Exploring the maximum efficiency in infrared-to-visible converting phosphors activated with Er and sensitized with Yb

Recently developed visible solid state light sources made by coating infrared light emitting diodes with infrared-to-visible converting phosphors appear technologically interesting. However, the maximum achievable performance of these devices has been uncertain since there was no accurate method for extrapolating the nonlinear characteristics of the phosphors. We have developed techniques for exploring the maximum possible efficiencies of these phosphors by exciting them with a tightly focused YA1G:Nd laser operating at 1.06µ. The results of a series of measurements on the brightest phosphor materials available indicate that the ultimate efficiencies are strongly dependent on the host crystal material. Of the materials we have examined, the most efficient hosts are barium yttium fluoride, BaYF5, and yttrium fluoride, YF3. The highest attainable efficiency of the latter, defined as the ratio of visible emitted power to incident infrared excitation power, is in excess of 24%. Based on these results and the known performance of GaAs infrared light emitting diodes, we conclude that the overall performance of infrared-to-visible converting phosphor-diode devices should be comparable to the direct visible emitting GaP devices.