- Email: [email protected]
The influence of growth conditions on the optical properties of barium betaborate single crystals
,. . . . . . . .
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 162 (1996) 89-94
The influence of growth conditions on the optical properties of barium betaborate single crystals A . M . L u g i n e t s a,.
S . A . Guretskii a A . P . G e s a A . S . M i l o v a n o v a L . V . M a r k o v a b V.S. Burak c
a Institute of Physics of Solids and Semiconductors, Academy of Sciences ofBelarus, P. Brovka Street 17, 220072 Minsk, Belarus b Byelorussian Powder Metallurgy Association. Platonova 41, 220600 Minsk, Belarus c Luikoo Heat and Mass Transfer Institute, Academy of Sciences of Belarus, 220072 Minsk, Belarus
Received 2 August 1995; accepted21 September 1995
Abstract High-temperature fluxed melt crystal growth in the Na20-B203-BaO system has been investigated. The width of concentration and temperature ranges of crystallization of the barium betaborate phase has been determined. /3-BaB204 single crystals were grown by the top-seeded solution growth method. An attempt is made to establish a correlation between the fluxed melt composition and the nonlinear properties of the /3-BaB204 single crystals at constant parameters of crystallization.
1. Introduction At present /3-BaB20 4 (BBO) is one of the most promising nonlinear optical (NLO) single crystals for use in ultra-violet high-power lasers [1]. The damage threshold of the NLO components is known to be the most critical parameter in high-power laser systems. Vulnerability to damage is associated with a relatively low value of optical damage threshold of BBO crystals, being several times lower than that of the volume damage threshold [2]. Since there is a strong tendency towards increasing laser power, and the volume damage threshold in BBO crystals is high, there is interest in means to increase the optical damage threshold. The top-seeded solution growth method (TSSG)
* Corresponding author.
was used to produce sufficiently large high-quality single crystals [3,4]. Based on data from previous work [5-7] a high-temperature fluxed melt in the N a 2 0 - B 2 0 3 - B a O system was used as the most appropriate for the experiments. The present paper records the results obtained in the investigation of the effect of the fluxed melt composition on the optical damage threshold, the conversion efficiency and the transmission spectrum of BBO crystals.
2. Experimental procedure The crystals were synthesized from a fluxed melt prepared by successive melting of Na2CO3, B203 and BaCO 3 in a platinum crucible with constant weight control. The crucibles employed varied from
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00742-3
90
A.M. Luginets et aL / Journal of Crystal Growth 162 (1996) 89-94
200 to 400 cm 3 in volume. The synthesis was carried out in a furnace with vertically positioned SiC heating elements. The temperature was controlled by a platinum-platinum-rhodium thermocouple and sustained with an accuracy not less than 0.1°C. The fluxed melt was repeatedly used in a series of experiments by adding crystal forming oxides in accordance with the weight of the extracted crystals. The f l - B a B 2 0 4 single crystals were grown by the top-seeded solution growth technique. Inclusion-free plates with [001] orientation of 7 - 1 0 mm in diameter and 1-2 mm in thickness were utilized as seeds. The crystals were grown from the fluxed melt top with a constant rate of crystal holder rotation and continuous reduction of temperature. The lifting of the growing crystal started as the boule diameter reached approximately 50 mm. After the synthesis the crystalline boule was cooled down to room temperature at a rate of 15-20°C/h. The main growth parameters are summarized in Table 1. For optical studies elements of the required orientation of 6 × 6 × 6 mm 3 in size were prepared. Mechanical treatment of the samples provided their high quality (the average height of irregularities was not more than 0.05/zm). The microstructure and surface morphology of the samples were examined with electron analysis performed on a Nanolab-9 SEM setup with a magnification up to 10000. The nonlinear optical measurements were performed utilizing STC SOLAR N d : Y A G lasers: LS 114 and LS 116 ( 0 = 1 mrad, ~-= 10 ns, A = 1.06 /zm, E l i 4 = 300 mJ, E l i 6 - - - 6 0 0 mJ). In measuring the conversion efficiency, the output energy density ranged from 100 to 120 M W / c m 2. The transmission spectrum was measured at room temperature on a Beckman spectrometer.
