Mirror, mirror on the wall leaves no reflection ... none at all

April 26, 2007
Researchers at Rensselaer Polytechnic Institute report they've created the world's first material that reflects virtually no light.

Silica nanorods deposited at a 45° angle on the surface of an aluminum-nitride substrate reflect virtually no ultraviolet, infrared, or visible light. The ability to eliminate reflections could improve the luminescence of LEDs, which could accelerate the replacement of conventional light sources by solid-state sources.

The team's nanorod coating has a refractive index of 1.05, which is extremely close to the refractive index of air and the lowest ever reported. Window glass, for comparison, has a refractive index of about 1.45.

According to E. Fred Schubert, a professor at Rensselaer, the team used a technique called oblique angle deposition to apply the coating on top of a thin film of aluminum nitride — a semiconducting material used in advanced LEDs. The process let them deposit silica nanorods at an angle of precisely 45°. From the side, deposited nanorods resemble slightly flattened blades of grass.

The Rensselaer coating reduces or even eliminates reflection at all wavelengths and incoming angles of light, reports Schubert. Conventional antireflection coatings work only at a single wavelength and only when the light source is positioned directly perpendicular to the material. The research could open the door for much brighter LEDs and a new class of light sources that adjust to specific environments.

"The coating could also have a major impact on solar cell performance by increasing by several percent the amount of light reaching the active region of a solar cell," says Schubert. "Conventional coatings are not appropriate for a broad spectral source like the sun. The sun emits in the ultraviolet, infrared, and visible spectral range. To use the energy provided by the sun, we won't want any energy reflected by the solar cell surface," he says.

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