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

Abstract

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 \ensuremath{\sim}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.