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Optical damage resistance in undoped LiNbO3 crystals
Optical Materials 16 (2001) 111±117
www.elsevier.nl/locate/optmat
Optical damage resistance in undoped LiNbO3 crystals M. Fontana *, K. Chah, M. Aillerie, R. Mouras, P. Bourson Laboratoire Mat eriaux Optiques a Propri et es Sp eci®ques, CLOES, Universit e de Metz and Sup elec, 2, rue E. Belin, 57070 Metz, France
Abstract Photo-induced birefringence changes are investigated on several undoped LiNbO3 crystals as a function of laserirradiation time and intensity. Comparison points out that the changes are the smallest in the crystals grown from high temperature top seeded solution (TSS). It is concluded that the optical damage resistance in undoped LiNbO3 crystals is increased with a decrease of the amount of intrinsic defects related to the non-stoichiometry. The necessity to carefully control the true crystal composition is stressed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.20; 61.80 Ba; 61.72-y
1. Introduction Despite its large electro-optical and non-linear optical coecients, LiNbO3 (LN) crystal presents a low optical damage threshold as the main disadvantage for its commercial use in electro-optic modulators, switches or optical parametric oscillators. This optical damage is mainly causd by photorefractive (PR) properties of the material which are generally attributed to the presence of traps and excited sites due to transition metal ions, especially iron. The PR optical damage can be observed by changes in the birefringence, particularly in the visible range, due to laser irradiation, even for a small power density. Some dierent ways have been undertaken to reduce the optical damage on the basis of the microscopic origin of PR properties. In iron-doped LN crystal, these eects can be well described by charge transport process in *
Corresponding author.
which Fe2 ion is an electron donor and Fe3 is an electron trap [1]. This PR one-centre model predicts a linear dependence of the photovoltaic current on the light density and iron concentration. In fact this behaviour was not proved by measurements on Fe-doped LN crystals under high light intensities [2] so that other impurities centres have to be taken into account in the charge transport model. As LiNbO3 crystals possess a large amount of defects related to the non-stoichiometry, the niobium antisites NbLi (i.e., niobium ions on Li sites) [3,4] have to be considered as secondary PR centres. Within this two-centre model [5], electrons are excited from Fe2 into Nb5 Li , which can be recombined with conduction electrons to give rise to Nb4 Li . This process explains the photovoltaic and thus the PR properties occurring in undoped LN crystals, which possess only a few ppm of iron. It is the reason why it is not possible to reduce the PR eect by removing iron only. As a consequence of this model, the enhancement of crystal resistance to optical damage needs reduction of the niobium antisite content.
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 6 6 - 5
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M. Fontana et al. / Optical Materials 16 (2001) 111±117
One method suggested for it is to introduce divalent (Mg2 ; Zn2 ) or trivalent (In3 ; Sc3 ) impurities which are known to incorporate the Li sites and thus to remove the Nb antisites. It has been shown that doping LN with 5% MgO leads to a drastic increase of optical damage resistance [6]. However it is dicult to grow doped crystals with sucient high optical quality and homogeneity [7], as required for their insertion in optical devices. The alternative way is to grow LN crystals with a composition close to the stoichiometry, and thus with a reduced amount of intrinsic defects. Recently, dierent crystal growth techniques have been shown to yield high-quality stoichiometric crystals [8±10]. The object of the paper is to investigate the PRinduced birefringence in various undoped crystals. Comparison is achieved between results obtained in congruent and stoichiometric crystals prepared with dierent techniques. The necessity to carefully control the crystal composition is pointed out.
2. Experimental Five LN crystals have been investigated in the present study. Three of them have a nominally stoichiometric composition i.e., Xc Li= Li Nb 50 mol% LiO2 and were prepared by different techniques which were recently shown to successfully provide samples with o-congruent composition. These crystals are labelled #1, #2 and #3 and have been grown respectively by top seeded solution (TSS) [10], Czochralski [9] and vapour transport equilibration (VTE) methods [8]. In the ®rst two techniques, K2 O was added to the high temperature solution and the congruent melt, respectively. It should be mentioned that K ions do not practically enter the samples. In principle nominally stoichiometric crystals possess the perfect lattice and the minimum concentration of intrinsic defects. The quality and properties of stoichiometric crystals is actually debated according to the growth method. Here this discussion is extended to the relation with the optical damage resistance.
