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Growth and optical properties of a new self-Q-switched laser crystal: Cr4+:Yb3Al5O12
Solid State Communications 129 (2004) 717–720 www.elsevier.com/locate/ssc
Growth and optical properties of a new self-Q-switched laser crystal: Cr4þ:Yb3Al5O12 Xiaodong Xu, Zhiwei Zhao*, Pingxin Song, Guoqing Zhou, Jun Xu, Peizhen Deng Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Crystal Centre, P.O. Box 800-211, Shanghai 201800, China Received 24 November 2003; accepted 15 December 2003 by P. Watcher
Abstract Cr4þ:YbAG (Cr4þ:Yb3Al5O12) crystal with a size up to F24 mm £ 30 mm was grown by the Czochralski method. In the absorption spectrum, there are two absorption bands at 939 and 969 nm, respectively, which are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switched laser output at 1.03 mm. A broad emission spectrum from 970 to 1100 nm was exhibited from 940 nm wavelength pumping. This crystal is promising as a self-Qswitched laser crystal used for compact, efficient, highly stable, passive self-Q-switched thin chip solid-state lasers. q 2003 Elsevier Ltd. All rights reserved. PACS: 81.10Fq; 42.70.Hj; 87.64.Ni Keywords: A. Cr4þ: A. YbAG crystal; A. Self-Q-switched laser crystal; B. Crystal growth; B. Optical properties
1. Introduction In recent years, Cr4þ-doped crystals have attracted a great deal of attention as passive Q-switches [1 – 6] in comparison with previously used saturable absorbers, such as dyes [7] and LiF:F-2 color center crystals [8]. Cr4þ-doped crystals are photochemically and thermally stable, have a high damage threshold, large absorption cross-section and low saturable intensity at the lasing wavelength. In addition, Cr4þ can be doped into gain medium to form self-Qswitched lasers [3,9 – 10]. As a result of the above advantages, Cr4þ-doped crystals become the most promising saturable absorbers for continuously pumped Qswitched lasers, particularly in the case of diode-pumped, passively Q-switched lasers. Recent development of InGaAs diode laser has stimulated interest in Yb3þ doped solid state materials to be used
* Corresponding author. Tel: þ86-21-69918482; fax: þ 86-2159928755. E-mail address: [email protected] (Z. Zhao). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.12.021
as gain for high efficiency, high power diode-pumped solid state lasers [11– 13]. The trivalent ytterbium ion’s simple [Xe]4f13 electronic structure allows for no excited state absorption, upconversion or concentration quenching. The small stokes shift (about 650 cm21) between absorption and emission reduces the thermal loading of the materials during laser operation. So the Yb3þ ion is favorable for laser diodepumped system [14]. Self-Q-switched Cr,Yb:YAG laser crystal combines the advantages of the gain medium, Yb3þ, and the saturable absorber, Cr4þ, and the laser has been demonstrated [15]. The thin-disk laser design requires laser crystals with high dopant concentrations to ensure a high absorption of the pump light within the thin crystal disk [16]. The stoichiometric crystal Yb3Al5O12 is a promising stoichiometric laser crystal compared with lowly-doped Yb:YAG [17]. Growth of Cr4þ-doped Yb3Al5O12 will potentially provide a passive Q-switched laser which is better than Cr,Yb:YAG crystal. This crystal has potential for compact, efficient, high stable diode-pumped solid-state laser devices. In this paper, we report the growth of Cr4þ:YbAG single crystal by the Czochralski method and its optical properties.
