Sep 25 2007
Some new technologies appear from nowhere. Others are heralded by such long fanfares that it seems they will never arrive. Electronic paper is surely in the second category. The idea of a display screen that has the clarity and flexibility of paper has been around for at least a decade. It has found a few niche, black-and-white applications in mobile phones and "electronic books". As a mass-market, full-colour product, however, it has conspicuously failed to show up.
Sooner or later someone will create the winning formula. And the latest attempt to do so, by a Canadian firm called Opalux, uses objects called photonic crystals to produce electronic paper that has bright, sharp colours.
Until now, attempts to make electronic paper colourful have used the traditional approach employed in liquid-crystal screens and cathode-ray tubes. They have, in other words, relied on pixels that have three subunits, each responsible for one of the primary colours. Varying the intensity of these primaries creates the illusion, from a distance, of the intended hue.
Doing things this way, however, exacts two prices: brightness and resolution. With only a third of the screen occupied by each primary, two-thirds of the potential output is wasted. For the same reason, the illuminated parts of the pixels are smaller and further apart than they could be, so the image is fuzzier. What is needed is a pixel that can change colour in its entirety. And that is what photonic crystals allow.
The defining characteristic of a crystal-any crystal-is that it is composed of regularly arranged components. In familiar crystals, such as salt, sugar and diamonds, these elements are atoms or molecules. But the elements of a crystal can be larger than that, as long as they are regular.
In a photonic crystal, the elements are about the same size as the wavelength of light-a few hundred nanometres (billionths of a metre). But for the crystal to do its job, those elements must also be electrical insulators.
The reason for this last requirement is that light is a form of electromagnetic radiation-in other words a wave in which electric and magnetic fields continuously leapfrog each other. Being insulators, the elements of a photonic crystal constrain this propagation, permitting some wavelengths to pass while denying passage to others, which are thus reflected. The result, since wavelength equates to colour, is that photonic crystals reflect coloured light.
Exactly what colour is reflected depends on the spacing between the elements. Alter the spacing and you alter the colour. And that is how Opalux's product, dubbed P-Ink, works.
In P-Ink, the elements of the crystals are tiny beads of silica, 200 nanometres in diameter. These are embedded in a spongy "electroactive" polymer. A pixel is built by sandwiching a layer of P-Ink, along with a small amount of electrolytic fluid, between two transparent electrodes. The pixels are then built into displays by embedding them and their electrical connections in a sheet of another polymer, this time transparent and flexible.
By applying a voltage across the electrodes of a pixel, the polymer can be made to absorb or expel some of the electrolyte and thus expand or contract. That alters the spacing between the elements of the crystal. The whole pixel thus changes colour-indeed, it can be tuned to reflect any desired colour.
That fact alone would make Opalux's technology worth considering for displays in general. But the third characteristic required of electronic paper, besides high resolution and flexibility, is that the image should remain when the power is switched off-and that, too, is the case for P-Ink. Once in the polymer, the electrolyte stays there until a newly applied voltage coaxes it out.
The one thing that may hold P-Ink back-and which has, in part, been responsible for the non-arrival of electronic paper more generally-is what is known as the refresh rate. This is the speed at which a pixel can change colour. For video that needs to happen at least 25 times a second if the eye is to be fooled into seeing continuous motion. The refresh rate for P-Ink at the moment is about once a second, so the trumpeters are not quite ready to put their instruments to their lips. But Opalux's scientists are working on it. If they can speed it up, expect another fanfare soon.