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Growth and Faraday rotation characteristics of Tb3−xNdxGa5O12 single crystal
Optical Materials 47 (2015) 157–160
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth and Faraday rotation characteristics of Tb3xNdxGa5O12 single crystal XiangYong Wang, Lei Yang, Zhe Chen, Jun Wang, Jiaqi Hong, Yaqi Wang, Chunjun Shi, Peixiong Zhang, Lianhan Zhang, Yin Hang ⇑ Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, No. 390, Qinghe Road, Jiading District, Shanghai, China
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 21 April 2015 Accepted 26 April 2015 Available online 9 June 2015 Keywords: Czochralski method Transmission spectrum Faraday effect Magneto-optical property
a b s t r a c t Tb3xNdxGa5O12 single crystal with dimension of U22 mm 28 mm and a good optical quality was grown by the Czochralski method. X-ray powder diffraction was carried out and lattice parameters were calculated, which showed that the single crystal belongs to cubic crystal system. The transmission spectrum in the wavelength range of 450–1500 nm, which indicated the crystal has low absorption coefficient at 900–1450 nm. The Verdet constants of Tb3xNdxGa5O12 at 532, 633 and 1064 nm wavelengths calculated by the extinction method are 225, 145 and 41 radm1 T1, respectively, which are larger than that of commercial TGG (Tb3Ga5O12) reported. The magneto-optical figures of merit of the crystal calculated is 3162°/dB at 1064 nm, and the extinction ratio is larger than that of commercial TGG. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Magneto-optical materials based on non-reciprocal Faraday Effect are fundamental components in optical communication, optical parameter amplifier, optical modulators, and high pulse energy and high average power lasers, etc. [1]. The principle behind is the single rotation sense of the polarization plane of light, independently of the propagation direction of light [2]. In optical communication system, Yttrium iron garnet (Y3Fe5O12:YIG) and Bi, Ce doped YIG crystals are the commonly used magnetooptical materials of optical isolators because of its large Faraday rotation angle, high transparency, and small absorption in the wavelengths of 1.2–5.0 lm. However, as the rapid development of fiber laser systems and the optical communication technology, the demand of high quality Magneto-optical material in the visible and near-infrared region has greatly increased. The conventional YIG crystals are not practical, because of its very poor transmittance in the visible and near-infrared spectral regions [3]. Therefore soon after the introduction of YIG and doped YIG as optical isolators, an intensive search for new magneto-optic crystals with similarly large Faraday rotation angle but a low optical absorption loss in the visible and near-infrared spectral regions began and led to numerous new materials based on rare-earth garnet single crystals [4]. According to research, rare earth with an unfilled 4f shell and unpaired 4f electrons exhibit a paramagnetism due to the transition ⇑ Corresponding author. E-mail address: [email protected] (Y. Hang). http://dx.doi.org/10.1016/j.optmat.2015.04.044 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
between 4fn ? 4fn15d1 [5], because their outer 4f electrons can be easily excited to the higher energy of 5d orbit under the magnetic field, leading to a strong Faraday effect [6,7]. Tb3+ yields to the largest Faraday rotation, so that electric dipole contribution Tb3+ dominates even over that of Fe3+ in magnetic terbium–iron [8], and hence shows the best magneto-optical properties [9,10]. Complex theoretical approaches have been done through the perturbation Hamiltonian, which takes into account the spin–orbit coupling, the crystal field, and the super-exchange interaction [11]. Terbium–aluminum garnet, Tb3Al5O12 (TAG), has been reported to show one of the promising paramagnetic magneto-optical materials in the visible-near IR spectral region, although it has a high light transmission coefficient and a high Verdet constant [12], its incongruent melting nature and unstable TAG phase in the process of growth hindered its industrial application. Recently, applied research on TGG (terbium–gallium garnet, Tb3Ga5O12) crystal properties has gained increased interest, because of its excellent optical quality and high thermal stability [13]. TGG has a high Verdet constant and low-absorption coefficient in visible and near-infrared spectral region [14]. Tb3+ has been pointed to show the best magneto-optical properties [10,15], and magneto-optic crystals contain Tb3+ ion were widely studied in recently years [16–18]. Studies have shown that Nd3+ substitution in garnet may induce a marked enhancement of the Faraday rotation although the magnetic moment of the Nd3+ ion in garnet is very small [19,20]. So Tb3xNdxGa5O12 can be a potential paramagnetism material for magneto-optical devices, such as optical isolators, optical modulators, etc. In this work, we report the growth, spectral and FR characteristics of Tb3xNdxGa5O12 for the first time.
