Sun 25 October 2020:
Most currently available mobile devices use red, green and blue organic light-emitting diode (OLED) screens manufactured by depositing dots of organic film through metal sheets with many holes punched in them. Such displays typically have a pixel density of 400 to 500 pixels per inch (PPI), while the size of the dots is limited by the thickness of the metal sheets.
In contrast, commercial TVs use white OLEDs with overlaid colour filters, which absorb more than 70% of the light emitted by the screen, requiring a large amount of electric power to function. Such displays usually have a pixel density of 100 to 200 PPI.
Now, a team of researchers from Stanford and Samsung have developed a new OLED display that uses OLED films to emit white light between two reflective layers – a silver film layer and a metasurface composed of microscopic pillars spaced less than a wavelength of light apart.
In the new display, clusters of these nano-pillars – 80 nanometres in height and 100 nanometres in width – assume the function of pixels, each measuring at 2.4 microns wide, or just under 1/10,000 of an inch.
Each pixel present on the metasurface is further divided into four subpixels of equal size, composed of nano-pillars which control incoming white light, such that each subpixel could reflect a specific light colour, depending on the amount of spacing between its nano-pillars.
Next, emitted light reflects back and forth between the display’s reflective layers, eventually escaping through the silver film and onto the surface. According to the researchers, this ensures twice the luminescence efficiency of currently existing white OLED’s, as well as higher colour purity.
“If you think of a musical instrument, you often see an acoustic cavity that sounds come out of that helps make a nice and beautiful pure tone,” senior author Mark Brongersma from Stanford University. “The same happens here with light – the different colours of light can resonate in these pixels.”
The authors claim that, at least theoretically, pixel density caps at around 20,000 PPI, yet pixels with dimensions below one micrometre are likely to result in incredibly sharp, but somewhat dim displays.
In the near term, displays of such densities could be used in VR headsets where extremely high resolutions are required to create the illusion of reality. Further down the line, metasurfaces could be deployed for light trapping purposes in solar cells and light sensors.
A paper detailing the new display was published in the journal Science.
This article is originally published in Technology.org
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