Upconversion in La F 3 : Tm 3 +

When excited by the 6471-\AA{} line from a krypton laser, blue and uv fluorescence has been observed in La${\mathrm{F}}_{3}$: ${\mathrm{Tm}}^{3+}$ at room temperature. The main emission features have been identified as originating from the $^{1}D_{2}$ and $^{1}G_{4}$ levels, reached by a stepwise excitation process. A set of accurate Judd-Ofelt $\ensuremath{\Omega}$ parameters for La${\mathrm{F}}_{3}$: ${\mathrm{Tm}}^{3+}$ has been derived from the measured lifetime of the $^{1}D_{2}$, $^{1}G_{4}$, $^{3}H_{4}$, and $^{3}F_{4}$ levels.