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X. Wang et al. / Optical Materials 47 (2015) 157–160
2. Experimental 2.1. Synthesis and crystal growth The polycrystalline material used for single crystal growth was prepared by the method of solid-state reaction. The doping concentration of Nd3+ in the raw material is 5 mol%. Stoichiometric amounts of commercially available Tb4O7(5N), Ga2O3(5N) and Nd2O3(5N) were weighed accurately and 1% excess Ga2O3 was added to compensate for its volatilization loss during the process of crystal growth, and subsequently mixed homogeneously. The uniformly mixed raw materials were pressed into tablets under 10 MPa pressure and then sintered at 1300 °C for 15 h in air. Finally, we obtained white Tb3xNdxGa5O12 polycrystalline material. The crystal was grown by the Czochralski method. An iridium crucible of 60 mm in diameter was utilized to grow the single crystal in Ar atmosphere with radio frequency (RF) induction heating. It was grown in the h1 1 1i direction at a pulling rate of 2.0–3.0 mm/h and a rotating rate of 10–15 r/min. When the growth process was over, the crystal was drawn out of the molten and cooled down to room temperature at a rate of 30–40 °C/h in order to prevent the crystal from cracking. And then we annealed the as-grown crystal once again in air at 1400 °C for 48 h to decrease the color center absorption. Finally, Tb3xNdxGa5O12 crystal with dimension U20 28 mm2 was obtained, as shown in Fig. 1(a), which is amaranth and crack free. 2.2. X-ray powder diffraction In order to confirm the structure of the as-grown crystal, the X-ray powder diffraction patterns of the single crystal were analyzed by a computer automated diffractometer (Rigaku D/max-3c) equipped with Cu Ka radiation (k = 1.54056 Å) at room
temperature. The measure diffraction data of Tb3xNdxGa5O12 were refined through an internal standard method with standard Si powder and were indexed by computer program. 2.3. Ions concentration measurement The concentrations of Nd, Tb and Ga in the as-grown crystal were measured by Inductively Coupled Plasmas (ICP) and Atomic Emission Spectrometer (ICP-AES, type: PerkinElmer, Optima 7000DV) using the solution samples prepared previously, and the segregation coefficient of Nd3+ was calculated. 2.4. Transmission spectrum and absorption coefficient A sample of Tb3xNdxGa5O12 with dimensions of 7 7 6 mm3 was cut from the as-grown crystal, and then ground and polished carefully to about 5 mm thickness for spectral measurement, as shown in Fig. 1(b). From the SEM image of the surface of the sample, as shown in Fig. 1(c), we found no micro-crack. The transmission spectrum was measured using a Perkin–Elmer Lambda 900 UV–vis–NIR spectrophotometer at wavelengths of 350–1500 nm at room temperature. And the absorption coefficient at 633 and 1064 nm was calculated. 2.5. Magneto-optical Faraday rotation measurement The magneto-optical Faraday rotation of the Tb3xNdxGa5O12 crystal was measured at room temperature by the extinction method using the magnet-optical test system [21]. In this measurement, lasers of 532, 633, 1064 nm wavelengths were used as the sources of the probe beam. The magnetic field could be adjusted from 0 to 1.2 T continuously. To investigate magneto-optical properties, magneto-optical figure of merit at 1064 nm was analyzed.
Fig. 1. Crystal of Tb3xNdxGa5O12 (a), the sample of the crystal (b), and the SEM morphology of the polished surface of sample (c).
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X. Wang et al. / Optical Materials 47 (2015) 157–160 Table 1 Element analysis results from ICP-AES.
3. Results and discussion 3.1. Lattice parameters The powder XRD pattern for Tb3xNdxGa5O12 is shown in Fig. 2. Using Jade program, the least-squares fit hexagonal lattice parameters of the as-grown crystal are: a = b = c = 12.347 Å, V = 1882.28 Å3. And the data is in accord with that of TGG obtained from the JCPDS standard card (JCPDS No. 88-0575), which indicates that the neodymium substitution in TGG does not induce an obvious change of the lattice parameters of TGG. Combining the XRD pattern with Jade 5.0, the relevant peaks are in good accordance with the ones for TGG crystal indicating that as-grown crystal belongs to cubic system garnet.
