Wednesday, June 12, 2013

Vanishing cats

Here’s my latest news story for Nature.

A new ‘invisibility cloak’ can hide animals

A cat climbs into a glass box and vanishes, while the scene behind the box remains perfectly visible through the glass. This latest addition to the science of invisibility cloaks is one of the simplest implementations so far, but there’s no denying its striking impact.

The ‘box of invisibility’ has been designed by a team of researchers at Zhejiang University in Hangzhou, China, led by Hongsheng Chen, and their coworkers. The box is basically a set of prisms made from high-quality optical glass that bend light around any object in the opening around which the prisms are arrayed [1].

As such, the trick is arguably closer to ‘disappearances’ staged in Victorian music hall using arrangements of slanted mirrors than to the modern use of substances called metamaterials to achieve invisibility by guiding light rays in unnatural ways.

But Chen and colleagues have forged a conceptual link between the two. Metamaterials – made from arrays of electrically conducting components that interact with light so as to create new optical effects such as negative refractive index – are needed if an invisibility cloak is to achieve ‘perfect’ cloaking, being invisible itself and preserving the phase relationships between the light waves moving through it [2].

Metamaterials that work at the wavelengths of visible light are very hard to make, however. Chen’s coworker Baile Zhang of Nanyang Technological University in Singapore [3], as well as John Pendry at Imperial College in London [4], and their coworkers have shown that a compromise of partial visible-light cloaking of macroscopic objects can be attained using blocks of transparent, optically anisotropic materials such as calcite crystal, in which light propagates at different speeds in different directions.

These partial cloaks will hide objects but remain visible themselves. “Everyone would like to have a cloak that hides big real world objects from visible light, but achieving this demands some compromises of the ideal theory”, Pendry explains.

He says that Chen and colleagues have now gone “further than most” with such compromises by abandoning any concern to preserve phase relationships in the transmitted light. “As a result the authors can report quite a large cloak that operates over most of the visible spectrum”, he says.

Chen and colleagues say that such a simplification is warranted for many applications, because there’s no need to preserve the phase. “Living creatures cannot sense the phase of light”, they say.

Chen and his coworker Bin Zheng first unveiled the principle last year with a hexagonal arrangement of triangular prisms that could hide small objects [5]. But he has now found a more spectacular demonstration of what this approach can achieve.

In the researchers’ first example, they use a similar but larger hexagon of prisms placed in a fish tank. As a fish swims through the central hole, it disappears while the pondweed behind the cloak remains perfectly visible.

The second example uses a square arrangement of eight prisms with a central cavity large enough for a cat to climb inside. The researchers project a movie of a field of flowers, with a butterfly flitting between them, onto a screen behind the cloak. Seen from the front, parts of the cat vanish as it sits in the cavity or pokes its head inside, while the scene behind can be seen through the glass.

As well as being visible themselves, these cloaks only work for certain viewing directions. All the same, the researchers say that they might find uses, for example in security and surveillance, where one might imagine hiding an observer in a glass compartment that looks empty.

1. Chen, H. et al., preprint (2013).
2. Schurig, D. et al., Science 314, 977-980 (2006).
3. Zhang, B., Luo, Y., Liu, X. & Barbastathis, G. Phys. Rev. Lett. 106, 033901 (2011).
4. Chen, X. et al., Nat. Commun. 2, 176 (2011).
5. Chen, H. & Zheng, B. Sci. Rep. 2, 255 (2012).

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