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1 This article was downloaded by: [Shur, Vladimir][NEICON Consortium] On: 24 June 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Ferroelectrics Publication details, including instructions for authors and subscription information: Abnormal Domain Growth in Lithium Niobate with Surface Layer Modified by Proton Exchange M. A. Dolbilov a ; E. I. Shishkin a ; V. YA. Shur a ; S. Tascu b ; P. Baldi b ; M. P. De Micheli b a Ferroelectric Laboratory, Ural State University, Ekaterinburg, Russia b Laboratoire de Physique de la Matière Condensée, Universite de Nice Sophia-Antipolis, Nice Cedex 2, France Online publication date: 23 June 2010 To cite this Article Dolbilov, M. A., Shishkin, E. I., Shur, V. YA., Tascu, S., Baldi, P. and De Micheli, M. P.(2010) 'Abnormal Domain Growth in Lithium Niobate with Surface Layer Modified by Proton Exchange', Ferroelectrics, 398: 1, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Ferroelectrics, 398: , 2010 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: / Abnormal Domain Growth in Lithium Niobate with Surface Layer Modified by Proton Exchange M. A. DOLBILOV, 1, E. I. SHISHKIN, 1 V. YA. SHUR, 1 S. TASCU, 2 P. BALDI, 2 AND M. P. DE MICHELI 2 1 Ferroelectric Laboratory, Ural State University , Ekaterinburg, Russia 2 Laboratoire de Physique de la Matière Condensée, Universite de Nice Sophia-Antipolis 06108, Nice Cedex 2, France I. Introduction Formation of abnormal domain shapes was studied in single crystalline LiNbO 3 with surface layer modified by proton exchange. It has been shown that the isolated domain shape is very sensitive to the electric field: 1) three-rayed stars and concave polygons appear in low field, 2) hexagonal domains form in moderate field, 3) oriented domain rays grow in high field. The transformation from concave to convex polygon and the fast growth of narrow domain rays in front of the growing domain wall were investigated. The obtained abnormal behavior was attributed to non-effective external screening of depolarization field caused by artificial dielectric layer. Keywords Lithium niobate; domain kinetics; polarization reversal; proton exchange; surface modification; domain shape; one-dimensional domain growth The self-organized formation of quasi-regular nanodomain patterns is very important both for understanding the domain structure evolution with high spatial resolution, and for creation of the nanoscale domain structures with controlled geometry, which is still problem challenging domain engineering in ferroelectrics. Recently appeared nanodomain engineering requires qualitatively new methods for production of the stable tailored domain structures with submicron periods which will allow to construct the new types of nonlinear optical devices for coherent light operation [1, 2]. It has been shown that the self-organized domain evolution can be realized under highly non-equilibrium switching conditions which are characterized by completely ineffective screening of depolarization field thus preventing classical sideways domain growth [3]. Polarization reversal in this case is governed by correlated nucleation effect leading to formation of quasi-regular domain structures consisting of isolated nanodomains [4, 5]. One of the possible ways to achieve these conditions can be satisfied by fabrication of the artificial surface dielectric layers using various methods. Recently, it has been shown that congruent lithium niobate (CLN) with PE surface layers demonstrates the original scenario of domain structure evolution during polarization reversal resulting in formation of self-assembled micro- and nanodomain patterns [4, 6]. Received September 16, 2009; in final form March 16, Corresponding author. mikhail.dolbilov@labfer.usu.ru 108

3 Domain Growth in Proton Exchanged Lithium Niobate 109 The proton exchange (PE) is a very popular and well elaborated technique widely used for integrated and nonlinear optical applications [7]. PE in CLN represents the partial substitution of Li by H in the surface layer prepared by annealing of the sample in acid [8, 9]. The dependence of the shape of growing isolated domains in CLN on the switching conditions has been demonstrated experimentally [10 12]. The variety of the domain shapes predicted for switching in non-equilibrium conditions arisen due to retardation of the depolarization field screening has been found experimentally. In this paper, we present the systematical experimental study of the domain shape evolution in uniform slow increasing field using in situ visualization of the domain evolution in CLN single crystals with PE layers on both polar surfaces. II. Experiment Both polar surfaces of the 0.5 mm thick CLN wafer cut perpendicular to polar axis were subjected to PE procedure which was carried out in benzoic