- Email: [email protected]
Growth and characterization of magneto-optical single-crystal ReYbBiIG with temperature-stabilized Faraday rotation
Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
Growth and characterization of magneto-optical single-crystal ReYbBiIG with temperature-stabilized Faraday rotation X.W. Zhang*, S.Y. Zhang, G.R. Han State Key Laboratory of Silicon Materials Science, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 13 September 2001; received in revised form 10 December 2001
Abstract The Bi and Re (Re:Tb, Ho, etc.) partially substituted yttrium iron garnet bulk single crystals of (ReYbBi)3Fe5O12 were grown by using Bi2O3/B2O3 as flux and accelerated crucible rotation technique for single-crystal growth. Results on the basis of temperature-changeable magneto-optical Faraday rotation spectra showed that their magneto-optical figures of merit were much higher than that of YIG in the wavelength range of 1.0–1.7 mm, while their Faraday rotation temperature coefficients were even smaller than that of YIG in the temperature range 10–801C. According to temperature compensation effect, magneto-optical materials with temperature-stabilized Faraday rotation were achieved through compounding two kinds of rare-earth ions with opposite temperature coefficients, indicating an advantage for Faraday rotator usage in optical isolator with temperature-stable isolation. r 2002 Elsevier Science B.V. All rights reserved. PACS: 76.30.Kg; 85.70.Sq; 78.20.Ls Keywords: Faraday rotation; Magneto-optical crystals; Temperature compensation effect; Rare-earth iron garnet; Optical isolator
1. Introduction Extensive investigations on magneto-optical materials and devices are essential for optical communication and information processing systems. To avoid unstable oscillation of laser diodes due to backward beam, an optical isolator with high quality is indispensable in optical fiber communication systems [1]. As a Faraday rotator in the central part of isolator, the magneto-optical *Corresponding author. Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: +86-571-7951649; fax: +86-571-7952341. E-mail address: [email protected] (X.W. Zhang).
material with large Faraday rotation angle yF ; small optical absorption loss and low saturated magnetizing field is required, together with low temperature and wavelength coefficients for practical application. Yttrium iron garnet (YIG) crystal has been widely utilized for Faraday rotator in optical isolators [2]. Although it exhibits high temperature stability due to its low temperature coefficient, its small Faraday rotation angle (yF ) in the near infrared region (yF B1801/cm, at l ¼ 1:55 mm) and high saturated magnetizing field (B1.4 105 A/m) are unfavorable to diminish isolator’s dimension. Compared with YIG, Bi-substituted rare-earth iron garnets such as GdBiIG have received much
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 0 2 7 - 6
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
68
more attention [3–5], mainly because of their higher magneto-optical figures of merit and lower saturated magnetizing field. Unfortunately, GdBiIG has a strong temperature dependence of Faraday rotation, which degrades the isolation ratio in the practical temperature range. In the present report, both the rare-earth ions Yb3+ and Re3+ (Tb3+, Ho3+, etc.) with opposite sign of temperature coefficients for Faraday rotation are compounded in Bi-substituted yttrium iron garnet. The temperature stability of Faraday rotation and magneto-optical properties of the new single crystal with composition of ReYbBiIG (Re:Tb, Ho, etc.) are investigated in the near infrared region (1.0–1.7 mm). It is verified that this kind of new crystal gives large Faraday rotation and very low temperature coefficient, which meets the requirements of highly qualified isolator for Faraday rotation quite well. 2. Experimental For growth of TbYbBiIG and HoYbBiIG single crystals with high Bi contents, Bi2O3 was used as the main flux [6]. After fine mixing and grinding, a mixture of Yb2O3, B2O3, Fe2O3, Bi2O3 and Tb4O7 (for TbYbBiIG growth) or Ho2O3 (for HoYbBiIG growth) was placed in a platinum crucible that was placed in a single-crystal furnace where the crucible may be in accelerated rotations in the opposite direction [7]. After heating to 12901C and maintaining the temperature for several hours, the crucible was placed in accelerated rotation and slowly cooled down to 9801C at a cooling rate of 0.5–31C/h. The crucible was then cooled down