Table 1 Growth parameters for BBO crystals grown by the TSSG technique Seed orientation, size
[001], 8.0 mm diameter, 2.0 mm height 2.0 rpm, clock-wise 30-40°C 925-850°C 30.0 ± 6.0°C/cm
Seed rotation Growth period Growth range Axial temperature gradient above melt Rate of temperature decrease during growth Pulling speed Cooling process Maximum crystal size (asgrown) Crucible
0.05-0.3°C/h 0.05 m m / h
15-ZS°C/h 60 mm diameter, 10.0 mm height Pt (99.99%), cylindrical (7080 mm diameter, 70-80 mm height)
Properties of BBO crystals Optical damage threshold (~- = 10 ns, A = 1.06/.tm)
Series I GW/cm 2 Series II GW/cm 2 Series II1 GW/cm 2 Series I Series II Series III Series I Series 11 Series 111
Conversion efficiency
Optical quality
0.9-1.4 > 2.0 0.06-0.1 13-18% 34-40% < 10% Transparent Transparent Inclusions
.~o
3. Results and discussion
Fig. 1 illustrates the region of primary crystallization and the temperature range of spontaneous crystallization of the /3-BaB20 a fluxed melt. This region involves three subregions depending on the quality of the grown crystals with constant crystallization parameters. The AB and CD curves represent the concentration ratio of the fluxed melt components such that the temperatures of spontaneous crystalliza-
/
VIII
\/.\Oo BaO 50
45
40 ]3203
Fig. 1. The region of primary crystallization of the fl-BaB204 phase in the N a 2 0 - B 2 0 3 - B a O system.
A.M. Luginets et al./ Journal of Crystal Growth 162 (1996) 89-94
tion correspond to the limiting values of the growth temperature interval. The lower limit of temperature is conditioned by the fluxed melt viscosity value compatible with crystal growth while the highest is due to the maximum available temperature of the f l , - B a B 2 0 4 phase existence. In the crystallization region considered, within the growth temperature range, a progressive variation of crystal quality was observed. The most perfect crystals were obtained from the fluxed melt corresponding to region II, the improvement in the crystal is observed as the concentration ratio of BaO and B203 approaches the stoichiometric value. The crystals demonstrated a habit typical of a hexagonal structure (Fig. 2) and a small amount of structure defects. The transmission spectrum, threshold power, and conversion efficiency of the frequency-doubled N d : Y A G laser (Table 1; Fig. 3) were consistent with the literature data [8] (Fig. 3). With increasing the amount of B203 (region I), the morphology of the crystals remained unchanged. No block-type structures or microcracks were present. However, the transmission spectrum revealed absorption in the ultraviolet region (UV), transmission being increased in proportion to the B203 con-
Fig. 2. BBO single crystal grown from the Na20-B203-BaO flux system.
i0050
91
2
50OWavelength 1000 (rim) Fig. 3. Characteristic transmission spectra for BBO single crystals grown from the fluxed melt with various component ratios: (1) stoichiometric ratio of B203 and BaO; (2) excess BaO; (3),(4) excess B203.
centration in the fluxed melt. From the nature of the transmission spectra and the observed 2- to 3-fold reduction in the conversion efficiency it can be assumed that the total of boron ions inclusioned in the single crystal lattice increased (Table 1). The threshold power in this case decreased insignificantly. The concentration of visible defects such as the block-type structures, large inclusions and large cracks increases considerably in the single crystals with the BaO concentration in the fluxed melt in region HI. The inclusions were arranged as concentric circles, their density growing from the edge towards the center. The chemical nature of the inclusions was similar to that of the fluxed melt used for the growth. Macrocracks were mainly formed parallel to the (001) crystallographic plane. The damage threshold of the best samples of this series did not exceed 100 M W / c m 2 with the transmission coefficient ranging from l0 to 20% for the best single crystal samples obtained from region II (Table 1). From the above it was concluded that the optical strength of BBO single crystals depended on the physical-chemical growth conditions. This is of obvious interest. Samples were prepared from the fluxed melt grown single crystals with various component ratios in accord with the crystallization region under consideration (Fig. 1). A considerable variation in the threshold damage powers was observed for the samples of the same series in experiments deter-
92
A.M. Luginets et al./Journal of Crystal Growth 162 (1996) 89-94
mined at various beam strengths. The optical damage threshold values of the investigated samples of different series are summarized in Table 1. From visual inspection of the damaged surface using an optical microscope is it seen that surface damage occurs in the form of separate spalls with different morphology and dimensions. Careful investigation of the damage morphology carried out with the electron microscope revealed the pronounced difference be-
tween the damaged optical surfaces. As seen from the micrographs (Fig. 4) the surface damages were cavities with uneven edges characteristic of thermal damage. Within the size of the irradiation spot of 1 mm in diameter small spalls of 50-180 /zm and large spalls of 200-400 /xm and more were observed. In samples of series II (Fig. 1, region II) with the highest beam strength the craters had the largest diameter and homogeneous morphological structure.
