As big as a picture window, yet thin enough to hang on the wall, today’s high-end display would be perfect if only it offered more detail and true color. Now researchers at the Swiss Federal Institute of Technology in Zurich propose to solve both these problems with moveable gratings that break white light into a rainbow and bend the right part of it to a spot on the screen. In a paper published in the 1 September edition of the journal Optics Letters, Manuel Aschwanden, a graduate student at the institute, and his advisor, nanotechnology professor Andreas Stemmer, predict that the technology will allow screens to reproduce every color visible to the human eye and offer print-quality resolution.
Optical gratings are regular arrays of fine lines that deflect light with a combination of diffraction and wave interference. Because each wavelength reacts differently, white light will separate into rays of its constituent hues, with each ray pointing a different angle. The problem lies in manipulating hundreds of thousands of gratings so that each one sends the right color to its pixel.
Aschwanden and Stemmer solve the problem with the help of the niftiest actuators around, the polymers known as artificial muscles. In their design, a wavy polymer sheet—looking much like a bunched-up shower curtain—lies between two electrodes. When the researchers apply a voltage, an electric field instantaneously presses the muscle’s waves flat, elongating it. If it is attached to a grating, it will then increase the widths of the grooves on the grating and the spaces between them, changing the diffraction angle. Because this angular change responds precisely to the applied voltage, the system can line up visible light of any wavelength with holes on a filter that allow tiny beams of light through to the pixel.
To create colors that are a mixture of the ones in the white light spectrum, the new system assigns three gratings to each pixel, just as a conventional display assigns a triad of red, blue, and green dots. But the gratings have a richer palette. The blue grating, for instance, can select any part of the blue spectrum, or all of it. This flexibility should enable the display to reproduce faithfully every color the human eye can see. That’s twice as many wavelengths as today’s best displays can muster.
The researchers say that the system should also vastly improve screen resolution. ”The smallest pixels we have been able to make measure 75 micrometers across,” says Aschwanden. ”I think 60 micrometers is the best you can achieve [using the manufacturing techniques described in the paper]. This works out to about 400 pixels per inch.” High-end LCDs have screen resolutions of around 200 pixels per inch.
The Swiss aren’t the first to try diffraction gratings, just the first to move them with muscles. Others have experimented with microelectromechanical actuators, based on piezoelectric crystals that expand in response to a change in voltage. Those systems, however, aren’t as tunable.
The visible spectrum stretches from about 380 to 780 nanometers, and if you divide the spectrum equally among three diffraction gratings near a pixel, each one would have to provide a 27 percent change in the diffraction angle of its beam. Aschwanden and Stemmer say their artificial muscle can manage a change of about 32 percent, more than enough to do the job. Piezoelectric-driven gratings can tune by less than 1 percent, allowing them to present only small slices of the spectrum.
There are other ways to do the job, involving moveable mirrors or acousto-optic deflectors, but Stemmer notes that devices based on those technologies must be manufactured inside clean rooms. ”In addition to requiring a specially equipped manufacturing facility, they’re silicon-based, so you need to use photomasks and to go through all the lithography steps, which not everybody has the resources to do,” Stemmer says. His polymer-powered gratings can be made without all that bother, so they should cost less.
To make the muscle, the scientists stretch an elastomer film to three times its length and three times its breadth, producing a film 62.5 µm thick. They mount it on a 3-by-3-millimeter holder and then stamp two 1-by-1-mm carbon black electrodes on the ends.
To make the grating, they apply a silicone-based elastomer film onto a mold formed in the diffraction pattern, and then spin the mold like a record until the film spreads out just far enough to make a layer 20 nm thick. Next, they sandwich the grating and the actuator together, bake them at 50 C for 60 minutes, and peel the cured film off the mold. Finally, they evaporate a 6-nm layer of gold onto the grating, more than doubling its reflectivity. Aschwanden was able to do all this in the nanotechnology department’s laboratory.
He has been working on his project for just a little over a year, and in the beginning he was concerned merely with making a demonstration model. He chose the reflective grating design because it was easiest to make. But manufacturers of displays would probably prefer a transmission grating, because they could illuminate it from the back rather than from the side. That would also make the gold coating unnecessary, cutting manufacturing costs still further.
The goal now is to lower the actuator’s drive wattage to the single digits, down from the 300 watts now required. ”The benchmark is essentially what you have for LCDs,” says Stemmer. This can be achieved by making the artificial muscle—and thus the grating structure—thinner.
New TV technologies have often taken a long time to catch on, in part because people hesitated to buy a display before suitable programming for it became available. Asked how long they thought it would take to see a commercially available display based on this technology, the researchers demurred, saying it depends on how manufacturers respond. ”We see ourselves as enablers of technology,” says Stemmer. ”All the rest, including getting it to market, is beyond our control.”