As mentioned above, PR properties depend on the iron content and the non-stoichiometry intrinsic defects. Therefore the photo-induced birefringence on undoped stoichiometric crystals #1, #2 and #3 are compared to that in an undoped congruent crystal as well as a 0.05% Fe-doped congruent sample. Y-cut plates of about 2 mm thick were especially prepared for the laser-induced birefringence change measurements. The photo-induced birefringence change is linked to the space charge ®eld Esc generated by laser irradiation in an electro-optic process expressed as dDn 12 n3e rc Esc ; where rc r33 no =ne r13 is the eective linear electro-optic coecient, which was recently shown to be strongly dependent on the crystal composition [11]. The dDn change was measured using a Senarmont compensation method and a modi®ed set-up described in Fig. 1. Optical damage is caused by Ar-laser emitting at k 514:5 nm with incident power varying between 40 and 400 mW. The beam was focused to obtain a 0.06 mm spot on the plate. A He±Ne laser (632.8 nm), with a low power (0.3 mW) is used as the probe beam. Its intensity is modulated by an ac voltage applied on a modulator. Its change of PRinduced phase shift between ordinary and extraordinary components of the probe beam can be determined by measuring the analyser rotation angle needed for compensation. The set-up using a modulation external to the crystal under study presents as main advantages [12] to permit the measurement of induced phase shift instead of corresponding intensity leading to an accurate determination of dDn [13,14], which is of great importance for the determination of small changes dDn ' 10 6 .
3. Results The dDn changes were monitored as a function of irradiation time, for various laser intensities. In
M. Fontana et al. / Optical Materials 16 (2001) 111±117
113
Fig. 1. Experimental set-up for measurement of birefringence change induced by argon laser.
Fig. 2 are shown the changes induced by relatively small power densities in Fe-doped crystal, undoped congruent and stoichiometric #1 crystals. The photo-induced birefringence in doped Fe± LN crystal increases very rapidly with irradiation
time. On the contrary, this raising is very smooth on non-intentionally doped crystals. This result re¯ects the iron content in the sample since initial slope of dDn plot versus time is nearly proportional to this concentration.
Fig. 2. Induced birefringence by Ar-laser at dierent light intensities in 0.05% Fe-doped, congruent and stoichiometric #1 LN crystals.
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M. Fontana et al. / Optical Materials 16 (2001) 111±117
Data are ®tted to ®rst-order exponential time response function given by dDn Dns 1
exp t=s;
where s is the characteristic time of the PR eect and Dns the saturated value of the birefringence change. We observe that Dns is nearly the same in the Fedoped crystal as in the undoped congruent crystal, for an irradiation 150 times larger. For the same laser intensity Dns is two times lower in the stoichiometric #1 crystal than in the congruent sample. These results corroborate the above description of the microscopic origin of the PR optical damage concerning the respective role of extrinsic (iron) and intrinsic defects. Fig. 3 shows the dependence of dDn on laser exposure time for dierent Ar-laser intensities. Results show that the stoichiometric crystal #1 is more resistant to the congruent sample, even for high laser irradiation. This is in agreement with previous data reported by Jermann et al. [15]. This conclusion is seemingly opposite to that of Furukawa et al. [16] who claimed that photo-
induced birefringence is larger in the stoichiometric crystal than in the congruent sample. In fact this contradiction is only apparent since low laser intensities (below 2 MW m 2 ) were used in the last investigation. Furthermore we point out that in the stoichiometric crystal labelled #1 as prepared by TSS technique, the saturated value is one order of magnitude smaller for the same laser irradiation than in the stoichiometric sample labelled #2 grown by Czochralski from K2 O added melt, which was also used by Jermann et al. [15]. This result was con®rmed by our measurements of dDn on dierent namely stoichiometric crystals performed by means of the same method and set-up. Results reported in Fig. 4 show various behaviours in the dependence of Dns on the laser power density. They indicate that the stoichiometric crystal #1 is more damage resistant than the stoichiometric crystals #2 and #3, respectively prepared by Czochralski and VTE methods. A possible reason for this signi®cant change according the crystal growth method is that the concentration of intrinsic defects, such as Nb
Fig. 3. Induced birefringence in congruent and stoichiometric #1 LN crystals irradiated by focused Ar-laser beam of 0.06 mm diameter with varying power from 40 up to 400 mW.