acid with 1% lithium benzoate at 300 C. Different exposures resulted in formation of so-called β 2 non-ferroelectric phase in the surface layers with thickness ranged from 0.5 to 3.0 µm [13]. It has been shown experimentally that the modified layers possess a step-like profile of refractive indices [14], thus representing well-defined uniform dielectric surface layers. Such layers are just appropriate for studying the influence of the uniform artificial surface dielectric gap on the domain evolution in CLN. The experimental setup based on polarizing microscope Carl Zeiss LMA 10 was used for in situ visualization of the domain kinetics during polarization reversal. The switching pulse of the external electric field was applied using uniform liquid electrodes (saturated LiCl aqueous solution) in order to invoke the polarization reversal of the studied CLN samples. The designed waveform of applied electric field (switching pulse) consisted of three stages: 1) rising front with a rate of kv/(mm s) from zero to kv/mm (slightly below the nominal threshold field of CLN), 2) main switching stage with slowly increasing field at a rate of kv/(mm s) up to kv/mm, 3) falling front witharateof2 10 kv/(mm s) to zero field. Polarization reversal mostly occurred at the second stage of the switching pulse. Switching current was measured for integral characterization of the polarization reversal. The switching current data were recorded simultaneously with the series of instantaneous domain patterns. III. Results and Discussion Typical shape of the switching pulse waveform and corresponding switching current for polarization reversal in CLN sample with 3-µm-thick PE layers on both polar surfaces are presented in Fig. 1. One can see that the polarization reversal occurred mostly at the second stage of the switching pulse. The switching current data can be divided in two basic inputs: 1) smooth monotonically increasing part demonstrating ohmic-like behavior and 2) noisy part consisting of the number of individual current spikes corresponding to fast rearrangements of the domain structure. The latter input represents the typical behavior for CLN when the polarization reversal is accompanied by jump-like motion of the domain walls strictly oriented along Y crystal directions [15, 16]. Although the switching current started to grow at the external field about 8 kv/mm (Fig. 1), the optically visible domains

4 110 M. A. Dolbilov et al. Figure 1. The waveform of applied external field and typical switching current data measured during polarization reversal in CLN with PE surface layers. (See Color Plate XII) appeared in the electroded area only at 20.9 kv/mm, which is just close to the well-known value of the threshold field for CLN (21 kv/mm) [5, 15, 17]. It has been found that in contrast with CLN crystals with classical hexagon domain growth [15, 17], the geometry of nucleated domains as well as dynamics of domain wall motion in PE-modified CLN samples crucially depend on the value of applied electric field (Figs. 2, 3). Formation of the isolated domains with three various shapes was revealed: 1) three-rayed stars and concave polygons appeared in low field range from 20.9 to 21.2 kv/mm (Fig. 2a), 2) hexagonal domains formed in moderate field range from 21.3 to 22.5 kv/mm (Fig. 2b), and 3) oriented domain rays started to grow in high field above 22.6 kv/mm (Fig. 2c). Polarization reversal in CLN sample with 3 µm thick PE layers started at 20.9 kv/mm by formation and growth of domains representing the three-rayed star shape (Figs. 2a and 3a). It must be pointed out that the rays of the domain stars are oriented strictly along Y directions. The detail analysis of the set of instantaneous domain images allows us to extract the field dependence of the sideways domain wall shift (distance from the star center) (Fig. 4). It is seen that at low field range the longitude growth of the star rays along Y + direction prevails with the average velocity about 7.1 ± 0.2 µm/s. In this case the domain walls are strictly oriented along Y directions (Fig. 5a) and the domain shape does not change. In moderate fields the shape of existing domain stars begins to change. The process of the shape transformation occurs through essential acceleration of the growth of three convex vertices along Y directions (the average velocity about 4.7 ± 0.2 µm/s) and essential deceleration of the longitude growth of the star rays along Y + direction with the average velocity about 2.2 ± 0.2 µm/s (Fig. 4). The change of domain wall orientation was caused Figure 2. Instantaneous domain patterns corresponding to different domain shapes appeared in CLN with PE surface layers in various field ranges: (a) low field (21.2 kv/mm), (b) moderate field (21.7 kv/mm), (c) high field (22.5 kv/mm). In situ visualization by polarizing microscopy.

