| The solid-state lasers working in the 1.8 ~ 2.0 μm band have been widely applied to the fields of medicine, communications and military, etc. Recently, large-size, high-quality and high optical performance laser materials have attracted more attention. Therefore, LiYF4 and LiLuF4 crystals with various Tm3+-doping concentrations are fabricated by Bridgman method in this paper. It aims to look for the optimum doping concentration of Tm3+ ions in LiYF4 and LiLuF4 crystals, the optimum doping material has the better laser performance in the range of 1.8~2.0 μm. Considering the energy level structure, cross-relaxation(3H6, 3H4'3F4, 3F4) and concentration quenching effect of Tm3+ ions, the important parameters of lasing performance are systematically analyzed, including luminescent property,decay lifetime and quantum efficiency.In chapter one, we show the research actuality of Tm3+ singly-doped and Tm3+/Ho3+ co-doped solid-state lasers. It indicates that no one has reported the optimum-doping concentration of Tm3+ in fluoride host, which outputs the superior laser performance in 1.8~2.0 μm band. Therefore, LiYF4 and LiLuF4 crystals with different Tm3+ doping concentrations are studied. The details of raw materials,synthesis procedures and the process parameters are described in this experiment of chapter two.Comparisons of optical properties among different Tm3+doping levels in LiYF4 and LiLuF4 crystals have been carried out in the third and the forth chapters, the crystal materials have potential applications in the research of 1.8~2.0 μm solid-state laser. According to Judd-Ofelt theory, we calculate the value of effective J–O parameters Ωeff(Ω6, Ω4, Ω2) and the radiative lifetime τrad of Tm3+:3F4 manifold in LiYF4 and LiLuF4 crystals with different Tm3+doping concentrations. It shows that the value of τrad is decreasing while the doping concentration of Tm3+ is increasing. In LiYF4 crystal, as the concentration of Tm3+ improved from 0.29 mol % to 3.49 mol %, the value of both emission intensity and emission lifetime increase firstly and then decrease rapidly. In LiYF4 crystal, the 1.28 mol% sample holds the best fluorescence performance. At this doping level, the lifetime is 17.68 ms, and the maximum emission cross-section reaches 3.76×10-21cm2 at 1.909 μm, and the largest quantum efficiency is 147%. In LiLuF4 crystal, with the doping level of Tm3+changes from 0.45 mol % to 3.25 mol %, the lifetime of Tm3+:3F4manifold is continuously reducing, the 0.45mol% doping one has the longest lifetime(12.78 ms).However, the emission intensity around 1.8 μm is first increasing and then decreasing with the doping level of Tm3+ increased to 3.25 mol%. In LiLuF4 crystal, the 0.90 mol% sample has the strongest fluorescence intensity around 1.8 μm. The lifetime of 0.90 mol% sample is 11.99 ms, and its maximum emission cross-section arrives at 3.76×10-21 cm2 at 1.89 μm, and its largest quantum efficiency is estimated to 120%.Based on the optimum Tm3+ doping concentration in LiYF4 crystal, chapter five gives various concentrations of Ho3+ ions(0.5, 1.01, 1.51 mol%) co-doped LiYF4 crystals. It suggests that 1.28mol%Tm3+/1.51 mol%Ho3+:LiYF4 one has the best fluorescence emission characteristics around 2.0 μm band. The maximum emission cross-section is 0.760×10–20 cm2 at 2.052 μm and its lifetime of 5I7 level reaches 21.97 ms.The results show that 1.28 mol%Tm3+:LiYF4、0.90 mol%Tm3+:LiLuF4 and 1.28 mol%Tm3+/1.51mol%Ho3+:LiYF4 samples have more advantages than others in laser material working in 1.8~2.0 μm band. It provides a better option for the excellent material which has large-size, high-quality and high optical performance.. |
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