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Mg co-doped LiNbO3 single crystal
Journal of Crystal Growth 244 (2002) 49–52
Segregation and laser properties of Er/Mg co-doped LiNbO3 single crystal Woo-Seok Yanga,*, Han-Young Leeb, Dae-Ho Yoona b
a Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-746, South Korea Korea Electronics Technology Institute, Optical Telecommunication Components Laboratory, Pyungtaek 451-865, South Korea
Received 22 May 2002; accepted 30 May 2002 Communicated by R. Kern
Abstract Crack-free homogeneous Er/Mg co-doped LiNbO3 single crystal fibers were grown by the micro-pulling down ( method. The structure as determined from XRD diffraction is hexagonal and lattice constants were a ¼ 5:1570 A, ( and a ¼ 5:1587 A, ( c ¼ 13:8578 A, ( for 0.6 mol% Er2O3/1 mol% MgO and 0.6 mol% Er2O3/3 mol% MgO c ¼ 13:8595 A co-doped LiNbO3 crystal, respectively. The segregation coefficient of Er3+ ions was in the range of 0.85–0.86 according to the MgO in the starting charge. Also, the transmission spectrum of the crystal was measured and the photoluminescence spectra were observed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10; 61.72 Keywords: A1. Impurities; A2. Growth from melt; B1. Oxides; B1. Rare earth compounds
1. Introduction The rapid development of the optical fiber as transmission medium has created an interest in fiber compatible optical device such as in-line optical amplifiers, modulators, optical isolators, signal processors, and coupler [1–3]. Er3+-doped LiNbO3 (Er:LiNbO3) crystal combining the good laser properties of Er3+ and excellent electro-optical and non-linear optical properties of LiNbO3 is a promising material for producing pure green lasers in a miniature laser that would be useful in practical application such *Corresponding author.
as laser medicine and high-speed photography [4–6]. Recently, Yoon et al. [7] have suggested that the LiNbO3 single crystal fiber has the potential to be of great use in optical devices due to growth of high-quality crystal without subgrain boundaries. However, the optical damage in LiNbO3 appeared to greatly reduce the efficiency and usefulness of the optical devices [8]. Also, application for optical devices has the need of highquality crystal. Zhong et al. [9] showed that LiNbO3 doped with about 5% or more magnesium oxide (MgO) exhibits a remarkably reduced photo-refractive response compared with undoped LiNbO3.
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 4 9 8 - 7
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52
Therefore, in order to have information about the MgO and Er2O3 co-doped LiNbO3 single crystal, we present here the results of properties (lattice parameter, segregation, transmission) for grown crystals by the micro-pulling down (m-PD) method.
2. Experimental procedure Er:MgO:LiNbO3 crystal fibers were grown by the m-PD method from a congruent melt containing with Er2O3 0.6 mol% and MgO 1 and 3 mol%. The m-PD method used is described in Ref. [10] in detail and is briefly explained here. The raw materials were melted in the Pt crucible and allowed to pass through the micro-nozzle. The single crystal fibers were formed by attaching the seed crystal to the pulled down at constant velocity. The alignment of the seed to the micro-nozzle was controlled by the micro XY stage. Crystal diameter was maintained constant by controlling the temperature of the main and after heaters during growth process. The crystal seed was oriented along the c-axis. Poling was carried out by the thermal electric effect during crystal growth. The crystal structure was measured by X-ray diffractometer (XRD, M18XHF, Mac Science Co.). Dopant concentration was measured with the JEOL JXA-8900R electron probe microanalysis (EPMA). Several test samples (2 mm thick) were cut from as-grown crystal fibers and polished for optical characterization. At room temperature, transmission spectrum of the crystal fibers was measured at a wavelength range of 380– 2000 nm. The resolution was fixed to 0.2 nm.
3. Results and discussion The grown crystals are transparent and crackfree and have a light yellow color which is characteristic of MgO doping. We have grown fibers of about 40 mm long and with diameter of about 1 mm.
