Non-reflective coating for optical devices

Materials World magazine
1 Apr 2007

An innovative coating that reflects virtually no light has been created by a team of researchers at the Rensselaer Polytechnic Institute, USA.

Most surfaces refract some light, which can have a negative effect on the performance of optical components and devices. The new material has a refractive index of 1.05, which is almost the same as air (window glass, by comparison, has an index of 1.45). This gives researchers improved control over the basic properties of light.

Using a technique called oblique angle deposition, the scientists, led by E Fred Schubert, Senior Constellation Professor, were able to position silica nanorods at an angle of precisely 45º onto a thin film of aluminium nitride, a semi-conducting material that is used in advanced light-emitting diodes (LEDs). By using low-reflective nanorods deposited at an angle from the lightwaves, researchers were able to greatly reduce or even eliminate reflection at all incoming angles of light. This has resulted in a material that sets a world record for decreased reflectivity, compared to conventional anti-reflection coatings, by an order of magnitude.

The oblique-angle deposition technique is not time-consuming, says Schubert.

‘The technical community will be able to easily reproduce our results.' It can also be used on a variety of substrates. Silica was chosen because it has a small refractive index to begin with, he says, ‘but for other applications, [for example] where conductivity is required, ITO [indium tin oxide] may be the next best material. So it depends on the application as to what material we would choose. ‘We will apply the new class of materials to solid-state lighting devices to attain higher efficiency and power,' adds Schubert.

The technique could also improve the luminance of LEDs, making them bright enough to replace conventional fluorescent and incandescent bulbs.

With a greater control over light, researchers may also be able to gain a better insight into the field of photonics, which would improve communication between electrical circuits in computing applications.