M. Fontana et al. / Optical Materials 16 (2001) 111±117
115
Fig. 4. Plots of saturated value of the induced birefringence Dns versus light intensity recorded on congruent and stoichiometric crystals #1, #2 and #3 grown respectively by high temperature top seeded solution growth with potassium, Czochralski with 6 wt% of potassium and vapour transport equilibration methods.
antisite and Li vacancy, is varying from a crystal to another. Several optical methods have been suggested to evaluate the true composition Xc of the LN crystal [17]. Among them, the measurement of the HWHM of the E TO1 Raman line can be successfully used [18,19]. This line is chosen because it is well resolved and very intense. Furthermore, since the corresponding vibrational mode consists into the vibration of Nb ions (on Nb sites) against oxygen octahedron, its damping is very sensitive to the amount of vacancies on Nb-site, due to Nb antisites (Nb on Li sites). In other terms, the width C of this phonon line is a signature of increasing disorder at Nb sites [19]. Furthermore Raman investigations on several crystals with varying composition revealed the presence in the spectra of extra bands attributed to density of states activated by defects [19]. Raman scattering measurements performed on the crystals labelled #1, #2 and #3 provide a signi®cant variation of the damping C, re¯ecting a dierence in the true crystal composition Xc among the three crystals. This proves that the crystal #1
prepared by TSS growth method has really a composition close to the stoichiometric and the crystals #2 and #3 which were previously claimed as stoichiometric, contain a non-negligible amount of intrinsic defects ( Fig. 5). The narrowest Raman line is recorded in the nearly stoichiometric crystal corresponding to the most ordered lattice. This con®rms earlier optical investigations, using UV absorption edge and OH vibrational band, that have shown that the optical quality and stoichiometry are improved by means of TSS method [10]. 4. Conclusion Our results allow to establish in undoped LiNbO3 crystals a correlation between the crystal quality revealed by the amount of intrinsic defects (especially Nb antisites) and the optical damage resistance. The PR optical damage is therefore linked to the presence of the intrinsic defects and disorder in the lattice, which can be removed by modifying the composition.
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M. Fontana et al. / Optical Materials 16 (2001) 111±117
Fig. 5. Plot of Raman linewidth versus crystal composition. The compositions of samples under study are thus estimated. The inset shows the Raman lineshape of phonon mode lying at 153 cm 1 when the composition varies between the congruent (Xc 48:5%) and nearly stoichiometric #3 (Xc 49:7%).
The resistance is smaller in o-congruent crystals than in commercial congruent samples but is enhanced in stoichiometric crystals, as grown by high temperature top seeded solution growth method.
Acknowledgements We are grateful to K. Polgar and G. Malovichko for providing some of the crystals used in the study.
References [1] P. G unter, J.P. Huignard (Eds.), Photorefractive Materials and their Applications, Springer, Heidelberg, 1988, p. 131. [2] O. Altho, A. Erdmann, L. Wiskolt, P. Hertel, Phys. Status Sol. (a) 128 (1991) K41. [3] P. Lerner, C. Legras, J.P. Dumas, J. Cryst. Growth 314 (1968) 231.
