A team of researchers at the University of Central Florida’s NanoScience Technology Center (Orlando, Florida) is developing a technology that could eventually triple the resolution of LCDs.
A recent article on this research published by the team is entitled “Actively addressed single pixel full-colour plasmonic display.” The article appeared in Nature Communications 8, Article number: 15209 (2017). The article was also published on-line on May 10, 2017 and can be found here. You may remember e-Skin Displays, which work on the same principle and based on published images, seem to be connected to the same scientific group.
First, a few words of background information. A conventional LCD is illuminated by a backlight that produces white light. Each pixel within such a typical LCD is composed of three subpixels with each subpixel filtering the white light so as to transmit one of the three primary colors. In operation, the LCD acts as a shutter that controls the transmission of each subpixel. By this means, it is possible to control the integrated brightness and color of each pixel.
The idea behind the new technology is to have each subpixel produce the full range of colors. These are two benefits to this approach. First, the resolution of the device is increased by a factor of three – each subpixel becoming, in effect, a pixel. Second, with every subpixel turned on and displaying any color or white, the resulting image has enhanced brightness as compared to the equivalent image produced by a conventional LCD.
The means developed by the team to enable each subpixel to produce the full range of colors is based on the use of large area nanoimprint lithography of nanostructures that are integrated into configurations and structures that use the technologies of current generation LCDs. More specifically, the means are based on the use of “a surface morphology-induced, polarization-dependent plasmonic resonance and a combination of interfacial and bulk LC effects. Each of these phenomenon dictate the color of the surface within different voltage regimes: bulk LC reorientation leading to polarization rotation in the low voltage regime, and surface LC reorientation leading to plasmonic resonance shifting at higher voltages.”
The technical article provides a great deal of detailed information on the technology and the means by which it is implemented. A brief overview of the technology is presented in the figure below.
A liquid crystal-plasmonic device, which works with ambient white light.
Light enters the device from the top and passes through a polarizer, the top substrate and a layer of high birefringence liquid crystal layer to impact and interact with a continuous aluminum nanostructure. The interaction of light with the last layer, a so-called plasmonic surface shows a polarization dependence that originates from surface roughness in the presence of an anisotropic media. The plasmonic surface absorbs some of the light while reflecting the balance back out of the device. The wavelength band of the absorbed light depends on the liquid crystal orientation near the substrate and the polarization of the incident light. By applying a field across the device, the orientation of the bulk and surface layers of liquid crystal can be changed within different regimes. The device acts as a polarization rotator at low voltages while shifting the plasmonic resonance of the aluminum nanostructure at higher voltages. The result is that the reflective band can be shifted from red to green to blue.
This color generation mechanism can be implemented in conjunction with various addressing schemes. A member of the team explained that “It allows you to leverage all the pre-existing decades of LCD technology. We don’t have to change all of the engineering that went into making that.”
To demonstrate the scalability and compatibility of their approach with existing LCD configurations and products, the team built a prototype device in which the technology was mated to a TFT array “dissected” from a commercially available LCD. The resulting device was able to present images and videos.
While this prototype demonstrated the ease by which LC-plasmonic technology could be integrated with existing TFT substrates, it also revealed several problems.
Achieved color gamut with a full color plasmic display
First, white light reflected from the TFT metal lines (meant for use in transmissive displays) and tended to wash out the colored light reflected from the underlying plasmonic surface One possible solution to this problem would be to superimpose a black matrix on the TFT metal lines which could help to improve colour gamut. A second possibility would be to fabricate the plasmonic surface on the glass of the TFT substrate itself .
Another problem was that the off-the-shelf TFT drivers did not provide a voltage that was high enough to cause the surface transition to green for the cell gap used. Future prototypes will need to have TFT drivers capable of providing a higher voltage and the device needs to use a smaller cell gap.
One of the current issues with this technology is the need for a good grayscale control to produce the full color gamut. As a reminder, in a normal LCD panel the applied voltage controls the pixel brightness but, here, the voltage controls the color. The authors discuss this in their article and mention that there is a way to do this with small voltage changes, but admit that this is a topic for further development.
A final issue was frame rate. The new method did not demonstrate a refresh rate that was comparable to that available in state-of-the-art commercial LCDs.
Despite these problems, the team believes that the technology described in their article can be further developed for use in transmissive and/or transflective displays. -Arthur Berman
University of Central Florida, Daniel Franklin, [email protected]
Analyst Comment
This article also had contributions from Norbert. (Man. Ed.)
New display technologies are always interesting to evaluate and require some imagination to see below the early promises and expected (and unexpected) pitfalls. The first LCD or OLED displays were nowhere near perfect color displays. Note that the prototype uses ITO, but this would need changing to make a flexible display. (NH)
