Nanomaterials Create Optical Coating That Is Almost Reflection Free

An optical coating engineered from nanomaterials has exhibited virtually no reflection, almost equaling the refractive index of air. Researchers at Rensselaer Polytechnic Institute in Troy, N.Y., reported in the March issue of Nature Photonics their development of the material, which could result in more efficient solar cells, brighter LEDs and perhaps even greater control over the properties of light itself.

Light reflects off different surfaces — from right to left, aluminum, silicon, aluminum nitride and a piece of aluminum nitride coated with a new antireflection material. Images reprinted with permission of Fred Schubert, Rensselaer Polytechnic Institute.

Although antireflection coatings are widely available, in most cases they function only at a single wavelength and only when the light source is perpendicular to the material.

The graded-index broadband omnidirectional coating was created using oblique-angle deposition, a technique commonly used to grow thin-film materials with a controllable porosity. Silica nanorods were deposited on an aluminum nitride substrate that was tilted at an angle close to 85°. The gradation in refractive index from one that matches air to one closely matching the substrate virtually eliminates Fresnel reflection.

Silica nanorods are deposited at an oblique angle on an aluminum nitride substrate.

The setup enabled the reflection of incoming light from a broad spectrum and many angles to be drastically reduced or even eliminated. In some scenarios, the coating achieved a refractive index of 1.05, which the researchers said is the lowest ever recorded for an optical thin-film material. Window glass, by comparison, has a refractive index of 1.45.

Experiments showed that the coating had very low reflectivity over the entire visible and near-IR spectrum. At wavelengths of ~400 nm, the reflectivity of the coating started to increase. The scientists also found that the coating had very low reflectivity at almost all incident angles, with the exception of those close to 90°.

The coating could improve the performance of solar cells by increasing the amount of light that reaches the cell. The sun emits energy over a broad spectral range, some of which is wasted with current solar cells. The coating also could reduce the reflection, and thus improve the luminance, of LEDs. This could accelerate their integration into a range of applications, including automobile headlights and solid-state lighting. The technology also could be reversed to make extremely high reflectance mirrors for use in telescopes and sensors.