naturally to room temperature. The single crystals were separated from the impurities by using diluted nitric acid. The obtained crystal exhibit shiny and regular surfaces and metal gloss, and the maximum dimension was about 7 6 4 mm3. They were identified as garnet phase mono-crystals by X-ray powder diffraction and directional X-ray diffraction along the /1 1 0S direction. The lattice constant was measured as a ¼ 1:2337 nm for TbYbBiIG and a ¼ 1:2354 nm for HoYbBiIG. The electron probe microanalysis (EPMA) gave the formula of Tb2.06Yb0.46Bi0.48Fe5O12 and Ho0.85Yb1.12Bi1.03Fe5O12, respectively. The magneto-optical Faraday rotation spectra of wavelength 1.0–1.7 mm were obtained on a CGX-1-type magneto-optical rotation spectrometer by using the modulated double-frequency method [8]. In preparing samples for magnetooptical measurements, we used X-ray diffraction to identify the crystal axis directions in bulk crystals of ReYbBiIG, then cut crystals along the (1 1 1) face and finely grind and polished into slice with 150 mm thickness. The measurement system is shown in Fig. 1. In our measurements, the polarizer and the polarization analyzer were placed with their direction of polarization at 901C to each other. The measurements were based on the system with photoelectric double-frequency signals. A halogen lamp was used for light source. 3. Results Fig. 2 shows the magneto-optical Faraday rotation spectra and optical absorption coefficient in
Fig. 1. Measuring system for the magneto-optical Faraday rotation spectra of wavelength 1.0–1.7 mm.
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
It shows that the figure of merit for TbYbBiIG is 111/dB at 1.31 mm wavelength and 181/dB at 1.55 mm wavelength, about two times larger than that of YIG. Measurement for HoYbBiIG also shows that its figure of merit is also much better than YIG. Therefore, it is concluded that the magneto-optical figure of merit of our new designed ReYbBiIG is superior to that of YIG. In order to measure the temperature stability of Faraday rotation yF for single crystal, a temperature changeable system is arranged at the measuring site. Fig. 4 shows the dependence of yF for
-1
(deg.dB )
16
TbYbBiIG
14 12 10
|θF|.L
-1
8 6
YIG
4 2 0.8
1.0
1.2
1.4
1.6
1.8
λ (µm) Fig. 3. Wavelength dependence of the figure of merit for TbYbBiIG and YIG.
8 -1
ð1Þ
18
2
L ¼ 10=d logðI0 =IÞðdBÞ:
20
-θF (× 10 deg.cm )
the near-infrared region of 1.0–1.7 mm for TbYbBiIG single crystal. The special Faraday rotation of the crystal under saturation magnetization was measured to be about –6501/cm at 1.31 mm wavelength and –5501/cm at 1.55 mm wavelength, which is about three times larger than that of YIG. The absorption coefficient of crystal is almost the same as that of YIG. In the same way, we obtained the special Faraday rotation of HoYbBiIG. It is about 3–4 times larger than that of YIG. Other optical absorption characters are almost the same as that of TbYbBiIG. Fig. 3 shows the wavelength dependence of the figure of merit for TbYbBiIG and YIG. L represents the optical absorption loss of a sample with thickness d:
69
7 6 5 4 3 2 0
16
100 80 60
8 40
6 4
20
2 0.8
1.0
1.2 1.4 λ ( µm)
1.6
60
80
T ( C) Fig. 4. Dependence of yF for TbYbBiIG on temperature at a wavelength of 1.55 mm.
α (cm -1)
- θF ( *102 deg.cm -1)
10
40 o
14 12
20
1.8
Fig. 2. Magneto-optical Faraday rotation spectra and optical absorption coefficient between 1.0–1.7 mm for TbYbBiIG single crystal.
TbYbBiIG on temperature (T) at l ¼ 1:55 mm when the sample’s environmental temperature increases from 51C to 801C. From Fig. 4, the Faraday rotation temperature coefficient of TbYbBiIG is obtained as dyF =dTE2:1 102 deg/mm/ K. According to the identification of temperature stability: S ¼ ð1=yF ÞðdyF Þ=dT;
ð2Þ
70
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
the temperature stability of TbYbBiIG is 3.6 104/K, even smaller than that of YIG (S ¼ 6:6 104 =K). In the same way, the temperature stability of HoYbBiIG crystal is obtained as 4.6 104/K, which is also smaller than that of YIG.
