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..

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.

The growth defects in #ux grown RbTiOAsO 4 crystal have been investigated by white-beam synchrotron radiation topography. It is found that growth sector boundaries are the primary growth defects and they show strong X-ray kinematical contrast in the topograph. All the dislocations are growth induced and appear only in the near capping region. The area away from the capping region is nearly dislocation free. Growth bands show weak contrast in the X-ray topograph, which implies that the impurity contents are relatively low in various growth sectors and each growth sector is uniform in composition. (1999 Elsevier Science B.V. All rights reserved.

The growth defects in flux-grown NdxYl-xAl3 (B03)4 (NYAB) single crystals have been investigated by means of chemical etching and synchrotron radiation techniques. The band-shaped and polysynthetic twins were detected by optical microscopy due to the difference in etching rates between the host crystal and the twin. In the meantime, the orientation and symmetry of the twins were analyzed from their etching patterns. A large number of etch pits with regular shapes were also found on the surface, and these etch pits correspond to outcrops of the dislocations originating from the seed crystal or inclusions. The growth defects in flux-grown NdxYl-xAl3 (B03)4 (NYAB) single crystals have been investigated by means of chemical etching and synchrotron radiation techniques．The band-shaped and polysynthetic twins were detected by optical microscopy due to the difference in etching rates between the host crystal and the twin. In the meantime, the orientation and symmetry of the twins were analyzed from their etching patterns. A large number of etch pits with regular shapes were also found on the surface, and these etch pits correspond to outcrops of the dislocations originating from the seed crystal or inclusions.

Studies of photochromic damage induced in KTiOPO4 single crystals by 514.5nm laser irradiation is reported. A dependence of photochromic damage on polarization was observed. Along the same propagation direction (y901j23:31), the gray-track susceptibility for 8z polarization irradiation is one order magnitude lower than that for >z polarization irradiation for KTP crystal. The results clearly show that in KTiOPO4, a correlation exists between the gray-track susceptibility and the ionic electrical conductivity. Due to a high ionic electrical conductivity along the polar z-axis, the optical absorption centers will be annihilated with 8z polarization irradiation as soon as they are produced. This results in a lower gray-track susceptibility for 8z polarization irradiation.