A printing method using nanostructured surfaces and laser pulses has yielded a microscopic image with a resolution of 127,000 dots per inch. The reproduction the "Mona Lisa," created by researchers from the Technical University of Denmark (DTU), is smaller than one pixel on an iPhone Retina display. This reproduction of the "Mona Lisa" is about 50 μm wide, or about 10,000 times smaller than the original painting. Courtesy of Anders Kristensen. Printing such microscopic images entailed a nanostructured surface consisting of rows with 100-nm-diameter columns. This metamaterial was then covered by 20 nm of aluminium. When a laser pulse was transmitted from nanocolumn to nanocolumn, each nanocolumn was heated locally, causing it to deform. The extent of column deformation determined which frequencies of light were reflected, meaning printing color could be controlled by varying laser intensity. Low-intensity pulses led to a minor deformation of nanocolumns, resulting in blue and purple reflections. Stronger pulses created a drastic deformation, which yielded orange and yellow tones. The temperature of the columns reached up to 1500 °C, but for only a few nanoseconds, preventing the extreme heat from spreading. Potential applications for the technology are broad, the researchers said. They have secured a patent and are focused on commercial development and applications. "It will be possible to save data invisible to the naked eye," said professor Anders Kristensen. "This includes serial numbers or bar codes of products and other information. The technology can also be used to combat fraud and forgery, as the products will be labelled in way that makes them very difficult to reproduce. It will be easier to determine whether the product is an original or a copy." The laser printing technology could also be used on a larger scale to personalize products such as mobile phones with unique decorations, as well as to replace conventional home and office laser printers. Foreign companies producing parts for cars, such as instrument panels and buttons, are also taking interest in the technology as a way to simplify production, the researchers said. Today, the large number of different instrument panels must be adapted to the various accessories that the car has, including air conditioning, USB outlets and cigarette lighters. Research results were published in Nature Nanotechnology (doi: 10.1038/nnano.2015.285).