Intensity-dependent upconversion efficiencies of Er3+ ions in heavy-metal fluoride glass

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Abstract
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 400 W/cm2 of absorbed 1.5-μm 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.