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Taking the Shine Off

1 March 2007 (All day)
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Rensselaer / Fred Schubert

Dark and darker.
From top, light reflecting off surfaces made from aluminum, silicon, and aluminum nitride. At bottom is a piece or aluminum nitride coated with the new antireflection material.

Narcissus beware! Scientists have created the world's first film that casts practically no reflection. A vast improvement over current nonreflective materials, the new technology could revolutionize solar cells, intensify light-emitting diodes, and possibly help solve mysteries in quantum mechanics by mimicking a "black body," an object that absorbs all light.

For years, scientists have been seeking ways to make certain materials less reflective. Solar cells, for example, would be far more efficient if they reflected less light and absorbed more of it as energy. To achieve this goal, researchers have concentrated on reducing a material's refractive index--a measure of its ability of to reflect light. Ice has a refractive index of 1.31, for example; air has an index of 1. But making low-reflective materials, which are also thin enough to serve as coatings, has proven tricky.

A team of electrical engineers led by E. Fred Schubert of the Rensselaer Polytechnic Institute in Troy, New York, approached the problem by creating a new class of highly porous nanomaterials. The researchers used an evaporation technique to deposit silica onto a semiconductor. The silica is arranged in such a way that the material becomes porous, letting in more light than do traditional antireflective coatings. What's more, the silica molecules are arranged at such an angle that they transmit more wavelengths of light than current coatings, as well as capturing more incident light. The new material has a refractive index of 1.05, the team reports in the March issue of Nature Photonics, the lowest ever reported for a thin film. "This was, so to speak, our dream goal," says Schubert. "It allows us to make materials in a range of refractive indices that weren't previously possible."

The true breakthrough of this material, says optical scientist Daniel Poitras of the Institute for Microstructural Sciences in Ontario, Canada, is its arrangement as a five-layer sheet. That's what allows it to capture far more light than traditional coatings, he says. Schubert adds that the new coating reflects no light across much of the visual spectrum, unlike others that work only at a single wavelength. That's a potential plus for solar cells, which could more efficiently use the sun's energy for a boost in performance. One concern, notes Poitras, is fragility. Such a porous material could degrade under certain conditions, such as humidity.

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