The RPI researchers made such a porous structure by depositing materials on a surface to create nanoscale rods. Tilting the surface makes it possible to grow the nanorods at an angle. The researchers found that by changing the angle of the nanorods, they can control the way the nanorods bend light–the index of refraction. Air has an index of refraction of very nearly one. The researchers were able to make a top layer of nanorods with what Schubert says is an unprecedented index of 1.05. (For comparison, glass has an index of refraction of 1.45, and a light-emitting semiconductor, aluminum nitride, has an index of about 2.05.) Each successive layer has a higher index of refraction until the last layer nearly matches the substrate. The top two layers incorporate glass nanorods. The bottom three are made of titania. The researchers tested the coating on aluminum nitride, but it should work on a variety of substrates, Schubert says.
“We have developed a new class of materials that has a refractive index that is lower than anything else–any other viable optical thin-film material that has been available in the past,” Schubert says. Since “everything in optics depends upon the refractive index,” he says it could have applications other than antireflective coatings. Indeed, the nanorods could be used to do the opposite, creating very highly reflective mirrors by pairing layers of nanorods that bend light very differently, rather than by creating a gradual transition.
Schubert is working with a spinoff company to commercialize the technology, and he anticipates that products could be available in three to five years. The technology will face competition with inexpensive conventional coatings as well as with other new nanostructured materials. “This is very elegant, beautiful work,” says Michael Rubner, a professor of materials science and engineering at MIT. “They’ve been able to get some exceptionally low refractive-index values for a coating. The key question is always going to be cost versus performance.”