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Growth and spectral properties of Er3+:Li6Y(BO3)3 crystal
Materials Letters 60 (2006) 418 – 421 www.elsevier.com/locate/matlet
Growth and spectral properties of Er 3+ :Li6Y(BO3)3 crystal Yuwei Zhao a,b , Xinghong Gong a , Yanfu Lin a , Zundu Luo a , Yidong Huang a,⁎ a
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 12 July 2005; accepted 3 September 2005 Available online 23 September 2005
Abstract Er3+-doped Li6Y(BO3)3 single crystal with dimensions of ϕ20 × 40 mm3 has been grown by the Czochralski method and the accurate concentration of Er3+ ions in the crystal was measured. The structure of the grown crystal was proved by X-ray powder diffraction and its hygroscopic behavior was also studied. The absorption spectrum, fluorescence spectrum, and decay curve of the crystal were measured at room temperature. The spectroscopic parameters of the crystal were compared with those of other Er3+-doped crystals. © 2005 Elsevier B.V. All rights reserved. Keywords: Crystal growth; Optical materials and properties; Er3+:Li6Y(BO3)3 crystal
1. Introduction
2. Experimental
In the past two decades, 1550 nm laser has attracted much attention because it is eye-safe and is located in the optical communication window. The emission from the 4I13 / 2 → 4I15 / 2 transition of Er3+ ions is around 1550 nm, therefore Er3+-doped materials have been widely investigated, such as Y3Al5O12 [1], Y3Ga5O12 [1], LiYF4 [2], YVO4 [3] crystals, and some glasses [4,5]. Li6Y(BO3)3 single crystal belongs to the monoclinic system with space group P21/c, and the cell parameters are as follows: a = 7.157(5) Å, b = 16.378(8)Å, c = 6.623 Å, β = 105.32(5)°, Z = 4, and the density of the crystal is 2.747 g/cm3 [6]. The growth and spectral properties of Nd3+:Li6Y(BO3)3 crystal have been reported and the results show that it has a weak concentration quenching effect and a low laser threshold owing to the existence of the covalent bonding in the BO3 group [6,7]. In this paper, the growth of the Er3+:Li6Y(BO3)3 crystal by the Czochralski method is reported. The spectral properties of the crystal have also been studied and compared with those of other Er3+-doped crystals.
Since the Li6Y(BO3)3 single crystal melts congruently at 860 °C, it can be grown by the Czochralski method. The raw materials were Er2O3, Y2O3 with 99.99% purity and Li2CO3,
⁎ Corresponding author. Tel.: +86 591 83776990; fax: +86 591 83714946. E-mail address: [email protected] (Y. Huang).
Fig. 1. Photograph of the as-grown Er3+:Li6Y(BO3)3 crystal. Inset shows the polished sample used in spectral experiments.
0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.007
Y. Zhao et al. / Materials Letters 60 (2006) 418–421 Table 1 Segregation coefficients of some Er3+-doped crystals Crystal
Ccry (mol%)
Cdiss (mol%)
K
Er3+:YAl3(BO3)4 [9] Er3+:KGd(WO4)2 [10] Er3+:LaGaO3 [11] Er3+:La3Ga5SiO14 [12] Er3+:Li6Y(BO3)3 a
1.18 1.92 0.15 0.41 1.88
1.3 2.4 0.5 2.0 2.0
0.91 0.80 0.30 0.21 0.94
a
This work.