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2. Experiments 2.1. Crystal growth Raw materials of 5 N pure Yb2O3, Al2O3, Cr2O3 and CaCO3 were weighed according to the stoichiometric composition. After the compounds were ground and thoroughly mixed, they were pressed into pieces. The pieces were heated to 1200 8C and kept at the temperature for 20 h. Then the mixture was charged into an Iridium crucible of 50 mm in diameter for crystal growth. The pulling and rotation rates were 1 mm/h and 10 – 20 rpm, respectively, under the nitrogen or argon atmosphere, respectively. The initial growth boundary in solid-melt was convex towards the melt so that dislocation and impurities were reduced or eliminated from the crystal. After that, the growth boundary became flat. In order to prevent the crystal from cracking, the crystal was cooled to room temperature slowly after growth. The crystal was blue-green and free from crack, inclusions and precipitations. When the 20 mW He– Ne laser beam passed through the as-grown Cr4þ:YbAG, the light beam was almost unseen, which indicated very few scattering particles in Cr4þ:YbAG crystal. The crystal changed to brown after annealing in oxygen atmosphere at 1400 8C for 24 h. 2.2. Sample preparation and measurements Samples for spectroscopic measurements were cut out of the boules and surfaces perpendicular to the k111l-growth axis were polished. The thickness of the samples was 0.35 mm. A JASCO V-570 UV/VIS/NIR Spectrophotometer was employed for acquisition of the absorption spectra at room temperature. The fluorescence spectra were acquired by a TRIAX 550 spectrophotometer with InGaAs LD as the pump source (excited at 940 nm). The decay time was measured by a computer controlled transient digitizer.
3. Results and discussion The room temperature absorption spectra of Cr4þ:YbAG are shown in Fig. 1. In Fig. 1, the data are actually two curves, one from a 0.35 mm sample cut from the top or firstto-freeze end of the Cr4þ:YbAG boule, and the other from a 0.35 mm sample cut from the boule bottom end. The perfect overlap of the curves indicates that there was essentially on concentration gradient for Cr4þ ions along the length of the boule and that the distribution coefficient for Cr4þ in YbAG must be close to unity. Fig. 2 shows the spectra of Cr4þ:YbAG crystal as-grown and after annealing at room temperature. In the wavelength range from 800 to 1200 nm, each absorption peak of Cr4þ:YbAG crystal increased in intensity and the absorption coefficients at 939 and 1030 nm increased from 90.9 to
Fig. 1. The absorption spectra of Cr4þ:YbAG crystal from top and bottom boule after annealing at room temperature.
92.6 cm21 and 15.6 to 17.5 cm21 after annealing. The absorption band of Cr4þ ranges from 900 to 1100 nm. The absorption bands of Yb3þ superimposed over new Cr4þ absorption band. The band centered at 1030 nm is believed to be caused by the 3A2 ! 3T1 transition of Cr4þ ions. The strong absorption at 939 and 969 nm is suitable for InGaAs diode laser pumping. At the same time, the sample color changed from blue-green to brown by annealing. These facts show that annealing increased Cr 4þ concentration dramatically. The absorption feature in the visible region is similar to that of chromium-doped YAG [18]. The absorption bands centered at 424 and 593 nm in Cr4þ:YbAG crystal are attributed to the 4A2 ! 4T1 and 4A2 ! 4T2 transitions of Cr3þ. After annealing, not only the intensity of the 4 A2 ! 4T1 and 4A2 ! 4T2 transitions absorption peaks of Cr3þ increased, but also the spectral shape in other wavelength regions also changed. The main band position moved from 424 to 478 nm and from 593 to 600 nm. Fig. 3 is calculated by subtracting the absorption spectrum of asgrown Cr4þ:YbAG from the absorption spectrum of annealed Cr4þ:YbAG crystal. In the oxidation process at high temperature, the concentration of Cr3þ would not be increased (Cr3þ ions were oxidized to Cr4þ ions). At the same time, the concentration of oxygen vacancy dropped. The bands centered at 480 and 601 nm in Fig. 3 are impossibly attributed to Cr3þ and color center, they may be possibly due to new octahedral Cr4þ center in the crystal after oxidization. The fluorescence spectra of Cr4þ:YbAG crystal asgrown and after annealing are shown in Fig. 3. From Fig. 4, we can see that the fluorescence intensity of Yb3þ in Cr4þ:YbAG after annealing is lower than that of Yb3þ in asgrown Cr4þ:YbAG. The fluorescence lifetime of Cr4þ:
X. Xu et al. / Solid State Communications 129 (2004) 717–720
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Fig. 4. The fluorescence spectra of Yb3þ in Cr4þ:YbAG crystal.
