Structural colouration works thanks to nanoscale pillar-like metallic, dielectric or hybrid structures reflecting light at different wavelengths. Similar structures are already found in nature – for example, a peacock’s tail feathers are brown, but on the microscale they reflect blue and green light.

A team led by Anders Kristensen of the Department of Micro- and Nanotechnology at the Technical University of Denmark has now created surfaces containing columns of plastic 60 nm high coated with a layer of 35 nm thick germanium, which is a high refractive-index dielectric material (an electrical insulator that can be polarized using an applied electric field).

“The germanium pillars can absorb a certain part of the electromagnetic spectrum and thus reflect the other parts,” explains team member and lead author of the study Xiaolong Zhu. “We change the colours being reflected by changing their geometry using a laser, which warms up a pillar and melts it into a sphere shape. Small spheres can absorb another part of the colour spectrum and reflect a different colour than the pillars (which look like disks from above). In this way, we can adjust the laser’s strength so that disks melt more or less or can even be ablated away, thus producing a group of colours that are reflected.”

A resolution of 100,000 dots per inch

Since the laser treatment is only applied to the nanostructures on the metasurface and only a single disk at the centre of the laser spot can absorb the laser energy efficiently and heat up to its melting point, neighbouring structures absorb less energy and so do not change their geometry, he tells We can thus control the colours being reflected on the single disk level, which gives us a resolution of 100,000 dots per inch (dpi), which is at the upper end of the limit for modern laser printers.

“The printed structure colours may replace any ink-based colour printing products,” says Zhu, “and products like outdoor posters and wallpapers, or even car coatings, could benefit.”

With their technique, the researchers have succeeded in producing almost all colours except green. They are now working on resolving this problem by studying dielectric materials other than germanium.

They report on their work in Science Advances DOI: 10.1126/sciadv.1602487.