42.1: Invited Paper: Review of the Properties of Up‐Conversion Phosphors for New Emissive Displays

Abstract

We review the properties of up-conversion materials which absorb near infrared light and re-emit in the visible and we assess their potential for a new display technology. Such phosphors have characteristics that are very appealing for displays: they cover a wide color gamut with very saturated colors, they can be operated at very high brightness without damage to the emitters, they have very long lifetimes and they have efficiencies comparable to those of existing technologies. . IntroductionThe concept of frequency upconversion (UC) of infrared-tovisible light in rare-earth (RE) doped materials was reported more than forty years ago for the first time [1] . The efficiency that was observed or expected for this process was low in singly doped media, but it was quickly noticed that the mechanism could be made one or two orders of magnitude more efficient by using ytterbium (Yb) as a sensitizer ion in addition to the active ion: erbium (Er), holmium (Ho), or thulium (Tm) [2][3][4][5][6][7] . Efficient UC materials were extensively investigated, as they could be used for several potentially important photonic applications including UC lasers (visible lasers that are pumped by infrared diode lasers), and displays. However, because no powerful source existed in the 980-nm region in order to excite such upconverters no practical product came out of the research. With the development of powerful 980-nm diode lasers for the telecommunication industry, a technology that appeared to be too inefficient in the past now has legitimate practical applications. Review of the resultsThe base materials we use are fluoride crystals doped with Yb 3+ and an active ion (Er 3+ or Tm 3+ ). In order to make this technology scalable, we grind those materials into powder and disperse them in a polymeric host (phosphorylated polymethylmethacrylate or p-PMMA) [8] . The preparation is then cast on a substrate and an infrared laser beam is scanned across the screen. No high voltage or vacuum tube is needed and the optical path can be folded in order to reduce the depth of the display. The low energy pump photons do not produce damage in the up-conversion materials so the screens do not suffer performance deterioration as do down-conversion displays (plasma technology) [9][10][11][12] .The physical process on which up-conversion is based is as follows: First, photons from the near-infrared pump source are absorbed by the Yb 3+ sensitizer ions. The absorbed energy is then transferred in two (or three) sequential steps to higher lying energy levels in the active ion (Er 3+ for green and red, and Tm 3+ for blue). Visible light is emitted through fluorescence for a duration of about 2 ms. This makes up-conversion materials suitable for video rates. With such a display, the viewer is never exposed to infrared laser light which is filtered out for safety purposes. He looks at fluorescence light which is not subject to speckle as with visible laser displays.The best performing materials to date present two narrow emission bands each in the visible. T...