Light reflects when it passes between two substances that differ in refractive index, or the speed of light through a material—the bigger the gap between the indexes, the brighter the reflection. The new coating has nearly the same refractive index as air. "A piece of glass would have a reflection, but this material doesn't," says Fred Schubert, an optical materials researcher at Rensselaer Polytechnic Institute in Troy, New York, whose group developed the coating. "It's wonderful." Sadly, he says it is not suitable for surfaces such as windshields and eyeglasses. Electronics researchers, though, have long sought to control reflection. In solar cells, for example, light bouncing off the surface of the cell reduces its efficiency, because the reflected light could have been converted to electricity. But constructing ultrathin anti-reflection coatings has proved challenging, Schubert says.
To create the material, he 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.
The key to the material's low refractive index is its porosity, Schubert says. The more easily a material's electrons are jostled by electromagnetic waves, the higher its index. But a porous material has fewer electrons to be jostled, he says, hence a lower index.
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, the group reports this week in Nature Photonics.
The graded steps had another notable effect: The coating works well for many wavelengths of light and for light coming in at a range of angles—though it works best at close to 30 degrees, the group found. Schubert says antireflection layers typically reduce the reflection of one wavelength of light at the expense of enhancing reflection at other wavelengths, and they tend to work only for light striking the surface perpendicularly.
"It's one of the highest quality antireflection coatings I've seen," says Steven DenBaars, an LED-lighting researcher at the University of California, Santa Barbara. "It's not the solution to high brightness for LEDs," he says, but it may bump up their output by 5 to 10 percent. The next step, he says: "They'll have to see what it does in a real device."