Float zone growth of rare-earth doped lutetium orthovanadate single crystals as new solid state laser materials

掺钕钒酸镥（Nd：LuVO_4）具有比传统掺钕稀土钒酸盐更大的吸收和发射截面，有望应用于激光二极管泵浦的固体激光器。由于Lu^<3+>的晶格常数半径与Tm^<3+>和Yb^<3 +>的晶格常数半径相当，因此LuVO_4也有望成为Tm和Yb的良好基质晶体。本文建立了用浮区法生长掺稀土（Nd，Tm和Yb）的LuVO_4单晶的工艺，并研究了其作为激光材料的光谱特性。生长条件为：熔区移动速度20-40 mm/h，旋转速度30-40 rpm，方向为[110]，气氛为氧气流。偏光显微镜证明，生长的晶体不含宏观缺陷，但生长条纹观察到只有在钕掺杂的晶体。有效的偏析效率被确定为接近1的Tm和Yb和约0.5的Nd。当Tm和Yb的掺杂量达到5at%，Nd的掺杂量达到1at%时，可重复地生长出直径为5 mm，长为50 mm的单晶，Nd：LuVO_4的吸收系数是其它稀土钒酸盐的1.4倍。关于Nd：在880 nm抽运下，LuVO_4（1at%）单晶表现出良好的激光性能，斜率效率为73%，阈值功率为7 mW。虽然Tm和Yb掺杂晶体的激光振荡实验正在进行中，但预计这些晶体基于其优异的光谱特性也表现出良好的激光性能。

Neodymium-doped lutetium orthovanadate (Nd : LuVO_4) has larger absorption and emission cross sections as compared with conventional Nd-doped rare earth orthovanadates and is expected to be applied for laser-diode pumped solid state lasers. LuVO_4 is also expected to be a good host crystal for Tm and Yb, since the tonic radius of Lu^<3+> is comparable to those of Tm^<3+> and Yb^<3+>. In this study, the growth technique of rare-earth (Nd, Tm and Yb) doped LuVO_4 single crystals by the floating zone technique has been established and some spectroscopic properties as a laser material have been investigated.Each dopant was added with 0.5-4 at% with respect to Lu. Growth conditions were as follows : traveling rate of the molten zone was 20-40 mm/h ; rotation rate was 30-40 rpm ; direction was [110] and atmosphere was an oxygen flow Any as-grown crystal was colored in yellowish ocher, but the coloring was completely eliminated by annealing at 1000 ℃ for 20h in air. Polarizing microscopy proved that the grown crystals contained no macroscopic defects but growth striations were observed only in Nd-doped crystals. Effective segregation efficient was determined to be close to unity for Tm and Yb and about 0.5 for Nd. At this stage, single crystals 5 mm in diameter and 50 mm long have been grown reproducibly for Tm-and Yb-doping of up to 5 at% and Nd-doping of up to 1 at%.The absorption coefficient of Nd : LuVO_4 was 1.4 times as high as those of other rare-earth orthovanadates. The Nd : LuVO_4 (1 at%) single crystal exhibited excellent laser performance with 880 nm pumping ; that is, the slope efficiency was 73 % and the threshold was 7 mW. Although the laser oscillation experiments for Tm and Yb-doped crystals are underway, it is expected that those crystals also exhibit good laser performance on the basis of their excellent spectroscopic properties.