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SBN single crystal fibers grown by micro-pulling down technique
Optical Materials 24 (2003) 419–424 www.elsevier.com/locate/optmat
SBN single crystal fibers grown by micro-pulling down technique A. El Hassouni a, K. Lebbou a,*, C. Goutaudier a, G. Boulon a, A. Yoshikawa b, T. Fukuda b a
Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon 1 University, CNRS UMR 5620, 69622 Villeurbanne Cedex, France b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan Received 8 December 2002; accepted 24 February 2003
Abstract SBN fibers single crystal have been grown using micro-pulling down (l-PD) technique. The grown fibers are oriented along the c-axis or a-axis as a function of initial seed. The diameters are approximately between 500 lm and 1 mm. The preliminary observation show that these fibers appear to be striation and cracks free. Ó 2003 Elsevier B.V. All rights reserved. Keywords: SBN; Tungsten; Fibers; Crystal; Diagram; l-PD
1. Introduction Strontium barium niobate (SBN) belongs to the tetragonal potassium structure [1] and present a wide range of solid solution in the SrNb2 O6 – BaNb2 O6 binary equilibrium diagram [2]. The structural formula of the compound taking into account the number of different positions (voids) and non-equivalence of the octahedra (NbO6 ) can be written as (A1 )2 (A2 )4 (C)4 (B1 )2 (B2 )8 O30 [3]. Positions A1 with coordination number 9 and A2 with coordination number 12 can be occupied by
*
Corresponding author. Tel.: +33-4-72-44-83-29; fax: +33-472-43-12-33. E-mail address: [email protected] (K. Lebbou).
Sr and Ba ions (altogether five ions for six A1 + A2 positions). Positions C with coordination number 9 are vacant, which provides electroneutrality. Ten NbO6 octahedra entering the unit cell composition contain (0 8) Nb ions in B2 and (0 2) ions in B1 positions. The Nb ions lie in the horizontal symmetry plane of oxygen octahedra. Voids A1, A2 and C are at distance c=2 from the plane occupied by niobium ions. Srx Ba1x Nb2 O6 is an attractive material for many applications such as electro-optic modulators, holographic storage [4] and beam steering [5]. Bulk SBN has an extremely high electro-optic coefficient [6] and is strongly photorefractive [7]. SBN crystals are more attractive for photorefractive applications than LiNbO3 offering the possibilities of much smaller devices it is a reason why much attention is now being paid to this material as
0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-3467(03)00156-3
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A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
potential product [8]. Unfortunately, the growth of optically homogeneous SBN crystals encounters substantial technological difficulties due to stochiometric violation and the occurrence of a high temperature phase transition between monoclinic and tetragonal symmetries of the crystal lattices [9]. Stoichiometry violation and a possible formation of second phase are responsible for optical inohomogeneities in the crystal. The optical inhomogeneities may also depend on the domain structure if the crystal is not single domain [10]. One of the major defect observed during crystal growth by conventional Czhocralski technique is the striations propagation associated with the variation of crystal composition upon fluctuation of instantaneous growth rate due to thermal variation at the crystallization front [11]. The shape of growth straie in the crystal is directly associated with the shape of the crystallization front. Striation is the typical optical defect common to solid solution crystals and is quite difficult to suppress in the case of SBN. One of the significant countermeasures to this problem is to grow crystals at the composition as close to a congruent melt as possible. Recently, a series of works to determine the accurate congruent melting composition has been successfully undertaken for the useful optic crystals, such LiNbO3 [12], LiTaO3 [13] and BNN [14–16]. SBN is a promising medium for holographic data storage. The use of fiber is very attractive because each fiber can store many independent hologram, for this purpose, fibers are preferred because they provide a significant increase in photorefractive performance. In this paper we present the stable fiber single crystal grown by micro-pulling down (l-PD) technique, the growth conditions and the fiber qualities will be discussed.