4. Discussion Relation of temperature and Faraday rotation is an intrinsic magneto-optical property of materials that largely influence the properties of magnetooptical apparatus, which is essentially originated from the temperature dependence of the saturation magnetization and some magneto-optical coefficients. For crystals with ferromagnetic garnet structure, their saturation magnetization is the vector sum of magnetization of three sub-lattices generated from magnetic ions at sites of oxygen ions composed tetrahedron, octahedron and dodecahedron. On the other hand, the contribution of each sub-lattice magnetization to magnetooptical Faraday rotation is given by different coefficients. Therefore, through changing each magnetization contribution of sub-lattice to the sum of magnetization, or through changing the type and quantity of substituted ion or rare-earth ion in sub-lattices, the temperature dependence of the Faraday rotation can be modulated. According to Booth’s results [9], yF BT relation of some important garnet materials can be classified into two groups (see Fig. 5). One group is like GdIG and YbIG, etc. Their temperature coefficients of yF are positive. The other is like TbIG, DyIG and YIG, etc. Their temperature coefficients of yF are negative. Therefore, it is speculated that through compounding the above two kinds of materials with opposite temperature coefficient signs, the materials with low temperature coefficient of yF could be obtained due to the temperature compensation effect. According to the above speculation, we have designed a new type of magneto-optical single crystal with composition of ReYbBiIG, in which Re represents one of the selectable doping rareearth ions with negative temperature coefficient such as Tb, Ho, etc. Since Re and Yb give opposite
Fig. 5. Dependence of yF on temperature for YIG, TbIG, DyIG, YbIG and GdIG at a wavelength of 1.55 mm (From Ref. [9]).
temperature dependence of Faraday rotation, the new single crystal is expected to exhibit low temperature coefficient. In addition, the smaller ionic radius of Yb and Re permits a larger Bi doping concentration, while the doping of Bi3+ on the dodecahedral site enhances the Faraday rotation greatly. In the early 1970s, Wittekoek [10] found that when the rare-earth ion in the rare-earth garnet was partially substituted by Bi ion, the Faraday rotation angle of crystals increased obviously and their magneto-optical properties were improved. Thereafter, Bi-substituted rare-earth iron garnets have received much attention. However, since the Bi2O3–Fe2O3–R2O3 ternary system itself may form many crystal phases that interfere strongly with each other, there is only a narrow phase area for the garnet phase in the ternary phase diagram. To solve this problem, we use Bi2O3/B2O3 system as the main flux for ReYbBiIG crystal growth. It is proven to reduce the absorption loss in the nearinfrared region and achieve a higher concentration of Bi3+. In addition, we employed the accelerated crucible rotation technique (ACRT) and controlled the cooling rate in the furnace to a very low level (o0.81C/h). Through the above technological improvement, we obtained perfect single crystal of ReYbBiIG with a size large enough for apparatus application. From our experimental results, magneto-optical figures of merit of ReYbBiIG bulk single crystals under saturation magnetization are several times
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
larger than that of YIG. Meanwhile, our specially designed ReYbBiIG crystals exhibit even higher temperature stability of Faraday rotation in a wide temperature range compared with that of YIG. Therefore, this new type of magneto-optical crystal is believed to be a good candidate for Faraday rotation isolator. As an example, we used HoYbBiIG as Faraday rotator to investigate the temperature dependence of the isolation for optical isolators. The optical arrangement was the same as for the compact isolator. In Fig. 6, temperature dependence of isolation using GdBiIG and HoYbBiIG crystals is compared. The isolation is estimated as follows:
71
In recent years, magneto-optical single-crystal films and their application in waveguide-type magneto-optical isolators have been extensively investigated [11–13]. The temperature dependence of the magneto-optical film on Faraday rotation is also sensitive to optical isolators. It is speculated that magneto-optical films of ReYbBiIG (Re: Ho, Tb, etc.) with very low temperature coefficient might be beneficial to waveguide type isolators. Further investigations are in progress in our group.
