Published online 11 August 2008 | Nature | doi:10.1038/news.2008.1033

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Reversing the prism

A slab of material that bends light the wrong way could herald true invisibility.

Materials that could one day make objects invisible to visible light have been devised by scientists at the University of California at Berkeley1,2.

metamaterialThis scanning electron microscope image shows a prism made from the 3-D metamaterial.X. Zhang / UC Berkeley

These artificial substances, called metamaterials, have a negative refractive index, so that light reflected from or passing through the metamaterial is bent the wrong way. Although invisibility is the most attention-grabbing application, metamaterials like this have many more immediate uses in optical technology, for example as powerful new lenses or for carrying light-based signals around microchips.

Previous metamaterials that control electromagnetic radiation have mostly worked for longer-wavelength radiation such as microwaves. But Xiang Zhang and his co-workers have made the first real chunks of metamaterials that work at visible and infrared wavelengths. "It's definitely an important step," says David Schurig, a specialist in optical metamaterials at North Carolina State University, Raleigh.

Optical metamaterials are built up from ‘artificial atoms’ — tiny metal circuits that absorb and re-radiate light in ways that normal materials cannot. In the microwave metamaterials pioneered by David Smith at Duke University in Durham, North Carolina, and his co-workers, the ‘atoms’ are metal loops and wires etched from thin films on circuit boards, which are then assembled into large arrays. Smith, along with Schurig and other colleagues, made a ring-shaped invisibility shield from such structures in 20063.

Although microwave metamaterials could find useful applications in microwave and radar technology (and are therefore of military interest), many of the most appealing uses of such fabrics involve visible light. But the wavelength of the electromagnetic radiation to which a metamaterial is sensitive is roughly equal to the size of its ‘atoms’. A visible-light metamaterial would need to have features just a micrometre or so in size.

That’s what Zhang and his colleagues have now made. In one version, their ‘atoms’ are stacks of very thin metal and insulating salt films, each just 30–50 nanometres thick1. The researchers then cut these multi-decker sandwiches up into a tiny mesh using a beam of high-energy ions to carve out rectangular holes. The resulting grid acts like an array of metamaterial atoms.

The Berkeley team showed that a prism made from this material has a negative refractive index for light of near-infrared wavelengths, which are used in today’s photonic technologies for transmitting information through optical fibres.

This isn’t the first metamaterial to work near visible wavelengths, but previous efforts have succeeded only in making them in flat sheets, not thick slabs4. Among other things, the sheets would probably produce a rather leaky invisibility cloak. "The thicker it is, the more effect you can have on the light," explains Schurig. "So you can do more with it."

Capacious cloak

In a second paper, published in Science2, the Berkeley team describe another optical metamaterial: an array of aligned silver wires just 60 nanometres wide (about 200 times thinner than a human hair), standing parallel like an orderly plantation of trees.

The wires are embedded in a matrix of aluminium oxide, which provides a mould for growing them in. The researchers perforated a film of the oxide with tiny pores using an electrochemical method to dissolve away narrow channels, which they then filled with silver. Slabs of this material have a negative refractive index for red light.

Zhang points out that both designs work for light over a broad range of wavelengths, unlike previous metamaterials, which tend to work only for specific ‘colours’. This broadband operation is essential for an invisibility shield that would provide cloaking over the entire visible spectrum.

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A true invisibility shield would work by smoothly bending the light rays around an object in the centre, like a river flowing around a rock, so that the light seems to pass straight through. This would require metamaterials with gradually varying optical properties, achieved by varying the size and shape of the metallic components in the ‘atoms’.

Next: the anti-cloak

These breakthroughs come hard on the heels of other discoveries on metamaterial invisibility cloaks. Baile Zhang and his coworkers at the Massachusetts Institute of Technology in Cambridge have suggested that light entering an invisibility shield is split into a rainbow of different colours, because of the differing extents to which each colour penetrates through the cloak’s wall5.

And Huanyang Chen of Shanghai Jiao Tong University in China and his colleagues have devised an ‘anti-cloak’ that will counteract the effects of an invisibility shield. An object covered by such an anti-cloak — which the researchers have described only in theory — will no longer be hidden when veiled by the shield. The work will be published soon in Optics Express6. 

  • References

    1. Valentine, J. et al. Nature advance online publication doi:10.1038/nature07247 (2008).
    2. Yao, J. et al. Science 321, 930 (2008).
    3. Schurig, D. et al. Science 314, 977-980 (2006). | Article | PubMed | ISI | ChemPort |
    4. Shalaev, V. M. Nature Photon. 1, 41–48 (2007).
    5. Zhang, B., Wu, B.-I., Chen, H. & Kong, J. A. Phys. Rev. Lett. 101, 063902 (2008). | Article |
    6. Chen, H., Luo, X., Ma, H. & Chan, C. T. Preprint at http://arxiv.org/abs/0807.4973 (2008).
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