Printing at its Ultimate Resolluti
The highest possible resolution images about *100,000 dots per inch have been achieved, and in full-colour, with a printing method that uses tiny pillars a few tens of nanometres tan.
The method could be used to print tiny watermarks or secret messages for security purposes, and to make high-density data-storage discs.
Each pixel in these ultra-resolution images is made up of four nanoscale posts capped with silver and gold nanodisks. By varying the diameters of the structures (which are tens of nanometres) and the spaces between them, it’s possible to control what colour of light they reflect. Researchers at the Agency for Science, Technology and Research (A”STAR) in Singapore used this effect, called structural colour, to come up with a full palette of colours.
Joel Yang, a materials scientist A*STAR, who led the study, first noticed the effect when looking at metal nano-particles under a light microscope. He said what they saw was, they could control the colours, from red to blue, by controlling the size of the particles. Depending on its size, a metal nano structure resonates with a particular wavelength of light much like a guitar string resonates at a particular frequency depending on its length. Light at the right wavelength causes electrons on the surface of the metal nanostructure to resonate, and this determines the colour the structure reflects. This effect is called plasmon resonance. Yang is the first to come up with a way to take advantage of it to print high-resolution, full-colour images.
Indelible Ink: For the initial demonstration of the Lena image, the researchers first used electron-beam lithography to pattern a silicon wafer with an array of posts made from an insulating material. Then, they deposited metal nano-disks on the posts and coated the wafer’s surface with metal. The metal coating on the wafer reflects the coloured light from the pillars, making the image bright. ‘The colours appeared all at once after the researchers applied the metal.
Yang’s stuctured-colour images have a resolution of about 100,000 dots per inch. By comparison, inkjet and laser printers produce ink spots that are micrometres in size, and their resolution tops out at around 10,000 dots per inch. In addition to resolution, another advantage to structural colour is image stability. The metal and insulating materials used to make these images are durable. They do not fade over time, unlike organic dyes and colourants.
The researchers have applied for a patent for the printing method, and hope
to commercialize microimages as
nano-scale watermarks, or for cryptography. The method for printing coloured spots very close together could also be used to encode ultra-dense optical data to disks similar to DVDs. Because these images cannot be rewritten, they’ would be most useful for storing archival informatio