5. Conclusion
2
isolationðl; TÞ ¼ 10 logftan ½451 þ yF ðl; TÞ g; ð3Þ where yF is the Faraday rotation angle which is optimized at 451 when l ¼ 1:55 mm and T0 ¼ 251C. From Fig. 6, the temperature range in which the isolation is higher than 40dB is 10 K for the temperature stabilized HoYbBiIG isolator and 50 K for the conventional GdYbBiIG. This result confirms that HoYbBiIG is an ideal temperature stabilized magneto-optical material for optical isolators. In our another report [7], HoYbBiIG isolator also exhibits very small wavelength dependence of Faraday rotation compared with the conventional GdBiIG crystal, indicating a potential application for wavelength divided multiplexing (WDM) optical fiber communication systems. 65
Acknowledgements
60
GdBiIG
55
Isolation (dB)
Through compounding YIG and ReIG (Re:Tb, Ho, etc.), and by partial substitution of Bi3+ with rare-earth iron garnet, a new type of magnetooptical single bulk crystal with composition ReYbBiIG (Re:Tb, Ho, etc.) is obtained. Bi2O3/ B2O3 is used as a flux and ACRT is adopted as single-crystal growth method. Results from temperature changeable magneto-optical Faraday rotation spectra verify that ReYbBiIG (Re:Tb, Ho, etc.) exhibit large Faraday rotation angle, low absorption loss and small Faraday rotation temperature coefficient compared with YIG, which is believed to be more suitable to meet the requirements of high-quality optical isolator with temperature-stable isolation.
This work was financially supported by the Foundation of National Natural Science (No. 69890230) and (No. 60077006).
HoYbBiIG
50 45 40 35
References
30 25 20 15 -20
0
20
40
60
80
o
T ( C) Fig. 6. Temperature dependence of the isolation for optical isolators using HoYbBiIG and GdBiIG as the Faraday rotator.
[1] G. Fischer, J. Opt. Commun. 8 (1987) 18. [2] K. Shiraishi, Appl. Opt. 24 (1985) 951. [3] K. Machida, Y. Asahara, H. Ishikawa, K. Nakajima, Y. Fujii, J. Appl. Phys. 61 (1987) 3256. [4] K. Matusuda, H. Minemoto, O. Kamada, IEEE Trans. Magn. MAG-23 (1987) 3479. [5] T. Takeuchi, S. Ito, I. Mikami, S. Taniguchi, J. Appl. Phys. 44 (1973) 4789.
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X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
[6] S. Durcok, E. Pollert, Z. Simsa, J.T. Hsu, Y.J. Tsou, Mater. Chem. Phys. 45 (1996) 124. [7] M. Huang, S. Zhang, H. Wu, Phys. Stat. Sol. (A) 163 (1997) 255. [8] S. Zhang, Z. Zhang, M. Huang, Y. Cao, W. Cai, Z. Xu, China Phys. 12 (1992) 740. [9] R.C. Booth, E.A.D. White, J. Phys. (D) 17 (1984) 579.
[10] S. Wittekoek, T.J.A. poma, J.M. Robertson, Phys. Rev. (B) 12 (1975) 2777. [11] K. Taki, Y. Miyaraki, Trans. IEEE Japan E71 (1988) 161. [12] K. Matsumoto, S. Sasaki, Y. Asahara, J. Magn. Magn. Mater. 451 (1992) 104. [13] M. Inoue, T. Fujii, J. Appl. Phys. 81 (1997) 5659.
Growth and characterization of magneto-optical single-crystal ReYbBiIG with temperature-stabilized Faraday rotation X.W. Zhang*, S.Y. Zhang, G.R. Han State Key Laboratory of Silicon Materials Science, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 13 September 2001; received in revised form 10 December 2001
Abstract The Bi and Re (Re:Tb, Ho, etc.) partially substituted yttrium iron garnet bulk single crystals of (ReYbBi)3Fe5O12 were grown by using Bi2O3/B2O3 as flux and accelerated crucible rotation technique for single-crystal growth. Results on the basis of temperature-changeable magneto-optical Faraday rotation spectra showed that their magneto-optical figures of merit were much higher than that of YIG in the wavelength range of 1.0–1.7 mm, while their Faraday rotation temperature coefficients were even smaller than that of YIG in the temperature range 10–801C. According to temperature compensation effect, magneto-optical materials with temperature-stabilized Faraday rotation were achieved through compounding two kinds of rare-earth ions with opposite temperature coefficients, indicating an advantage for Faraday rotator usage in optical isolator with temperature-stable isolation. r 2002 Elsevier Science B.V. All rights reserved. PACS: 76.30.Kg; 85.70.Sq; 78.20.Ls Keywords: Faraday rotation; Magneto-optical crystals; Temperature compensation effect; Rare-earth iron garnet; Optical isolator
1. Introduction Extensive investigations on magneto-optical materials and devices are essential for optical communication and information processing systems. To avoid unstable oscillation of laser diodes due to backward beam, an optical isolator with high quality is indispensable in optical fiber communication systems [1]. As a Faraday rotator in the central part of isolator, the magneto-optical *Corresponding author. Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: +86-571-7951649; fax: +86-571-7952341. E-mail address: [email protected] (X.W. Zhang).
