Light control

A team of researchers from Rensselaer Polytechnic Institute has created the world’s first material that reflects virtually no light.

Reporting in the March issue of Nature Photonics, they describe an optical coating made from the material that enables improved control over the basic properties of light. The research could open the door to much brighter LEDs, more efficient solar cells, and a new class of "smart" light sources that adjust to specific environments, among many other potential applications.

Most surfaces reflect some light. The new material has almost the same refractive index as air, making it an ideal building block for anti-reflection coatings. It is said to set a world record by decreasing the reflectivity compared to conventional anti-reflection coatings by an order of magnitude.

A fundamental property called 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,”’ said E. Fred Schubert, the Wellfleet Senior Constellation Professor of the Future Chips Constellation at Rensselaer and senior author of the paper.

Schubert and his colleagues have created a material with a refractive index of 1.05, which is close to the refractive index of air and the lowest ever reported. Window glass, for comparison, has a refractive index of about 1.45.

Scientists have attempted for years to create materials that can eliminate unwanted reflections, which can 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 said. ‘If we had such a material, we could do incredible new things in optics and photonics.’

From left, light reflecting off surfaces made from aluminium, silicon, and aluminium nitride. On the right is a piece of aluminium nitride coated with the new anti-reflection material

Using a technique called oblique angle deposition, the researchers deposited silica nanorods at an angle of precisely 45 degrees on top of a thin film of aluminium nitride, which is 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, Schubert said. 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 optical coating could find use in just about any application where light travels into or out of a material, such as more efficient solar cells, brighter LEDs, "Smart" lighting, and optical interconnects.

With regard to solar cells, the new coating could increase the amount of light reaching the active region of a solar cell by several percent, which could have a major impact on its performance. ‘Conventional coatings are not appropriate for a broad spectral source like the sun,’ Schubert said. ‘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.’

Schubert and his colleagues 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 aluminium nitride.