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

为了开发高质量的微拉下法制备铌酸锂钾单晶纤维，研究了微拉下法制备的铌酸锂钾单晶的质量和二次谐波的产生。虽然有一些关于利用Cz或TSSG方法生长KLN的报道，但尚未获得实际使用的具有足够尺寸和良好成分均匀性的块状晶体。通过mu-PD法生长出高质量的KLN单晶。晶体直径控制良好，无裂纹的最大下拉速率可达120 mm/h。得到的KLN单晶是无色的矩形纤维，具有发育良好的[100]和[001]面。透射到a平面的光在交叉偏振片下均匀熄灭，表明晶体具有较高的光学均匀性。测量了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匀性。另一方面，相位匹配波长沿径向变化较大。相位匹配波长在光纤截面中心处最小，向外围增大。偏振透过率分析表明，SHG波长的变化是由于异常光折射率的变化引起的。x射线形貌图显示，内部应变呈对角线分布，//[101]。这种现象在生长较快的晶体中更为明显。成分不均匀性和/或内部应变被认为是导致折射率变化的可能原因。应通过优化生长速度、温度梯度和半月板形状等生长条件来避免。

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.