Bright constant color upconversion based on dual 980 and 1550 nm excitation of SrF2:Yb3+, Er3+ and β-NaYF4:Yb3+, Er3+ micropowders-considerations for persistence of vision displays

Upconversion phosphors are of interest for 2D head-up and 3D volumetric displays. These exploit persistence of vision; fast scanning of the near-infrared excitation means that the material at each emission point is only excited for a short fraction of the time it takes to trace the whole image. To achieve an average luminance on the order of 100 cd m(-2) (necessary for the display to be visible in ambient indoor lighting), the luminance during excitation must be several orders of magnitude higher than its time-averaged value. For this purpose, efficient energy-transfer upconversion materials such as the benchmark beta-NaYF4:Yb3+,Er3+ are of obvious interest. However, under 980 nm excitation the perceived color of their emission varies with their luminance, limiting their applicability for a grayscale display. We demonstrate that under dual 980 nm and 1550 nm excitation, a constant green color can be maintained at CIE coordinates (0.31, 0.66), as the luminance is varied. At moderate power densities of 100 W cm(-2) at each excitation wavelength, a luminance of 8 x 10(6) cd m(-2) can be achieved. Conservatively considering that the luminance will be reduced by 5 orders of magnitude in a laser scanning display, this material will still be bright enough to be viewed in indoor ambient light. We also investigate SrF2:Yb3+,Er3+, and find color tuning from the red to green is possible under dual wavelength excitation, but only at limited luminance, so color tuning in this manner would only be appropriate for displays viewed in a dark environment.