H3BO3 with analar grade purity, and they were weighed according to the following reaction: 3Li2CO3 + 0.01Er2O3 + 0.49Y2O3 + 3H3BO3 → Er0.02Li6Y0.98 (BO3)3 + 4.5H2O↑ + 3CO2↑. Considering the evaporation of B2O3 during the growth process, an excess quantity of H3BO3 (about 1 wt.%) was added to the starting components. After all of the chemicals were ground and mixed thoroughly in an agate mortar, they were pressed into tablets and put into a platinum crucible. These tablets were heated up to 750 °C and kept at this temperature for 2 days to prepare Er0.02Li6Y0.98(BO3)3 polycrystalline material through the method of solid-state reaction. The Er3+:Li6Y(BO3)3 crystal was grown in the air atmosphere by the Czochralski method in a 2 kHz radio frequency furnace heating a platinum crucible containing Er0.02Li6Y0.98(BO3)3 polycrystalline material. The size of the platinum crucible was 50 mm in diameter and 35 mm in height. The temperature-controlled apparatus was a EUROTHERM 818 Controller/Programmer with a precision of ± 0.5 °C. The polycrystalline material was heated up to a
419
temperature of 30 °C higher than the melting point of 860 °C and had been kept for 2 h so as to be melted completely and homogeneously. An un-oriented single crystal bar obtained from spontaneous nucleation with dimensions of 5 × 3 × 6 mm3 was used as the seed. The pulling and crystal rotation rates were 0.5– 1 mm/h and 7.5–10 rpm, respectively. When the growth process ended, the crystal was drawn above the surface of the solution and cooled down to room temperature at a rate of 5–20 °C/h. The concentration of Er3+ ions in the grown crystal was determined to be 1.88 at.% by inductively coupled plasma atomic emission spectrometry (ICP-AES).Phase identification of the as-grown crystal was performed by X-ray powder diffraction (XRD), and the data were collected on a diffractometer (Rigaku, D/Max 2500) equipped with CuKα radiation (λ = 1.54056 Å) using a Ni-filtered Cu-target tube at room temperature. Intensities for the diffraction peaks were recorded in a range of 10–80° (2θ) with a step of 0.02° and a scan speed of 5°/min. The hygroscopic behavior of the crystal was roughly studied by measuring the transmittance of a polished crystal, which was placed in a cabinet with invariable relative humidity of 40% and temperature of 25 °C when it was out of experiment. The transmittance at the wavelength of 1100 nm was recorded with a spectrophotometer (Perkin-Elmer, Lambda 900) at room temperature. The experiment was carried out every 10 days. The absorption spectrum was measured from 190 to 1700 nm at room temperature using the same spectrophotometer in the hygroscopic measurements. Excited at the wavelength of 964 nm, the room temperature fluorescence spectrum of the 4 I13 / 2 → 4I15 / 2 transition was recorded by another spectrophotometer (FL920, Edinburg). The fluorescence decay curve at 1535 nm was recorded by the same spectrophotometer while the sample was excited by a microsecond flash lamp (μF900,
Fig. 2. Comparison of the XRD patterns of the Er3+:Li6Y(BO3)3 crystal and the pure Li6Y(BO3)3 crystal.
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Y. Zhao et al. / Materials Letters 60 (2006) 418–421
Table 2 Time dependent transmittance of polished Er3+:Li6Y(BO3)3 crystal Days after polished
Transmittance (%)
0 10 20 30
87.5 85.7 82.4 81.0
Edinburgh) at 1480 nm, and the signals were detected with a NIR PMT (R5509, Hamamatsu).