[4] S.C. Abrahams, P. Marsh, Acta Crystallogr. B 42 (1986) 61. [5] F. Jermann an, J. Otten, J. Opt. Soc. Am. B 10 (1993) 2085. [6] G.-G. Zhong, J. Jian, Z.-K. Wu, IEEE Cat no. 80 CH 1561-0, 1980, p. 631. [7] Y. Furukawa, A. Yokotani, T. Yasaki, H. Yoshida, K. Yoshida, F. Nitanda, M. Sato, J. Appl. Phys. 69 (1991) 3372. [8] P.F. Bordui, R.G. Norwood, D.H. Jundt, M.M. Fejer, J. Appl. Phys. 71 (1992) 875. [9] G. Malovichko, V.G. Grachev, L.P. Yurchenko, V.Ya. Proshko, E.P. Kokanyan, V.T. Gabrielyan, Phys. Status Sol (a) 133 (1992) K29. [10] K. Polgar, A. Peter, L. Kovacs, G. Corradi, Zs. Szaller, J. Cryst. Growth 177 (1997) 211. [11] K. Chah, M.D. Fontana, M. Aillerie, P. Bourson, G. Malovichko, Appl. Phys. B 67 (1) (1998) 65. [12] M. Aillerie, F. Abdi, M.D. Fontana, N. Theofanous, E. Abarkan, Scient. Instr. 71 (2000) 1627. [13] N.G. Theofanous, M. Aillerie, M.D. Fontana, G.E. Alexakis, Rev. Sci. Instrum. 68 (5) (1997) 2138. [14] M. Aillerie, N. Theofanous, M.D. Fontana, Appl. Phys. B 70 (2000) 317. [15] F. Jermann, M. Simon, E. Kratzig, J. Opt. Soc. Am. 12 (1995) 7273.
M. Fontana et al. / Optical Materials 16 (2001) 111±117 [16] Y. Furukawa, M. Sato, K. Kitamura, Y. Yajima, M. Minakata, J. Appl. Phys. 72 (1992) 3250. [17] M. W ohlecke, G. Corradi, K. Betzler, Appl. Phys. B 63 (1996) 323.
117
[18] U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, M. W ohlecke, Appl. Phys. A 56 (1993) 311. [19] A. Ridah, M.D. Fontana, P. Bourson, G. Malovichko, J. Phys. Condens. Matter 9 (1997) 9687.
www.elsevier.nl/locate/optmat
Optical damage resistance in undoped LiNbO3 crystals M. Fontana *, K. Chah, M. Aillerie, R. Mouras, P. Bourson Laboratoire Mat eriaux Optiques a Propri et es Sp eci®ques, CLOES, Universit e de Metz and Sup elec, 2, rue E. Belin, 57070 Metz, France
Abstract Photo-induced birefringence changes are investigated on several undoped LiNbO3 crystals as a function of laserirradiation time and intensity. Comparison points out that the changes are the smallest in the crystals grown from high temperature top seeded solution (TSS). It is concluded that the optical damage resistance in undoped LiNbO3 crystals is increased with a decrease of the amount of intrinsic defects related to the non-stoichiometry. The necessity to carefully control the true crystal composition is stressed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.20; 61.80 Ba; 61.72-y
1. Introduction Despite its large electro-optical and non-linear optical coecients, LiNbO3 (LN) crystal presents a low optical damage threshold as the main disadvantage for its commercial use in electro-optic modulators, switches or optical parametric oscillators. This optical damage is mainly causd by photorefractive (PR) properties of the material which are generally attributed to the presence of traps and excited sites due to transition metal ions, especially iron. The PR optical damage can be observed by changes in the birefringence, particularly in the visible range, due to laser irradiation, even for a small power density. Some dierent ways have been undertaken to reduce the optical damage on the basis of the microscopic origin of PR properties. In iron-doped LN crystal, these eects can be well described by charge transport process in *
Corresponding author.
which Fe2 ion is an electron donor and Fe3 is an electron trap [1]. This PR one-centre model predicts a linear dependence of the photovoltaic current on the light density and iron concentration. In fact this behaviour was not proved by measurements on Fe-doped LN crystals under high light intensities [2] so that other impurities centres have to be taken into account in the charge transport model. As LiNbO3 crystals possess a large amount of defects related to the non-stoichiometry, the niobium antisites NbLi (i.e., niobium ions on Li sites) [3,4] have to be considered as secondary PR centres. Within this two-centre model [5], electrons are excited from Fe2 into Nb5 Li , which can be recombined with conduction electrons to give rise to Nb4 Li . This process explains the photovoltaic and thus the PR properties occurring in undoped LN crystals, which possess only a few ppm of iron. It is the reason why it is not possible to reduce the PR eect by removing iron only. As a consequence of this model, the enhancement of crystal resistance to optical damage needs reduction of the niobium antisite content.
