Growth of Fibrous Single Crystals of Oxides by the Floating Zone Method

在各种光学器件中，期望利用光纤晶体封装激光束，实现有效的激光振荡或波长转换。本研究选择了作为激光二极管抽运激光材料而备受关注的掺钕正钒酸钇（Nd: YVO_4）作为模型材料，将生长体晶体的浮区法应用于氧化物光纤晶体的生长。考察了获得较薄晶体的生长条件，阐明了晶体直径与质量的关系。为了保持稳定的熔融区，进料直径应随晶体直径的减小而减小，进料直径与晶体直径的比值应小于3。在小于1mm的情况下。比例应该是2。为了实现这些条件，采用了直径均匀的单晶进料棒，在经过区道后进行切削和磨削。得到的最小直径为0.8 mm。用四椭球型仪器可以得到更薄的晶体。与现有装置相比，具有更高的收敛效率。在3 mmphi以下的晶体中，位错密度急剧下降，随着晶体的继续生长，位错会自发地减少。由于晶体直径的减小，热应力的减小有助于质量的提高。本研究建立了一种用浮区法生长氧化物纤维晶体的基本技术，随着晶体直径的减小，晶体质量得到了提高。该技术也适用于其他难以用常规方法生长的纤维晶体的生长。

In various optical devices, efficient laser oscillation or wavelength conversion is expected using a fiber crystal, in which laser beam is encapsulated. In this study, the floating zone method, which had been employed to grow bulk crystals, was applied to grow fiber crystals of oxides Neodymium-doped yttrium orthovanadeate (Nd : YVO_4), which has attracted the attention as a laser material with laser-diode pumping, was chosen as a model material. Growth conditions to obtain thinner crystals were examined and the relation between the crystal diameter and quality was clarified.To keep a stable molten zone, the feed diameter should be decreased with decreasing crystal diameters and the ratio of the feed diameter to the crystal one should be less than three. In the case of less than 1 mmphi. the ratio should be about two. To realize these conditions, a single crystalline feed rod with a uniform diameter was used by cutting and grinding after the zone pass. The minimum diameter obtained was 0.8 mm. Thinner crystals are expected by using a four-ellipsoidal type apparatus. Which has much higher convergence efficiency than in the present apparatus. A drastic decrease in dislocation density was found in the crystals below 3 mmphi, in which dislocations were spontaneously decreased with continuing crystal growth. The decrease in the thermal stresses because of the decrease in the crystal diameter would contribute to the quality improvement.In this study, a basic technique for growing fiber crystals of oxides by the floating zone method was established and the quality of the crystals was improved with decreasing crystal diameters. This technique is also applicable to grow other kinds of fiber crystals that were difficult to grow by the conventional methods.