Everyone, one hopes, is well aware by now of metamaterial - remarkable conceptual stuff which might be used in coming years to make invisibility cloaks; or more realistically, invisible sheds. Few, however, have spotted the critical flaw in a metamaterial cloak, shed or cladding - people so concealed would no more be able to see out than those outside could see in.
But that problem may have been dealt with in advance by a team of top Chinese invisibility boffins.
"Cloaking is an important problem since invisibility can help survival in a hostile environment," said Huanyang Chen of Shanghai Jiao Tong University in China.
According to Chen, the currently-envisaged metamaterials work by bending light around the enclosed area. Thus, light cannot penetrate a metamaterial, and so it cannot be seen through.
A harassed family man lurking in his invisible garden shed might not be troubled by an inability to see out, but troops in an invisibility-clad tank or boy wizards reconnoitring the girls' showers no doubt would. They could, of course, simply peer out through a small peephole; but Chen and his crew of Shanghai vanishment boffins - along with colleagues at the Hong Kong University of Science and Technology - have come up with a more elegant solution.
More switched-on readers will have guessed the answer. All you need, needless to say, is a piece of "anisotropic negative refractive index material that is impedance matched to the positive refractive index of the invisibility cloak". Pleasingly, Chen has chosen to dub this "the anti-cloak", rather than ANRIMIMPRIIC or similar.
The anti-cloak, if pressed against the inside of the invisible shed, would permit outside light (which would normally travel around to be emitted from the far wall) to penetrate to the inside, so letting a clandesto-shed denizen see out. If we've grasped the concept properly*, the anti-cloak window would be apparent from outside, but principally as a dark patch on the opposite wall of the shed.
The findings are published in a new paper: The Anti-Cloak, Huanyang Chen et al, Optics Express, Vol. 16, Issue 19, September 15, 2008, pp. 14577 – 14582. ®
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