Fig. 4. Morphology of typical damages of the optical surface for BBO single crystals grown from the fluxed melt with various component ratios: (a) at excess B203; (b) at excess BaO.
A.M. Luginets et al. / Journal of Crystal Growth 162 (1996) 89- 94
93
4/ m Fig. 5. Microphotographs of foreign bodies on the optical surface of a BBO single crystal.
The smaller sized spalls, encountered more rarely, had a considerably lower damage threshold as compared to that for large craters. The cause of the optical surface damage in this case was, as a rule, the focusing effect of microscopic inclusions on the surface acting as microlenses (Fig. 5). In the samples of series I (Fig. 1, region I) an appreciable decrease in the surface damage threshold was observed. This reduction was accompanied by the formation of smaller craters as compared to those in series II samples. The spalls typical of this series exhibited an extended shape with a pronounced block boundary along the crater (Fig. 4a). The damaged surface morphology in the best samples of series III (Fig. 1, region III), as compared to those of series II, were characterized by deep craters and a marked blocktype structure (Fig. 4b). Additionally, an increase of bulk absorption was often the decisive factor responsible for the sample damage. In this case, the surface and bulk damage occurred practically simultaneously at a lower threshold of laser damage.
fled. The quality of single crystals has been found to depend on the concentration ratio of crystal forming oxides. The defective structure of the crystals was evidenced by developing blocks, structural and nonstructural impurities and cracks. Structural impurities were the defects typical of single crystals grown from the fluxed melt with an excess amount of B203. This gave rise to absorption lines in the UV spectrum range and to reduced conversion efficiency. The samples grown from the fluxed melt with an excess amount of BaO had a well developed block structure and large quantities of both structural and nonstructural impurities. Cracking on cooling, due to high internal strain, and abrupt deterioration of the optical properties did not allow crystals of this series to be further used. The surface optical damage threshold has been found to be dependent both on the availability of absorbing inclusions and on a block crystal structure.
Acknowledgements 4. Conclusion The region of /3-BaB204 monophase crystallization in the N a 2 0 - B 2 0 3 - B a O system has been speci-
The authors express their thanks to Professor N.I. Leoniuk for useful comments. The article was written with the support of the Soros Foundation.
94
A.M. Luginets et al./Journal of Crystal Growth 162 (1996) 89-94
References [1] G.D. Goodno, Z. Guo, R.J.D. Miller, I.J. Miller, J.W. Montgomery, S.R. Adhav and R.S. Adhav, Appl. Phys. 66 (1995) 1575. [2] Qiguang Tan, Hongwei Mao, Shujie Lin, Hui Chen, Shaofan Lu, Dingyuan Tang and Tomoya Ogawa, J. Crystal Growth 141 (1994) 393. [3] R.S, Feigelson, R.J. Raymakers and R.K. Route, Progr. Crystal Growth Characterization 20 (1990) 115.
[4] W.R. Bosenberg, R.L. Lane and C.L. Tang, J. Crystal Growth 108 (1991) 394. [5] A. Jiang, F. Cheng, Q. Lin, Z. Cheng and Y. Zheng, J. Crystal Growth 79 (1986) 963. [6] L.K. Cheng, W.R. Bosenberg and C.L. Tang, J. Crystal Growth 89 (1988) 553. [7] R.S. Feigelson, R.J. Raymakers and R.K. Route, J. Crystal Growth 97 (1989) 352. [8] J.T. Lin, Opt. Quantum Electron. 22 0990) 283.
