AN APPLICATION OF X-RAY TOPOGRAPHY AND DIFFRACTION: PERIODIC INVERSION DOMAINS IN NON-LINEAR OPTICAL MATERIALS. Z. W. Hu^* , P. A. Thomas^||, ^||Department of Physics, University of Warwick, Coventry, CV4 7AL, UK, ^*Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OXI 3PU, UK

Periodic inversion domains or twins formed in non-linear optical materials by periodic-poling techniques, which are used for generating quasi-phased second harmonic generation, are investigated by synchrotron transmission topography, multiple-crystal topography and diffraction. Using a high-resolution X-ray diffractometer with a triple-bounce analyser attached, the inversion domains in KTiOPO₄ (KTP) and the inversion domain walls in LiNbO₃ (LN) have been, respectively, imaged with a high spatial resolution mode, furthermore, diffraction contrast subtly dependent on choice of reflection and position has been revealed. With reciprocal-space mapping, the different broadening effects of the widths of diffraction profiles along [[Delta]]q_|| and [[Delta]]q for KTP and the extended diffuse scattering tails along [[Delta]]q_|| for LN have been extracted and shown to relate to the lattice tilting or strain induced in the domain-inversion processing through high angular resolution topography. Synchrotron Laue topography has been carried out and the contrast of the domain walls observed in KTP is suggested to be the result of interbranch scattering caused by the phase-shift between the X-ray wavefields in neighbouring domains. Differences in topographic images of the domain-inverted structures between KTP and LN are discussed. The results of the above artificial inversion domains are compared with those of the naturally occurring inversion domains and the similarities and differences between them are examined in the context of diffraction topography. Finally, the observation of the evolution of periodic inversion domains of KTP with changing temperature is briefly discussed.

On leave from the National Laboratory of Solid Sate Microstructures, Nanjing Universiyty, P. R. China