Researchers in Singapore have developed an innovative inkless printing method that uses metal nanostructures to build sharp full-colour images at a resolution of 100,000 dots per inch (dpi) – 10 times the current best resolution. Scaled up, the technique could find applications in anti-counterfeiting, high-density optical data storage or in transmitting hidden messages.

Even with today's best optical microscopes, there is a hard resolution limit – half the wavelength of the light used for imaging – that dictates how close two juxtaposed colour pixels can be while still being distinguishable from each other. Any closer than this "optical diffraction limit" and the light reflecting from the two elements diffracts, overlaps and the colours blur into one.

For mid-spectrum visible light of about 500 nm, colour pixels are bound by the optical diffraction limit to a minimum size of 250 nm – or a resolution of about 100,000 dpi. But even the best industrial inkjet and laserjet printers struggle to achieve resolutions one-tenth that fine, because of their micron-scale ink spots.

Stained-glass inspiration

Recently, nanotechnologists have looked to the medieval art of glass staining for inspiration. Here, metal-dust additives create characteristically vivid colours when light hits the metal particles and certain wavelengths are absorbed to excite plasmons – coherent oscillations of conduction electrons on the metal's surface. Based on this knowledge, reflective metal films dotted with selective light-transmitting holes have already been successfully used to make micron-sized colour pixels, but no one has hit upon a method to shrink them any further, until now.

Finely tuned reflectors

Karthik Kumar and colleagues from Singapore's Agency for Science, Technology and Research combined the idea of nanohole reflectors with another tried-and-tested idea – arrays of isolated metal nanoparticles that absorb or reflect different wavelengths of light according to their diameters. Using electron-beam lithography, they etched silicon-oxide pillars, tens of nanometres wide, on top of a silicon substrate. Next they used a metal evaporation technique to deposit an ultrathin film of plasmonically active silver (15 nm) and gold (5 nm) on to the tips of the pillars and the substrate.

The raised nanodiscs were arranged two-by-two on the back reflector, in pixels measuring 250 nm. Different colours were encoded into each pixel by altering the diameter of the nanodiscs (from 50–140 nm) and the distance between them (30–20 nm). By tweaking the diameters of the discs, the researchers were able to control the frequency of the plasmon resonances across their surfaces – in much the same way that altering the length of a violin string alters its resonant frequencies – and thus control which wavelengths of light were removed from the incident light and which were reflected.

"The distance between the structures also seems to make a difference in what we think is the two structures coupling with one another," explains Kumar. "Basically they seem to be talking to each other at these small distances, and that is why we also see that we need to have a small group of these structures in order to be able to see the colours effectively."

Striking demonstration

In demonstrating their novel technique, the team perfectly reproduced the "Lena test image" – a cropped image of a 1972 Playboy centrefold used extensively as an image-processing standard – in all her detailed colour and tone, small enough to fit on a human cell – just 50 µm across.

"[We] built a database of colour that corresponded to a specific nanostructure pattern, size and spacing. These nanostructures were then positioned accordingly," explains co-author Joel Yang, also from the Agency for Science, Technology and Research, adding that the colours appeared "all at once, almost like magic" when the metal film was applied.

"With the ability to accurately position these extremely small colour dots, we were able to demonstrate the highest theoretical print-colour resolution of 100,000 dpi," says Kumar. "As far as we could see, we were getting all the colours of the rainbow," he adds, although mixing in different amounts of other metals such as gold should provide more of the warm red and yellow hues.

Prospects and applications

Possible applications of the technology range from security tags and secret messages, to high-density optical data storage along the lines of Blu-ray discs. According to Kumar, "Reflector colour displays – something like a Kindle in colour should be possible for this technology as well."

The team has already tried substituting the silicon oxide for quartz, and are now investigating various polymers and alternative lithography techniques that might make the whole architecture easier and more economical to mass-produce.

The research appears in Nature Nanotechnology.