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Optical coating with refractive index of 1.05 |
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Mar 02, 2007 at 09:36 AM |
 Rensselaer Polytechnic Institute (RPI) scientists have produced an optical coating with almost the same refractive index as air (Nature Photonics, March 2007). This means that its light reflection properties are practically zero. RPI claims a world record by decreasing the reflectivity compared to conventional anti-reflection coatings by an order of magnitude.
An optical coating made from the material that enables vastly improved control over the basic properties of light, the team reports. Potential applications of anti-reflection coatings made from the material include much brighter LEDs, more efficient solar cells, and a new class of "smart" light sources that adjust to specific environments.
The new material 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.
The refractive index governs the amount of light a material reflects, as well as other optical properties such as diffraction, refraction, and the speed of light inside the material. "The refractive index is the most fundamental quantity in optics and photonics. It goes all the way back to Isaac Newton, who called it the ‘optical density,'" comments E. Fred Schubert, the Wellfleet Senior Constellation Professor of the Future Chips Constellation at Rensselaer and senior author of the paper.
Many scientists have searched for materials that can eliminate unwanted reflections that degrade the performance of various optical components and devices. "We started thinking, there is no viable material available in the refractive index range 1.0-1.4," Schubert says. "If we had such a material, we could do incredible new things in optics and photonics."
The material is produced using a technique called oblique angle deposition, which puts silica nanorods at a 45 degree angle on top of a thin film of aluminum nitride, a semiconducting material used in advanced light-emitting diodes (LEDs). From the side, the films look much like the cross section of a piece of lawn turf with the blades slightly flattened.
The technique allows the researchers to strongly reduce or even eliminate reflection at all wavelengths and incoming angles of light. Conventional anti-reflection coatings, although widely used, work only at a single wavelength and when the light source is positioned directly perpendicular to the material.
The new coating could be used to increase the amount of light reaching the active region of a solar cell by several percent, boosting performance. "Conventional coatings are not appropriate for a broad spectral source like the sun," Schubert says. "The sun emits light in the ultraviolet, infrared, and visible spectral range. To use all the energy provided by the sun, we don't want any energy reflected by the solar cell surface."
In addition, current LEDs are not yet bright enough to replace the standard light bulb. Eliminating reflection could improve the luminance of LEDs, which could accelerate the replacement of conventional light sources by solid-state sources. In the other direction, the team thinks it possible to precisely control a material's refractive index thereby making extremely high-reflectance mirrors.
Schubert and his coworkers have only made several samples of the new material to prove it can be done, but the oblique angle evaporation technique is already widely used in industry, and the design can be applied to any type of substrate — not just an expensive semiconductor such as aluminum nitride.
The research is funded primarily by the National Science Foundation, with additional support from the U.S. Department of Energy, the U.S. Army Research Office, the New York State Office of Science, Technology and Academic Research (NYSTAR), Sandia National Laboratories, and the Samsung Advanced Institute of Technology in Korea. The substrates were provided by Crystal IS, a manufacturer of single-crystal aluminum nitride substrates for the production of high-power, high-temperature, and optoelectronic devices such as blue and ultraviolet lasers.
By Dr Mike Cooke
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Lithography 
