Arrangement of glass prisms routes light around an object but cannot hide itself.
A fish swims into an enclosure and disappears, while the pondweed behind it remains perfectly visible. A cat climbs into a glass box and vanishes — again, the scene behind the box remains visible through the glass. The latest addition to the science of invisibility cloaks is one of the simplest so far, but there is no denying its striking impact.
The ‘box of invisibility’ has been designed by a team of researchers led by Hongsheng Chen at Zhejiang University in Hangzhou, China. The box is basically an array of prisms made from high-quality optical glass that make up an enclosure and bend light around any object inside. The researchers describe their methods in a paper posted online on the preprint server arXiv1.
Chen and his colleague Bin Zheng first unveiled their principle last year, with six wedge-shaped prisms arranged to form a hexagon that could hide small objects2. In the latest work, the team has used a similar but larger hexagon of prisms placed in a fish tank. As a fish swims through the central hole, it disappears, leaving the weed behind it visible.
A second demonstration of the concept in the same paper uses a rectangular arrangement of eight prisms with a central cavity large enough for a cat to climb inside. The researchers project a film through the cloak onto a screen behind it. Seen from the front, parts of the cat vanish as it sits in the cavity or pokes its head inside, while the scene behind — a field of flowers, with a butterfly flitting between them — can be seen through the glass (see figure and video).
But the cloaks themselves are visible, and work from only certain viewing directions. A sophisticated observer would also be able to tell that different rays of light have travelled different distances inside the prism array.
As such, the trick is arguably closer to ‘disappearances’ staged in Victorian music halls, which used arrangements of slanted mirrors, than to the modern use of substances called metamaterials that achieve invisibility by guiding light rays in unnatural ways. These are materials made from arrays of electrically conducting components that interact with light to create exotic optical effects such as negative refraction, in which light is refracted in the opposite direction to normal. In principle, metamaterials can create ‘perfect’ cloaking — which makes the cloak itself invisible, as well as its contents3. Metamaterials that work at the wavelengths of visible light are very hard to make, however.
Teams led by applied physicist Baile Zhang of Nanyang Technological University in Singapore, a co-author of the latest study, and John Pendry, a theoretical physicist at Imperial College London, have previously shown4,5 that a compromise of partial visible-light cloaking of objects a few millimetres in size can be attained using blocks of transparent materials such as calcite crystal, in which light propagates at different speeds in different directions.
These partial cloaks will hide objects but, like the prisms, 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,” says Pendry.
He adds that Chen and his 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,” says Pendry.
Chen and his team say that such a simplification is fine for many applications, for example in security and surveillance, where one might imagine hiding an observer in a glass compartment that looks empty.
Chen, H. et al. Preprint available at http://arxiv.org/abs/1306.1780 (2013).
Chen, H. & Zheng, B. Sci. Rep. 2, 255 (2012).
Schurig, D. et al. Science 314, 977-980 (2006).
Zhang, B., Luo, Y., Liu, X. & Barbastathis, G. Phys. Rev. Lett. 106, 033901 (2011).
Chen, X. et al. Nat. Commun. 2, 176 (2011).
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Ball, P. ‘Invisibility cloak’ hides cats and fish. Nature (2013). https://doi.org/10.1038/nature.2013.13184