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Published online: 1 March 2007; | doi:10.1038/news070226-11 None more blackEngineers make the most anti-reflective coating yet.Dave Mosher
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Layers of tilted silicon nanorods make a soft landing for light. Fred Schubert/Rensselaer Polytechnic Institute |
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Shiny metal objects have been reduced to a dull, record-breaking black by a super anti-reflective coating. As Nigel Tufnel from This is Spinal Tap says in the film: "It's like, how much more black could this be? And the answer is none. None more black."
Today's anti-reflective coatings reduce the reflectivity of an object to less than 1%, which is pretty good. But they often miss a few chunks of the visible spectrum. Magnesium fluoride anti-reflective coating applied to camera lenses, for example, prevents oranges, yellows, greens and blues from bouncing away (which helps to funnel more light into the lens), but misses violet and red wavelengths to create purple reflections.
"At some wavelengths, today's coatings are effective," says Fred Schubert, a member of the research team at the Rensselaer Polytechnic Institute in Troy, New York state. "But over all visible wavelengths, our coating is 10 times better than the best anti-reflective coating out there."
The team reports its work in the March issue of Nature Photonics1.
Light as air
Reflection occurs when light hits a material with a dramatically different 'refractive index'. In simple terms, a refractive index tells how fast light can travel through a material — the denser the material is, the slower light travels, and the higher the index. This is part of the reason why diamonds sparkle so brilliantly — they are made of very dense material and have a refractive index of 2.4, in comparison to air's 1.0.
The new coating has a refractive index of 1.05, cutting reflection to less than 0.1%. On a clear material, such as glass, that makes for a perfectly clear surface without glare. On an opaque material, it makes for the blackest black yet.
The coating is built of silica rods, each 2,000 times thinner than a human hair, sticking up at an angle from the surface like a slightly flattened lawn. Layers of these nanorods can be stacked up on each other, each a little bit less crowded than the one below.
The rods are deposited in a chamber that Schubert says is similar to a cathode ray tube found in old TV sets. The electron gun in the tube allows the coating's density — and refractive index — to be controlled with extreme precision. The gradual changes in layer density gently guide light towards the material, instead of having it slam into a hard, reflective surface.
Apply to surface
Anti-reflective coatings are currently used on everything from prescription glasses to computer monitors, telescopes and solar panels. Sometimes the goal is to prevent annoying glare, sometimes it's there to ensure that the material gathers as much light energy as possible. Either way, says Schubert, the new coating should perform better than what's out there already.
So far they have tested the coating on aluminum nitride, a promising light-emitting diode (LED) material. Cutting down on reflection can brighten outgoing light by ensuring that none of it is bounced off in a non-useful direction. "LEDs are incredibly energy-efficient light sources, but with our material they could make them 40% more efficient," Schubert says.
Objects that don't reflect light may also be of use to quantum physicists investigating mysterious black-body radiation. "Our anti-reflective coatings allow someone to get closer to black-body radiation than ever before," Schubert says.
But the coating is more complex than other anti-reflectives, which usually rely on one layer to get rid of reflection and can't achieve an air-like refractive index.
So will this super anti-glare coating make it onto prescription glasses any time soon? Kevin Robbie, a material physicist at Queen's University in Kingston, Canada, says a big hurdle for the coating is its fragility. "It could get wet, but rubbing it would destroy the coating," he says.
Jay Senkevich, a materials scientist with the high-tech microelectronics company Brewer Science in Rolla, Missouri, has other concerns. "The problem is manufacturing this stuff," he says; depositing the rods at a tilt is, he says, a difficult thing and can only be done on flat surfaces. "If things get too complex, it's hard to make money off of it." Schubert, however, thinks there's hope for the intricate coating.
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References
- Xi J.-Q., et al. Nature Photonics, 1 . 176 - 179 (2007). | Article |
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