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

在各种光学设备中，期望使用封装了激光光束的光纤晶体来实现高效的激光振荡或波长转换。本研究以激光二极管泵浦的激光材料--掺钕钒酸钇(ND：YVO_4)为模型材料，采用浮区法生长块体晶体，并进行了实验研究。考察了获得较细晶体的生长条件，明确了晶体直径与晶体质量的关系；为了保持稳定的熔区，喂料直径应随晶体直径的减小而减小，喂料直径与晶体直径之比应小于3。在小于1 Mmphi的情况下。这一比例应该是两倍左右。为了实现这些条件，使用了直径均匀的单晶进给杆，并在分区孔型后进行了切割和磨削。得到的最小直径为0.8 mm。通过使用四个椭球体类型的仪器，预计晶体会变薄。其收敛效率比目前的装置高得多。在3Mmphi以下的晶体中，位错密度急剧下降，随着晶体的继续生长，位错密度自发减少。本研究建立了浮区法生长氧化物纤维晶体的基本工艺，并通过减小晶体直径来提高晶体的质量。这项技术也适用于用常规方法难以生长的其他类型的光纤晶体。

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.