3. Results and discussion Fig. 1 shows the as-grown Er3+:Li6Y(BO3)3 crystal with dimensions of ϕ20 × 40 mm3. A high optical quality sample with size up to 4 × 4 × 4 mm3 in arbitrary orientation could be cut from the as-grown crystal, which is shown in the inset of Fig. 1. Through the concentration of Er3+ ions in the grown crystal measured by ICP-AES, the segregation coefficient K is calculated to be 0.94 according to the following equation [8] K ¼ Ccry =Cdiss where Ccry and Cdiss are the Er3+ concentration in the grown crystal and the borate crystalline substances of the melt, respectively. The value of K close to unit means that the Er3+ ions can distribute in the bulk crystal homogeneously and the Er3+:Li6Y(BO3)3 crystal may be grown easily with high optical quality. The segregation coefficients of Er3+ ions in some crystals [9–12] are compared in Table 1. The XRD pattern is shown in Fig. 2. It can be seen that the crystal has the same structure as the pure Li6Y(BO3)3 crystal [13]. The results of the hygroscopic behavior measurements were listed in Table 2. It can be seen from this table that the transmittance decreases slightly by about 2.2% every 10 days. The results indicate that the Er3+-doped Li6Y(BO3)3 single crystal is slightly hygroscopic, whereas the Ce3+:Li6Gd(BO3)3 crystal is not hygroscopic [14] though the two crystals are iso-structural. However, the growth of Ce3+:Li6Gd(BO3)3 crystal was carried out in evacuated and backfilled argon atmosphere whereas the growth of Er3+:Li6Y(BO3)3 crystal was performed in air atmosphere. It is still in progress to determine the relationship between hygroscopic behavior and the growth atmosphere.
Fig. 3. Absorption spectrum of Er3+:Li6Y(BO3)3 crystal at room temperature.
Fig. 4. Fluorescence spectrum of Er3+:Li6Y(BO3)3 crystal at room temperature under excitation oat 964 nm.
The absorption spectrum with resolution of 1.0 nm is shown in Fig. 3. Some excited manifolds related to the major absorption bands are also marked in the figure. The FWHM of the absorption band around 964 nm, which is related to the 4I15 / 2 → 4I11 / 2 transition, is about 16 nm. It indicates that the crystal is suitable for InGaAs diode-laser pumping and tolerant of the wavelength shift of the diode-laser. The fluorescence spectrum with resolution of 1.0 nm is shown in Fig. 4. The range of the spectrum is from 1460 to 1650 nm and the strongest emission peak is located at 1535 nm. The spectrum is similar to that of Er3+:YAG [1], in which the laser emission has been realized. The sharp spectral lines at room temperature imply that the optical quality of the grown Er3+:Li6Y(BO3)3 crystal is high and makes the inhomogeneous broadening of the line small. From the fluorescence decay curve shown in Fig. 5, the fluorescence lifetime of the 4I13 / 2 manifold, which is the upper level for 1550 nm laser emission, is estimated to be 0.448 ms. It is comparable to the 0.68 ms of Er3+:LaSc3(BO3)4 [15] but is much shorter than the 6.5 ms of Er3+:YAG [1]. The result can be explained by the high nonradiative transition probability comes from the high phonon energy of 1400 cm− 1 in the borate crystal [16].
Fig. 5. Fluorescence decay curve (dotted line) of Er3+:Li6Y(BO3)3 crystal at room temperature under excitation at 1480 nm. The fitting result of single exponential decay (solid line) is 0.448 ms.
Y. Zhao et al. / Materials Letters 60 (2006) 418–421
4. Conclusion The Er3+:Li6Y(BO3)3 single crystal was grown by the Czochralski method. The hygroscopic behavior of the crystal was also studied by the measurement of transmittance. The absorption and fluorescence spectra of the crystal were measured at room temperature. The fluorescence lifetime of the 4I13 / 2 manifold was estimated by fitting the recorded fluorescence decay curve. The detailed spectral analysis and evaluation of the Er3+-doped Li6Y (BO3)3 crystal as a 1550 nm laser crystal are in process. Acknowledgements This work has been supported by the National Natural Science Foundation of China (Grant No. 50590405 and 50372068) and the Major Programs of Science and Technology Foundation of Fujian Province (Grant No. 2004HZ01-1). References [1] H. Stange, K. Petermann, G. Huber, E.W. Ducaynski, Appl. Phys., B 49 (1989) 269. [2] R. Brede, T. Danger, E. Heumann, G. Huber, Appl. Phys. Lett. 62 (1993) 541.