0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 6 6 - 5
112
M. Fontana et al. / Optical Materials 16 (2001) 111±117
One method suggested for it is to introduce divalent (Mg2 ; Zn2 ) or trivalent (In3 ; Sc3 ) impurities which are known to incorporate the Li sites and thus to remove the Nb antisites. It has been shown that doping LN with 5% MgO leads to a drastic increase of optical damage resistance [6]. However it is dicult to grow doped crystals with sucient high optical quality and homogeneity [7], as required for their insertion in optical devices. The alternative way is to grow LN crystals with a composition close to the stoichiometry, and thus with a reduced amount of intrinsic defects. Recently, dierent crystal growth techniques have been shown to yield high-quality stoichiometric crystals [8±10]. The object of the paper is to investigate the PRinduced birefringence in various undoped crystals. Comparison is achieved between results obtained in congruent and stoichiometric crystals prepared with dierent techniques. The necessity to carefully control the crystal composition is pointed out.
2. Experimental Five LN crystals have been investigated in the present study. Three of them have a nominally stoichiometric composition i.e., Xc Li= Li Nb 50 mol% LiO2 and were prepared by different techniques which were recently shown to successfully provide samples with o-congruent composition. These crystals are labelled #1, #2 and #3 and have been grown respectively by top seeded solution (TSS) [10], Czochralski [9] and vapour transport equilibration (VTE) methods [8]. In the ®rst two techniques, K2 O was added to the high temperature solution and the congruent melt, respectively. It should be mentioned that K ions do not practically enter the samples. In principle nominally stoichiometric crystals possess the perfect lattice and the minimum concentration of intrinsic defects. The quality and properties of stoichiometric crystals is actually debated according to the growth method. Here this discussion is extended to the relation with the optical damage resistance.
As mentioned above, PR properties depend on the iron content and the non-stoichiometry intrinsic defects. Therefore the photo-induced birefringence on undoped stoichiometric crystals #1, #2 and #3 are compared to that in an undoped congruent crystal as well as a 0.05% Fe-doped congruent sample. Y-cut plates of about 2 mm thick were especially prepared for the laser-induced birefringence change measurements. The photo-induced birefringence change is linked to the space charge ®eld Esc generated by laser irradiation in an electro-optic process expressed as dDn 12 n3e rc Esc ; where rc r33 no =ne r13 is the eective linear electro-optic coecient, which was recently shown to be strongly dependent on the crystal composition [11]. The dDn change was measured using a Senarmont compensation method and a modi®ed set-up described in Fig. 1. Optical damage is caused by Ar-laser emitting at k 514:5 nm with incident power varying between 40 and 400 mW. The beam was focused to obtain a 0.06 mm spot on the plate. A He±Ne laser (632.8 nm), with a low power (0.3 mW) is used as the probe beam. Its intensity is modulated by an ac voltage applied on a modulator. Its change of PRinduced phase shift between ordinary and extraordinary components of the probe beam can be determined by measuring the analyser rotation angle needed for compensation. The set-up using a modulation external to the crystal under study presents as main advantages [12] to permit the measurement of induced phase shift instead of corresponding intensity leading to an accurate determination of dDn [13,14], which is of great importance for the determination of small changes dDn ' 10 6 .
3. Results The dDn changes were monitored as a function of irradiation time, for various laser intensities. In
M. Fontana et al. / Optical Materials 16 (2001) 111±117
113
Fig. 1. Experimental set-up for measurement of birefringence change induced by argon laser.
