The ultraviolet upconversion luminescence of the ion sensitized by the ion in fluoride glass as excited by a diode laser was studied. Two typical ultraviolet upconversion luminescence lines positioned at 290.6 and were found and could be identified as the fluorescence transitions of and , respectively. The elaborative measurement of the variation of upconversion luminescence intensity F as a function of the pumping laser power P has proven that the luminescence lines result from five-photon and six-photon upconversion processes, respectively. To our best knowledge, the finding of six-photon ultraviolet upconversion luminescence is original. Moreover, a novel characteristic upconversion luminescence saturation phenomenon was observed, as the curve of upconversion fluorescence versus pumping laser power is a straight line, while the slopes of these plots are apparently smaller than the value determined by the normal multiphoton relation. When the pumping laser spot was increased, the slopes were enhanced noticeably from small values to those that are equal to or near what a normal multiphoton relation would determine. The mechanism of this characteristic saturation phenomenon is the energy expansion resulting from the energy resonant migration among ions.
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