Investigation of KLN single crystals grown by micro-pulling-down method for blue SHG applications

为了研制高质量的微下拉(MU-PD)法生长的KLN单晶纤维，对晶体的生长质量和二次谐波(SHG)进行了研究。虽然已有一些关于CZ法和TSSG法生长KLN的报道，但尚未获得尺寸足够、成分均一性好的块体晶体用于实际应用。用Mu-Pd法生长了高质量的KLN单晶。晶体直径控制良好，最大无裂纹下拉速率达到120 mm/h，得到的KLN单晶为无色矩形纤维，[100]和[001]面发达。在交叉偏振器的作用下，a面的透射光被均匀熄灭，表明晶体具有较高的光学均匀性。测量了位相匹配波长沿下拉轴的分布。KLN晶体沿下拉轴具有良好的成分均一性。另一方面，位相匹配波长沿径向变化较大。位相匹配波长在光纤截面中心附近最小，向周边逐渐增大。偏振透过率分析表明，倍频波长的变化是由于非常光的折射率变化引起的。X射线形貌图像显示，内部应变沿对角方向保持，//[101]。这种现象在生长速度较快的晶体中观察到的更为明显。成分不均匀和/或内部应变被认为是折射率变化的可能原因。应通过优化生长速度、温度梯度和半月面形状等生长条件来避免它们。

In order to develop for high quality potassium lithium niobate (KLN) single crystals fibers grown by micro-pulling-down (mu-PD) method, the quality of grown crystals and second-harmonic-generation (SHG) were investigated.Although there are several reports concerning growth of KLN by means of Cz or TSSG methods, bulk crystals with sufficient size and good compositional homogeneity for a practical use have not been obtained yet. High quality KLN single crystals were grown via a mu-PD method. Crystal diameter was well controlled, and the maximum pulling-down rate without cracks was attained to be 120 mm/h. The KLN single crystals were obtained as colorless rectangular-fibers with well developed [100] and [001] facet planes. The transmitted light to an a-plane extinguished uniformly under the crossed polarizers indicting high optical uniformity of the crystal.Distribution of phase-matching wavelength along the pulling-down axis was measured. A KLN crystal had a good compositional homogeneity along the pulling-down axis. On the other hand, relatively large variation of phase-matching wavelength is found along the radial direction. The phase-matching wavelength shows its minimum around the center of the section of the fiber and increase toward the periphery. Polarized transmittance analysis reveals that the SHG wavelength variation is due to the change of refractive index for an extraordinary ray. X-ray topographic images showed that the internal strain was diagonally remained, //[101]. This phenomena was more obviously obsered in a faster-grown crystal. Compositional inhomogeneity and/or internal strain is considered to be the possible cause of refractive-index change. They should be avoided by optimizing growth conditions such as growth velocity, temperature gradient and meniscus shape.