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 162 (1996) 89-94
The influence of growth conditions on the optical properties of barium betaborate single crystals A . M . L u g i n e t s a,.
S . A . Guretskii a A . P . G e s a A . S . M i l o v a n o v a L . V . M a r k o v a b V.S. Burak c
a Institute of Physics of Solids and Semiconductors, Academy of Sciences ofBelarus, P. Brovka Street 17, 220072 Minsk, Belarus b Byelorussian Powder Metallurgy Association. Platonova 41, 220600 Minsk, Belarus c Luikoo Heat and Mass Transfer Institute, Academy of Sciences of Belarus, 220072 Minsk, Belarus
Received 2 August 1995; accepted21 September 1995
Abstract High-temperature fluxed melt crystal growth in the Na20-B203-BaO system has been investigated. The width of concentration and temperature ranges of crystallization of the barium betaborate phase has been determined. /3-BaB204 single crystals were grown by the top-seeded solution growth method. An attempt is made to establish a correlation between the fluxed melt composition and the nonlinear properties of the /3-BaB204 single crystals at constant parameters of crystallization.
1. Introduction At present /3-BaB20 4 (BBO) is one of the most promising nonlinear optical (NLO) single crystals for use in ultra-violet high-power lasers [1]. The damage threshold of the NLO components is known to be the most critical parameter in high-power laser systems. Vulnerability to damage is associated with a relatively low value of optical damage threshold of BBO crystals, being several times lower than that of the volume damage threshold [2]. Since there is a strong tendency towards increasing laser power, and the volume damage threshold in BBO crystals is high, there is interest in means to increase the optical damage threshold. The top-seeded solution growth method (TSSG)
* Corresponding author.
was used to produce sufficiently large high-quality single crystals [3,4]. Based on data from previous work [5-7] a high-temperature fluxed melt in the N a 2 0 - B 2 0 3 - B a O system was used as the most appropriate for the experiments. The present paper records the results obtained in the investigation of the effect of the fluxed melt composition on the optical damage threshold, the conversion efficiency and the transmission spectrum of BBO crystals.
2. Experimental procedure The crystals were synthesized from a fluxed melt prepared by successive melting of Na2CO3, B203 and BaCO 3 in a platinum crucible with constant weight control. The crucibles employed varied from
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00742-3
90
A.M. Luginets et aL / Journal of Crystal Growth 162 (1996) 89-94
200 to 400 cm 3 in volume. The synthesis was carried out in a furnace with vertically positioned SiC heating elements. The temperature was controlled by a platinum-platinum-rhodium thermocouple and sustained with an accuracy not less than 0.1°C. The fluxed melt was repeatedly used in a series of experiments by adding crystal forming oxides in accordance with the weight of the extracted crystals. The f l - B a B 2 0 4 single crystals were grown by the top-seeded solution growth technique. Inclusion-free plates with [001] orientation of 7 - 1 0 mm in diameter and 1-2 mm in thickness were utilized as seeds. The crystals were grown from the fluxed melt top with a constant rate of crystal holder rotation and continuous reduction of temperature. The lifting of the growing crystal started as the boule diameter reached approximately 50 mm. After the synthesis the crystalline boule was cooled down to room temperature at a rate of 15-20°C/h. The main growth parameters are summarized in Table 1. For optical studies elements of the required orientation of 6 × 6 × 6 mm 3 in size were prepared. Mechanical treatment of the samples provided their high quality (the average height of irregularities was not more than 0.05/zm). The microstructure and surface morphology of the samples were examined with electron analysis performed on a Nanolab-9 SEM setup with a magnification up to 10000. The nonlinear optical measurements were performed utilizing STC SOLAR N d : Y A G lasers: LS 114 and LS 116 ( 0 = 1 mrad, ~-= 10 ns, A = 1.06 /zm, E l i 4 = 300 mJ, E l i 6 - - - 6 0 0 mJ). In measuring the conversion efficiency, the output energy density ranged from 100 to 120 M W / c m 2. The transmission spectrum was measured at room temperature on a Beckman spectrometer.
