PHASE RETRIEVAL BY COMBINED BRAGG AND FRESNEL X-RAY DIFFRACTION IMAGING

Petra Rejmánková-Pernot¹, Peter Cloetens¹, José Baruchel¹, Jean-Pierre Guigay¹,
Paul Moretti²

¹European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France
e-mail: rejma@esrf.fr

²Laboratoire de Physico-Chimie des Matériaux Luminescents, Université Claude Bernard-Lyon I, F­69622 Villeurbanne, France

Keywords: coherence, ferroelectric domains, periodically poled lithium niobate

Lithium niobate can realize second-harmonic generation devices, with enhanced efficiency when periodic reversal of the sign of the nonlinear optical coefficient d33 occurs in the phase matching period [1]. This periodically inverted domain structure can be produced by applying an external electric field using periodically patterned electrodes [2]. Various methods revealing ferroelectric domains in LiNbO3 were reported: chemical etching, electron microscopy, pyroelectrically induced electron emission and X-ray topography.

Free space propagation is increasingly applied to perform phase sensitive X-ray imaging, taking advantage of the coherence properties of third generation synchrotron beams [3,4]. In the present work, Fresnel and Bragg diffraction are simultaneously used to visualize ferroelectric domains within a periodically poled lithium niobate crystal, using a simple white beam section topography set-up. Bragg diffraction introduces a phase shift between the waves diffracted in adjacent domains due to the phase difference between their structure factors. Fresnel diffraction and its specific manifestation for periodic objects, the ëTalbot effectí[5], determine the intensity variations after free space propagation. Through the measurement of the observed contrast as a function of propagation distance, one can directly obtain the phase difference between the structure factors of reflections hkl and -h,-k,-l [6]. This opens the way 1) to an efficient visualization, within the bulk, of these technologically important domains and 2) to a direct determination of the phase difference between structure factors. Consequently, any tine variations of the atomic positions as a function of an external parameter (e.g. electric field or temperature) can be studied.

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[6] P. Rejmánková-Pernot et al., submitted