Nanostructures allow specific filtering of colours

An international team of engineers, led by Jinghua Teng and Yan Jun Liu at the A*STAR Institute of Materials Research and Engineering in Singapore, has developed arrays of silver nanoscale pillars that can selectively reflect light of any desired colour. A beam of sunlight is a mixture of different-coloured light, including all the colours of the rainbow. Filtering or blocking a specific colour, or colours, is often important in photography, colour displays and other imaging techniques. According to the researchers, the discovery has enabled them to select various colours by varying the size of the pillars.

The stained glass in the windows of a church owes its colour in part to an effect called surface plasmon resonance: light passing through the window interacts with electrons in the nanometre-sized metallic impurities that are trapped in the glass.

Light of a specific colour, or wavelength, forces these electrons to quickly oscillate. In turn, the oscillating electrons enhance the amount of light transmitted through the glass at this wavelength. Teng, Liu and their co-workers were able to transfer this plasmonic effect from light-transmitting windows to light-reflecting mirrors. "Our compact reflectors could be used for applications including colour coding, anti-counterfeiting and product branding," indicated Teng.

The researchers deposited 6nm of titanium, followed by 180nm of silver on a quartz substrate. Onto the silver layer, they etched arrays of cylinders with diameters of 300-500nm and a centre-to-centre separation of 320-540nm. The resulting gap between some of the pillars was as small as 20nm. To achieve these tiny features, the team used a technique called electron-beam lithography: they scanned a beam of electrons to pattern the required features onto a protective layer placed on top of the silver. Then, they used a stream of charged ion atoms to mill the exposed metal and create the nanopillars.

After construction, Teng, Liu and their team shone white light onto each of the arrays and measured the wavelength of the reflected radiation. Arrays of cylinders of 500nm in diameter and separated by 40nm appeared red because they predominantly reflected light with a wavelength of 630nm. Similarly, pillars with a diameter of 300nm and a separation of 20nm appeared blue as they reflected light with a 490nm wavelength.

"We are now working to further develop this technique to create large-area colour displays," stated Teng. "We also aim to develop applications and collaborations with industry."