Table 1 Growth parameters for BBO crystals grown by the TSSG technique Seed orientation, size
[001], 8.0 mm diameter, 2.0 mm height 2.0 rpm, clock-wise 30-40°C 925-850°C 30.0 ± 6.0°C/cm
Seed rotation Growth period Growth range Axial temperature gradient above melt Rate of temperature decrease during growth Pulling speed Cooling process Maximum crystal size (asgrown) Crucible
0.05-0.3°C/h 0.05 m m / h
15-ZS°C/h 60 mm diameter, 10.0 mm height Pt (99.99%), cylindrical (7080 mm diameter, 70-80 mm height)
Properties of BBO crystals Optical damage threshold (~- = 10 ns, A = 1.06/.tm)
Series I GW/cm 2 Series II GW/cm 2 Series II1 GW/cm 2 Series I Series II Series III Series I Series 11 Series 111
Conversion efficiency
Optical quality
0.9-1.4 > 2.0 0.06-0.1 13-18% 34-40% < 10% Transparent Transparent Inclusions
.~o
3. Results and discussion
Fig. 1 illustrates the region of primary crystallization and the temperature range of spontaneous crystallization of the /3-BaB20 a fluxed melt. This region involves three subregions depending on the quality of the grown crystals with constant crystallization parameters. The AB and CD curves represent the concentration ratio of the fluxed melt components such that the temperatures of spontaneous crystalliza-
/
VIII
\/.\Oo BaO 50
45
40 ]3203
Fig. 1. The region of primary crystallization of the fl-BaB204 phase in the N a 2 0 - B 2 0 3 - B a O system.
A.M. Luginets et al./ Journal of Crystal Growth 162 (1996) 89-94
tion correspond to the limiting values of the growth temperature interval. The lower limit of temperature is conditioned by the fluxed melt viscosity value compatible with crystal growth while the highest is due to the maximum available temperature of the f l , - B a B 2 0 4 phase existence. In the crystallization region considered, within the growth temperature range, a progressive variation of crystal quality was observed. The most perfect crystals were obtained from the fluxed melt corresponding to region II, the improvement in the crystal is observed as the concentration ratio of BaO and B203 approaches the stoichiometric value. The crystals demonstrated a habit typical of a hexagonal structure (Fig. 2) and a small amount of structure defects. The transmission spectrum, threshold power, and conversion efficiency of the frequency-doubled N d : Y A G laser (Table 1; Fig. 3) were consistent with the literature data [8] (Fig. 3). With increasing the amount of B203 (region I), the morphology of the crystals remained unchanged. No block-type structures or microcracks were present. However, the transmission spectrum revealed absorption in the ultraviolet region (UV), transmission being increased in proportion to the B203 con-
Fig. 2. BBO single crystal grown from the Na20-B203-BaO flux system.
i0050
91
2
50OWavelength 1000 (rim) Fig. 3. Characteristic transmission spectra for BBO single crystals grown from the fluxed melt with various component ratios: (1) stoichiometric ratio of B203 and BaO; (2) excess BaO; (3),(4) excess B203.
centration in the fluxed melt. From the nature of the transmission spectra and the observed 2- to 3-fold reduction in the conversion efficiency it can be assumed that the total of boron ions inclusioned in the single crystal lattice increased (Table 1). The threshold power in this case decreased insignificantly. The concentration of visible defects such as the block-type structures, large inclusions and large cracks increases considerably in the single crystals with the BaO concentration in the fluxed melt in region HI. The inclusions were arranged as concentric circles, their density growing from the edge towards the center. The chemical nature of the inclusions was similar to that of the fluxed melt used for the growth. Macrocracks were mainly formed parallel to the (001) crystallographic plane. The damage threshold of the best samples of this series did not exceed 100 M W / c m 2 with the transmission coefficient ranging from l0 to 20% for the best single crystal samples obtained from region II (Table 1). From the above it was concluded that the optical strength of BBO single crystals depended on the physical-chemical growth conditions. This is of obvious interest. Samples were prepared from the fluxed melt grown single crystals with various component ratios in accord with the crystallization region under consideration (Fig. 1). A considerable variation in the threshold damage powers was observed for the samples of the same series in experiments deter-
92
A.M. Luginets et al./Journal of Crystal Growth 162 (1996) 89-94
mined at various beam strengths. The optical damage threshold values of the investigated samples of different series are summarized in Table 1. From visual inspection of the damaged surface using an optical microscope is it seen that surface damage occurs in the form of separate spalls with different morphology and dimensions. Careful investigation of the damage morphology carried out with the electron microscope revealed the pronounced difference be-
tween the damaged optical surfaces. As seen from the micrographs (Fig. 4) the surface damages were cavities with uneven edges characteristic of thermal damage. Within the size of the irradiation spot of 1 mm in diameter small spalls of 50-180 /zm and large spalls of 200-400 /xm and more were observed. In samples of series II (Fig. 1, region II) with the highest beam strength the craters had the largest diameter and homogeneous morphological structure.
