Images made up of metal-nanostructure pixels could be used for security or optical data storage. 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 nanometers) and the spaces between them, it’s possible to control what color of light they reflect. Researchers at the Agency for Science, Technology and Research (A*STAR) in Singapore used this effect, called structural color, to come up with a full palette of colors. As a proof of principle, they printed a 50×50-micrometer version of the ‘Lena’ test image, a richly colored portrait of a woman that is commonly used as a printing standard.
Joel Yang, a materials scientist A*STAR, who led the study, first noticed the effect when looking at metal nanoparticles under a light microscope. “We saw that we could control the colors, from red to blue, by controlling the size of the particles,” he says. Depending on its size, a metal nanostructure 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 color the structure reflects. This effect, called plasmon resonance, is well known to physicists. Yang is the first to come up with a way to take advantage of it to print high-resolution, full-color images, says Jay Guo, an engineer at the University of Michigan in Ann Arbor, who was not involved with the work.