The X-ray diffraction pattern of the grown Er2O3 0.6/MgO 1 mol% doped LiNbO3 and Er2O3 0.6/MgO 3 mol% doped LiNbO3 crystal fibers at room temperature is shown in Fig. 1. The crystal structure is hexagonal and the lattice constant a increases with increasing the MgO content, but c decreases as shown in Table 1. The chemical composition of the grown crystal fibers measured by EPMA along the growth axis is shown in Fig. 2. Also, the distribution of Er and Mg ions across the cross-section of crystals is observed as shown in Fig. 3. The axial and radial distribution of doping ions was comparatively homogeneous. The segregation coefficient of Er3+ ions measured by the EPMA method was in the range of
40k
0.6 mol% Er2O3 3 mol% MgO
30k 20k
Intensity (Arb. Unit)
50
10k 0
40k
0.6 mol% Er2O3 1 mol% MgO
30k 20k 10k 0 20
40
60
80
2θ (deg.)
Fig. 1. The powder XRD patterns of 1 mol% MgO/ 0.6 mol% Er2O3 co-doped LiNbO3 and 3 mol% MgO/ 0.6 mol% Er2O3 co-doped LiNbO3 crystal fibers at room temperature.
Table 1 Lattice constants of Er:MgO:LiNbO3 crystal fibers Lattice constants
Er:Mg:LiNbO3 (0.6:1:48.6:51.4)
Er:Mg:LiNbO3 (0.6:3:48.6:51.4)
( a (A) ( c (A) c=a
5.1570 13.8595 2.6875
5.1587 13.8578 2.6862
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52 1.0
1.2
1 mol% (MgO) 3 mol% (MgO)
MgO 1 mol% MgO 3 mol%
Mg Concentration (wt%)
0.9
1.0
Mg concentration (wt%)
51
0.8
0.6
0.4
0.8 0.7 0.6 0.5 0.4 0.3
0.2
0.2 0.0 0
5
10
15
20
25
30
35
Distance (mm)
(a)
-4
-2
0
2
4
Position along the cross-section (mm)
(a)
2.0
2.0
1:0.6 mol% (Mg:Er) 3:0.6 mol% (Mg:Er)
1.5
Er Concentration (wt%)
Er concentration (wt%)
1:0.6 mol% (Mg:Er) 3:0.6 mol% (Mg:Er)
1.0
0.5
1.5
1.0
0.5
0.0 0
(b)
2
4
6
8
10
12
14
16
0.0
Distance (mm)
Fig. 2. Distribution of Mg (a) and Er (b) concentrations in MgO and Er2O3 co-doped LiNbO3 along the growth axis.
0.85–0.86 according to the Mg concentration in the starting charge. Also, the segregation coefficient of Mg ions was about 1.02 according to MgO concentrations in the melt. It can be explained with a view of growth method because the nature convection was restricted in the micro-nozzle and the Er2O3 and MgO constituent in Er:MgO:LiNbO3 melt became unity along the growth axis leading to the homogeneous composition throughout the crystal. The transmission spectra of Er2O3 0.6/MgO 1 and 3 mol% doped LiNbO3 crystal fibers at room temperature are shown in Fig. 4. Several energy bands are observed in the 370– 1800 nm region [11]. The observed energy bands
-4
(b)
-2
0
2
4
Position along the cross-section (mm)
Fig. 3. Distribution of Mg (a) and Er (b) concentration on the cross-section of a Er:MgO:LiNbO3 fiber.
are due to the f11 electronic configuration of Er3+, that is the transition from the ground state 4I15/2 to the excited states which are created from the 4f11 electron configuration [12]. Also, the transmission spectra is similar to those observed in MgO 1 mol% doped crystal containing Er3+ ions, but relative absorption was different. For example, three intense lines are observed around 1530, 520 and 390 nm in Er:MgO:LiNbO3 crystal fibers according to MgO concentration which are due to 4I15/2-4I13/2, 4I15/2-2H11/2 and 4I15/2-4G11/2 transitions, respectively. From a viewpoint of reducing the photorefractive effect, relative absorption spectrum
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52
52 250
0.6:1 mol% (Er:Mg) 0.6:3 mol% (Er:Mg)
Transmission (a.u.)