Fig. 2 are shown the changes induced by relatively small power densities in Fe-doped crystal, undoped congruent and stoichiometric #1 crystals. The photo-induced birefringence in doped Fe± LN crystal increases very rapidly with irradiation
time. On the contrary, this raising is very smooth on non-intentionally doped crystals. This result re¯ects the iron content in the sample since initial slope of dDn plot versus time is nearly proportional to this concentration.
Fig. 2. Induced birefringence by Ar-laser at dierent light intensities in 0.05% Fe-doped, congruent and stoichiometric #1 LN crystals.
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M. Fontana et al. / Optical Materials 16 (2001) 111±117
Data are ®tted to ®rst-order exponential time response function given by dDn Dns 1
exp t=s;
where s is the characteristic time of the PR eect and Dns the saturated value of the birefringence change. We observe that Dns is nearly the same in the Fedoped crystal as in the undoped congruent crystal, for an irradiation 150 times larger. For the same laser intensity Dns is two times lower in the stoichiometric #1 crystal than in the congruent sample. These results corroborate the above description of the microscopic origin of the PR optical damage concerning the respective role of extrinsic (iron) and intrinsic defects. Fig. 3 shows the dependence of dDn on laser exposure time for dierent Ar-laser intensities. Results show that the stoichiometric crystal #1 is more resistant to the congruent sample, even for high laser irradiation. This is in agreement with previous data reported by Jermann et al. [15]. This conclusion is seemingly opposite to that of Furukawa et al. [16] who claimed that photo-
induced birefringence is larger in the stoichiometric crystal than in the congruent sample. In fact this contradiction is only apparent since low laser intensities (below 2 MW m 2 ) were used in the last investigation. Furthermore we point out that in the stoichiometric crystal labelled #1 as prepared by TSS technique, the saturated value is one order of magnitude smaller for the same laser irradiation than in the stoichiometric sample labelled #2 grown by Czochralski from K2 O added melt, which was also used by Jermann et al. [15]. This result was con®rmed by our measurements of dDn on dierent namely stoichiometric crystals performed by means of the same method and set-up. Results reported in Fig. 4 show various behaviours in the dependence of Dns on the laser power density. They indicate that the stoichiometric crystal #1 is more damage resistant than the stoichiometric crystals #2 and #3, respectively prepared by Czochralski and VTE methods. A possible reason for this signi®cant change according the crystal growth method is that the concentration of intrinsic defects, such as Nb
Fig. 3. Induced birefringence in congruent and stoichiometric #1 LN crystals irradiated by focused Ar-laser beam of 0.06 mm diameter with varying power from 40 up to 400 mW.
M. Fontana et al. / Optical Materials 16 (2001) 111±117
115
Fig. 4. Plots of saturated value of the induced birefringence Dns versus light intensity recorded on congruent and stoichiometric crystals #1, #2 and #3 grown respectively by high temperature top seeded solution growth with potassium, Czochralski with 6 wt% of potassium and vapour transport equilibration methods.
antisite and Li vacancy, is varying from a crystal to another. Several optical methods have been suggested to evaluate the true composition Xc of the LN crystal [17]. Among them, the measurement of the HWHM of the E TO1 Raman line can be successfully used [18,19]. This line is chosen because it is well resolved and very intense. Furthermore, since the corresponding vibrational mode consists into the vibration of Nb ions (on Nb sites) against oxygen octahedron, its damping is very sensitive to the amount of vacancies on Nb-site, due to Nb antisites (Nb on Li sites). In other terms, the width C of this phonon line is a signature of increasing disorder at Nb sites [19]. Furthermore Raman investigations on several crystals with varying composition revealed the presence in the spectra of extra bands attributed to density of states activated by defects [19]. Raman scattering measurements performed on the crystals labelled #1, #2 and #3 provide a signi®cant variation of the damping C, re¯ecting a dierence in the true crystal composition Xc among the three crystals. This proves that the crystal #1
prepared by TSS growth method has really a composition close to the stoichiometric and the crystals #2 and #3 which were previously claimed as stoichiometric, contain a non-negligible amount of intrinsic defects ( Fig. 5). The narrowest Raman line is recorded in the nearly stoichiometric crystal corresponding to the most ordered lattice. This con®rms earlier optical investigations, using UV absorption edge and OH vibrational band, that have shown that the optical quality and stoichiometry are improved by means of TSS method [10]. 4. Conclusion Our results allow to establish in undoped LiNbO3 crystals a correlation between the crystal quality revealed by the amount of intrinsic defects (especially Nb antisites) and the optical damage resistance. The PR optical damage is therefore linked to the presence of the intrinsic defects and disorder in the lattice, which can be removed by modifying the composition.