pressed under 1 kgf cm2 into discs of 15 mm diameter which were placed into an alumina crucible. The pellets were sintered at 900 °C for 10 h and at 1250 °C for 10 h in air atmosphere, then repowdered and subjected to the same treatment a second time. The identification of the phases was carried out by X-ray powder diffraction analysis. The nominal composition of Sr, Ba and Nb elements was determined by plasma emission spectroscopy. Quantitative analyses for Sr, Ba and Nb along the pulled fibers were performed by X-ray energy dispersion, ZAF correction is used, where Z is the atomic number; A, the absorption correction factor and F , the fluorescence correction factor. Measurements were obtained within an accuracy of 2%. For crystal growth, the melt was contained in a crucible made of Pt/Rh plate of 0.1 mm thickness and Pt tube of 0.4–0.8 mm outer diameter and 0.01–0.05 mm wall thickness. Fig. 1 shows the l-PD apparatus used. The technique, involves growing a fiber through a micro-nozzle by pulling in the downward direction, as shown in Fig. 1. The growth equipment consists of a Pt/Rh crucible (10 3 2 mm3 ) directly heated resistively, an after-heater made from Pt wire, an annealing furnace, and lowering mechanism with a microX –Y stage. The raw materials were melted in the Pt crucible and allowed to pass through the micro-nozzle. The fiber was formed by attaching
Row materials
(-)
(+)
Crucible
After heater
2. Experimental Sr1x Bax Nb2 O6 ceramics compounds belong to the ternary SrO–BaO–NbO1:5 equilibrium diagram were used as starting material for the melt preparation. The ceramics were prepared by solid state reaction from mixtures of SrCO3 , Nb2 O5 and BaCO3 powders (99.99% purity), cold
Seed Grown fiber
Fig. 1. Schematic illustration of the l-PD technique.
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
421
Fig. 3. Single crystal grown fiber from Sr0:85 Ba0:15 Nb2 O6 .
velocity. The alignment of the seed and the micro-nozzle was controlled by the micro-X –Y stage. The crystal growth was performed using two kind of seeds [1 0 0] and [0 0 1]. The crystals are grown in air atmosphere with pulling rates between 0.5 and 1 mm/min and
Fig. 2. Single crystal grown from Sr1x Bax Nb2 O6 : (a) x ¼ 0:3, (b) x ¼ 0:4 and (c) x ¼ 0:6.
the seed to the tip of the micro-nozzle (Fig. 1) and slowly pulling it downward with a constant
5.0
5.0
(a)
4.5 4.0
Sr Ba
4.0
3.5
3.5
3.0
3.0
% variation
% Variation
(b)
4.5
Sr Ba
2.5 2.0 1.5
2.5 2.0 1.5
1.0
1.0
0.5
0.5 0.0
0.0 0
2
4
6
8
10
12
14
16
18
0
20
2
4
6
8
10
12
14
16
18
20
Fiber length (mm)
Fiber Length (mm) 5.0
(c)
4.5
Sr Ba
4.0
% Variation
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
2
4
6
8
10
12
14
16
18
20
Fiber length (mm)
Fig. 4. Variation of the Sr1x Bax Nb2 O6 crystal composition along the growth direction: (a) x ¼ 0:3, (b) x ¼ 0:4 and (c) x ¼ 0:6.
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A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
annealed at 1000 °C for 12 h to reduce residual stress formed during growth.
3. Results Srx Ba1x Nb2 O6 (0:2 6 x 6 0:8) was grown from the melt, from x ¼ 0:3 to x ¼ 0:6, as shown in Fig. 2 no cracks were observed during growth process. The crystals were colorless and transparent independent of the melt composition, with uniform shape and free of macroscopic defects such as cracks, bubbles or inclusions. The cracks and diameter variation were observed only for SBN with x < 0:2 (Fig. 3). The typical size of crystals was 0.5–1 mm in cross-section, depending on diameter of the nozzle and a few centimeters in length. In the main, about 95 vol.% was crystallized into the single crystals. It was also possible to grow SBN single crystals fibers with high pulling rate of about 1.5 mm/min. Phase homogeneity of the crystals was studied by X-ray diffraction (XRD) powder analysis at room temperature. The crystals were SBN single phase with tetragonal structure (JCPDS 39-0265). Fig. 4 shows the crystal composition as a function of distance along the growth axis. The uniformity of
the composition of the crystals was found to be high. The deviation from the average data for Sr, Ba, and Nb concentrations within the same crystals did not exceed about 2%, which corresponds to the experimental error of the EPMA measurements. The Sr0:2 Ba0:8 Nb2 O6 fiber shows the {1 1 0} facet and striation presence Fig. 5, but we did not see the {1 2 0}, {1 0 0} and {1 3 0} facets as it was already mentioned by Abrahams et al. [17], the same results was observed in the case of Sr0:1 Ba0:8 Nb2 O6 . It is clear that an increase of Ba concentration in the Sr0:2 Ba0:8 Nb2 O6 composition increase the tendency of the growth stability. The crystal growth of fiber with Sr concentration less than 2 are difficult, especially the diameter is not stable, cracks are observed and a disconnection tendency of the molten zone from the seed is observed. It is very difficult to investigate the effect of each growth condition and SBN composition on the crystal quality by changing each individual condition, especially for the material pulled by l-PD. Therefore we studied the effect of the composition in the ternary equilibrium diagram (SrO–BaO–NbO2:5 ) of one growth condition (pulling rate 0.5 mm/min) on a given aspect of crystal quality, on the basis of different samples grown by l-PD. Fig. 6 shows
Fig. 5. Sr0:2 Ba0:8 Nb2 O6 fiber showing the presence of striation, bubbles and facets.
