Published online 14 November 2007 | Nature | doi:10.1038/news.2007.246

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How to trap a rainbow

Imagined material could soon be a reality.

Different wavelengths of light would be trapped at different spots in this future material.B. STAROSTA

The Mother Superior in the Sound of Musicasked: "how do you hold a moonbeam in your hand?" The answer to this immortal question is probably still "you can’t"; but physicists may have solved the equally ponderable conundrum of how to hold a rainbow.

In research published in Nature,1 Ortwin Hess at the University of Surrey, Guildford, UK, and his colleagues have combined two fairly whacky physics research areas to come up with a material that, theoretically, should be able to slow light to a standstill. The material would halt each frequency of light, and hence colour, at slightly different places, to make what Hess calls a "trapped rainbow".

The work combines knowledge about how to slow and halt light, with work on 'metamaterials' — materials made from repeated structures of a similar size to the wavelength of incoming radiation, such as light. Metamaterials can produce some strange effects, the most exciting of which, so far, has been an invisibility cloak2.

Light, made of photons, is hard to tame. Photons have no charge and therefore cannot deflected by magnetic fields. So slowing them down and manipulating them like we do electrons is tricky: “Photons are very elusive,” says Hess. So far, to get light to slow down has required the use of very cold gases, in cumbersome cryogenic experiments. And if stopped, the photons can be kept from wriggling away for only a few microseconds.

If light could ever be stopped entirely, new possibilities would open up for data storage. At the moment, processing data with optical signals is limited by how quickly the signal can be interpreted. If the signal could be slowed, more information could be processed without overloading the system.

Guide that wave

Hess modelled how light would be affected if it travelled through a waveguide made from a material that had a core with a negative refractive index, which was sandwiched between two materials with normal, positive, refractive indices. A material with a negative refractive index bends an incoming wave in the opposite direction as would be expected in a normal material. And a waveguide, as the name suggests, is a structure that shepherds a wave in a particular direction, a simple example being an optical fibre.

Hess looked at one particular 'mode' of light, and examined the 'group velocity' of the light, which is essentially the speed of the wave as it travels down the waveguide. The result of his calculations showed that the group velocity depends on the thickness of the waveguide. The reason for this correlation wasn’t obvious to Hess: "I didn’t understand it at first," he says. But after a thorough crunch through the maths he quickly became convinced.

If true, the effect means that it should be possible to control the group velocity, of light simply by changing the thickness of the waveguide. At some point the thickness reaches a critical point that makes the group velocity zero, and so the wave comes to a halt.

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This critical thickness differs depending on the wavelength of the light. But a wedge-shaped waveguide should stop them all. The shortest wavelength, blue, will be trapped by the thicker part of the guide, and red by the thinner part: a device that would trap a visible rainbow would be a wedge about 55 micrometres long and 0.8 to 1.4 micrometres thick.

The impossible dream

Such a material might sound like science fiction. But other odd metamaterial inventions have been born from similar theoretical beginnings.

When the possibility that a metamaterial could make something invisible was first mooted, experimentalists quickly took up the challenge, and within six months made the theory a reality.

Hess hopes that the same will happen for his trapped rainbow, although he thinks that the first practical devices are likely to be for longer wavelength waves — infrared or microwaves — because these require larger devices that are easier to build.

As far as applications go, Hess thinks that if the speed of light can be altered at will, optical data will be much easier to handle. “There will be applications in storing information long enough to do something with it,” he says. At the moment, says Hess, the huge broadband advantage of light is lost when it is used to transfer data simply as an on or off signal. The trapped rainbow effect opens up possibilities to transfer more data in a more complex way. 

  • References

    1. Tsakmakidis, K. L., Boardman, A. D. & Hess, O. Nature 450, 397-401 (2007). | Article |
    2. Schurig D., et al. Science 314, 977-980 (2006). | Article | PubMed | ISI | ChemPort |
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