Sample
Tb3+ (wt%)
Nd3+ (wt%)
Ga3+ (wt%)
1# 2# 3#
48.07 47.79 50.44
1.10 0.96 1.00
36.94 36.94 38.81
Relative error of the data 3–5%.
3.2. Ions concentration and segregation coefficient of Nd3+ The element analysis results of the crystal are list in Table 1. And the three samples were cut from the top of the as-grown crystal. The average contents of Tb3+, Nd3+ and Ga3+ are 48.76%, 1.02% and 37.48%, respectively. The formula of the segregation coefficient is as follows: ke = Cs/Cl, where Cs is the doped-ion concentration in the crystal and Cl is the doped-ion concentration in the melt, and the segregation coefficient of Nd3+ calculated is 0.45. 3.3. Transmission spectrum and absorption coefficient Fig. 3. Transmission spectrum of Tb3xNdxGa5O12 crystal.
It is important for a good magneto-optical material to have a high transmittance in some specific range such as 532 nm, 633 nm, 1064 nm. The transmission spectrum of Tb3xNdxGa5O12 is displayed in Fig. 3. Because of the strong absorption of Nd3+ in the visible spectral region, the transmittance of the as-grown crystal 633 nm is about 5% lower than that at 1064 nm. However, this measured transmission spectrum includes the reflection loss. Excluding the impact of reflection loss, the absorption coefficient a is calculated by the following formula:
T¼
ð1 RÞ2 expðadÞ 1 R2 expð2adÞ
ð1Þ
where T is the transmission coefficient, d is the specimen thickness and R is the dispersion of the reflection coefficient, which can be calculated from the reflective index. Fig. 4 shows the absorption
coefficient spectrum of Tb3xNdxGa5O12. The absorption coefficients calculated at 633 and 1064 nm are 0.1 and 0.01 cm1, respectively. 3.4. The Faraday rotation and magneto-optical figure of merit As for paramagnetic materials under magnetic saturation, the Faraday rotation h is lineal ratio to magnetic induction intensity B when the length of sample L is fixed. Faraday rotation in a paramagnetic material is given by the Eq. (2).
h¼V HL
ð2Þ
where h is the rotation angle, L is the length of the light path in the medium, H is the magnetic field applied along the light beam and V is the Verdet constant. The relations between the Faraday rotation
Fig. 2. Powder XRD pattern of Tb3xNdxGa5O12 crystal.
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X. Wang et al. / Optical Materials 47 (2015) 157–160 Table 2 Verdet constants of Tb3xNdxGa5O12 and TGG crystals. Crystal
Tb3xNdxGa5O12
Wavelength (nm) Verdet constants (rad/mT)
532 225
L¼
10 I0 lg ¼ 4:343a d I
633 145
TGG 1064 41
532 190
633 134
1064 40
ð4Þ
According to the previous data, the intrinsic absorption coefficients of Tb3xNdxGa5O12 crystal are about 0.1 and 0.01 cm1 at 633 and 1064 nm. And the magneto-optical figures of merit of the crystal at 633 and 1064 nm are 1065 and 3162°/dB at 1.2 T, respectively. 4. Conclusion Fig. 4. The absorption coefficient spectrum of Tb3xNdxGa5O12 crystal.
of Tb3xNdxGa5O12 crystal and magnetic field are linear in the range of 0–1.2 T at different wavelength, as shown in Fig. 5. The straight lines were fitted using a computer program and the Verdet constants can be calculated by the slope of straight lines. They are listed in Table 2 along with the Verdet constants of TGG from Refs. [22,23]. In the Tb3xNdxGa5O12 lattice, Tb3+ and Nd3+ are not adjacent, so super-exchange interaction exists between Tb3+ and Nd3+. And the results indicate that the super-exchange interaction between Nd3+ and Tb3+ may contribute to the enhancement of the Verdet constant for Tb3xNdxGa5O12.The extinction ratio of the as-grown crystal is up to 43.5 dB tested by a homemade instrument at 1064 nm. To our knowledge, the value is over the highest value of commercial TGG (40 dB). Magneto-optical figure of merit, defined by the ratio of Faraday rotation angle and optical absorption loss, can generally represent the magneto-optical properties of materials and is defined by Eq. (3) [24]:
F¼
jhF j L
ð3Þ
where hF (deg/cm) and L(dB) correspond to the ratio of FR and optical absorption loss, respectively. L is defined by Eq. (4), where I0 and I are the incident and transmission intensity, respectively, and a is the optical absorption coefficient.
