The Study On Crystal Growth And Upconversion Luminescence Of ZnWO₄:Tm～(3+)

The upconversion luminescence has been found for forty years, but it was not put into extensive use due to low efficiency. Because of urgent demand for all-solid-state lasers, especially the emergence of high power laser diode as suitable sources, the upconversion materials have attracted renewed interest as one of promising routes for achieving LD- pumped, efficient, compact long-life all-solid-state lasers emitting in visible spectral region (VSR). In the paper, the historical and recent progress of the upconversion materials is reviewed, and four mechanisms producing upconversion and the factors affecting luminescence intensity are discussed. ZnWO4 crystal belongs to monoclinic system, space group P2/c, weak crystal field intensity, which are advantageous to upconverslon. A series of different Tm3~ ion-doped concentrations ZnWO4 single crystals and TmhYb3 ion-codoped were grown with Czochralski method after carefully designing temperature field and choosing technique parameters A (1 30 /cm, rotation rate 4mmh and pulling rate 250r.p.m. Annealing in alumina powder which the phase transforms in high temperature increased the transmission in VSR . The result of density experiment showed thulium ions substituted the partial sites of zinc ions to form permutable solid-solution. The distribution coefficient of thulium ion in zinc tungstate, about 0.27, was obtained by ICP-AES method. Absorption spectra of Tm:ZW and Tm,Yb:ZW measured at room temperature exhibited that they were fit to be pumped by 808nm LD due to high to 411cm energy splitting of 3H4 level correspondent to 26.7nm peak width which attributed to the low symmetry, and the energy level distribution of the thulium ion and ytterbium ion in zinc tungstate were confirmed, and the absorption cross section of thulium ion was 1.58 X 1020cm2. In Tm:ZW, pumped by 808nm LD, three upconversion peaks at 404,486 and 695nm can be observed, which attributed to transitions of b3H6,3H6 and 3F3 3H6, respectively. The slope a of the three luminescence intensity versus pump power in double-logarithmic were 1.2,0.8 and 1.2, respectively. But in codoped sample, the intensity of 486nm luminescence increased above two magnitude orders and was the strongest peak, this due to resonance energy transfer from ytterbium ions to thulium ions, a were 2.44,1.81 and 1.13, respectively. The emission cross section of thulium ion was 3.99 X 10cm2.