Laser power density dependent energy transfer between Tm 3+ and Tb 3+ : tunable upconversion emissions in NaYF 4 :Tm 3+ ,Tb 3+ ,Yb 3+ microcrystals.

Energy transfer between Tm ³⁺ and Tb ³⁺ dependent on the power density of pump laser was investigated in NaYF ₄ : Tb ³⁺ ,Tm ³⁺ ,Yb ³⁺ microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb ³⁺ -Tm ³⁺ -Yb ³⁺ 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 ³⁺ and Tb ³⁺ . When the power density of pump laser was low, the energy transfer process from Tm ³⁺ ( ³ F ₄ ) to Tb ³⁺ ( ⁷ F ₀ ) occurred efficiently. Upconversion processes in Tm ³⁺ were inhibited, only visible emissions from Tb ³⁺ with green color were observed. When the power density increased, energy transfer from the ³ F ₄ (Tm ³⁺ ) to ⁷ F ₀ level (Tb ³⁺ ) was restrained and population on high energy levels of Tm ³⁺ was increased. Contribution of upconversion emissions from Tm ³⁺ 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 ³⁺ will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb ³⁺ -Tm ³⁺ -Yb ³⁺ doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.