the following laser printer is printing a picture of the Mona Lisa:

unlike previous printers, this printer does not need ink. Yes, there is no need for ink cartridges or toner, as long as the laser irradiation, you will get a variety of colors. The effect of printing looks like this:

Why don't you need ink? Because this printer prints structural colors.

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colors can be rendered in two ways: pigments and structural colors. Our skin, as well as the various pigments attached to the paper, all come from chemical pigments. There are also many colors that are not determined by pigments, but by special microstructure. For example, the sparkling blue on butterfly wings comes from a special surface structure and the action of light.

(picture source asknature.org)

in the laboratory, scientists also use a variety of microstructure to create artificial structural colors, as do printing techniques that do not require ink. Here, the "paper" used for printing actually has a special microstructure: on the plastic substrate, there are neatly arranged nanometer "small columns" covered with a thin layer of germanium.

this special nanostructure can interact with light to show color, and different intensity of laser can also change the local microstructure, thus printing different colors. Low-intensity laser can produce blue, while increasing intensity can produce red and yellow. the laser processes the microstructure one by one, and finally prints the whole color picture.

what are the benefits of this? First of all, compared with the pigment substance, the structural color is more stable and is not easy to fade after printing. Moreover, because the microstructure of the color is very small, the picture printing can also achieve a very high resolution and can make a very fine image. For example, the following printed color images are all as wide as a human hair:

but this ink-free printing technique also has its drawbacks: it can't print green. Red and blue are not bright enough, and they look a little strange. At present, the researchers continue to improve the microstructure of these colors, hoping to achieve better printing results.

Source:

https://www.newscientist.com/article/2130151-lasers-print-ultra-high-res-images-narrower-than-a-human-hair/

http://advances.sciencemag.org/content/3/5/e1602487