[Study on ultraviolet upconversion emissions of Gd3+ induced by Tm3+ under 980 nm excitation]

Series of Tm3+/Yb3+ co-doped GdF3 powders were synthesized through an easy and mild hydrothermal method. The phase and purity of powders were characterized by powder X-ray diffraction (XRD) (Rigaku RU-200b). The morphologies of the samples were characterized by field emission scanning electron microscopy (FE SEM) (Hitachi S-4800). The ultraviolet (UV) up-conversion (UC)emission spectra were recorded by a fluorescence spectrophotometer (Hitachi F-4500) with a 980 nm semiconductor continuous wave laser diode as the excitation source. And the luminescent dynamics was measured by excitation with 980 nm using an optical parameter oscillator (OPO) laser pumped by a pulsed Nd : YAG laser with a pulse duration of 10 ns, repetition frequency of 10 Hz, and the signal was recorded by using a monochromator and an oscillograph. Under 980 nm excitation, Gd3+, acting as a kind of host ion in the studied system, and its UV UC emissions were observed and studied. The luminescent dynamics of the characteristic emission of Gd3+ (311.6 nm, 6P7/2 --> 8S7/2) was explored and studied. The luminescent dynamics analysis results indicated that, on UV UC emissions of Gd3+, Yb3+ ions served as primary sensitizer ions successively transferring energy to Tm3+ to populate the 3P2 level. Then, Tm3+ ions served as secondary sensitizer ions transferring energy to populate the multiple 6 I(J) states of Gd3+ 3P2 --> 3H6 (Tm3+): 8S7/2 --> 6 I(J) (Gd3+). Further, 6D(J) levels were populated through other energy transfer processes between Gd3+ and Yb3+ or Tm3+. Finally, UV UC emissions from the excited 6D9/2, 6 I(J), 6P5/2, and 6P7/2 states to the ground state 8S7/2 were observed. Meanwhile, Tm3+ acted as activator in its own UC emissions, and the article did not put emphasis on those except the 3P2 and 1 I6 levels to the ground state 3 H6 transitions. Especially, the dependences of UV UC emissions of Gd3+ on the Yb3+ concentrations, the Tm3+ concentrations, the annealing temperatures, and the excitation power densities of the 980 nm semiconductor continuous wave laser diode were studied, too.