200
150
line corresponding to the 4S3/2-4I15/2 transition, as observed around the UV, NIR regions under the 514 nm excitation (Fig. 5). The emission intensity for the 3 mol% MgO-doped crystals was about 1 time larger than that for the 1 mol% MgO-doped crystals.
100
4. Summary 50
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Wavelength (nm)
Fig. 4. Transmission spectrum of Er3+ ions in 1 and 3 mol% MgO co-doped LiNbO3 crystal fibers.
Intensity (a.u.)
4
0.6:1 mol% (Er:Mg) 0.6:3 mol% (Er:Mg)
4
S 3/2 -> I15/2
Er:MgO:LiNbO3 single crystal fibers were grown according to MgO concentration, which means that the very little segregation occurred the MgO content, but c decreases. The emission intensity for the 3 mol% MgOdoped crystals was about 1 time larger than that for the 1 mol% MgO-doped crystals. The Er:MgO:LiNbO3 crystal grown with added MgO of 3 mol% was observed the progressive photo-refractive resistance more than grown crystal from 1 mol% MgO and 0.6 mol% Er2O3 codoped LiNbO3 congruent melt.
References 2
4
H 11/2 -> I15/2
4
500
550
600
4
F 9/2 -> I15/2
650
700
Wavelength (nm) Fig. 5. PL spectra of MgO:Er:LiNbO3 crystal fibers in the range of 500–700 nm pumped by Ar-ion laser 514.5 nm, 150 mW at room temperature.
according to MgO content, the Er:MgO:LiNbO3 crystal grown with added MgO of 3 mol% was observed the progressive photo-refractive resistance more than grown crystal from 1 mol% MgO and 0.6 mol% Er2O3 co-doped LiNbO3 congruent melt. The spectra of crystal fibers have shown an energy band emission spectrum with the strongest
[1] M. Saifi, B. Dubois, E.M. Vogel, J. Mater. Res. 1 (1986) 452. [2] O. Witte, H. Stolz, W. von der Osten, J. Phys. D 29 (1996) 561. [3] A.S. Oilveira, M.T. de Arajo, A.S. Gouveia-Neto, Appl. Phys. Lett. 72 (1998) 753. [4] J. Zheng, Y. Lu, G. Luo, J. Ma, Y. Lu, Appl. Phys. Lett. 72 (1998) 808. [5] T. Nolte, T. Pawlik, J.M. Spaeth, Solid State Commun. 104 (1997) 535. [6] J.J. Ju, T.Y. Kwon, S.I. Yun, Opt. Sci. New Technol. 27 (1996) 618. [7] D.H. Yoon, I. Yonenaga, T. Fukuda, N. Ohnishi, J. Crystal Growth 142 (1994) 339. [8] T.Y. Fan, A. Cordova-Plaza, M.J.F. Digonnet, R.L. Byer, H.J.J. Shaw, Opt. Am. 3 (1) (1986) 140. [9] G. Zhong, J. Jia, Z. Wu, in: Proceedings of the 11th International Quantum Electronics Conference, 1998, San Francisco, IEEE Catalog No. 80CH 1561-0, p. 631 [10] D.H. Yoon, P. Rudolph, T. Fukuda, J. Crystal Growth 144 (1994) 207. [11] W.S. Yang, S.J. Suh, J.H. Lee, T. Fukuda, D.H. Yoon, J. Korean Assoc. Crystal Growth 9 (1999) 377. [12] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (1995) 2102.
