Laser power density dependent energy transfer between Tm³⁺ and Tb³⁺: tunable upconversion emissions in NaYF₄:Tm³⁺,Tb³⁺,Yb³⁺ microcrystals.

Energy transfer between Tm³⁺ and Tb³⁺ dependent on the power density of pump laser was investigated in NaYF₄: Tb³⁺,Tm³⁺,Yb³⁺ microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb³⁺-Tm³⁺-Yb³⁺ 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 Tm³⁺ and Tb³⁺. When the power density of pump laser was low, the energy transfer process from Tm³⁺ (³F₄) to Tb³⁺ (⁷F₀) occurred efficiently. Upconversion processes in Tm³⁺ were inhibited, only visible emissions from Tb³⁺ with green color were observed. When the power density increased, energy transfer from the ³F₄ (Tm³⁺) to ⁷F₀ level (Tb³⁺) was restrained and population on high energy levels of Tm³⁺ was increased. Contribution of upconversion emissions from Tm³⁺ 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 Tm³⁺ will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb³⁺-Tm³⁺-Yb³⁺ doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.