421
[3] I. Sokolska, E. Heumann, S. Kuck, T. Lukasiewicz, Appl. Phys., B 71 (2000) 893. [4] Q. Wang, N.K. Dutta, R. Ahrens, J. Appl. Phys. 95 (2004) 402. [5] Y. Huang, M. Mortier, F. Auzel, Opt. Mater. 17 (2001) 501. [6] Z. Luo, H. Zhang, Y. Huang, M. Qiu, Y. Huang, C. Tu, A. Jiang, Cryst. Res. Technol. 26 (1991) K5. [7] Y. Huang, C. Tu, Z. Luo, G. Chen, Opt. Commun. 92 (1992) 57. [8] E.V. Koporulina, N.I. Leonyuk, O.V. Pilipenko, A.V. Mokhov, G. Bocelli, L. Righi, Mater. Lett. 47 (2001) 145. [9] W. You, Y. Lin, Y. Chen, Z. Luo, Y. Huang, J. Cryst. Growth 270 (2004) 481. [10] M.C. Pujol, M. Rico, C. Zaldo, V. Nikolov, X. Solans, M. Aguilo, F. Diaz, Appl. Phys., B 68 (1999) 187. [11] I. Sokolska, Appl. Phys., B 71 (2000) 157. [12] Z. Wang, D. Yuan, X. Shi, X. Cheng, D. Xu, M. Lv, L. Pan, S. Guo, J. Cryst. Growth 257 (2003) 141. [13] C. Tu, A. Jiang, Z. Luo, Chin. J. Struct. Chem. 8 (1989) 215. [14] J.P. Chaminade, O. Viraphong, F. Guillen, C. Fouassieer, B. Czirr, IEEE Trans. Nucl. Sci. 48 (2001) 1158. [15] V.A. Lebedev, V.F. Pisarenko, N.V. Selina, A.A. Perfilin, M.G. Brik, Opt. Mater. 14 (2000) 121. [16] D. Jaque, M.O. Ramirez, L.E. Bausa, J.G. Sole, E. Cavalli, A. Spheghini, M. Betinelli, Phys. Rev., B 68 (2003) 035118.
Growth and spectral properties of Er 3+ :Li6Y(BO3)3 crystal Yuwei Zhao a,b , Xinghong Gong a , Yanfu Lin a , Zundu Luo a , Yidong Huang a,⁎ a
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 12 July 2005; accepted 3 September 2005 Available online 23 September 2005
Abstract Er3+-doped Li6Y(BO3)3 single crystal with dimensions of ϕ20 × 40 mm3 has been grown by the Czochralski method and the accurate concentration of Er3+ ions in the crystal was measured. The structure of the grown crystal was proved by X-ray powder diffraction and its hygroscopic behavior was also studied. The absorption spectrum, fluorescence spectrum, and decay curve of the crystal were measured at room temperature. The spectroscopic parameters of the crystal were compared with those of other Er3+-doped crystals. © 2005 Elsevier B.V. All rights reserved. Keywords: Crystal growth; Optical materials and properties; Er3+:Li6Y(BO3)3 crystal
1. Introduction
2. Experimental
In the past two decades, 1550 nm laser has attracted much attention because it is eye-safe and is located in the optical communication window. The emission from the 4I13 / 2 → 4I15 / 2 transition of Er3+ ions is around 1550 nm, therefore Er3+-doped materials have been widely investigated, such as Y3Al5O12 [1], Y3Ga5O12 [1], LiYF4 [2], YVO4 [3] crystals, and some glasses [4,5]. Li6Y(BO3)3 single crystal belongs to the monoclinic system with space group P21/c, and the cell parameters are as follows: a = 7.157(5) Å, b = 16.378(8)Å, c = 6.623 Å, β = 105.32(5)°, Z = 4, and the density of the crystal is 2.747 g/cm3 [6]. The growth and spectral properties of Nd3+:Li6Y(BO3)3 crystal have been reported and the results show that it has a weak concentration quenching effect and a low laser threshold owing to the existence of the covalent bonding in the BO3 group [6,7]. In this paper, the growth of the Er3+:Li6Y(BO3)3 crystal by the Czochralski method is reported. The spectral properties of the crystal have also been studied and compared with those of other Er3+-doped crystals.
