A team from Sweden has created a micro display with the smallest ever pixels, it claims.
It is a reflective display that uses sub-wavelength structures that act as meta-materials to create a colour image.
Chalmers University of Technology, the University of Gothenburg and Uppsala University collaborated on the proof-of-concept device, which uses a three-level geometric hierarchy to create CMY pixels.
The smallest units of this hierarchy are nano-scale discs etched into tungsten trioxide epitaxy on a metal surface.
At the second level, discs with identical diameter and spacing are made in patches, with the diameter of the discs in the patch, and the distance between discs, within the patch together optimised to reflect a particular colour.
For example: 220nm diameter discs with 200nm gaps are needed for a red patch, while 260nm discs with 200nm gaps are needed for green.
Three types of patch are used, which are nominally red, green and blue reflecting, but instead of covering large areas and emitting those colours, each patch was made as a long strip two discs wide.
Cyan, magenta and yellow sub-pixels were needed in the final reflective display and, in this meta-material world, those colours were achieved by the final level of hierarchy: repeating alternating rows of two coloured strips with just the right spacing to form an optical grating.
For example, a rectangular yellow sub-pixel was made with alternating red and green strips separated by 380nm gaps, and magenta needed blue and red strips spaced 80nm apart.
Using this technology, pixels measuring ~560nm across can be achieved, equating to 25,000dpi, according to the researchers.
“This means that each pixel roughly corresponds to a single photoreceptor in the eye,” said Chalmers chemical engineer Professor Andreas Dahlin. “Humans cannot perceive a higher resolution than this.”
Electrical control comes from submerging the whole display in an electrolyte which allows the WO3 discs to be reversibly switched from a coloured state to a black state by bias voltages that move lithium ions in and out of the tungsten oxide.
±4V was sufficient to switch the state of a pixel in a 40ms.
The demonstrators have no active matrix nor per-pixel control, and instead shared two control electrodes across all pixels resulting in simultaneous operation across the whole display area.
To demonstrate display resolution, a substrate was patterned with with patches and sub-pixels that created ~2μm pixels pre-set to display a 1.9mm tall reproduction of Gustav Klimt’s The Kiss.
Far more information is freely available in the Nature paper ‘Video‐rate tunable colour electronic paper with human resolution‘.