YbAG after annealing is 180 ms, lower than that of as-grown Cr4þ:YbAG which is 432 ms. The great change my be related to the increase of Cr4þ concentration that results in the ground state absorption and the concentration quenching of Yb3þ in Cr4þ:YbAG after annealing. Although the fluorescence lifetime of Cr4þ:YbAG crystal is lower that that of YbAG crystal at 1031 nm, the absorption and fluorescence spectra show that Cr4þ:YbAG crystal combines the saturable absorber and gain medium into one and it can be a self-Q-switched laser crystal if the crystal is pumped with high power.
4. Conclusion
Fig. 2. The absorption spectra of Cr4þ:YbAG crystal as-grown and after annealing at room temperature. (a) 800 – 1200 nm. (b) 300–800 nm.
Cr4þ:YbAG crystal with dimension F24 mm £ 30 mm has been grown by Czochralski method. The absorption spectra, emission spectra and fluorescence lifetimes of asgrown and annealed Cr,Yb:YAG crystal have been investigated at room temperature. The absorption spectra of Cr4þ:YbAG have six absorption bands. In the visible region, after annealing, the absorption spectra moved from 424 and 593 nm to 478 and 600 nm, respectively. The absorption intensity increased after samples were annealed. The fluorescence intensity was reduced and the emission lifetime was shortened after annealing. The absorption and fluorescence spectra show that Cr4þ:YbAG crystal combines the saturable absorber and gain medium into one and it can be a self-Q-switched laser crystal if the crystal is pumped with high power.
Acknowledgements
Fig. 3. Absorption of Cr4þ centers in oxidized Cr4þ:YbAG crystal.
This work is supported by the High Technology and Development Project of the People’s Republic of China (Grant No. 2002AA311030)
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X. Xu et al. / Solid State Communications 129 (2004) 717–720
References [1] P. Yankov, J. Phys. D 27 (1994) 1118. [2] Y. Shimony, Z. Burshtein, Y. Kalisky, IEEE J. Quantum Electron. 31 (1995) 1738. [3] S. Zhou, K.K. Lee, Y.C. Chen, Opt. Lett. 18 (1993) 511. [4] W. Chen, K. Spariosu, R. Stultz, Opt. Commun. 104 (1993) 71. [5] Y.K. Kuo, M.F. Huang, M. Birnbaum, IEEE J. Quantum Electron. 31 (1995) 657. [6] M.I. Demchuk, V.P. Mikhailov, N.I. Zhavoronkov, N.V. Kileshov, P.V. Prokoshin, K.V. Yumasgev, M.G. Livshits, V.I. Minkov, Opt. Lett. 17 (1992) 929. [7] X. Zhang, S. Zhao, Q. Wang, Y. Liu, J. Wang, IEEE J. Quantum Electron. 30 (1994) 905. [8] J.A. Morris, C.R. Pollock, Opt. Lett. 15 (1990) 440. [9] P. Wang, S.H. Zhou, K.K. Lee, Y.C. Chen, Opt. Commun. 114 (1995) 439.
[10] J. Dong, P.Z. Deng, Y.T. Lu, Y.H. Zhang, Y.P. Liu, J. Xu, W. Chen, Opt. Lett. 25 (2000) 1101. [11] P. Lacovara, H.K. Choi, C.A. Wang, R.L. Aggarwal, T.Y. Fan, Opt. Lett. 16 (1991) 1089. [12] H.W. Qiu, P.Z. Yang, J. Dong, et al., Mater. Lett. 55 (2002) 1. [13] J. Saikawa, S. Kurimura, I. Shoji, T. Taira, Opt. Mater. (2003) 19169. [14] X.D. Xu, Z.W. Zhao, J. Xu, P.Z. Deng, J. Cryst. Growth 255 (2003) 338. [15] J. Dong, P.Z. Deng, Y.P. Liu, Y.H. Zhang, G.S. Huang, F.X. Gan, Chin. Phys. Lett. 19 (2002) 342. [16] F.D. Patel, E.C. Honea, J. Speth, S.A. Payne, R. Hutcheson, R. Equall, IEEE J. Quantum Electron. 37 (2001) 135. [17] X.D. Xu, Z.W. Zhao, J. Xu, P.Z. Deng, J. Cryst. Growth 257 (2003) 272. [18] J. Dong, P.Z. Deng, J. Xu, Opt. Commun. (1999) 170255.
