Blue upconversion with excitation into Tm ions at 780 nm in Yb- and Tm-codoped fluoride crystals

Strong blue emissions have been observed in fluoride crystals, such as ${\mathrm{LiYF}}_4$, ${\mathrm{BaY}}_2$ ${\mathrm{F}}_8$, and ${\mathrm{KYF}}_4$, codoped with ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ when excited into the ${\mathrm{Tm}}^{3+}$ ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$ state at ∼780 nm. Energy transfer from ${\mathrm{Tm}}^{3+}$ to ${\mathrm{Yb}}^{3+}$ ions followed by the transfer from ${\mathrm{Yb}}^{3+}$ to ${\mathrm{Tm}}^{3+}$ was demonstrated to be responsible for the upconversion process. A pumping scheme is proposed based on this upconversion mechanism for blue-laser applications using these materials.