Enhancement of optical nonlinearity of heavymetal oxide glasses by replacing lead and bismuth with thallium Appl.The upconversion of infrared radiation to shorter wavelengths by Er3 + ions in heavy-metal fluoride glasses, some doped with Yb 3 + , 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/cm* or higher, depending on emission wavelength. For 4 mol % ErF, in BaF,/ThF,, the 41, ,,* to 41,,,2 transition is over 10% efficient for 400 W/cm* of absorbed 1.5pm power, i.e., for 41,,,, to 41,,,, excitation. A rate equation model and Judd-Gfelt analysis of data for the glasses doped with just Er" + provide an understanding of the upconversion process while yielding values for Er" + energytransfer rates and excited-state populations. These results could be used to develop glasses for lasing and as infrared optical detectors.
The dependence of the M-center reorientation efficiency on M + concentration indicates that at low temperatures, M centers reorient indirectly by absorbing light, forming M + centers which then absorb and rotate. Reoriented M centers are produced only after reoriented M + centers capture electrons. We further show that M + centers reorient with high efficiency during the radiationless transition, 2pir u~* 2pcr u o
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