14 August 2012
A nanoscale image (a) before the addition of metal and (b) after adding metal layers to the nanostructures in specific patterns (Images: Agency for Science, Technology and Research)
A new way of printing colours at the highest resolution permitted by the laws of physics could be used to create secure watermarks or high-density data storage – not to mention some great-looking pictures.
Joel Yang and colleagues at the Agency for Science, Technology and Research (A*STAR) in Singapore, who created the new technique, were inspired by the colours in stained-glass windows. These are normally made by adding metallic fragments to glass, with light scattering off nanoparticles in the metal to produce a range of colour.
Zooming in, the specular reflection at the corner of the eye shows the refined colour detail that the new method is able to achieve. The region indicated (right) is made up of nanostructures as observed in the electron micrograph
Each “pixel” in the image is actually made up of four nanoscale cylinders coated in silver and gold. The colour they produce depends on both the diameter and spacing of the cylinders, allowing Yang to “print” a full-colour image just by carving out cylinders at the right scale.
The team tried out their method by printing a 50×50 micrometre copy of “Lena”, a photograph of a woman commonly used in image processing tests. This image has a resolution of around 100,000 dots per inches (dpi), compared to the 10,000 dpi images produced by regular printing methods such as inkjet and laser printers.
Yang and colleagues choose this figure because it corresponds to a fundamental optical limit. Visible light, with an average wavelength of 500 nanometres, can only distinguish between objects that are half that distance apart – any closer than 250 nanometres, and the two would blur into one.
If the new method can be scaled up to print at regular sizes the resulting images will be of incredibly high quality. Alternatively, Yang and colleagues could apply their printing technique to creating tiny watermarks for security. The method of printing nanostructures very close together could also be used to create high-density versions of optical storage discs such as DVDs.