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Creating vibrant optical colours with ultra-thin silicon films

24 March 2015

Technology that creates a rainbow of optical colours with ultra-thin layers of silicon, could have applications in jewellery, automotive interior trim and wearable displays.

Seyed Sadreddin Mirshafieyan (left) with Professor Junpeng Guo present a disc showing a rainbow of optical colours created with ultra-thin layers of silicon (photo: Michael Mercier/UAH)

A research group at the University of Alabama in Huntsville (UAH) has demonstrated how vibrant optical colours can be generated from ultra-thin, single-layer silicon films deposited on a thin aluminium film surface.

In this low-cost process, the optical colours are controlled by the thickness of the silicon films, which ranges from 20 to 200nm. The silicon colour coating process can be applied on almost any material surface; in fact, the team has coloured quarter coins, turning them into a variety of colours.

The group's choice of silicon was influenced by the fact that it is an indirect band-gap semiconductor material with both high index of refraction and low optical absorption in the visible spectrum.

"The combination of high index of refraction and low absorption enables strong optical wave interference inside ultra-thin silicon films, a physical process that results in colours,” says UAH Professor Junpeng Guo, who has published the result with his graduate student, Seyed Sadreddin Mirshafieyan, in a recent issue of the journal, Optics Express.

colours seen from flowers in nature and chemical materials are caused by wavelength selective light absorption in organic molecules. Currently, colours on computer and iPhone screens come from dye materials pre-placed on the pixels. colours of chemical dyes only work in a limited range of temperatures around room temperature. The demonstrated silicon colours can sustain high temperatures and harsh environment. 

“The reason these colours are so vibrant, is because one wavelength of light is completely absorbed,” says Professor Guo, “and the colours are very durable."

Professor Guo adds that a lot of colours seen in nature are due to wavelength selective light absorption in organic molecules which cannot withstand high temperatures. Ultraviolet light destroys organic dye molecules over time, leading to colour change and fading.

The new technology may hold promise for many applications such as for jewellery, automotive interior trim, aviation, signage, coloured keypads, electronics and wearable displays.

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