Laser printing has taken a large step forward into the diminutive realm of nanotechnology with recognizable, albeit imperfect, reproductions of images so small that they could fit comfortably on the finest human hair.
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| A “Mini Lisa”, approximately 5 millimeters across Image Credit: Zhu, et al. Technical University of Denmark |
A team of researchers at the Technical University of Denmark’s (DTU) Nanotech and Fotonik departments has innovated a new laser-printing technology that is able to achieve a resolution of 127,000 dots per inch. For comparison, consider that most print publications use a resolution of 300 dots per inch, and high-end laser printers provide resolution as high as 2,400 dots per inch. The printout of Mona Lisa shown above is about 5 millimeters from edge to edge, but the DTU team has successfully printed images a hundred times smaller, coming in at 50 micrometers in width. The finest grade of human hair (so-called flaxen hair) can be as thin as 17 micrometers across according to the Physics Factbook, giving it a circumference greater than the width of the images below.
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| The white scale bar is 10 micrometers long; these images wouldn’t even wrap once around the thinnest human hair. Image Credit: Zhu, et al. Technical University of Denmark |
Although the DTU laser technology is referred to as laser-printer technology, it functions almost nothing like a conventional laser printer. The laser beam in commercial laser printers is an intermediary that creates an electrostatic charge, which then bonds ink onto the page where it dries. DTU’s laser, on the other hand, etches images directly onto the printed surface.
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| To see the individual pixels that make up the grid, you need a scanning electron microscope. Image Credit: Zhu, et al. Technical University of Denmark |
The DTU laser requires a special surface to print on, etching images onto a grid of raised columns (pictured right) that comprise the image’s “pixels”. Each column is only 100 nanometers in diameter, less than 1/100th the size of the finest human hair, and overlaid with a 20 nanometer-thick coating of aluminum. The laser printer creates differences in color by melting individual columns to different depths, which correspond to different colors. Relatively high-intensity laser pulses—sometimes reaching temperatures as high as 1,500 degrees Celsius—create deeper warps in the columns, which then reflect orange and yellow colors. Relatively low-intensity laser pulses create shallower warps in the columns, which then reflect blue and purple colors. The laser beam fires briefly, for just a few nanoseconds, to keep warping from spreading beyond just one column at a time.
“Every time you make a slight change to the column geometry, you change the way it absorbs light,” DTU Fotonik Professor of Photonics Engineering N. Asger Mortensen said in a DTU press release. “The light which is not absorbed is the colour that our eyes see. If the column absorbs all the blue light, for example, the red light will remain, making the surface appear red.”
Such high-quality reproduction at such a minute scale is not just a spectacle but could find use in security and authentication practices. “It will be possible to save data invisible to the naked eye,” DTU Nanotech Professor of Micro- and Nanotechnology Anders Kristensen (who was on the team that developed the printing technology) said in a statement. “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 technology has already been patented and the DTU team is now trying to develop it into a viable replacement for commercial laser printers.