Segregation and laser properties of Er/Mg co-doped LiNbO3 single crystal Woo-Seok Yanga,*, Han-Young Leeb, Dae-Ho Yoona b
a Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-746, South Korea Korea Electronics Technology Institute, Optical Telecommunication Components Laboratory, Pyungtaek 451-865, South Korea
Received 22 May 2002; accepted 30 May 2002 Communicated by R. Kern
Abstract Crack-free homogeneous Er/Mg co-doped LiNbO3 single crystal fibers were grown by the micro-pulling down ( method. The structure as determined from XRD diffraction is hexagonal and lattice constants were a ¼ 5:1570 A, ( and a ¼ 5:1587 A, ( c ¼ 13:8578 A, ( for 0.6 mol% Er2O3/1 mol% MgO and 0.6 mol% Er2O3/3 mol% MgO c ¼ 13:8595 A co-doped LiNbO3 crystal, respectively. The segregation coefficient of Er3+ ions was in the range of 0.85–0.86 according to the MgO in the starting charge. Also, the transmission spectrum of the crystal was measured and the photoluminescence spectra were observed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10; 61.72 Keywords: A1. Impurities; A2. Growth from melt; B1. Oxides; B1. Rare earth compounds
1. Introduction The rapid development of the optical fiber as transmission medium has created an interest in fiber compatible optical device such as in-line optical amplifiers, modulators, optical isolators, signal processors, and coupler [1–3]. Er3+-doped LiNbO3 (Er:LiNbO3) crystal combining the good laser properties of Er3+ and excellent electro-optical and non-linear optical properties of LiNbO3 is a promising material for producing pure green lasers in a miniature laser that would be useful in practical application such *Corresponding author.
as laser medicine and high-speed photography [4–6]. Recently, Yoon et al. [7] have suggested that the LiNbO3 single crystal fiber has the potential to be of great use in optical devices due to growth of high-quality crystal without subgrain boundaries. However, the optical damage in LiNbO3 appeared to greatly reduce the efficiency and usefulness of the optical devices [8]. Also, application for optical devices has the need of highquality crystal. Zhong et al. [9] showed that LiNbO3 doped with about 5% or more magnesium oxide (MgO) exhibits a remarkably reduced photo-refractive response compared with undoped LiNbO3.
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 4 9 8 - 7
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52
Therefore, in order to have information about the MgO and Er2O3 co-doped LiNbO3 single crystal, we present here the results of properties (lattice parameter, segregation, transmission) for grown crystals by the micro-pulling down (m-PD) method.
2. Experimental procedure Er:MgO:LiNbO3 crystal fibers were grown by the m-PD method from a congruent melt containing with Er2O3 0.6 mol% and MgO 1 and 3 mol%. The m-PD method used is described in Ref. [10] in detail and is briefly explained here. The raw materials were melted in the Pt crucible and allowed to pass through the micro-nozzle. The single crystal fibers were formed by attaching the seed crystal to the pulled down at constant velocity. The alignment of the seed to the micro-nozzle was controlled by the micro XY stage. Crystal diameter was maintained constant by controlling the temperature of the main and after heaters during growth process. The crystal seed was oriented along the c-axis. Poling was carried out by the thermal electric effect during crystal growth. The crystal structure was measured by X-ray diffractometer (XRD, M18XHF, Mac Science Co.). Dopant concentration was measured with the JEOL JXA-8900R electron probe microanalysis (EPMA). Several test samples (2 mm thick) were cut from as-grown crystal fibers and polished for optical characterization. At room temperature, transmission spectrum of the crystal fibers was measured at a wavelength range of 380– 2000 nm. The resolution was fixed to 0.2 nm.
3. Results and discussion The grown crystals are transparent and crackfree and have a light yellow color which is characteristic of MgO doping. We have grown fibers of about 40 mm long and with diameter of about 1 mm.
