Research On The Growth And Properties Of Chromium-Doped Germanate Laser Crystals

The study of laser medium,the core component of tunable lasers,is of great significance because of its wide applications in medicine,optical communication,and laser measurement.Chromium-doped germanate laser crystals are a potential tunable laser material,however,because this type of tunable laser crystals are difficult to grow,the structure and optical properties of the crystals have not been studied systematically enough,which to a certain extent hinders the process of their practical application on laser output.In this thesis,the growth process and properties of germanate host crystals（LiInGeO₄,Li₂TiGeO₅and Li₂CaGeO₄）doped with transition metal Cr ions,respectively,are investigated.Three kinds of germanate crystals,Cr⁴⁺:LiInGeO₄,Cr⁴⁺:Li₂TiGeO₅and Cr⁴⁺:Li₂CaGeO₄,were grown,and the growth process,structure,density and optical properties of the three crystals were systematically investigated,thus solving the difficult problem of growing chromium-doped germanate crystals.The main research work of this thesis are as follows.Three kinds of Cr⁴⁺:LiInGeO₄,Cr⁴⁺:Li₂TiGeO₅and Cr⁴⁺:Li₂CaGeO₄chromium-doped germanate crystal growth materials（polycrystalline materials）were synthesized by high-temperature solid-phase method,and a series of properties were characterized.The synthesis temperature of the three polycrystalline materials was 800～900℃,and the synthesis time was 5 h.Three types of Cr⁴⁺:LiInGeO₄,Cr⁴⁺:Li₂TiGeO₅and Cr⁴⁺:Li₂CaGeO₄chromium-doped germanate laser crystals were prepared by the top seed crystal pullng method.Based on the crystal growth mechanism and repeated experimental exploration,the temperature field position and the optimal process parameters for crystal growth were analyzed and determined,with the rotational speed of 20～22 rad/min,the cooling rate of 0.1～0.2℃/h and the pulling speed of 1～1.5 mm/d.The physical structure aspects such as X-ray diffraction analysis,density test and Cr ion valence state of the three grown crystals were studied.The analysis revealed that the Cr⁴⁺:LiInGeO₄crystals belong to the orthogonal crystal system,and the Cr⁴⁺:Li₂TiGeO₅and Cr⁴⁺:Li₂CaGeO₄crystals both belong to the tetragonal crystal system.With the increase of Cr ion doping concentration,the lattice constant increased and the density showed a gradual decrease,which indicates the successful doping of Cr ions.X-ray photoelectron scanning energy spectra showed that in three germanate laser crystals of0.4%Cr:LiInGeO₄,0.6%Cr:Li₂TiGeO₅and 6%Cr:Li₂CaGeO₄,the Cr ions were mainly in the+4 and+6 valence states,where the Cr⁶⁺ions were due to the further oxidation of the Cr⁴⁺ions in air.The energy level structure and optical properties of Cr ion combined with different host crystals were investigated by measuring the absorption spectrum,fluorescence emission spectrum and fluorescence lifetime of the three grown crystals.In the absorption spectra of all three crystals,the absorption peaks of Cr⁴⁺ions were shown.It was also found that Cr ions showed broadband emission behavior in the near-infrared band in the range of1000～1500 nm in the fluorescence emission spectra of all three crystals,where in x%Cr:LiInGeO₄（x=0.2,0.4,0.6,0.8）crystals,the strongest emission peak was located at 1156nm(λ_ex=808 nm)when x=0.4;in x%Cr:Li₂TiGeO₅（x=0.2,0.4,0.6,0.8）crystals,the strongest fluorescence emission peak was located at 1140 nm(λ_ex=824 nm)and 1234 nm(λ_ex=980 nm),respectively,when x=0.6;in x%Cr:Li₂CaGeO₄（x=4,5,6,7）crystals,the the strongest fluorescence emission peak was located at 1197 nm（λex=824 nm）and 1192nm（λex=980 nm）,respectively,when x=6,and this phenomenon was attributed to the³T₂→³A₂jump of Cr⁴⁺ions.The quenching mechanism of the concentration of Cr⁴⁺ions in three kinds of crystals has been further discussed,which was considered as dipole-dipole interaction.It was found that the fluorescence lifetimes of all three crystals belonged to theμs level,and all of them showed a decreasing trend with the increasing doping concentration of Cr⁴⁺ions.