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M. Fontana et al. / Optical Materials 16 (2001) 111±117
Fig. 5. Plot of Raman linewidth versus crystal composition. The compositions of samples under study are thus estimated. The inset shows the Raman lineshape of phonon mode lying at 153 cm 1 when the composition varies between the congruent (Xc 48:5%) and nearly stoichiometric #3 (Xc 49:7%).
The resistance is smaller in o-congruent crystals than in commercial congruent samples but is enhanced in stoichiometric crystals, as grown by high temperature top seeded solution growth method.
Acknowledgements We are grateful to K. Polgar and G. Malovichko for providing some of the crystals used in the study.
References [1] P. G unter, J.P. Huignard (Eds.), Photorefractive Materials and their Applications, Springer, Heidelberg, 1988, p. 131. [2] O. Altho, A. Erdmann, L. Wiskolt, P. Hertel, Phys. Status Sol. (a) 128 (1991) K41. [3] P. Lerner, C. Legras, J.P. Dumas, J. Cryst. Growth 314 (1968) 231.
[4] S.C. Abrahams, P. Marsh, Acta Crystallogr. B 42 (1986) 61. [5] F. Jermann an, J. Otten, J. Opt. Soc. Am. B 10 (1993) 2085. [6] G.-G. Zhong, J. Jian, Z.-K. Wu, IEEE Cat no. 80 CH 1561-0, 1980, p. 631. [7] Y. Furukawa, A. Yokotani, T. Yasaki, H. Yoshida, K. Yoshida, F. Nitanda, M. Sato, J. Appl. Phys. 69 (1991) 3372. [8] P.F. Bordui, R.G. Norwood, D.H. Jundt, M.M. Fejer, J. Appl. Phys. 71 (1992) 875. [9] G. Malovichko, V.G. Grachev, L.P. Yurchenko, V.Ya. Proshko, E.P. Kokanyan, V.T. Gabrielyan, Phys. Status Sol (a) 133 (1992) K29. [10] K. Polgar, A. Peter, L. Kovacs, G. Corradi, Zs. Szaller, J. Cryst. Growth 177 (1997) 211. [11] K. Chah, M.D. Fontana, M. Aillerie, P. Bourson, G. Malovichko, Appl. Phys. B 67 (1) (1998) 65. [12] M. Aillerie, F. Abdi, M.D. Fontana, N. Theofanous, E. Abarkan, Scient. Instr. 71 (2000) 1627. [13] N.G. Theofanous, M. Aillerie, M.D. Fontana, G.E. Alexakis, Rev. Sci. Instrum. 68 (5) (1997) 2138. [14] M. Aillerie, N. Theofanous, M.D. Fontana, Appl. Phys. B 70 (2000) 317. [15] F. Jermann, M. Simon, E. Kratzig, J. Opt. Soc. Am. 12 (1995) 7273.
M. Fontana et al. / Optical Materials 16 (2001) 111±117 [16] Y. Furukawa, M. Sato, K. Kitamura, Y. Yajima, M. Minakata, J. Appl. Phys. 72 (1992) 3250. [17] M. W ohlecke, G. Corradi, K. Betzler, Appl. Phys. B 63 (1996) 323.
117
[18] U. Schlarb, S. Klauer, M. Wesselmann, K. Betzler, M. W ohlecke, Appl. Phys. A 56 (1993) 311. [19] A. Ridah, M.D. Fontana, P. Bourson, G. Malovichko, J. Phys. Condens. Matter 9 (1997) 9687.