Since the Li6Y(BO3)3 single crystal melts congruently at 860 °C, it can be grown by the Czochralski method. The raw materials were Er2O3, Y2O3 with 99.99% purity and Li2CO3,
⁎ Corresponding author. Tel.: +86 591 83776990; fax: +86 591 83714946. E-mail address: [email protected] (Y. Huang).
Fig. 1. Photograph of the as-grown Er3+:Li6Y(BO3)3 crystal. Inset shows the polished sample used in spectral experiments.
0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.007
Y. Zhao et al. / Materials Letters 60 (2006) 418–421 Table 1 Segregation coefficients of some Er3+-doped crystals Crystal
Ccry (mol%)
Cdiss (mol%)
K
Er3+:YAl3(BO3)4 [9] Er3+:KGd(WO4)2 [10] Er3+:LaGaO3 [11] Er3+:La3Ga5SiO14 [12] Er3+:Li6Y(BO3)3 a
1.18 1.92 0.15 0.41 1.88
1.3 2.4 0.5 2.0 2.0
0.91 0.80 0.30 0.21 0.94
a
This work.
H3BO3 with analar grade purity, and they were weighed according to the following reaction: 3Li2CO3 + 0.01Er2O3 + 0.49Y2O3 + 3H3BO3 → Er0.02Li6Y0.98 (BO3)3 + 4.5H2O↑ + 3CO2↑. Considering the evaporation of B2O3 during the growth process, an excess quantity of H3BO3 (about 1 wt.%) was added to the starting components. After all of the chemicals were ground and mixed thoroughly in an agate mortar, they were pressed into tablets and put into a platinum crucible. These tablets were heated up to 750 °C and kept at this temperature for 2 days to prepare Er0.02Li6Y0.98(BO3)3 polycrystalline material through the method of solid-state reaction. The Er3+:Li6Y(BO3)3 crystal was grown in the air atmosphere by the Czochralski method in a 2 kHz radio frequency furnace heating a platinum crucible containing Er0.02Li6Y0.98(BO3)3 polycrystalline material. The size of the platinum crucible was 50 mm in diameter and 35 mm in height. The temperature-controlled apparatus was a EUROTHERM 818 Controller/Programmer with a precision of ± 0.5 °C. The polycrystalline material was heated up to a
419
temperature of 30 °C higher than the melting point of 860 °C and had been kept for 2 h so as to be melted completely and homogeneously. An un-oriented single crystal bar obtained from spontaneous nucleation with dimensions of 5 × 3 × 6 mm3 was used as the seed. The pulling and crystal rotation rates were 0.5– 1 mm/h and 7.5–10 rpm, respectively. When the growth process ended, the crystal was drawn above the surface of the solution and cooled down to room temperature at a rate of 5–20 °C/h. The concentration of Er3+ ions in the grown crystal was determined to be 1.88 at.% by inductively coupled plasma atomic emission spectrometry (ICP-AES).Phase identification of the as-grown crystal was performed by X-ray powder diffraction (XRD), and the data were collected on a diffractometer (Rigaku, D/Max 2500) equipped with CuKα radiation (λ = 1.54056 Å) using a Ni-filtered Cu-target tube at room temperature. Intensities for the diffraction peaks were recorded in a range of 10–80° (2θ) with a step of 0.02° and a scan speed of 5°/min. The hygroscopic behavior of the crystal was roughly studied by measuring the transmittance of a polished crystal, which was placed in a cabinet with invariable relative humidity of 40% and temperature of 25 °C when it was out of experiment. The transmittance at the wavelength of 1100 nm was recorded with a spectrophotometer (Perkin-Elmer, Lambda 900) at room temperature. The experiment was carried out every 10 days. The absorption spectrum was measured from 190 to 1700 nm at room temperature using the same spectrophotometer in the hygroscopic measurements. Excited at the wavelength of 964 nm, the room temperature fluorescence spectrum of the 4 I13 / 2 → 4I15 / 2 transition was recorded by another spectrophotometer (FL920, Edinburg). The fluorescence decay curve at 1535 nm was recorded by the same spectrophotometer while the sample was excited by a microsecond flash lamp (μF900,
Fig. 2. Comparison of the XRD patterns of the Er3+:Li6Y(BO3)3 crystal and the pure Li6Y(BO3)3 crystal.
