Energy up-conversion in the visible to ultraviolet range in ${\mathrm{LaF}}_3$:(${\mathrm{Pr}}^{3+}$,${\mathrm{Nd}}^{3+}$)

We report the observation, at low temperature, of ultraviolet fluorescence arising from state ${}_{}{}^4{}_{}{}^{}\mathrm{D}_{3/2}^{}{}_{}{}^{}$ of ${\mathrm{Nd}}^{3+}$, in ${\mathrm{LaF}}_3$ containing impurities of Pr and Nd, following the irradiation of the sample by a laser pulse resonant with the ${}_{}{}^3{}_{}{}^{}\mathrm{H}_4^{}{}_{}{}^{}$ to ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ ${\mathrm{Pr}}^{3+}$ transition. The mechanism responsible for the up-conversion effect is shown to be photon addition by energy transfer, within a ${\mathrm{Pr}}^{3+}$-${\mathrm{Pr}}^{3+}$-${\mathrm{Nd}}^{3+}$ triad. The up-conversion process is observed in a (1 at.?Pr+0.5 at. ?d)–doped sample and in a 0.1 at.?Nd–doped crystal where Pr impurities are present as unintentional traces. The different behaviors obtained from the two samples are discussed.