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

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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.