The world's most anti-reflective coating

Researchers from the Rensselaer Polytechnic Institute (RPI) have created a material that reflects virtually no light -- or absorbs it like a black hole. They've built this new material by stacking layers of 'silica nanorods.' And they reached a refractive index of 1.05. This can be compared with 1.0 for air or 2.4 for diamonds. This coating, which is effective for all wavelengths, could be used for brighter light-emitting diodes (LEDs), more efficient solar cells, and new classes of 'smart' light sources that adjust to their environments. But don't expect to see your next glasses or the windshields of your cars protected with this coating: it's too sensitive to water and it would be destroyed by rain.

This new material has been developed by a team led by E. Fred Schubert, "the Wellfleet Senior Constellation Professor of the Future Chips Constellation at RPI. And it will be described in a paper to appear in the March 2007 issue of Nature Photonics which is not available online yet.

The image on the left shows various reflecting surfaces. The top three images show respectively "light reflecting off surfaces made from aluminum, silicon, and aluminum nitride. At bottom is a piece of aluminum nitride coated with the new anti-reflection material" designed at RPI (Credit: Fred Schubert, RPI). Here is a link to a larger version.

In "None more black," Dave Mosher writes for Nature that "today's anti-reflective coatings reduce the reflectivity of an object to less than 1%, which is pretty good." But according to Schubert, "over all visible wavelengths, our coating is 10 times better than the best anti-reflective coating out there."

And in "No Glare There," JR Minkel tells in Scientific American how the new material was created. [Schubert] "and his colleagues deposited silicon dioxide or titanium dioxide vapor on aluminum nitride, a transparent semiconductor used in high-tech LEDs. By tilting the aluminum nitride wafer, the group coaxed the vapor to condense into a forest of tiny rods, each about 50 nanometers wide and tilted at an angle" [of precisely 45 degrees.]

Nature gives more explanations about this coating. "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.

This is how they reduced the refractive index, as explains Scientific American. "By stacking five layers of nanorods, each one less porous than the one below it, they varied the coating's refractive index in a series of steps, going from 2.03 at the bottom (closely matching the wafer) to 1.05 at the top (compared with 1.0 for air). Because each step is small, total reflection was reduced to as little as 0.1 percent."

And if this new coating, which works best at around 30° Celsius, can be destroyed by water, what could be its possible applications. According to the RPI news release, they could include brighter LEDs, 'Smart' lighting, optical interconnects or high-reflectance mirrors.

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