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
423
Fig. 6. Relationships between crystal growth conditions (materials composition) and crystals qualities in the ternary equilibrium diagram of the system (SrO–BaO–NbO2:5 ).
the relationships between the crystal fiber qualities and the starting composition. It is clear that the fibers belongs to the CD line are more better than the fibers belongs to the EF line. The growth process is easier in the cases of CD fibers. The fibers are single crystal, homogeneous, cracks, bubbles and striations free. The crystallization percentage is higher (more than 95%) and the crucible is empty. In the case of EF fibers, the crystals are not transparent, the growth conditions are note stable (unstable diameter, cracks and bubbles are observed) and the material flow on the crucible. The growth conditions are stable and the crystal qualities are relatively improved along AB direction.
4. Conclusion We introduced a good single crystal fiber grown by l-PD technique. We have optimized the growth composition to perform the best single crystal fibers. The range of the best growth condition is determined. The increases of Ba concentration stabilize the SBN growth process and improve the crystal qualities in SBN materials. This grown fibers for Srx Ba1x Nb2 O6 (0:3 6 x 6 0:6)
are striation free. This results is very promised to perform a fibers for photorefractive application. The optical properties study will be initiated soon. References [1] P.V. Lenzo, G.E. Spenser, A.A. Ballman, Appl. Phys. Lett. 11 (1967) 23. [2] J.R. Carruthers, M. Crasso, Solid State Sci. 117 (1970) 1426. [3] A.M. Prokhorov, Yu.S. Kuzminov, Book, Edi Adam Hilger, 1990, p. 81. [4] L. Hesselink, M.C. Bashaw, Opt. Quant. Electron. 25 (1993) S611. [5] L.B. Aronson, L. Hesselink, Opt. Lett. 15 (1990) 30. [6] R.A. Vasquez, M.D. Ewbank, R.R. Neurgaonkar, Opt. Commun. 80 (1991) 235. [7] G.A. Rakuljic, A. Yariv, R.R. Neurganonkar, Opt. Eng. 25 (1986) 1212. [8] A. ElHassouni, K. Lebbou, A. Yoshikawa, T. Fukuda, G. Boulon, M.Th. Cohen-Adad, The 1st Asian Conference on crystal growth and crystal technology (CGCT-1), August 29–September 1, 2000, Sendai, Japan. [9] K. Megumi, N. Nagatsuma, Y. Kashiwada, Y. Furuhata, J. Mat. Sci. 11 (1976) 1583. [10] Y.L. Kopylov, V.B. Kravchenko, V.P. Moiseev, Kris. Tech. 14 (1979) 697. [11] Y. Ito, H. Kozuka, Y. Kashiwada, Y. Furuhata, Jpn. J. Appl. Phys. 14 (1975) 1443.
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A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
[12] Y. Furukawa, M. Sato, F. Nitanda, K. Ito, J. Cryst. Growth 99 (1990) 832. [13] T. Lukaseiwicz, W.R. Romanowski, J. Sokolska, M. Swirkowicz, S. Golab, Z. Galazka, Cryst. Res. Technol. 36 (2001) 127. [14] K. Lebbou, A. Yoshikawa, T. Fukuda, M.Th. Cohen Adad, G. Boulon, Mater. Res. Bull. 8 (2000) 35.
[15] K. Lebbou, H. Itagaki, A. Yoshikawa, T. Fukuda, F. Carillo-Romo, G. Boulon, A. Brenier, M.Th. CohenAdad, J. Cryst. Growth 210 (2000) 655–662. [16] K.Lebbou,H.Itagaki,A.Yoshikawa,T.Fukuda,G.Boulon, M.Th. Cohen-Adad, J. Cryst. Growth 224 (2001) 59–66. [17] S.C. Abrahams, J.M. Reddy, J.L. Bernstien, J. Phys. Chem. Solids 27 (1966) 999.
