INTENSITY-DEPENDENT UPCONVERSION EFFICIENCIES OF ER-3+ IONS IN HEAVY-METAL FLUORIDE GLASS

The upconversion of infrared radiation to shorter wavelengths by Er3+ ions in heavy-metal fluoride glasses, some doped with Yb3+, has been studied over a wide intensity range and for various excitation wavelengths. The upconversion efficiency measured at room temperature varies linearly or quadratically with intensity and saturates at 100 W/cm2 or higher, depending on emission wavelength. For 4 mol% ErF3 in BaF2/ThF4, the 4I11/2 to 4I15/2 transition is over 10% efficient for 40 W/cm2 of absorbed 1.5-mu-m power, i.e. for 4I15/2 to 4I13/2 excitation. A rate equation model and Judd-Ofelt analysis of data for the glasses doped with just Er3+ provide an understanding of the upconversion process while yielding values for Er3+ energy-transfer rates and excited-state populations. These results could be used to develop glasses for lasing and as infrared optical detectors.