Fig. 5. Faraday rotation of Tb3xNdxGa5O12 in different wavelengths at room temperature.
The Tb3xNdxGa5O12 crystal has been grown through the Czochralski method. It belongs to cubic system, and the segregation coefficient of Nd3+ is 0.45. The as-grown crystal has a low absorption at 900–1450 nm region and has larger Verdet constant than that of commercial TGG crystal. The magneto-optical figure of merit at 1064 nm is 3162°/dB at 1.2 T, close to that of commercial TGG crystal. The extinction ratio is up to 43.5 dB and is over the highest value of commercial TGG. So Tb3xNdxGa5O12 crystal could be a promising candidate material for magneto-optical devices in the wavelength range of 900–1450 nm. We are working on the development of the quality of Tb3xNdxGa5O12 and what the optimized concentration is. Acknowledgment This work was supported by Nature Science Foundation of China (No. 51472257). References [1] H. Lin, S.M. Zhou, H. Teng, J. Opt. Mater. 33 (2011) 1833–1836. [2] V. Vasyliev, E.G. Villora, M. Nakamura, Y. Sugahara, K.i. Shimamura, J. Opt. Express 20 (2012) 14460–14470. [3] W.J. Zhang, F.Y. Guo, J.Z. Chen, J. Cryst. Growth 306 (2007) 195–199. [4] X. Zhang, W.H. Zhang, Q.P. Wan, F.Y. Guo, et al., J. Opt. Mater. 37 (2014) 188– 192. [5] N. Sawanobori, N. Mori, et al., J. New Glass 18 (2003) 5–9. [6] M.J. Weber, R. Morgret, S.Y. Leung, J.A. Griffin, D. Gabbe, A. Linz, J. Appl. Phys. 49 (1978) 3464. [7] K. Sato, K. Yamaguchi, et al., Phys. Rev. B 63 (2001) 104416. [8] W.A. Crossley, R.W. Cooper, Phys. Rev. 181 (1964) 896. [9] S.B. Berger, C.B. Rubinstein, C.R. Kurkjian, A.W. Treptow, Phys. Rev. 133 (1964) A723. [10] D.R. Macfarlane, C.R. Bradbury, P.J. Newman, J. Javorniczky, J. Non-Cryst. Solids 213 (1997) 199–204. [11] J.H. Yang, Y. Xu, G.Y. Zhang, J. Phys. Phys. 75 (1994) 6798. [12] M. Geho, T. Skijima, T. Fujii, J. Cryst. Growth 275 (2005) e663. [13] Hidetsugu Yoshida, Koji Tsubakimoto, Yasushi Fujimito, et al., J. Opt. Express 19 (2011) 15181–15187. [14] V.I. Chani, A. Yoshikawa, H. Machida, T. Satoh, T. Fukuda, J. Cryst. Growth 210 (2000) 633. [15] S.B. Berger, C.B. Rubinstein, C.R. Kurkjian, A.W. Treptow, Phys. Rev. 133 (3A) (1964) A723–A727. [16] F.Y. Guo, R.R. Zhang, Z.H. Cui, C.C. Liu, J.Z. Chen, J. Opt. Mater. 35 (2012) 227– 230. [17] F. Guo, J. Ru, H. Li, N. Zhuang, B. Zhao, J. Chen, J. Appl. Phys. B 94 (2009) 437– 441. [18] J.Y. Lu, F.Y. Guo, J.Z. Chen, J. Cryst. Growth 314 (2011) 157–162. [19] F. Zhang, Y. Xu, J.H. Yang, M. Guillot, J. Condens. Matter 12 (2000) 7287–7294. [20] F. Zhang, Y. Xu, M. Guillot, J.H. Yang, J. Appl. Phys. 103 (2008) 07D307. [21] J. Liu, F. Guo, B. Zhao, N. Zhuang, Y. Chen, Z. Gao, J. Chen, J. Cryst. Growth 310 (2008) 2613–2616. [22] D.J. Dentz, R.C. Puttbach, R.R. Belt, AIP Conf. Proc. 18 (1974) 954. [23] M.Y.A. Raja, D. Allen, W. Sisk, Appl. Phys. Lett. 67 (1995) 2123. [24] M. Huang, Z.-C. Xu, Thin Solid Films 450 (2004) 324.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth and Faraday rotation characteristics of Tb3xNdxGa5O12 single crystal XiangYong Wang, Lei Yang, Zhe Chen, Jun Wang, Jiaqi Hong, Yaqi Wang, Chunjun Shi, Peixiong Zhang, Lianhan Zhang, Yin Hang ⇑ Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, No. 390, Qinghe Road, Jiading District, Shanghai, China
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 21 April 2015 Accepted 26 April 2015 Available online 9 June 2015 Keywords: Czochralski method Transmission spectrum Faraday effect Magneto-optical property