Growth and optical properties of a new self-Q-switched laser crystal: Cr4þ:Yb3Al5O12 Xiaodong Xu, Zhiwei Zhao*, Pingxin Song, Guoqing Zhou, Jun Xu, Peizhen Deng Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Crystal Centre, P.O. Box 800-211, Shanghai 201800, China Received 24 November 2003; accepted 15 December 2003 by P. Watcher
Abstract Cr4þ:YbAG (Cr4þ:Yb3Al5O12) crystal with a size up to F24 mm £ 30 mm was grown by the Czochralski method. In the absorption spectrum, there are two absorption bands at 939 and 969 nm, respectively, which are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switched laser output at 1.03 mm. A broad emission spectrum from 970 to 1100 nm was exhibited from 940 nm wavelength pumping. This crystal is promising as a self-Qswitched laser crystal used for compact, efficient, highly stable, passive self-Q-switched thin chip solid-state lasers. q 2003 Elsevier Ltd. All rights reserved. PACS: 81.10Fq; 42.70.Hj; 87.64.Ni Keywords: A. Cr4þ: A. YbAG crystal; A. Self-Q-switched laser crystal; B. Crystal growth; B. Optical properties
1. Introduction In recent years, Cr4þ-doped crystals have attracted a great deal of attention as passive Q-switches [1 – 6] in comparison with previously used saturable absorbers, such as dyes [7] and LiF:F-2 color center crystals [8]. Cr4þ-doped crystals are photochemically and thermally stable, have a high damage threshold, large absorption cross-section and low saturable intensity at the lasing wavelength. In addition, Cr4þ can be doped into gain medium to form self-Qswitched lasers [3,9 – 10]. As a result of the above advantages, Cr4þ-doped crystals become the most promising saturable absorbers for continuously pumped Qswitched lasers, particularly in the case of diode-pumped, passively Q-switched lasers. Recent development of InGaAs diode laser has stimulated interest in Yb3þ doped solid state materials to be used
* Corresponding author. Tel: þ86-21-69918482; fax: þ 86-2159928755. E-mail address: [email protected] (Z. Zhao). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.12.021
as gain for high efficiency, high power diode-pumped solid state lasers [11– 13]. The trivalent ytterbium ion’s simple [Xe]4f13 electronic structure allows for no excited state absorption, upconversion or concentration quenching. The small stokes shift (about 650 cm21) between absorption and emission reduces the thermal loading of the materials during laser operation. So the Yb3þ ion is favorable for laser diodepumped system [14]. Self-Q-switched Cr,Yb:YAG laser crystal combines the advantages of the gain medium, Yb3þ, and the saturable absorber, Cr4þ, and the laser has been demonstrated [15]. The thin-disk laser design requires laser crystals with high dopant concentrations to ensure a high absorption of the pump light within the thin crystal disk [16]. The stoichiometric crystal Yb3Al5O12 is a promising stoichiometric laser crystal compared with lowly-doped Yb:YAG [17]. Growth of Cr4þ-doped Yb3Al5O12 will potentially provide a passive Q-switched laser which is better than Cr,Yb:YAG crystal. This crystal has potential for compact, efficient, high stable diode-pumped solid-state laser devices. In this paper, we report the growth of Cr4þ:YbAG single crystal by the Czochralski method and its optical properties.