The X-ray diffraction pattern of the grown Er2O3 0.6/MgO 1 mol% doped LiNbO3 and Er2O3 0.6/MgO 3 mol% doped LiNbO3 crystal fibers at room temperature is shown in Fig. 1. The crystal structure is hexagonal and the lattice constant a increases with increasing the MgO content, but c decreases as shown in Table 1. The chemical composition of the grown crystal fibers measured by EPMA along the growth axis is shown in Fig. 2. Also, the distribution of Er and Mg ions across the cross-section of crystals is observed as shown in Fig. 3. The axial and radial distribution of doping ions was comparatively homogeneous. The segregation coefficient of Er3+ ions measured by the EPMA method was in the range of
40k
0.6 mol% Er2O3 3 mol% MgO
30k 20k
Intensity (Arb. Unit)
50
10k 0
40k
0.6 mol% Er2O3 1 mol% MgO
30k 20k 10k 0 20
40
60
80
2θ (deg.)
Fig. 1. The powder XRD patterns of 1 mol% MgO/ 0.6 mol% Er2O3 co-doped LiNbO3 and 3 mol% MgO/ 0.6 mol% Er2O3 co-doped LiNbO3 crystal fibers at room temperature.
Table 1 Lattice constants of Er:MgO:LiNbO3 crystal fibers Lattice constants
Er:Mg:LiNbO3 (0.6:1:48.6:51.4)
Er:Mg:LiNbO3 (0.6:3:48.6:51.4)
( a (A) ( c (A) c=a
5.1570 13.8595 2.6875
5.1587 13.8578 2.6862
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52 1.0
1.2
1 mol% (MgO) 3 mol% (MgO)
MgO 1 mol% MgO 3 mol%
Mg Concentration (wt%)
0.9
1.0
Mg concentration (wt%)
51
0.8
0.6
0.4
0.8 0.7 0.6 0.5 0.4 0.3
0.2
0.2 0.0 0
5
10
15
20
25
30
35
Distance (mm)
(a)
-4
-2
0
2
4
Position along the cross-section (mm)
(a)
2.0
2.0
1:0.6 mol% (Mg:Er) 3:0.6 mol% (Mg:Er)
1.5
Er Concentration (wt%)
Er concentration (wt%)
1:0.6 mol% (Mg:Er) 3:0.6 mol% (Mg:Er)
1.0
0.5
1.5
1.0
0.5
0.0 0
(b)
2
4
6
8
10
12
14
16
0.0
Distance (mm)
Fig. 2. Distribution of Mg (a) and Er (b) concentrations in MgO and Er2O3 co-doped LiNbO3 along the growth axis.
0.85–0.86 according to the Mg concentration in the starting charge. Also, the segregation coefficient of Mg ions was about 1.02 according to MgO concentrations in the melt. It can be explained with a view of growth method because the nature convection was restricted in the micro-nozzle and the Er2O3 and MgO constituent in Er:MgO:LiNbO3 melt became unity along the growth axis leading to the homogeneous composition throughout the crystal. The transmission spectra of Er2O3 0.6/MgO 1 and 3 mol% doped LiNbO3 crystal fibers at room temperature are shown in Fig. 4. Several energy bands are observed in the 370– 1800 nm region [11]. The observed energy bands
-4
(b)
-2
0
2
4
Position along the cross-section (mm)
Fig. 3. Distribution of Mg (a) and Er (b) concentration on the cross-section of a Er:MgO:LiNbO3 fiber.
are due to the f11 electronic configuration of Er3+, that is the transition from the ground state 4I15/2 to the excited states which are created from the 4f11 electron configuration [12]. Also, the transmission spectra is similar to those observed in MgO 1 mol% doped crystal containing Er3+ ions, but relative absorption was different. For example, three intense lines are observed around 1530, 520 and 390 nm in Er:MgO:LiNbO3 crystal fibers according to MgO concentration which are due to 4I15/2-4I13/2, 4I15/2-2H11/2 and 4I15/2-4G11/2 transitions, respectively. From a viewpoint of reducing the photorefractive effect, relative absorption spectrum
W.-S. Yang et al. / Journal of Crystal Growth 244 (2002) 49–52
52 250
0.6:1 mol% (Er:Mg) 0.6:3 mol% (Er:Mg)
Transmission (a.u.)