material with large Faraday rotation angle yF ; small optical absorption loss and low saturated magnetizing field is required, together with low temperature and wavelength coefficients for practical application. Yttrium iron garnet (YIG) crystal has been widely utilized for Faraday rotator in optical isolators [2]. Although it exhibits high temperature stability due to its low temperature coefficient, its small Faraday rotation angle (yF ) in the near infrared region (yF B1801/cm, at l ¼ 1:55 mm) and high saturated magnetizing field (B1.4 105 A/m) are unfavorable to diminish isolator’s dimension. Compared with YIG, Bi-substituted rare-earth iron garnets such as GdBiIG have received much
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 0 2 7 - 6
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
68
more attention [3–5], mainly because of their higher magneto-optical figures of merit and lower saturated magnetizing field. Unfortunately, GdBiIG has a strong temperature dependence of Faraday rotation, which degrades the isolation ratio in the practical temperature range. In the present report, both the rare-earth ions Yb3+ and Re3+ (Tb3+, Ho3+, etc.) with opposite sign of temperature coefficients for Faraday rotation are compounded in Bi-substituted yttrium iron garnet. The temperature stability of Faraday rotation and magneto-optical properties of the new single crystal with composition of ReYbBiIG (Re:Tb, Ho, etc.) are investigated in the near infrared region (1.0–1.7 mm). It is verified that this kind of new crystal gives large Faraday rotation and very low temperature coefficient, which meets the requirements of highly qualified isolator for Faraday rotation quite well. 2. Experimental For growth of TbYbBiIG and HoYbBiIG single crystals with high Bi contents, Bi2O3 was used as the main flux [6]. After fine mixing and grinding, a mixture of Yb2O3, B2O3, Fe2O3, Bi2O3 and Tb4O7 (for TbYbBiIG growth) or Ho2O3 (for HoYbBiIG growth) was placed in a platinum crucible that was placed in a single-crystal furnace where the crucible may be in accelerated rotations in the opposite direction [7]. After heating to 12901C and maintaining the temperature for several hours, the crucible was placed in accelerated rotation and slowly cooled down to 9801C at a cooling rate of 0.5–31C/h. The crucible was then cooled down
naturally to room temperature. The single crystals were separated from the impurities by using diluted nitric acid. The obtained crystal exhibit shiny and regular surfaces and metal gloss, and the maximum dimension was about 7 6 4 mm3. They were identified as garnet phase mono-crystals by X-ray powder diffraction and directional X-ray diffraction along the /1 1 0S direction. The lattice constant was measured as a ¼ 1:2337 nm for TbYbBiIG and a ¼ 1:2354 nm for HoYbBiIG. The electron probe microanalysis (EPMA) gave the formula of Tb2.06Yb0.46Bi0.48Fe5O12 and Ho0.85Yb1.12Bi1.03Fe5O12, respectively. The magneto-optical Faraday rotation spectra of wavelength 1.0–1.7 mm were obtained on a CGX-1-type magneto-optical rotation spectrometer by using the modulated double-frequency method [8]. In preparing samples for magnetooptical measurements, we used X-ray diffraction to identify the crystal axis directions in bulk crystals of ReYbBiIG, then cut crystals along the (1 1 1) face and finely grind and polished into slice with 150 mm thickness. The measurement system is shown in Fig. 1. In our measurements, the polarizer and the polarization analyzer were placed with their direction of polarization at 901C to each other. The measurements were based on the system with photoelectric double-frequency signals. A halogen lamp was used for light source. 3. Results Fig. 2 shows the magneto-optical Faraday rotation spectra and optical absorption coefficient in
Fig. 1. Measuring system for the magneto-optical Faraday rotation spectra of wavelength 1.0–1.7 mm.
