaks are ever to be used to shield tanks, steer microwaves in space or hide humans.
Cloak begone
Now Huanyang Chen鈥檚 team at in China and colleagues at the 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 鈥渁nti-cloak鈥� 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 like 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 (see image, right).
鈥楴ice twist鈥�
at the University of Liverpool, UK, thinks the idea is an interesting one.
鈥淚t鈥檚 a nice twist on Pendry鈥檚 work,鈥� he says. But 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.
鈥淭he cloak needs to be anisotropic, heterogeneous and have a negative index of refraction,鈥� 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 鈥渨rong鈥� way compared to all natural materials.
鈥淭hat鈥檚 an absolute killer combination in terms of metamaterial technology. It鈥檚 very hard to make an invisibility cloak 鈥� it鈥檚 going to be twice as hard to make an anti-cloak.鈥�
A paper on the anti-cloak will appear in a forthcoming issue of . A pre-print is available .
