Joel Yang of the Agency for Science, Technology and Research (A*STAR) in Singapore is head of a team which has created a printed image at the highest possible resolution, an incredible 100,000 dots per inch. The team's discovery was reported in Nature and has huge implications for the fields of security and data storage.
Yang's team found that they were able to produce the ultra-high resolution image by exploiting the light-reflecting properties of metal nanoparticles. Nanoparticles are billionths of a metre, or millionths of a millimetre across – a DNA helix, for example is 2 nanometres wide. When studying the nanoparticles under a microscope Yang and his team found that varying the size of the particles altered the wavelength, and therefore the colour, of the light that they reflected.
There are two different types of colour that we can see; pigmented and structural. Pigmented colour is created when materials absorb some wavelengths of light and reflect others, so we see the reflected wavelengths as a particular colour. Toner particles are pigmented so cyan toner, for example, absorbs all wavelengths except blue, which is reflected into our eyes. Structural colour is created when light hits a prism and different wavelengths are bent at different angles. This effect is responsible for the iridescence in a soap bubble or a butterfly wing and the colour we see is dependent on the size and shape of the reflecting surface and the angle at which the light hits it.
Yang and his team were able to manipulate this property in microscopic discs of gold and silver to create their image. The pixels in the image are made up of four pillars, each just tens of nanometres tall and topped with the silver and gold nanodiscs. By varying the size of each disc and the distance between them, Yang’s team could determine the colour of each pixel. They then printed a 50x50 micrometre test image, of the "Lena" portrait, a picture widely used as a printing standard.
The resolution of the Lena image is a staggering 100,000 dots per inch, ten times the maximum resolution possible with a laser or inkjet printer and the highest resolution that it’s possible to achieve. The reason for this limit is an effect called the diffraction limit, which means that objects cannot be closer than 250 nanometres apart without looking smudged, even under the most powerful microscope. This is still roughly 1000 times smaller than the smallest object than can be seen by the human eye.
One of the advantages of structural over pigmented colour is that structural colour doesn't fade over time, so the images are durable. The process is slow and is currently impractical over larger areas but Yang is working on ways to commercialise the printing method and sees it being used to create nanoscale watermarks, or to encrypt information. It may also be possible use the technique to create ultra-high density optical discs to archive large amounts of data.
Will the technique ever have a practical application in the world of commercial and business printing? Given that the resolution of the images is higher than the human eye can detect, it seems unlikely but new technology often ends up being used in ways never imagined by its creators.
If you're really interested in the science behind the A*STAR team's discovery, you can read the original paper in Nature Nanotechnology.
by Anthony Morgan