Appl. Phys. A 66, 511–514 (1998)
Applied Physics A
Materials
Science & Processing
Springer-Verlag 1998
Photorefractivedetection of antiparallel ferroelectric domainsin
BaTiO
3
andBaTiO
3
:Cocrystals
P. Mathey
1
, P. Jullien
1
, P. Lompr´e
1
,D.Rytz
2
1
Mat
´
eriaux pour l’Optique Non-Lin
´
eaire, Laboratoire de Physique de l’Universit
´
e de Bourgogne, UPRES-A CNRS 5027, 9 Avenue Alain Savary, B.P. 400,
21011 Dijon Cedex, France
(Fax: +33-3/80-39-59-61, E-mail: pmathey@u-bourgogne.fr)
2
Forschungsinstitut für mineralische und metallische Werkstoffe, Edelsteine/Edelmetalle GMBH, Struthstrasse 2, Wackenmühle, 55743 Idar-Oberstein,
Germany
Received: 11 July 1997/Accepted: 19 November 1997
Abstract. An all-optical method involving one coherent
beam of light and based on photorefractive wave mixing is
used to reveal antiparallel ferroelectric domains in one pure,
and two cobalt-doped, barium titanate crystals (BaTiO
3
).
Rod-shaped domains with square cross sections are revealed.
PACS: 61.72; 77.80.D; 42.70.N
The interest in photorefractive materials and their applica-
tions in holographic memories, optical storage, phase conju-
gation, novelty filtering, and image amplification make these
materials more and more attractive [1–4]. Among them, are
a large variety of ferroelectrics: KNbO
3
, LiNbO
3
, BaTiO
3
,
and SBN. In these materials, a crucial point is that all the
ferroelectric domains must be aligned along the same direc-
tion. Indeed, the single-domain feature of a photorefractive
crystal is a critical factor that must be fulfilled to reach an ef-
ficient energy transfer in beam coupling configurations used
in most of the aforementioned applications. The physics of
the photorefractive effect is now well understood. Two co-
herent beams interfere inside the material. A photorefractive
grating results from a photoinduced charge redistribution in
the bulk of the material. The associated photoinduced elec-
tric field, in turn, generates a change in the refractive index.
Each beam is diffracted by the index grating and interferes
constructively or destructively with the incident beam. The
consequence is that the intensity of one beam is decreased
while the intensity of the other beam is amplified. Let us re-
call that the direction of the photorefractive energy transfer
from a weak beam (the probe) to the pump beam depends on
the orientation of the polar axis. Keeping this in mind, we
can see that the consequence of misaligned domains is dras-
tic. There are regions in the bulk where the probe beam is
amplified and others where the beam is depleted. To take this
fact into account, the so-called “poling factor” F has been in-
cluded in the beam coupling gain Γ by Klein to adjust the
experimental gain values [5]:
Γ =
2πn
3
λ
Fr
eff
E
SC
. (1)
n is the refractive index, r
eff
is the combination of electro-
optic tensor components for a fully poled crystal, λ is the
wavelength, and E
SC
is the modulus of the space charge field.
The 90
◦
ferroelectric domains can be revealed with
microscopy observations whereas the 180
◦
domains can
not be detected for the crystals mentioned above neither
with a polarizing microscope, nor with crossed polariz-
ers. Polarization topography based on two-beam coup-
ling has been developed to reveal ferroelectric domains in
Ba
0.39
Sr
0.61
Nb
2
O
6
:Ce [6,7]. The method requires a strong
constant electric field (900kV/m). Effortshave been made to
visualize the domains at the crystal surfaces by using acids
that etch the crystal c faces. The domains in the bulk can
be revealed by second-harmonic generation or X-ray diffrac-
tion [8–10]. Unfortunately, these techniques can damage the
crystals or they need sophisticated instruments.
Methods based on optical principles involving simple ar-
rangements and low intensities (some mW/cm
2
) are therefore
particularly suitable.
1 Experimental setup and results
A non-destructive method to reveal the 180
◦
ferroelectric do-
mains in the crystal has been proposed recently [11,12]. The
method is based on energy transfer between two beams. The
crystallographic orientation is chosen to deplete the signal
wave so that, in a single-domain crystal, the depletion should
be uniform. The presence of antiparallel domains implies
signal amplification and the generation of a non-uniformly
depleted probe beam. This is evidenced by the appearance
of bright spots on the depleted signal beam. One drawback
of this method is the presence of interference fringes re-
sulting from the signal-beam multiple reflections from the