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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Lett. 62, 435 (1993).
[3] A. Snigirev et al., Rev. Sci. Instrum. 66, 5486
(1995).
[4] P. Cloetens et al., J. Phys. D: Appl. Phys. 29,
133 (1996)
[5] H. F. Talbot, Phil. Mag. third series 9, 401 (1836).
[6] P. Rejmánková-Pernot et al., submitted