In a major breakthrough in optical science, researchers at Rensselaer Polytechnic Institute (RPI), in Troy, NY, and semiconductor maker Crystal IS, in Green Island, NY, have developed a new type of nanostructured antireflection coating that can virtually eliminate reflections, potentially leading to remarkable improvements in optical devices.
The coating material has very low refractive index which will tremendously reduce reflection losses and increase the efficiency of optical devices. Fred Schubert, professor of physics and electrical, computer, and systems engineering at RPI says that the total reflection is 10 times less than it is with current coatings.
The coating would result in a 40 percent improvement in the amount of light absorbed by LEDs. This antireflection coating is made possible by creating a multilayered, porous coating that eases the transition as light moves from air into a solid material or as light is emitted from a semiconductor in an LED. Each successive layer bends light more as light moves from air into a substrate. The porous structure is made by growing nanoscale rods projected at specific angles on a tilted surface.
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.