420
Y. Zhao et al. / Materials Letters 60 (2006) 418–421
Table 2 Time dependent transmittance of polished Er3+:Li6Y(BO3)3 crystal Days after polished
Transmittance (%)
0 10 20 30
87.5 85.7 82.4 81.0
Edinburgh) at 1480 nm, and the signals were detected with a NIR PMT (R5509, Hamamatsu).
3. Results and discussion Fig. 1 shows the as-grown Er3+:Li6Y(BO3)3 crystal with dimensions of ϕ20 × 40 mm3. A high optical quality sample with size up to 4 × 4 × 4 mm3 in arbitrary orientation could be cut from the as-grown crystal, which is shown in the inset of Fig. 1. Through the concentration of Er3+ ions in the grown crystal measured by ICP-AES, the segregation coefficient K is calculated to be 0.94 according to the following equation [8] K ¼ Ccry =Cdiss where Ccry and Cdiss are the Er3+ concentration in the grown crystal and the borate crystalline substances of the melt, respectively. The value of K close to unit means that the Er3+ ions can distribute in the bulk crystal homogeneously and the Er3+:Li6Y(BO3)3 crystal may be grown easily with high optical quality. The segregation coefficients of Er3+ ions in some crystals [9–12] are compared in Table 1. The XRD pattern is shown in Fig. 2. It can be seen that the crystal has the same structure as the pure Li6Y(BO3)3 crystal [13]. The results of the hygroscopic behavior measurements were listed in Table 2. It can be seen from this table that the transmittance decreases slightly by about 2.2% every 10 days. The results indicate that the Er3+-doped Li6Y(BO3)3 single crystal is slightly hygroscopic, whereas the Ce3+:Li6Gd(BO3)3 crystal is not hygroscopic [14] though the two crystals are iso-structural. However, the growth of Ce3+:Li6Gd(BO3)3 crystal was carried out in evacuated and backfilled argon atmosphere whereas the growth of Er3+:Li6Y(BO3)3 crystal was performed in air atmosphere. It is still in progress to determine the relationship between hygroscopic behavior and the growth atmosphere.
Fig. 3. Absorption spectrum of Er3+:Li6Y(BO3)3 crystal at room temperature.
Fig. 4. Fluorescence spectrum of Er3+:Li6Y(BO3)3 crystal at room temperature under excitation oat 964 nm.
The absorption spectrum with resolution of 1.0 nm is shown in Fig. 3. Some excited manifolds related to the major absorption bands are also marked in the figure. The FWHM of the absorption band around 964 nm, which is related to the 4I15 / 2 → 4I11 / 2 transition, is about 16 nm. It indicates that the crystal is suitable for InGaAs diode-laser pumping and tolerant of the wavelength shift of the diode-laser. The fluorescence spectrum with resolution of 1.0 nm is shown in Fig. 4. The range of the spectrum is from 1460 to 1650 nm and the strongest emission peak is located at 1535 nm. The spectrum is similar to that of Er3+:YAG [1], in which the laser emission has been realized. The sharp spectral lines at room temperature imply that the optical quality of the grown Er3+:Li6Y(BO3)3 crystal is high and makes the inhomogeneous broadening of the line small. From the fluorescence decay curve shown in Fig. 5, the fluorescence lifetime of the 4I13 / 2 manifold, which is the upper level for 1550 nm laser emission, is estimated to be 0.448 ms. It is comparable to the 0.68 ms of Er3+:LaSc3(BO3)4 [15] but is much shorter than the 6.5 ms of Er3+:YAG [1]. The result can be explained by the high nonradiative transition probability comes from the high phonon energy of 1400 cm− 1 in the borate crystal [16].