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
It shows that the figure of merit for TbYbBiIG is 111/dB at 1.31 mm wavelength and 181/dB at 1.55 mm wavelength, about two times larger than that of YIG. Measurement for HoYbBiIG also shows that its figure of merit is also much better than YIG. Therefore, it is concluded that the magneto-optical figure of merit of our new designed ReYbBiIG is superior to that of YIG. In order to measure the temperature stability of Faraday rotation yF for single crystal, a temperature changeable system is arranged at the measuring site. Fig. 4 shows the dependence of yF for
-1
(deg.dB )
16
TbYbBiIG
14 12 10
|θF|.L
-1
8 6
YIG
4 2 0.8
1.0
1.2
1.4
1.6
1.8
λ (µm) Fig. 3. Wavelength dependence of the figure of merit for TbYbBiIG and YIG.
8 -1
ð1Þ
18
2
L ¼ 10=d logðI0 =IÞðdBÞ:
20
-θF (× 10 deg.cm )
the near-infrared region of 1.0–1.7 mm for TbYbBiIG single crystal. The special Faraday rotation of the crystal under saturation magnetization was measured to be about –6501/cm at 1.31 mm wavelength and –5501/cm at 1.55 mm wavelength, which is about three times larger than that of YIG. The absorption coefficient of crystal is almost the same as that of YIG. In the same way, we obtained the special Faraday rotation of HoYbBiIG. It is about 3–4 times larger than that of YIG. Other optical absorption characters are almost the same as that of TbYbBiIG. Fig. 3 shows the wavelength dependence of the figure of merit for TbYbBiIG and YIG. L represents the optical absorption loss of a sample with thickness d:
69
7 6 5 4 3 2 0
16
100 80 60
8 40
6 4
20
2 0.8
1.0
1.2 1.4 λ ( µm)
1.6
60
80
T ( C) Fig. 4. Dependence of yF for TbYbBiIG on temperature at a wavelength of 1.55 mm.
α (cm -1)
- θF ( *102 deg.cm -1)
10
40 o
14 12
20
1.8
Fig. 2. Magneto-optical Faraday rotation spectra and optical absorption coefficient between 1.0–1.7 mm for TbYbBiIG single crystal.
TbYbBiIG on temperature (T) at l ¼ 1:55 mm when the sample’s environmental temperature increases from 51C to 801C. From Fig. 4, the Faraday rotation temperature coefficient of TbYbBiIG is obtained as dyF =dTE2:1 102 deg/mm/ K. According to the identification of temperature stability: S ¼ ð1=yF ÞðdyF Þ=dT;
ð2Þ
70
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
the temperature stability of TbYbBiIG is 3.6 104/K, even smaller than that of YIG (S ¼ 6:6 104 =K). In the same way, the temperature stability of HoYbBiIG crystal is obtained as 4.6 104/K, which is also smaller than that of YIG.
4. Discussion Relation of temperature and Faraday rotation is an intrinsic magneto-optical property of materials that largely influence the properties of magnetooptical apparatus, which is essentially originated from the temperature dependence of the saturation magnetization and some magneto-optical coefficients. For crystals with ferromagnetic garnet structure, their saturation magnetization is the vector sum of magnetization of three sub-lattices generated from magnetic ions at sites of oxygen ions composed tetrahedron, octahedron and dodecahedron. On the other hand, the contribution of each sub-lattice magnetization to magnetooptical Faraday rotation is given by different coefficients. Therefore, through changing each magnetization contribution of sub-lattice to the sum of magnetization, or through changing the type and quantity of substituted ion or rare-earth ion in sub-lattices, the temperature dependence of the Faraday rotation can be modulated. According to Booth’s results [9], yF BT relation of some important garnet materials can be classified into two groups (see Fig. 5). One group is like GdIG and YbIG, etc. Their temperature coefficients of yF are positive. The other is like TbIG, DyIG and YIG, etc. Their temperature coefficients of yF are negative. Therefore, it is speculated that through compounding the above two kinds of materials with opposite temperature coefficient signs, the materials with low temperature coefficient of yF could be obtained due to the temperature compensation effect. According to the above speculation, we have designed a new type of magneto-optical single crystal with composition of ReYbBiIG, in which Re represents one of the selectable doping rareearth ions with negative temperature coefficient such as Tb, Ho, etc. Since Re and Yb give opposite
Fig. 5. Dependence of yF on temperature for YIG, TbIG, DyIG, YbIG and GdIG at a wavelength of 1.55 mm (From Ref. [9]).