a b s t r a c t Tb3xNdxGa5O12 single crystal with dimension of U22 mm 28 mm and a good optical quality was grown by the Czochralski method. X-ray powder diffraction was carried out and lattice parameters were calculated, which showed that the single crystal belongs to cubic crystal system. The transmission spectrum in the wavelength range of 450–1500 nm, which indicated the crystal has low absorption coefficient at 900–1450 nm. The Verdet constants of Tb3xNdxGa5O12 at 532, 633 and 1064 nm wavelengths calculated by the extinction method are 225, 145 and 41 radm1 T1, respectively, which are larger than that of commercial TGG (Tb3Ga5O12) reported. The magneto-optical figures of merit of the crystal calculated is 3162°/dB at 1064 nm, and the extinction ratio is larger than that of commercial TGG. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Magneto-optical materials based on non-reciprocal Faraday Effect are fundamental components in optical communication, optical parameter amplifier, optical modulators, and high pulse energy and high average power lasers, etc. [1]. The principle behind is the single rotation sense of the polarization plane of light, independently of the propagation direction of light [2]. In optical communication system, Yttrium iron garnet (Y3Fe5O12:YIG) and Bi, Ce doped YIG crystals are the commonly used magnetooptical materials of optical isolators because of its large Faraday rotation angle, high transparency, and small absorption in the wavelengths of 1.2–5.0 lm. However, as the rapid development of fiber laser systems and the optical communication technology, the demand of high quality Magneto-optical material in the visible and near-infrared region has greatly increased. The conventional YIG crystals are not practical, because of its very poor transmittance in the visible and near-infrared spectral regions [3]. Therefore soon after the introduction of YIG and doped YIG as optical isolators, an intensive search for new magneto-optic crystals with similarly large Faraday rotation angle but a low optical absorption loss in the visible and near-infrared spectral regions began and led to numerous new materials based on rare-earth garnet single crystals [4]. According to research, rare earth with an unfilled 4f shell and unpaired 4f electrons exhibit a paramagnetism due to the transition ⇑ Corresponding author. E-mail address: [email protected] (Y. Hang). http://dx.doi.org/10.1016/j.optmat.2015.04.044 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.
between 4fn ? 4fn15d1 [5], because their outer 4f electrons can be easily excited to the higher energy of 5d orbit under the magnetic field, leading to a strong Faraday effect [6,7]. Tb3+ yields to the largest Faraday rotation, so that electric dipole contribution Tb3+ dominates even over that of Fe3+ in magnetic terbium–iron [8], and hence shows the best magneto-optical properties [9,10]. Complex theoretical approaches have been done through the perturbation Hamiltonian, which takes into account the spin–orbit coupling, the crystal field, and the super-exchange interaction [11]. Terbium–aluminum garnet, Tb3Al5O12 (TAG), has been reported to show one of the promising paramagnetic magneto-optical materials in the visible-near IR spectral region, although it has a high light transmission coefficient and a high Verdet constant [12], its incongruent melting nature and unstable TAG phase in the process of growth hindered its industrial application. Recently, applied research on TGG (terbium–gallium garnet, Tb3Ga5O12) crystal properties has gained increased interest, because of its excellent optical quality and high thermal stability [13]. TGG has a high Verdet constant and low-absorption coefficient in visible and near-infrared spectral region [14]. Tb3+ has been pointed to show the best magneto-optical properties [10,15], and magneto-optic crystals contain Tb3+ ion were widely studied in recently years [16–18]. Studies have shown that Nd3+ substitution in garnet may induce a marked enhancement of the Faraday rotation although the magnetic moment of the Nd3+ ion in garnet is very small [19,20]. So Tb3xNdxGa5O12 can be a potential paramagnetism material for magneto-optical devices, such as optical isolators, optical modulators, etc. In this work, we report the growth, spectral and FR characteristics of Tb3xNdxGa5O12 for the first time.