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X. Xu et al. / Solid State Communications 129 (2004) 717–720
2. Experiments 2.1. Crystal growth Raw materials of 5 N pure Yb2O3, Al2O3, Cr2O3 and CaCO3 were weighed according to the stoichiometric composition. After the compounds were ground and thoroughly mixed, they were pressed into pieces. The pieces were heated to 1200 8C and kept at the temperature for 20 h. Then the mixture was charged into an Iridium crucible of 50 mm in diameter for crystal growth. The pulling and rotation rates were 1 mm/h and 10 – 20 rpm, respectively, under the nitrogen or argon atmosphere, respectively. The initial growth boundary in solid-melt was convex towards the melt so that dislocation and impurities were reduced or eliminated from the crystal. After that, the growth boundary became flat. In order to prevent the crystal from cracking, the crystal was cooled to room temperature slowly after growth. The crystal was blue-green and free from crack, inclusions and precipitations. When the 20 mW He– Ne laser beam passed through the as-grown Cr4þ:YbAG, the light beam was almost unseen, which indicated very few scattering particles in Cr4þ:YbAG crystal. The crystal changed to brown after annealing in oxygen atmosphere at 1400 8C for 24 h. 2.2. Sample preparation and measurements Samples for spectroscopic measurements were cut out of the boules and surfaces perpendicular to the k111l-growth axis were polished. The thickness of the samples was 0.35 mm. A JASCO V-570 UV/VIS/NIR Spectrophotometer was employed for acquisition of the absorption spectra at room temperature. The fluorescence spectra were acquired by a TRIAX 550 spectrophotometer with InGaAs LD as the pump source (excited at 940 nm). The decay time was measured by a computer controlled transient digitizer.
3. Results and discussion The room temperature absorption spectra of Cr4þ:YbAG are shown in Fig. 1. In Fig. 1, the data are actually two curves, one from a 0.35 mm sample cut from the top or firstto-freeze end of the Cr4þ:YbAG boule, and the other from a 0.35 mm sample cut from the boule bottom end. The perfect overlap of the curves indicates that there was essentially on concentration gradient for Cr4þ ions along the length of the boule and that the distribution coefficient for Cr4þ in YbAG must be close to unity. Fig. 2 shows the spectra of Cr4þ:YbAG crystal as-grown and after annealing at room temperature. In the wavelength range from 800 to 1200 nm, each absorption peak of Cr4þ:YbAG crystal increased in intensity and the absorption coefficients at 939 and 1030 nm increased from 90.9 to
Fig. 1. The absorption spectra of Cr4þ:YbAG crystal from top and bottom boule after annealing at room temperature.
92.6 cm21 and 15.6 to 17.5 cm21 after annealing. The absorption band of Cr4þ ranges from 900 to 1100 nm. The absorption bands of Yb3þ superimposed over new Cr4þ absorption band. The band centered at 1030 nm is believed to be caused by the 3A2 ! 3T1 transition of Cr4þ ions. The strong absorption at 939 and 969 nm is suitable for InGaAs diode laser pumping. At the same time, the sample color changed from blue-green to brown by annealing. These facts show that annealing increased Cr 4þ concentration dramatically. The absorption feature in the visible region is similar to that of chromium-doped YAG [18]. The absorption bands centered at 424 and 593 nm in Cr4þ:YbAG crystal are attributed to the 4A2 ! 4T1 and 4A2 ! 4T2 transitions of Cr3þ. After annealing, not only the intensity of the 4 A2 ! 4T1 and 4A2 ! 4T2 transitions absorption peaks of Cr3þ increased, but also the spectral shape in other wavelength regions also changed. The main band position moved from 424 to 478 nm and from 593 to 600 nm. Fig. 3 is calculated by subtracting the absorption spectrum of asgrown Cr4þ:YbAG from the absorption spectrum of annealed Cr4þ:YbAG crystal. In the oxidation process at high temperature, the concentration of Cr3þ would not be increased (Cr3þ ions were oxidized to Cr4þ ions). At the same time, the concentration of oxygen vacancy dropped. The bands centered at 480 and 601 nm in Fig. 3 are impossibly attributed to Cr3þ and color center, they may be possibly due to new octahedral Cr4þ center in the crystal after oxidization. The fluorescence spectra of Cr4þ:YbAG crystal asgrown and after annealing are shown in Fig. 3. From Fig. 4, we can see that the fluorescence intensity of Yb3þ in Cr4þ:YbAG after annealing is lower than that of Yb3þ in asgrown Cr4þ:YbAG. The fluorescence lifetime of Cr4þ:
X. Xu et al. / Solid State Communications 129 (2004) 717–720
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Fig. 4. The fluorescence spectra of Yb3þ in Cr4þ:YbAG crystal.