temperature dependence of Faraday rotation, the new single crystal is expected to exhibit low temperature coefficient. In addition, the smaller ionic radius of Yb and Re permits a larger Bi doping concentration, while the doping of Bi3+ on the dodecahedral site enhances the Faraday rotation greatly. In the early 1970s, Wittekoek [10] found that when the rare-earth ion in the rare-earth garnet was partially substituted by Bi ion, the Faraday rotation angle of crystals increased obviously and their magneto-optical properties were improved. Thereafter, Bi-substituted rare-earth iron garnets have received much attention. However, since the Bi2O3–Fe2O3–R2O3 ternary system itself may form many crystal phases that interfere strongly with each other, there is only a narrow phase area for the garnet phase in the ternary phase diagram. To solve this problem, we use Bi2O3/B2O3 system as the main flux for ReYbBiIG crystal growth. It is proven to reduce the absorption loss in the nearinfrared region and achieve a higher concentration of Bi3+. In addition, we employed the accelerated crucible rotation technique (ACRT) and controlled the cooling rate in the furnace to a very low level (o0.81C/h). Through the above technological improvement, we obtained perfect single crystal of ReYbBiIG with a size large enough for apparatus application. From our experimental results, magneto-optical figures of merit of ReYbBiIG bulk single crystals under saturation magnetization are several times
X.W. Zhang et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 67–72
larger than that of YIG. Meanwhile, our specially designed ReYbBiIG crystals exhibit even higher temperature stability of Faraday rotation in a wide temperature range compared with that of YIG. Therefore, this new type of magneto-optical crystal is believed to be a good candidate for Faraday rotation isolator. As an example, we used HoYbBiIG as Faraday rotator to investigate the temperature dependence of the isolation for optical isolators. The optical arrangement was the same as for the compact isolator. In Fig. 6, temperature dependence of isolation using GdBiIG and HoYbBiIG crystals is compared. The isolation is estimated as follows:
71
In recent years, magneto-optical single-crystal films and their application in waveguide-type magneto-optical isolators have been extensively investigated [11–13]. The temperature dependence of the magneto-optical film on Faraday rotation is also sensitive to optical isolators. It is speculated that magneto-optical films of ReYbBiIG (Re: Ho, Tb, etc.) with very low temperature coefficient might be beneficial to waveguide type isolators. Further investigations are in progress in our group.
5. Conclusion
2
isolationðl; TÞ ¼ 10 logftan ½451 þ yF ðl; TÞ g; ð3Þ where yF is the Faraday rotation angle which is optimized at 451 when l ¼ 1:55 mm and T0 ¼ 251C. From Fig. 6, the temperature range in which the isolation is higher than 40dB is 10 K for the temperature stabilized HoYbBiIG isolator and 50 K for the conventional GdYbBiIG. This result confirms that HoYbBiIG is an ideal temperature stabilized magneto-optical material for optical isolators. In our another report [7], HoYbBiIG isolator also exhibits very small wavelength dependence of Faraday rotation compared with the conventional GdBiIG crystal, indicating a potential application for wavelength divided multiplexing (WDM) optical fiber communication systems. 65
Acknowledgements
60
GdBiIG
55
Isolation (dB)
Through compounding YIG and ReIG (Re:Tb, Ho, etc.), and by partial substitution of Bi3+ with rare-earth iron garnet, a new type of magnetooptical single bulk crystal with composition ReYbBiIG (Re:Tb, Ho, etc.) is obtained. Bi2O3/ B2O3 is used as a flux and ACRT is adopted as single-crystal growth method. Results from temperature changeable magneto-optical Faraday rotation spectra verify that ReYbBiIG (Re:Tb, Ho, etc.) exhibit large Faraday rotation angle, low absorption loss and small Faraday rotation temperature coefficient compared with YIG, which is believed to be more suitable to meet the requirements of high-quality optical isolator with temperature-stable isolation.
This work was financially supported by the Foundation of National Natural Science (No. 69890230) and (No. 60077006).
HoYbBiIG
50 45 40 35
References
30 25 20 15 -20
0
20
40
60
80
o
T ( C) Fig. 6. Temperature dependence of the isolation for optical isolators using HoYbBiIG and GdBiIG as the Faraday rotator.
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