200
150
line corresponding to the 4S3/2-4I15/2 transition, as observed around the UV, NIR regions under the 514 nm excitation (Fig. 5). The emission intensity for the 3 mol% MgO-doped crystals was about 1 time larger than that for the 1 mol% MgO-doped crystals.
100
4. Summary 50
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
Wavelength (nm)
Fig. 4. Transmission spectrum of Er3+ ions in 1 and 3 mol% MgO co-doped LiNbO3 crystal fibers.
Intensity (a.u.)
4
0.6:1 mol% (Er:Mg) 0.6:3 mol% (Er:Mg)
4
S 3/2 -> I15/2
Er:MgO:LiNbO3 single crystal fibers were grown according to MgO concentration, which means that the very little segregation occurred the MgO content, but c decreases. The emission intensity for the 3 mol% MgOdoped crystals was about 1 time larger than that for the 1 mol% MgO-doped crystals. The Er:MgO:LiNbO3 crystal grown with added MgO of 3 mol% was observed the progressive photo-refractive resistance more than grown crystal from 1 mol% MgO and 0.6 mol% Er2O3 codoped LiNbO3 congruent melt.
References 2
4
H 11/2 -> I15/2
4
500
550
600
4
F 9/2 -> I15/2
650
700
Wavelength (nm) Fig. 5. PL spectra of MgO:Er:LiNbO3 crystal fibers in the range of 500–700 nm pumped by Ar-ion laser 514.5 nm, 150 mW at room temperature.
according to MgO content, the Er:MgO:LiNbO3 crystal grown with added MgO of 3 mol% was observed the progressive photo-refractive resistance more than grown crystal from 1 mol% MgO and 0.6 mol% Er2O3 co-doped LiNbO3 congruent melt. The spectra of crystal fibers have shown an energy band emission spectrum with the strongest
[1] M. Saifi, B. Dubois, E.M. Vogel, J. Mater. Res. 1 (1986) 452. [2] O. Witte, H. Stolz, W. von der Osten, J. Phys. D 29 (1996) 561. [3] A.S. Oilveira, M.T. de Arajo, A.S. Gouveia-Neto, Appl. Phys. Lett. 72 (1998) 753. [4] J. Zheng, Y. Lu, G. Luo, J. Ma, Y. Lu, Appl. Phys. Lett. 72 (1998) 808. [5] T. Nolte, T. Pawlik, J.M. Spaeth, Solid State Commun. 104 (1997) 535. [6] J.J. Ju, T.Y. Kwon, S.I. Yun, Opt. Sci. New Technol. 27 (1996) 618. [7] D.H. Yoon, I. Yonenaga, T. Fukuda, N. Ohnishi, J. Crystal Growth 142 (1994) 339. [8] T.Y. Fan, A. Cordova-Plaza, M.J.F. Digonnet, R.L. Byer, H.J.J. Shaw, Opt. Am. 3 (1) (1986) 140. [9] G. Zhong, J. Jia, Z. Wu, in: Proceedings of the 11th International Quantum Electronics Conference, 1998, San Francisco, IEEE Catalog No. 80CH 1561-0, p. 631 [10] D.H. Yoon, P. Rudolph, T. Fukuda, J. Crystal Growth 144 (1994) 207. [11] W.S. Yang, S.J. Suh, J.H. Lee, T. Fukuda, D.H. Yoon, J. Korean Assoc. Crystal Growth 9 (1999) 377. [12] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, W.R. Bosenberg, J.W. Pierce, J. Opt. Soc. Am. B 12 (1995) 2102.