SBN single crystal fibers grown by micro-pulling down technique A. El Hassouni a, K. Lebbou a,*, C. Goutaudier a, G. Boulon a, A. Yoshikawa b, T. Fukuda b a
Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon 1 University, CNRS UMR 5620, 69622 Villeurbanne Cedex, France b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan Received 8 December 2002; accepted 24 February 2003
Abstract SBN fibers single crystal have been grown using micro-pulling down (l-PD) technique. The grown fibers are oriented along the c-axis or a-axis as a function of initial seed. The diameters are approximately between 500 lm and 1 mm. The preliminary observation show that these fibers appear to be striation and cracks free. Ó 2003 Elsevier B.V. All rights reserved. Keywords: SBN; Tungsten; Fibers; Crystal; Diagram; l-PD
1. Introduction Strontium barium niobate (SBN) belongs to the tetragonal potassium structure [1] and present a wide range of solid solution in the SrNb2 O6 – BaNb2 O6 binary equilibrium diagram [2]. The structural formula of the compound taking into account the number of different positions (voids) and non-equivalence of the octahedra (NbO6 ) can be written as (A1 )2 (A2 )4 (C)4 (B1 )2 (B2 )8 O30 [3]. Positions A1 with coordination number 9 and A2 with coordination number 12 can be occupied by
*
Corresponding author. Tel.: +33-4-72-44-83-29; fax: +33-472-43-12-33. E-mail address: [email protected] (K. Lebbou).
Sr and Ba ions (altogether five ions for six A1 + A2 positions). Positions C with coordination number 9 are vacant, which provides electroneutrality. Ten NbO6 octahedra entering the unit cell composition contain (0 8) Nb ions in B2 and (0 2) ions in B1 positions. The Nb ions lie in the horizontal symmetry plane of oxygen octahedra. Voids A1, A2 and C are at distance c=2 from the plane occupied by niobium ions. Srx Ba1x Nb2 O6 is an attractive material for many applications such as electro-optic modulators, holographic storage [4] and beam steering [5]. Bulk SBN has an extremely high electro-optic coefficient [6] and is strongly photorefractive [7]. SBN crystals are more attractive for photorefractive applications than LiNbO3 offering the possibilities of much smaller devices it is a reason why much attention is now being paid to this material as
0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-3467(03)00156-3
420
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
potential product [8]. Unfortunately, the growth of optically homogeneous SBN crystals encounters substantial technological difficulties due to stochiometric violation and the occurrence of a high temperature phase transition between monoclinic and tetragonal symmetries of the crystal lattices [9]. Stoichiometry violation and a possible formation of second phase are responsible for optical inohomogeneities in the crystal. The optical inhomogeneities may also depend on the domain structure if the crystal is not single domain [10]. One of the major defect observed during crystal growth by conventional Czhocralski technique is the striations propagation associated with the variation of crystal composition upon fluctuation of instantaneous growth rate due to thermal variation at the crystallization front [11]. The shape of growth straie in the crystal is directly associated with the shape of the crystallization front. Striation is the typical optical defect common to solid solution crystals and is quite difficult to suppress in the case of SBN. One of the significant countermeasures to this problem is to grow crystals at the composition as close to a congruent melt as possible. Recently, a series of works to determine the accurate congruent melting composition has been successfully undertaken for the useful optic crystals, such LiNbO3 [12], LiTaO3 [13] and BNN [14–16]. SBN is a promising medium for holographic data storage. The use of fiber is very attractive because each fiber can store many independent hologram, for this purpose, fibers are preferred because they provide a significant increase in photorefractive performance. In this paper we present the stable fiber single crystal grown by micro-pulling down (l-PD) technique, the growth conditions and the fiber qualities will be discussed.