Fig. 5. Fluorescence decay curve (dotted line) of Er3+:Li6Y(BO3)3 crystal at room temperature under excitation at 1480 nm. The fitting result of single exponential decay (solid line) is 0.448 ms.
Y. Zhao et al. / Materials Letters 60 (2006) 418–421
4. Conclusion The Er3+:Li6Y(BO3)3 single crystal was grown by the Czochralski method. The hygroscopic behavior of the crystal was also studied by the measurement of transmittance. The absorption and fluorescence spectra of the crystal were measured at room temperature. The fluorescence lifetime of the 4I13 / 2 manifold was estimated by fitting the recorded fluorescence decay curve. The detailed spectral analysis and evaluation of the Er3+-doped Li6Y (BO3)3 crystal as a 1550 nm laser crystal are in process. Acknowledgements This work has been supported by the National Natural Science Foundation of China (Grant No. 50590405 and 50372068) and the Major Programs of Science and Technology Foundation of Fujian Province (Grant No. 2004HZ01-1). References [1] H. Stange, K. Petermann, G. Huber, E.W. Ducaynski, Appl. Phys., B 49 (1989) 269. [2] R. Brede, T. Danger, E. Heumann, G. Huber, Appl. Phys. Lett. 62 (1993) 541.
421
[3] I. Sokolska, E. Heumann, S. Kuck, T. Lukasiewicz, Appl. Phys., B 71 (2000) 893. [4] Q. Wang, N.K. Dutta, R. Ahrens, J. Appl. Phys. 95 (2004) 402. [5] Y. Huang, M. Mortier, F. Auzel, Opt. Mater. 17 (2001) 501. [6] Z. Luo, H. Zhang, Y. Huang, M. Qiu, Y. Huang, C. Tu, A. Jiang, Cryst. Res. Technol. 26 (1991) K5. [7] Y. Huang, C. Tu, Z. Luo, G. Chen, Opt. Commun. 92 (1992) 57. [8] E.V. Koporulina, N.I. Leonyuk, O.V. Pilipenko, A.V. Mokhov, G. Bocelli, L. Righi, Mater. Lett. 47 (2001) 145. [9] W. You, Y. Lin, Y. Chen, Z. Luo, Y. Huang, J. Cryst. Growth 270 (2004) 481. [10] M.C. Pujol, M. Rico, C. Zaldo, V. Nikolov, X. Solans, M. Aguilo, F. Diaz, Appl. Phys., B 68 (1999) 187. [11] I. Sokolska, Appl. Phys., B 71 (2000) 157. [12] Z. Wang, D. Yuan, X. Shi, X. Cheng, D. Xu, M. Lv, L. Pan, S. Guo, J. Cryst. Growth 257 (2003) 141. [13] C. Tu, A. Jiang, Z. Luo, Chin. J. Struct. Chem. 8 (1989) 215. [14] J.P. Chaminade, O. Viraphong, F. Guillen, C. Fouassieer, B. Czirr, IEEE Trans. Nucl. Sci. 48 (2001) 1158. [15] V.A. Lebedev, V.F. Pisarenko, N.V. Selina, A.A. Perfilin, M.G. Brik, Opt. Mater. 14 (2000) 121. [16] D. Jaque, M.O. Ramirez, L.E. Bausa, J.G. Sole, E. Cavalli, A. Spheghini, M. Betinelli, Phys. Rev., B 68 (2003) 035118.