Fig. 4. Morphology of typical damages of the optical surface for BBO single crystals grown from the fluxed melt with various component ratios: (a) at excess B203; (b) at excess BaO.
A.M. Luginets et al. / Journal of Crystal Growth 162 (1996) 89- 94
93
4/ m Fig. 5. Microphotographs of foreign bodies on the optical surface of a BBO single crystal.
The smaller sized spalls, encountered more rarely, had a considerably lower damage threshold as compared to that for large craters. The cause of the optical surface damage in this case was, as a rule, the focusing effect of microscopic inclusions on the surface acting as microlenses (Fig. 5). In the samples of series I (Fig. 1, region I) an appreciable decrease in the surface damage threshold was observed. This reduction was accompanied by the formation of smaller craters as compared to those in series II samples. The spalls typical of this series exhibited an extended shape with a pronounced block boundary along the crater (Fig. 4a). The damaged surface morphology in the best samples of series III (Fig. 1, region III), as compared to those of series II, were characterized by deep craters and a marked blocktype structure (Fig. 4b). Additionally, an increase of bulk absorption was often the decisive factor responsible for the sample damage. In this case, the surface and bulk damage occurred practically simultaneously at a lower threshold of laser damage.
fled. The quality of single crystals has been found to depend on the concentration ratio of crystal forming oxides. The defective structure of the crystals was evidenced by developing blocks, structural and nonstructural impurities and cracks. Structural impurities were the defects typical of single crystals grown from the fluxed melt with an excess amount of B203. This gave rise to absorption lines in the UV spectrum range and to reduced conversion efficiency. The samples grown from the fluxed melt with an excess amount of BaO had a well developed block structure and large quantities of both structural and nonstructural impurities. Cracking on cooling, due to high internal strain, and abrupt deterioration of the optical properties did not allow crystals of this series to be further used. The surface optical damage threshold has been found to be dependent both on the availability of absorbing inclusions and on a block crystal structure.
Acknowledgements 4. Conclusion The region of /3-BaB204 monophase crystallization in the N a 2 0 - B 2 0 3 - B a O system has been speci-
The authors express their thanks to Professor N.I. Leoniuk for useful comments. The article was written with the support of the Soros Foundation.
94
A.M. Luginets et al./Journal of Crystal Growth 162 (1996) 89-94
References [1] G.D. Goodno, Z. Guo, R.J.D. Miller, I.J. Miller, J.W. Montgomery, S.R. Adhav and R.S. Adhav, Appl. Phys. 66 (1995) 1575. [2] Qiguang Tan, Hongwei Mao, Shujie Lin, Hui Chen, Shaofan Lu, Dingyuan Tang and Tomoya Ogawa, J. Crystal Growth 141 (1994) 393. [3] R.S, Feigelson, R.J. Raymakers and R.K. Route, Progr. Crystal Growth Characterization 20 (1990) 115.
[4] W.R. Bosenberg, R.L. Lane and C.L. Tang, J. Crystal Growth 108 (1991) 394. [5] A. Jiang, F. Cheng, Q. Lin, Z. Cheng and Y. Zheng, J. Crystal Growth 79 (1986) 963. [6] L.K. Cheng, W.R. Bosenberg and C.L. Tang, J. Crystal Growth 89 (1988) 553. [7] R.S. Feigelson, R.J. Raymakers and R.K. Route, J. Crystal Growth 97 (1989) 352. [8] J.T. Lin, Opt. Quantum Electron. 22 0990) 283.