Laser power density dependent energy transfer between Tm and Tb: tunable upconversion emissions in NaYF:Tm,Tb,Yb microcrystals.

Energy transfer between Tm and Tb dependent on the power density of pump laser was investigated in NaYF: Tb,Tm,Yb microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb-Tm-Yb doped samples exhibited intense visible emissions with tunable color between green and blue. The ratio of blue and green emission were determined by energy transfer between Tm and Tb. When the power density of pump laser was low, the energy transfer process from Tm (F) to Tb (F) occurred efficiently. Upconversion processes in Tm were inhibited, only visible emissions from Tb with green color were observed. When the power density increased, energy transfer from the F (Tm) to F level (Tb) was restrained and population on high energy levels of Tm was increased. Contribution of upconversion emissions from Tm gradually became dominant. The emission color was tuned from green to blue with increasing the power density. Energy transfer processes between low-lying levels of activators, such as Tm will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb-Tm-Yb doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.