158
X. Wang et al. / Optical Materials 47 (2015) 157–160
2. Experimental 2.1. Synthesis and crystal growth The polycrystalline material used for single crystal growth was prepared by the method of solid-state reaction. The doping concentration of Nd3+ in the raw material is 5 mol%. Stoichiometric amounts of commercially available Tb4O7(5N), Ga2O3(5N) and Nd2O3(5N) were weighed accurately and 1% excess Ga2O3 was added to compensate for its volatilization loss during the process of crystal growth, and subsequently mixed homogeneously. The uniformly mixed raw materials were pressed into tablets under 10 MPa pressure and then sintered at 1300 °C for 15 h in air. Finally, we obtained white Tb3xNdxGa5O12 polycrystalline material. The crystal was grown by the Czochralski method. An iridium crucible of 60 mm in diameter was utilized to grow the single crystal in Ar atmosphere with radio frequency (RF) induction heating. It was grown in the h1 1 1i direction at a pulling rate of 2.0–3.0 mm/h and a rotating rate of 10–15 r/min. When the growth process was over, the crystal was drawn out of the molten and cooled down to room temperature at a rate of 30–40 °C/h in order to prevent the crystal from cracking. And then we annealed the as-grown crystal once again in air at 1400 °C for 48 h to decrease the color center absorption. Finally, Tb3xNdxGa5O12 crystal with dimension U20 28 mm2 was obtained, as shown in Fig. 1(a), which is amaranth and crack free. 2.2. X-ray powder diffraction In order to confirm the structure of the as-grown crystal, the X-ray powder diffraction patterns of the single crystal were analyzed by a computer automated diffractometer (Rigaku D/max-3c) equipped with Cu Ka radiation (k = 1.54056 Å) at room
temperature. The measure diffraction data of Tb3xNdxGa5O12 were refined through an internal standard method with standard Si powder and were indexed by computer program. 2.3. Ions concentration measurement The concentrations of Nd, Tb and Ga in the as-grown crystal were measured by Inductively Coupled Plasmas (ICP) and Atomic Emission Spectrometer (ICP-AES, type: PerkinElmer, Optima 7000DV) using the solution samples prepared previously, and the segregation coefficient of Nd3+ was calculated. 2.4. Transmission spectrum and absorption coefficient A sample of Tb3xNdxGa5O12 with dimensions of 7 7 6 mm3 was cut from the as-grown crystal, and then ground and polished carefully to about 5 mm thickness for spectral measurement, as shown in Fig. 1(b). From the SEM image of the surface of the sample, as shown in Fig. 1(c), we found no micro-crack. The transmission spectrum was measured using a Perkin–Elmer Lambda 900 UV–vis–NIR spectrophotometer at wavelengths of 350–1500 nm at room temperature. And the absorption coefficient at 633 and 1064 nm was calculated. 2.5. Magneto-optical Faraday rotation measurement The magneto-optical Faraday rotation of the Tb3xNdxGa5O12 crystal was measured at room temperature by the extinction method using the magnet-optical test system [21]. In this measurement, lasers of 532, 633, 1064 nm wavelengths were used as the sources of the probe beam. The magnetic field could be adjusted from 0 to 1.2 T continuously. To investigate magneto-optical properties, magneto-optical figure of merit at 1064 nm was analyzed.
Fig. 1. Crystal of Tb3xNdxGa5O12 (a), the sample of the crystal (b), and the SEM morphology of the polished surface of sample (c).
