Study On The Growth And Upconversion Properties Of Yb:Ho:LiNbO₃ Single Crystals

A series of Yb: Ho: LiNbO₃ single crystals with different Yb³⁺ (0.5, 1.5, 2.0, 2.5, 3.0 mol%) doping concentrations and a constant Ho³⁺ (0.1mo%) concentration were grown by the Czochralski method. The crystals had no macroscopic defect and exhibit good optical properties. In the process of crystal growth, temperature gradient is 35-40℃/cm, pulling speed is 0.5-2.5mm/h and rotating speed is 22-26r/min. These optimum parameters were selected to maintain the flat growth interface, and the heterogeneity component is decreased.The results of X-ray diffraction indicated that there is no additional peaks appeared in Yb: Ho: LiNbO₃ single crystals. All these crystals are a single phase of LiNbO₃ with trigonal structure. Comparing with the standard card of lithium niobate, the diffeencees between doped crystals and standed LiNbO₃ crystal showed that the lattice and cell volumes was changed slightly as a result of the different ionic radius between doped and replaced ions. The cell volumes of doped LiNbO₃ were decreased, which might be attributed to the exsisted impurities of LiNbO₃ crystals resulting from the doping ions of Yb³⁺ or Ho³⁺. UV-visible absorption spectras show that the absorption of these doped crystals, compared with pure LiNbO₃ crystal, intensity increases. So that the occupying mechanism of the dopants was found. Ho³⁺ and Yb³⁺ prefers to occupy the sites of NbLi4+. The UV absorption edge shift to red and then to blue. Besides, combined with the IR absorption spectra, The IR absorption spectras of the Yb:Ho:LiNbO₃ in the wave number range of 3200–3800 cm?1 show that as the concentration of Yb³⁺ increases, the OH? absorption decreases. The reduction intensity of OH- absorption peaks indicates that the decrease of the phonon energy and hence results in the increase of the luminous efficiency increased.Under 980 nm excitation, green and red emission bands are observed in these samples, respectively. The up-conversion spectroscopies of Yb: Ho: LiNbO₃ crystals were tested to obtain the power-upconversion curves which were used to analyze the up-conversion mechanism. According to the experimental results, Power dependence results indicate a dominnent two-photon population process of the emitting levels. In addition, the intensity of the red and green upconversion fluorescence emissions increase with Yb³⁺ concentrations of 0-2.0 mol% due to an increased Yb³⁺ sensitization, but decrease attribute to cooperative sensitization between two Yb³⁺.