Tapering Waveguide Captures a Rainbow 72
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by
kdawson
from the you-fight-with-light-surely-that-is-forbidden dept.
from the you-fight-with-light-surely-that-is-forbidden dept.
SubComdTaco passes along news of researchers in the US who have trapped a rainbow in a tapering waveguide. The research is described (PDF) on the arXiv. "In 2007, Ortwin Hess of the University of Surrey in Guildford, UK, and colleagues proposed a technique to trap light inside a tapering waveguide [made of metamaterials]... 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.' ... 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-mm-diameter lens with a gold film..., 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... gilded waveguide, a trapped rainbow formed inside. This could be seen as a series of colored rings when the lens was viewed from above with a microscope: the visible light leaked through the thin gold film."
I'm interested to see what journal that gets in (Score:4, Insightful)
It was a very simple experiment to perform. It doesn't make any measurement of the group velocity or demonstration of trapped light (which would typically involve releasing it controllably and detecting it). The original proposal involved meta-materials to achieve a region with a negative index of refraction to use as the waveguide. They could then (hopefully) manipulate the meta-material to controllably store and retrieve light.
It seems this experiment used a simple meta-material the consisted of the glass surfaces, the 30-nm gold coating and the air gap in a Newton's rings setup. They may even have had the gold coated lens lying around and did the experiment over lunch (which just involved taking a picture). I don't think it's all that interesting until they get storage and retrieval.
RE: Newtons Rings type experiment. (Score:2, Insightful)
The effect described is based on the distance between two very reflective surfaces being smaller than the wavelength of light involved, thus preventing the light from travelling further down the waveguide. The taper on the waveguide means that as you go to shorter wavelengths of light, it can travel further, thus generating a 'trapped rainbow' of visible light inside the waveguide.
A key difference to note is that the fringe pattern generated by an interferometer type setup repeats itself as you increase/decrease the distance between the two reflective surfaces, so generating a series of lines or concentric circles. The setup with the 'trapped rainbow' will create a single rainbow pattern.