159
X. Wang et al. / Optical Materials 47 (2015) 157–160 Table 1 Element analysis results from ICP-AES.
3. Results and discussion 3.1. Lattice parameters The powder XRD pattern for Tb3xNdxGa5O12 is shown in Fig. 2. Using Jade program, the least-squares fit hexagonal lattice parameters of the as-grown crystal are: a = b = c = 12.347 Å, V = 1882.28 Å3. And the data is in accord with that of TGG obtained from the JCPDS standard card (JCPDS No. 88-0575), which indicates that the neodymium substitution in TGG does not induce an obvious change of the lattice parameters of TGG. Combining the XRD pattern with Jade 5.0, the relevant peaks are in good accordance with the ones for TGG crystal indicating that as-grown crystal belongs to cubic system garnet.
Sample
Tb3+ (wt%)
Nd3+ (wt%)
Ga3+ (wt%)
1# 2# 3#
48.07 47.79 50.44
1.10 0.96 1.00
36.94 36.94 38.81
Relative error of the data 3–5%.
3.2. Ions concentration and segregation coefficient of Nd3+ The element analysis results of the crystal are list in Table 1. And the three samples were cut from the top of the as-grown crystal. The average contents of Tb3+, Nd3+ and Ga3+ are 48.76%, 1.02% and 37.48%, respectively. The formula of the segregation coefficient is as follows: ke = Cs/Cl, where Cs is the doped-ion concentration in the crystal and Cl is the doped-ion concentration in the melt, and the segregation coefficient of Nd3+ calculated is 0.45. 3.3. Transmission spectrum and absorption coefficient Fig. 3. Transmission spectrum of Tb3xNdxGa5O12 crystal.
It is important for a good magneto-optical material to have a high transmittance in some specific range such as 532 nm, 633 nm, 1064 nm. The transmission spectrum of Tb3xNdxGa5O12 is displayed in Fig. 3. Because of the strong absorption of Nd3+ in the visible spectral region, the transmittance of the as-grown crystal 633 nm is about 5% lower than that at 1064 nm. However, this measured transmission spectrum includes the reflection loss. Excluding the impact of reflection loss, the absorption coefficient a is calculated by the following formula:
T¼
ð1 RÞ2 expðadÞ 1 R2 expð2adÞ
ð1Þ
where T is the transmission coefficient, d is the specimen thickness and R is the dispersion of the reflection coefficient, which can be calculated from the reflective index. Fig. 4 shows the absorption
coefficient spectrum of Tb3xNdxGa5O12. The absorption coefficients calculated at 633 and 1064 nm are 0.1 and 0.01 cm1, respectively. 3.4. The Faraday rotation and magneto-optical figure of merit As for paramagnetic materials under magnetic saturation, the Faraday rotation h is lineal ratio to magnetic induction intensity B when the length of sample L is fixed. Faraday rotation in a paramagnetic material is given by the Eq. (2).
h¼V HL
ð2Þ
where h is the rotation angle, L is the length of the light path in the medium, H is the magnetic field applied along the light beam and V is the Verdet constant. The relations between the Faraday rotation
Fig. 2. Powder XRD pattern of Tb3xNdxGa5O12 crystal.
160
X. Wang et al. / Optical Materials 47 (2015) 157–160 Table 2 Verdet constants of Tb3xNdxGa5O12 and TGG crystals. Crystal
Tb3xNdxGa5O12
Wavelength (nm) Verdet constants (rad/mT)
532 225
L¼
10 I0 lg ¼ 4:343a d I
633 145
TGG 1064 41
532 190
633 134
1064 40
ð4Þ
According to the previous data, the intrinsic absorption coefficients of Tb3xNdxGa5O12 crystal are about 0.1 and 0.01 cm1 at 633 and 1064 nm. And the magneto-optical figures of merit of the crystal at 633 and 1064 nm are 1065 and 3162°/dB at 1.2 T, respectively. 4. Conclusion Fig. 4. The absorption coefficient spectrum of Tb3xNdxGa5O12 crystal.
of Tb3xNdxGa5O12 crystal and magnetic field are linear in the range of 0–1.2 T at different wavelength, as shown in Fig. 5. The straight lines were fitted using a computer program and the Verdet constants can be calculated by the slope of straight lines. They are listed in Table 2 along with the Verdet constants of TGG from Refs. [22,23]. In the Tb3xNdxGa5O12 lattice, Tb3+ and Nd3+ are not adjacent, so super-exchange interaction exists between Tb3+ and Nd3+. And the results indicate that the super-exchange interaction between Nd3+ and Tb3+ may contribute to the enhancement of the Verdet constant for Tb3xNdxGa5O12.The extinction ratio of the as-grown crystal is up to 43.5 dB tested by a homemade instrument at 1064 nm. To our knowledge, the value is over the highest value of commercial TGG (40 dB). Magneto-optical figure of merit, defined by the ratio of Faraday rotation angle and optical absorption loss, can generally represent the magneto-optical properties of materials and is defined by Eq. (3) [24]:
F¼
jhF j L
ð3Þ
where hF (deg/cm) and L(dB) correspond to the ratio of FR and optical absorption loss, respectively. L is defined by Eq. (4), where I0 and I are the incident and transmission intensity, respectively, and a is the optical absorption coefficient.
