The growth defects in Nd: YCa4O (BO3) 3 (Nd:YCOB) crystals were investigated by transmission synchrotron topography. It was found that growth striations were the primary defects in Nd: YCOB crystals. Grown-in dislocations, mosaic blocks and inclusions were also observed in the crystals. The effect of temperature field on the formation of growth defects in the crystals was discussed.

The growth twinning in self-frequency-doubling laser crystal NdxGd1 xAl3 (BO3) 4 (NGAB) has been observed with white-beam synchrotron radiation topography (WBSRT). It is shown that the growth twinning in flux-grown NGAB crystal is the 180 rotation twin in which the twofold symmetric operation axis is parallel to the [0001] direction and this kind of twinning often shows the black-and-white contrast in X-ray topography. It is believed that the growth twinning in NGAB was induced by growth bands formed during crystal growth.

Using chemical etching. method, the growth twins in self-frequency doubling laser crystal YbxYl-xAl3 (B03)4 have been observed. The etching pits on both sides of growth twin boundaries in the (101l) slice are ofthe triangles with different orientations. The structure ofgrowth twins is investigated by trallsmission synchrotron topography. In the transmission synchrotron topograph, the growth twins are visible not bv domain contrast 'but by' boundary contrast', i.e. the twins appear in the topograph in form of X-rav kinematical diffraction contrast due to the lattice strain stemming from the impurity incorporaton in the boundaries. The growth twins in YbxY1-xAl3 (B03)4 crystal are of inversion types, since no domain cnntrast Was observed..

High purity KTiOPO4 (KTP) crystals have been grown using a flux method. The quality, SHG e ciency and damage threshold of KTP crystals grown from high purity reagents were compared with those grown from commercial reagents. Experimental results indicate that KTP crystals using a high purity charge contain less impurities and possess higher SHG efficiency and damage threshold than do crystals grown from commercial reagents. It is proposed that decreasing the concentrations of impurities is an effective way of raising the damage threshold for KTP crystals.

YbxY1 xAl3 (BO3) 4 (x 0:1, 0.07 and 0) crystals have been grown by the flux method. The growth defects of YbxY1 xAl3 (BO3) 4 crystals were detected by X-ray topography. It is found that the perfection of YbYAB crystal with low Yb dopant is better than that with high Yb dopant. In Yb0. 1Y0. 9Al3 (BO3) 4 crystal, growth bands, growth boundaries, grown-in dislocations and inclusions were observed. However, the densities of growth defects for Yb0. 07Y0. 93Al3 (BO3) 4 and YAl3 (BO3) 4 are low and no obvious inclusions are observed in these crystals. In addition, growth twins were detected in YbxY1 xAl3 (BO3) 4 crystal by using the chemical etching method. It is found that the rowth twins occur frequently in Yb0. 1Y0. 9Al3 (BO3) 4 crystal whereas no growth twin appears in YAl3 (BO3) 4 crystal. Based on the experimental observations, the formation mechanism of growth twins is discussed. In the meantime, the e ective measures for reducing the growth twins and defects are proposed.