Physics & Math

Oh, to catch a rainbow. Well, it's been done for the first time ever – and with just a simple lens and a plate of glass at that. The technique could be used to store information using light, a boon for optical computing and telecommunications.

All-optical computing devices promise to be faster and more efficient than current technology, but they suffer from the drawback that signals have to be converted back and forth from optical to electrical. The ability to "slow" light to a crawl or even trap it helps, as information in the light can then be manipulated directly.

In 2007, Ortwin Hess of the University of Surrey in Guildford, UK, and colleagues proposed a technique to trap light inside a tapering waveguide, which is a structure that guides light waves down its length. The waveguide in question would use metamaterials – exotic materials that can bend light sharply.

The idea is that as the waveguide tapers, the components of the light are made to stop in turn at ever narrower points. That's because any given component of the light cannot pass through an opening that's smaller than its wavelength. This leads to a "trapped rainbow".

Gilded waveguide

While numerical models showed that such waveguides would work in theory, making them out of metamaterials remained a distant dream. Now Vera Smolyaninova of Towson University in Baltimore, Maryland, and colleagues have used a convex lens to create the tapered waveguide and trap a rainbow of light.

They coated one side of a 4.5-millimetre-diameter lens with a gold film 30 nanometre thick, and laid the lens – gold-side down – on a flat glass slide which was also coated with film of gold. Viewed side-on, the space between the curved lens and the flat slide was a layer of air that narrowed to zero thickness where the lens touched the slide – essentially a tapered waveguide.

When they shone a multi-wavelength laser beam at the open end of the gilded waveguide, a trapped rainbow formed inside. This could be seen as a series of coloured rings when the lens was viewed from above with a microscope: the visible light leaked through the thin gold film.

Simply amazing

Shorter-wavelength green light was trapped at a point where the taper became too thin for it to penetrate the waveguide. Longer-wavelength red light was trapped further out, where the taper was thicker, with intermediate wavelengths in between (www.arxiv.org/abs/0911.4464).

"I think it's beautiful that we can create such complex phenomena using a very, very simple configuration," says Smolyaninova. "It's amazing."

Hess agrees. He is delighted to see his theoretical prediction validated and impressed by the simplicity of the experiment. Setting the lens on the slide, he says, "is a very, very elegant way of tapering".

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