5 Domain Growth in Proton Exchanged Lithium Niobate 111 Figure 3. Transformation of the isolated domain shape in increasing electric field in CLN with PE surface layers. In situ visualization by polarizing microscopy. Figure 4. Time dependence of the domain wall shift in Y + (diamonds) and Y (circles) directions for switching in slowly increasing electric field in CLN with PE surface layers. (See Color Plate XIII) Figure 5. Scheme of the domain shape transformation for switching in slowly increasing electric field in CLN with PE surface layers.

6 112 M. A. Dolbilov et al. by growth prevalence of three convex vertices (Fig. 5b). The process of transformation to regular hexagon is obtained at 21.9 kv/mm. In this case the distance from the domain center to all vertices became equal (Figs. 4 and 5c). The different stages of domain shape transformation are demonstrated in Fig. 3. Finally, the further increasing of the external field above 22.5 kv/mm led to the instability of the domain wall shape and arising of individual strictly Y-oriented narrow domain rays (Fig. 2c). Their growth velocities (longitude growth of the domain ray along Y + direction) ranged from 20 to 60 µm/s exceed essentially the sideways wall motion velocity of hexagon domain. IV. Explanation The observed behavior can be easily explained in terms of competition between switching rate and screening efficiency [15, 18]. The former is determined by the value of the acting field, while the latter strongly depends on the thickness of the surface dielectric layers. The thicker the modified PE layers are, the less effective the fast external screening of depolarization field is resulting in suppression of the conventional domain walls sideways motion [5]. The growth of the isolated domain in LN is based on the predetermined nucleation mechanism, which means that the generation of the steps (2D nucleation) along the wall is spatially inhomogeneous [15, 19]. During domain growth the switching field at the wall is decreased due to incomplete screening of depolarization field. For polygon domains the screening retardation leads to formation of the field singularities at the vertices leading to increase of the step generation rate. Thus, in LN crystal with C 3V symmetry the predetermined nucleation at three Y non-adjacent polygon vertices must be realized. The generated steps propagated by 1D nucleation along the existing domain wall in three Y directions. The classical hexagon shape is obtained when the step propagation rate exceeds essentially the step generation rate. For highly non-equilibrium switching conditions, the step propagation can be completely suppressed and step generation at the three vertices being the only switching mechanism leads to growth of domain rays in three Y + directions (Figs. 2a and 3a). Thus, the growth of three-rayed domain stars is observed in low fields. Increasing of the applied field stimulates the step propagation, thus the velocity of wall motion in Y direction increases in moderate field range (Fig. 3). The various effects of loss of the domain wall shape stability are observed when switching is realized under highly non-equilibrium conditions. The uncompensated depolarization field suppresses the domains broadening by the sideways motion. Nevertheless, the alternative mechanism of the broadening through formation of fingers representing rays oriented along Y direction can be observed. The individual fingers have been generated by perturbation at the domain wall (Fig. 2c) [20]. V. Conclusion In conclusion, the growth of isolated domains was studied in the single crystals of CLN with surface layers modified by proton exchange. Evolution of the domain structure in uniform electric field was studied in situ using conventional polarizing microscopy. It has been shown that the geometry of nucleated domains as well as dynamics of domain wall motion crucially depend on the value of applied electric field. Three regimes corresponding to growth of: 1) three-rayed domain stars, 2) hexagonal domains, and 3) irregularly shaped

