Researchers at the University of Central Florida in Orlando have developed a new type of laser-based display. The early-stage work uses a near-infrared laser to upconvert doped polymer materials embedded in a screen so they emit light in red, green, and blue wavelengths.
The display medium is a transparent polymer, phosphorylated polymethylmetharcrylate (p-PMMA), that contains particles of crystals doped with rare earth ions. The host crystal, NaYF4, is doped with four rare earth ions. Yb3+ ions are used to absorb light from a commercially available diode laser emitting nearly 975 nm, and to transfer that energy to the other dopant ions. Co-dopant Tm3+, for example, absorbs this energy and re-emits blue light at around 480 nm. Likewise, Ho3+ and Er3+ ions are used to re-emit green light at around 550 nm and red light at around 660 nm.
The most cost-effective way to make a screen is to grind these dopants into a powder of tiny 10-µm crystals and then evenly disperse them within the polymer medium. This way, the display medium can be formed in any desired shape, it can be transparent or reflective, and it can be affixed to any desired substrate.
To produce an image, an infrared laser is scanned over the screen. This requires the conversion of an incoming digital signal into laser scanning and intensity modulation signals. The modulated laser is then scanned over the screen to control the emission from the red, green, or blue doped crystals. Currently, the pumped spot size is about 1 mm in diameter, or around the size of a high-definition TV pixel on a screen that's a meter wide.
The approach uses mature and inexpensive near-infrared laser diodes and avoids the high cost of producing red, green, and blue light separately. This approach also eliminates speckle, which can result from direct laser-display systems.
So far, a research team led by Michael Bass has demonstrated the technology. While it shows promise, there are hurdles to overcome. For instance, the pumped crystals emit light in all directions instead of only into the viewing hemisphere. A way must be found to collect this other half of the emitted light and re-direct it toward viewers. "We are working to optimize the coupling of display medium with the substrate selection, along with additional coatings, to help direct the light toward the observer," Bass says.
Using a 0.6-W pump laser at 970 nm, green light is quite visible in fully lit rooms. "The efficiency is hard to define," Bass adds, "but it's very good and will get better with proper display medium design as we improve on the very simple version we have tested." Calibrated detectors for measuring lumen output will soon be delivered to researchers to aid in the efficiency measurements.
In addition, the group is looking at different materials. It has developed three materials as blue emitters below 480 nm, and it is investigating the possibility of polymers producing even shorter wavelengths. The researchers also have a 660-nm red-emitting material and two in the green, at 540 and 550 nm.
The group needs financial support, though, to continue development efforts. "We are open to partners and are actively looking for companies that want to work with us to develop this display technology," Bass notes. "We believe the lack of a vacuum tube and high-voltage electronics offers significant improvements in the areas of display safety, cost, complexity, and size." This sounds interesting.
