The infrared to ultraviolet upconversion emissions of Tm3+ I61→F43 (346nm) and D21→H63 (362nm) transitions, originating from the five- and four-photon upconversion processes, respectively, were observed in the Tm3+∕Yb3+ codoped precursor glass and glass ceramic containing β-YF3 nanocrystals. The ultraviolet luminescence of the glass ceramic is 30 times stronger than that of the precursor glass, which could be attributed to the decreased probability of the F23→F43 transition and the increased cross relaxation of F23+H43→H63+D21 resulted from the partition of rare earth ions into nanocrystals.
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Chen et al. (2007) studied this question.