pressed under 1 kgf cm2 into discs of 15 mm diameter which were placed into an alumina crucible. The pellets were sintered at 900 °C for 10 h and at 1250 °C for 10 h in air atmosphere, then repowdered and subjected to the same treatment a second time. The identification of the phases was carried out by X-ray powder diffraction analysis. The nominal composition of Sr, Ba and Nb elements was determined by plasma emission spectroscopy. Quantitative analyses for Sr, Ba and Nb along the pulled fibers were performed by X-ray energy dispersion, ZAF correction is used, where Z is the atomic number; A, the absorption correction factor and F , the fluorescence correction factor. Measurements were obtained within an accuracy of 2%. For crystal growth, the melt was contained in a crucible made of Pt/Rh plate of 0.1 mm thickness and Pt tube of 0.4–0.8 mm outer diameter and 0.01–0.05 mm wall thickness. Fig. 1 shows the l-PD apparatus used. The technique, involves growing a fiber through a micro-nozzle by pulling in the downward direction, as shown in Fig. 1. The growth equipment consists of a Pt/Rh crucible (10 3 2 mm3 ) directly heated resistively, an after-heater made from Pt wire, an annealing furnace, and lowering mechanism with a microX –Y stage. The raw materials were melted in the Pt crucible and allowed to pass through the micro-nozzle. The fiber was formed by attaching
Row materials
(-)
(+)
Crucible
After heater
2. Experimental Sr1x Bax Nb2 O6 ceramics compounds belong to the ternary SrO–BaO–NbO1:5 equilibrium diagram were used as starting material for the melt preparation. The ceramics were prepared by solid state reaction from mixtures of SrCO3 , Nb2 O5 and BaCO3 powders (99.99% purity), cold
Seed Grown fiber
Fig. 1. Schematic illustration of the l-PD technique.
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
421
Fig. 3. Single crystal grown fiber from Sr0:85 Ba0:15 Nb2 O6 .
velocity. The alignment of the seed and the micro-nozzle was controlled by the micro-X –Y stage. The crystal growth was performed using two kind of seeds [1 0 0] and [0 0 1]. The crystals are grown in air atmosphere with pulling rates between 0.5 and 1 mm/min and
Fig. 2. Single crystal grown from Sr1x Bax Nb2 O6 : (a) x ¼ 0:3, (b) x ¼ 0:4 and (c) x ¼ 0:6.
the seed to the tip of the micro-nozzle (Fig. 1) and slowly pulling it downward with a constant
5.0
5.0
(a)
4.5 4.0
Sr Ba
4.0
3.5
3.5
3.0
3.0
% variation
% Variation
(b)
4.5
Sr Ba
2.5 2.0 1.5
2.5 2.0 1.5
1.0
1.0
0.5
0.5 0.0
0.0 0
2
4
6
8
10
12
14
16
18
0
20
2
4
6
8
10
12
14
16
18
20
Fiber length (mm)
Fiber Length (mm) 5.0
(c)
4.5
Sr Ba
4.0
% Variation
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
2
4
6
8
10
12
14
16
18
20
Fiber length (mm)
Fig. 4. Variation of the Sr1x Bax Nb2 O6 crystal composition along the growth direction: (a) x ¼ 0:3, (b) x ¼ 0:4 and (c) x ¼ 0:6.
422
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
annealed at 1000 °C for 12 h to reduce residual stress formed during growth.
3. Results Srx Ba1x Nb2 O6 (0:2 6 x 6 0:8) was grown from the melt, from x ¼ 0:3 to x ¼ 0:6, as shown in Fig. 2 no cracks were observed during growth process. The crystals were colorless and transparent independent of the melt composition, with uniform shape and free of macroscopic defects such as cracks, bubbles or inclusions. The cracks and diameter variation were observed only for SBN with x < 0:2 (Fig. 3). The typical size of crystals was 0.5–1 mm in cross-section, depending on diameter of the nozzle and a few centimeters in length. In the main, about 95 vol.% was crystallized into the single crystals. It was also possible to grow SBN single crystals fibers with high pulling rate of about 1.5 mm/min. Phase homogeneity of the crystals was studied by X-ray diffraction (XRD) powder analysis at room temperature. The crystals were SBN single phase with tetragonal structure (JCPDS 39-0265). Fig. 4 shows the crystal composition as a function of distance along the growth axis. The uniformity of
the composition of the crystals was found to be high. The deviation from the average data for Sr, Ba, and Nb concentrations within the same crystals did not exceed about 2%, which corresponds to the experimental error of the EPMA measurements. The Sr0:2 Ba0:8 Nb2 O6 fiber shows the {1 1 0} facet and striation presence Fig. 5, but we did not see the {1 2 0}, {1 0 0} and {1 3 0} facets as it was already mentioned by Abrahams et al. [17], the same results was observed in the case of Sr0:1 Ba0:8 Nb2 O6 . It is clear that an increase of Ba concentration in the Sr0:2 Ba0:8 Nb2 O6 composition increase the tendency of the growth stability. The crystal growth of fiber with Sr concentration less than 2 are difficult, especially the diameter is not stable, cracks are observed and a disconnection tendency of the molten zone from the seed is observed. It is very difficult to investigate the effect of each growth condition and SBN composition on the crystal quality by changing each individual condition, especially for the material pulled by l-PD. Therefore we studied the effect of the composition in the ternary equilibrium diagram (SrO–BaO–NbO2:5 ) of one growth condition (pulling rate 0.5 mm/min) on a given aspect of crystal quality, on the basis of different samples grown by l-PD. Fig. 6 shows
Fig. 5. Sr0:2 Ba0:8 Nb2 O6 fiber showing the presence of striation, bubbles and facets.