7 Domain Growth in Proton Exchanged Lithium Niobate 113 domains with fingers, were distinguished and quantitatively characterized. The observed domain shape transformations were attributed to the changes of the ratio between step generation and step propagation rates. It has been confirmed that non-effective external screening of depolarization field plays the crucial role in formation and growth of domains in ferroelectrics with artificial surface dielectric layers. Acknowledgments The research was made possible in part by RFBR (grants ofr, r-ofr, r-Ural-a, a), by RFBR-CNRS (grant ), U.S. CRDF BRHE & FAE (PhD Award RNP /Y4-P-05-11), by Federal Agency of Education (contract P870), by Federal Agency of Science and Innovation (contracts and ). References 1. R. L. Byer, Quasi-phasematched Nonlinear Interactions and Devices, J. Nonlinear Optic. Phys. &Mater.6, (1997). 2. D. S. Hum and M. M. Fejer, Quasi-phasematching, C. R. Physiques 8, (2007). 3. V. Ya. Shur, Correlated Nucleation and Self-Organized Kinetics of Ferroelectric Domains, in Nucleation Theory and Applications, (ed. by J. W. P. Schmelzer, WILEY-VCH, Weinheim, 2005). 4. V. Ya. Shur, E. I. Shishkin, E. L. Rumyantsev, E. V. Nikolaeva, A. G. Shur, R. Batchko, M. Fejer, K. Gallo, S. Kurimura, K. Terabe, and K. Kitamura, Self-Organization in LiNbO 3 and LiTaO 3 : Formation of Micro- and Nano-Scale Domain Patterns, Ferroelectrics 304, (2004). 5. V. Ya. Shur, Nano- and Micro-Domain Engineering in Normal and Relaxor Ferroelectrics, in Handbook of advanced dielectric, piezoelectric and ferroelectric materials. Synthesis, properties and applications, (ed. by Z.-G. Ye, Woodhead Publishing Ltd., 2008). 6. M. A. Dolbilov, V. Ya. Shur, E. I. Shishkin, M. F. Sarmanova, E. V. Nikolaeva, S. Tascu, P. Baldi, and M. P. De Micheli, Influence of Surface Layers Modified by Proton Exchange on Domain Kinetics of Lithium Niobate, Ferroelectrics 374, (2008). 7. Yu.N. Korkishko and V. A. Fedorov, Ion Exchange in Single Crystals for Integrated Optics and Optoelectronics (Cambridge International Science, Cambridge, UK, 1999). 8. J. L. Jackel, R. E. Rice, and J. J. Veslka, Proton Exchange for High-Index Waveguides in LiNbO 3, Appl. Phys. Lett. 41, (1982). 9. M. De Micheli, J. Botineau, S. Neveu, P. Sibillot, D. B. Ostrowsky, and M. Papuchon, Independent Control of Index and Profiles in Proton-Exchanged Lithium Niobate Guides, Opt. Lett. 8, (1983). 10. A. P. Chernykh, V. Ya. Shur, E. V. Nikolaeva, E. I. Shishkin, A. G. Shur, K. Terabe, S. Kurimura, K. Kitamura, and K. Gallo, Shapes of Isolated Domains and Field Induced Evolution of Regular and Random 2D Domain Structures in LiNbO 3 and LiTaO 3, Material Science & Engineering B 120, (2005). 11. A. I. Lobov, V. Ya. Shur, I. S. Baturin, E. I. Shishkin, D. K. Kuznetsov, A. G. Shur, M. A. Dolbilov, and K. Gallo, Field Induced Evolution of Regular and Random 2D Domain Structures and Shape of Isolated Domains in LiNbO 3 and LiTaO 3, Ferroelectrics 341, (2006). 12. V. Ya. Shur, A. I. Lobov, A. G. Shur, E. L. Rumyantsev, and K. Gallo, Shape Evolution of Isolated Micro-Domains in Lithium Niobate, Ferroelectrics 360, (2007). 13. M. P. De Micheli, Fabrication and Characterization of Proton Exchanged Waveguides in Periodically Poled Congruent Lithium Niobate, Ferroelectrics 340, (2006). 14. Yu. N. Korkishko, V. A. Fedorov, M. P. De Micheli, P. Baldi, K. El Hadi, and A. Leycuras, Relationships Between Structural and Optical Properties of Proton-Exchanged Waveguides on Z-Cut Lithium Niobate, Applied Optics 35, (1996).

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