Electronic paper displays from photonic crystals
Scientists in Canada have used photonic crystals to create flexible electronic-paper displays with improved colour and resolution.
Normally, electronic paper displays involve electrical manipulation of black or white polyethylene or titanium dioxide particles within tiny microcapsules. Colour images are created by limiting each pixel to a single primary colour. Varying the intensity of each pixel generates different colours, but obtaining an intense colour on the entire display is difficult as only one-third of the pixels could create that colour.
Opalux, a company based in Toronto, Canada, has created P-Ink, a technology that employs photonic crystals and the light reflected between them. Each pixel in the display contains hundreds of silica spheres that are around 200nm in diameter and embedded in a polymer containing iron atoms. These are sandwiched between a pair of electrodes along with an electrolyte fluid.
When voltage is applied to the electrodes, the electrolyte is drawn into the iron polymer, causing it to expand and the silica beads to be pushed apart. This changes their refractive index, allowing different colours to be achieved. Once a pixel has been tuned to a colour, it can hold that image for days without requiring a power source.
‘We are using microAmps/cm2 while applying a low voltage (under two volts),’ says Dr Andre Arsenault, Chief Technology Officer of Opalux. ‘The base materials we are using are cheap, and the processes we are developing should be low cost. We believe that our price points will be competitive.’
The company has demonstrated 0.3mm pixels – same size as many LCD displays – to independently generate a range of colours.
‘Our switching speeds are a little bit below one second at the moment,’ says Arsenault. ‘Clearly this is too slow for video, but the applications we are targeting [including large area digital advertising screens] do not require video speed.’
Dr Sergei Romanov, Senior Researcher in the Photonic Nanostructure Group at University College Cork, Ireland, says ‘The idea of applying 3D colloidal crystals with variable lattice parameters to e-paper displays is a fantastic example of clever material engineering to materialise very simple physical phenomenon of light diffraction.
‘The principal drawback of the proposed display is the narrow viewing angle for each single colour. In particular, each colour is stable within approximately plus/minus five degrees from certain directions.’
Romanov explains that scanning from higher degrees could distort the colour. The long switching times of the pixels, and the potential for the iron polymer to degrade, leading to reduced switching cycles, are also areas that need to be addressed. Nevertheless, ‘a number of suggested innovations [by P-Ink], like the use of diffraction instead of filtering and enabling the same pixel to display different colours, makes this work very important for the field,’ he adds.