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Inversion of domain structure

Control of a scanning electron microscope to reverse the domain structure
of ferroelectric crystals
Anett NAGYVÁRADI*, Dr. Gábor ALMÁSI** * Pollack Mihály Faculty of Engineering, University of Pécs **Institute of Physics, University of Pécs
Our goal is to develop a device which is able to create a patterned surface charge density
distribution on ferroelectric crystals to produce controlled domain structure ready for light
frequency conversion.
There are nonlinear optical methods, with which one can achieve this frequency conversion
like second harmonic generation sum- and difference frequency generation or optical
parametric oscillation. These techniques require an effective phase matching of the pumping
and generated electromagnetic fields. The most effective technique for maintaining the phase
of these two fields with different wavelength is the quasi phase matching which applies a
nonlinear crystal with periodically modified domain structure. Crystals with periodically
modified domain structure are called periodically polarised crystals, in our case periodically
polarised lithium-niobate (PPLN).
The stoichiometric LiNbO3 is a ferroelectic crystal with an extremely low coercive field
strength (Ec is in the range of 100 Vcm-1).The coercive field is a critical measure of the poling
process: the smaller the coercive field strength the easier to modify the domain structure.
There are several techniques to produce periodically poled lithium niobate: one of these is the
electron beam bombardment.
The charges of the surface electrons generate an electric field across the LiNbO3 and
spontaneous polarisation reversal follows due to the displacement of the ions in the crystal
lattice structure: Li ions move in the z direction into oxygen triangles and Nb ion
displacements are also induced. This phenomenon is achieved if an electric field with an
intensity superior to the coercive field is applied in the direction opposite to spontaneous
polarisation of the crystal.
We generate the surface charge pattern with the help of the electron beam of an electron
microscope,
where the electron beam deflection is controlled by a microcontroller to produce
the necessary surface charge pattern.
The control program was written in C programming language and was simulated by Keil
MicroVision software.
The poling process is modelled by a 3D finite element numerical program (FlexPDE 5.0) to
clarify the role of the dielectrical properties of the crystal in details. With the simulation of the
process one can optimise the domain inversion process to achieve high spatial resolution and
contrast of the produced PPLN device.

References

[ l ]
C. Restoina,*, C. Darraud-Taupiaca, J.L. Decossasa, J.C. Vareillea, J. Hauden, Ferroelectric-domain-inverted gratings by electron beam on LiNbO3, Materials Science in Semiconductor Processing 3 (2000) 405±407 [ 2] Xijun Lia,_, Kazuya Terabea, Hideki Hatanoa,b, Kenji Kitamura, Domain patterning in LiNbO3 and LiTaO3 by focused electron beam, Journal of Crystal Growth 292 (2006) 324–327

Source: http://e-oktat.pmmf.hu/webgui/www/uploads/images/948/Nagyvaradi_XXV_int_wiss_kolloquium_Schweinfurt_2007_abstract.pdf

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