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+ (F-3(4)) to Tb3+ (F-7(0)) 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 F-3(4) (Tm3+) to F-7(0) 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. (C) 2016 Optical Society of America