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Trapped rainbow promises all-optical processing

16 Nov 2007

All-optical computers and ultrafast optical communications are the ultimate goal of a computer analysis that combines slow light with metamaterials.

Computer simulations of light traveling through a metamaterial suggest for the first time how light could be stopped and stored in a solid-state device at room temperature. What's more, the analysis by researchers at Surrey University in the UK indicates that light over a range of frequencies could be trapped like a rainbow, which is critical for potential applications in optical communications and all-optical computing. (Nature 450 397)

"Until now, stopping light inside a solid-state device was thought to be infeasible," Ortwin Hess from the University of Surrey, told optics.org. The only method proposed for completely stopping and storing light required extremely low temperatures and trapped atomic gases - which would need extensive experimental effort. "Clearly, while the physics behind that work was very exciting, in practical real-world applications we need solid-state materials and room temperatures," said Hess.

According to Hess, the ability to slow down light at major interconnection points in optical networks could increase data rates by a factor of 1000. What's more, finding a way to stop and store photons could lead to all-optical memories and, ultimately, all-optical or quantum computers.

The UK group performed a computer analysis of light rays passing through a metamaterial comprising a tapered, negatively-refracting waveguide sandwiched between two positively-refracting materials. "The metamaterial contains tiny metallic inclusions of various shapes and arrangements," explained Hess. "As light passes through these structures, oscillating electric currents are set up, which modify the way the light travels through the material."

As light travels through a negatively refracting waveguide, the phase shift at each reflection is negative. This means that the rays of light bounce between the limits of the waveguide in a backwards direction instead of forwards, as they would in a positively refracting material.

The key finding is that if such a waveguide gets narrower, the light covers less and less distance between reflections, and eventually the backwards jumps will dominate - so the group velocity is stopped in its tracks. "A ray of light with a given frequency propagating in a wedge-shaped metamaterial layer will gradually become slower and slower before stopping completely at a particular critical width of the tapered layer," explained Hess. "A wavelength at a different frequency is shown to stop at a correspondingly different point inside the layer."

As a result, light of different frequencies becomes spatially separated: the blue component would be stopped first and red last, with a "trapped rainbow" in between. This, says Hess, means that an optical storage device based on the idea could be used to separately process different components of light.

The team now hopes to construct this material for a real-life demonstration. "We are in contact with a number of experimental groups that are interested in taking on the challenge to realize our scheme, and we would be delighted to start new collaborative links with other interested experimental groups," commented Hess.

Metamaterials meet slow light

According to Hess, this is the first time that two major themes of contemporary photonics research have been brought together. "Metamaterials and slow light research have been conducted by two rather separate communities," he told optics.org. "Consequently, slow light has not been in the centre of attention in the metamaterials community, and vice versa."

Metamaterials are artificial structures that are engineered to have a negative index of refraction, allowing them to offer unprecedented control over the flow of light. For instance, metamaterials can be used to create a "super" lens with unlimited resolution, in theory even down to the molecular level. More recently, metamaterials have been used to create an invisibility cloak, albeit so far at longer wavelengths.

“Stopping light inside a solid-state device was thought to be infeasible.”

In contrast, the impetus for research into slow light has been the prospect of all-optical data processing. In communications networks, for example, the ability to slow down light at network nodes could avoid the need to convert all the optical information into electrical signals and back again.

According to Hess, controlling the traffic at these interconnections in the optical domain -- by decelerating or holding some data packets while letting others pass through -- could speed up optical networks by a factor of 1000. Meanwhile, finding a way to stop and store photons in an "optical capacitor" will be crucial for building integrated optical memories that can data in future all-optical or hybrid computers.

Researchers has already succeeded in slowing light down in photonic crystal structures, while a technique known as electromagnetically-induced transparency (EIT) has been found to stop light altogether in gases of ultracold atoms. But EIT cannot currently be performed in a room-temperature gas, and both EIT and photonic crystals are unable to slow light at anything other than a narrow range of frequencies. This limits their usefulness in communications, which are typically broadband.

In contrast, the analysis by Hess and colleagues show that a metamaterial structure can stop light over a range of frequencies. They have also demonstrated that light can efficiently "enter" such a structure before being trapped and stored. While the current analysis assumed light with a frequency of 1 THz, the team believes that its results are equally applicable to radiation from the microwave to the ultraviolet part of the electromagetic spectrum.

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