YbAG after annealing is 180 ms, lower than that of as-grown Cr4þ:YbAG which is 432 ms. The great change my be related to the increase of Cr4þ concentration that results in the ground state absorption and the concentration quenching of Yb3þ in Cr4þ:YbAG after annealing. Although the fluorescence lifetime of Cr4þ:YbAG crystal is lower that that of YbAG crystal at 1031 nm, the absorption and fluorescence spectra show that Cr4þ:YbAG crystal combines the saturable absorber and gain medium into one and it can be a self-Q-switched laser crystal if the crystal is pumped with high power.
4. Conclusion
Fig. 2. The absorption spectra of Cr4þ:YbAG crystal as-grown and after annealing at room temperature. (a) 800 – 1200 nm. (b) 300–800 nm.
Cr4þ:YbAG crystal with dimension F24 mm £ 30 mm has been grown by Czochralski method. The absorption spectra, emission spectra and fluorescence lifetimes of asgrown and annealed Cr,Yb:YAG crystal have been investigated at room temperature. The absorption spectra of Cr4þ:YbAG have six absorption bands. In the visible region, after annealing, the absorption spectra moved from 424 and 593 nm to 478 and 600 nm, respectively. The absorption intensity increased after samples were annealed. The fluorescence intensity was reduced and the emission lifetime was shortened after annealing. The absorption and fluorescence spectra show that Cr4þ:YbAG crystal combines the saturable absorber and gain medium into one and it can be a self-Q-switched laser crystal if the crystal is pumped with high power.
Acknowledgements
Fig. 3. Absorption of Cr4þ centers in oxidized Cr4þ:YbAG crystal.
This work is supported by the High Technology and Development Project of the People’s Republic of China (Grant No. 2002AA311030)
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X. Xu et al. / Solid State Communications 129 (2004) 717–720
References [1] P. Yankov, J. Phys. D 27 (1994) 1118. [2] Y. Shimony, Z. Burshtein, Y. Kalisky, IEEE J. Quantum Electron. 31 (1995) 1738. [3] S. Zhou, K.K. Lee, Y.C. Chen, Opt. Lett. 18 (1993) 511. [4] W. Chen, K. Spariosu, R. Stultz, Opt. Commun. 104 (1993) 71. [5] Y.K. Kuo, M.F. Huang, M. Birnbaum, IEEE J. Quantum Electron. 31 (1995) 657. [6] M.I. Demchuk, V.P. Mikhailov, N.I. Zhavoronkov, N.V. Kileshov, P.V. Prokoshin, K.V. Yumasgev, M.G. Livshits, V.I. Minkov, Opt. Lett. 17 (1992) 929. [7] X. Zhang, S. Zhao, Q. Wang, Y. Liu, J. Wang, IEEE J. Quantum Electron. 30 (1994) 905. [8] J.A. Morris, C.R. Pollock, Opt. Lett. 15 (1990) 440. [9] P. Wang, S.H. Zhou, K.K. Lee, Y.C. Chen, Opt. Commun. 114 (1995) 439.
[10] J. Dong, P.Z. Deng, Y.T. Lu, Y.H. Zhang, Y.P. Liu, J. Xu, W. Chen, Opt. Lett. 25 (2000) 1101. [11] P. Lacovara, H.K. Choi, C.A. Wang, R.L. Aggarwal, T.Y. Fan, Opt. Lett. 16 (1991) 1089. [12] H.W. Qiu, P.Z. Yang, J. Dong, et al., Mater. Lett. 55 (2002) 1. [13] J. Saikawa, S. Kurimura, I. Shoji, T. Taira, Opt. Mater. (2003) 19169. [14] X.D. Xu, Z.W. Zhao, J. Xu, P.Z. Deng, J. Cryst. Growth 255 (2003) 338. [15] J. Dong, P.Z. Deng, Y.P. Liu, Y.H. Zhang, G.S. Huang, F.X. Gan, Chin. Phys. Lett. 19 (2002) 342. [16] F.D. Patel, E.C. Honea, J. Speth, S.A. Payne, R. Hutcheson, R. Equall, IEEE J. Quantum Electron. 37 (2001) 135. [17] X.D. Xu, Z.W. Zhao, J. Xu, P.Z. Deng, J. Cryst. Growth 257 (2003) 272. [18] J. Dong, P.Z. Deng, J. Xu, Opt. Commun. (1999) 170255.