A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
423
Fig. 6. Relationships between crystal growth conditions (materials composition) and crystals qualities in the ternary equilibrium diagram of the system (SrO–BaO–NbO2:5 ).
the relationships between the crystal fiber qualities and the starting composition. It is clear that the fibers belongs to the CD line are more better than the fibers belongs to the EF line. The growth process is easier in the cases of CD fibers. The fibers are single crystal, homogeneous, cracks, bubbles and striations free. The crystallization percentage is higher (more than 95%) and the crucible is empty. In the case of EF fibers, the crystals are not transparent, the growth conditions are note stable (unstable diameter, cracks and bubbles are observed) and the material flow on the crucible. The growth conditions are stable and the crystal qualities are relatively improved along AB direction.
4. Conclusion We introduced a good single crystal fiber grown by l-PD technique. We have optimized the growth composition to perform the best single crystal fibers. The range of the best growth condition is determined. The increases of Ba concentration stabilize the SBN growth process and improve the crystal qualities in SBN materials. This grown fibers for Srx Ba1x Nb2 O6 (0:3 6 x 6 0:6)
are striation free. This results is very promised to perform a fibers for photorefractive application. The optical properties study will be initiated soon. References [1] P.V. Lenzo, G.E. Spenser, A.A. Ballman, Appl. Phys. Lett. 11 (1967) 23. [2] J.R. Carruthers, M. Crasso, Solid State Sci. 117 (1970) 1426. [3] A.M. Prokhorov, Yu.S. Kuzminov, Book, Edi Adam Hilger, 1990, p. 81. [4] L. Hesselink, M.C. Bashaw, Opt. Quant. Electron. 25 (1993) S611. [5] L.B. Aronson, L. Hesselink, Opt. Lett. 15 (1990) 30. [6] R.A. Vasquez, M.D. Ewbank, R.R. Neurgaonkar, Opt. Commun. 80 (1991) 235. [7] G.A. Rakuljic, A. Yariv, R.R. Neurganonkar, Opt. Eng. 25 (1986) 1212. [8] A. ElHassouni, K. Lebbou, A. Yoshikawa, T. Fukuda, G. Boulon, M.Th. Cohen-Adad, The 1st Asian Conference on crystal growth and crystal technology (CGCT-1), August 29–September 1, 2000, Sendai, Japan. [9] K. Megumi, N. Nagatsuma, Y. Kashiwada, Y. Furuhata, J. Mat. Sci. 11 (1976) 1583. [10] Y.L. Kopylov, V.B. Kravchenko, V.P. Moiseev, Kris. Tech. 14 (1979) 697. [11] Y. Ito, H. Kozuka, Y. Kashiwada, Y. Furuhata, Jpn. J. Appl. Phys. 14 (1975) 1443.
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A. El Hassouni et al. / Optical Materials 24 (2003) 419–424
[12] Y. Furukawa, M. Sato, F. Nitanda, K. Ito, J. Cryst. Growth 99 (1990) 832. [13] T. Lukaseiwicz, W.R. Romanowski, J. Sokolska, M. Swirkowicz, S. Golab, Z. Galazka, Cryst. Res. Technol. 36 (2001) 127. [14] K. Lebbou, A. Yoshikawa, T. Fukuda, M.Th. Cohen Adad, G. Boulon, Mater. Res. Bull. 8 (2000) 35.
[15] K. Lebbou, H. Itagaki, A. Yoshikawa, T. Fukuda, F. Carillo-Romo, G. Boulon, A. Brenier, M.Th. CohenAdad, J. Cryst. Growth 210 (2000) 655–662. [16] K.Lebbou,H.Itagaki,A.Yoshikawa,T.Fukuda,G.Boulon, M.Th. Cohen-Adad, J. Cryst. Growth 224 (2001) 59–66. [17] S.C. Abrahams, J.M. Reddy, J.L. Bernstien, J. Phys. Chem. Solids 27 (1966) 999.