What's at the end of the rainbow?

British scientists have revealed a system called 'trapping rainbows' that may be able to slow down, stop and even capture light.

This technique would allow the use of light rather than electrons to store memory in devices such as computers, enabling an increase in operating capacity of 1000% by using light's broad spectrum rather than single electrons.

Slow light could also be used to increase the speed of optical networks, such as the internet. At major interconnection points - where billions of optical data packets arrive simultaneously - it would be useful to control this traffic optically, by slowing some data packets to let others through.

The system has been developed by Prof Ortwin Hess and his PhD student Kosmas Tsakmakidis - of the Advanced Technology Institute & Department of Physics at the University of Surrey, southern England - and Prof Alan Boardman from Salford University, Manchester.

Previous attempts to slow and capture light have involved extremely low or cryogenic temperatures that have been costly and have only worked with one frequency at a time.

The technique proposed by Hess and Tsakmakidis involves using negative refractive index metamaterials along with the exploitation of the Goos Hänchen effect. This shows that when light hits an object or an interface between two media, it does not bounce back immediately but seems to travel very slightly along that object or - in the case of metamaterials - travels very slightly backwards along the object.

Metamaterial is matter that gains its properties from its structure rather than directly from its composition.

It is of particular importance in electromagnetism (especially optics and photonics). Metamaterials show promise for a variety of optical and microwave applications such as new types of beam steerers, modulators, band-pass filters, lenses, microwave couplers and antenna radomes.

Hess's theory shows that if you create a tapered layer of glass surrounded by two suitable layers of negative refractive index metamaterials, a packet of white light injected into this prism from the wide end will be stopped at some point in the prism.

Because different component 'colours' of white light have different frequencies, each individual frequency would therefore be stopped at a different stage down the taper, so creating the 'trapped rainbow'.

The negative index metamaterials that allow for unprecedented control over the flow of light have a sub-structure with tiny metallic components much smaller than the wavelength of the light and have recently been demonstrated experimentally for terahertz and infrared wavelengths.

"Our trapped rainbow bridges the exciting fields of metamaterials with slow light research. It may open the way to the long-awaited realisation of an 'optical capacitor'," Hess said.

"Clearly, the macroscopic control and storage of photons will conceivably find applications in optical data processing and storage, a multitude of hybrid photonic devices to be used in optical fibre communication networks and integrated photonic signal processors as well as become a key component in the realisation of quantum optical memories."

"It may further herald a new realm of photonics with direct application of the trapped rainbow storage of light in a huge variety of scientific and consumer fields."