Fig. 5. Faraday rotation of Tb3xNdxGa5O12 in different wavelengths at room temperature.
The Tb3xNdxGa5O12 crystal has been grown through the Czochralski method. It belongs to cubic system, and the segregation coefficient of Nd3+ is 0.45. The as-grown crystal has a low absorption at 900–1450 nm region and has larger Verdet constant than that of commercial TGG crystal. The magneto-optical figure of merit at 1064 nm is 3162°/dB at 1.2 T, close to that of commercial TGG crystal. The extinction ratio is up to 43.5 dB and is over the highest value of commercial TGG. So Tb3xNdxGa5O12 crystal could be a promising candidate material for magneto-optical devices in the wavelength range of 900–1450 nm. We are working on the development of the quality of Tb3xNdxGa5O12 and what the optimized concentration is. Acknowledgment This work was supported by Nature Science Foundation of China (No. 51472257). References [1] H. Lin, S.M. Zhou, H. Teng, J. Opt. Mater. 33 (2011) 1833–1836. [2] V. Vasyliev, E.G. Villora, M. Nakamura, Y. Sugahara, K.i. Shimamura, J. Opt. Express 20 (2012) 14460–14470. [3] W.J. Zhang, F.Y. Guo, J.Z. Chen, J. Cryst. Growth 306 (2007) 195–199. [4] X. Zhang, W.H. Zhang, Q.P. Wan, F.Y. Guo, et al., J. Opt. Mater. 37 (2014) 188– 192. [5] N. Sawanobori, N. Mori, et al., J. New Glass 18 (2003) 5–9. [6] M.J. Weber, R. Morgret, S.Y. Leung, J.A. Griffin, D. Gabbe, A. Linz, J. Appl. Phys. 49 (1978) 3464. [7] K. Sato, K. Yamaguchi, et al., Phys. Rev. B 63 (2001) 104416. [8] W.A. Crossley, R.W. Cooper, Phys. Rev. 181 (1964) 896. [9] S.B. Berger, C.B. Rubinstein, C.R. Kurkjian, A.W. Treptow, Phys. Rev. 133 (1964) A723. [10] D.R. Macfarlane, C.R. Bradbury, P.J. Newman, J. Javorniczky, J. Non-Cryst. Solids 213 (1997) 199–204. [11] J.H. Yang, Y. Xu, G.Y. Zhang, J. Phys. Phys. 75 (1994) 6798. [12] M. Geho, T. Skijima, T. Fujii, J. Cryst. Growth 275 (2005) e663. [13] Hidetsugu Yoshida, Koji Tsubakimoto, Yasushi Fujimito, et al., J. Opt. Express 19 (2011) 15181–15187. [14] V.I. Chani, A. Yoshikawa, H. Machida, T. Satoh, T. Fukuda, J. Cryst. Growth 210 (2000) 633. [15] S.B. Berger, C.B. Rubinstein, C.R. Kurkjian, A.W. Treptow, Phys. Rev. 133 (3A) (1964) A723–A727. [16] F.Y. Guo, R.R. Zhang, Z.H. Cui, C.C. Liu, J.Z. Chen, J. Opt. Mater. 35 (2012) 227– 230. [17] F. Guo, J. Ru, H. Li, N. Zhuang, B. Zhao, J. Chen, J. Appl. Phys. B 94 (2009) 437– 441. [18] J.Y. Lu, F.Y. Guo, J.Z. Chen, J. Cryst. Growth 314 (2011) 157–162. [19] F. Zhang, Y. Xu, J.H. Yang, M. Guillot, J. Condens. Matter 12 (2000) 7287–7294. [20] F. Zhang, Y. Xu, M. Guillot, J.H. Yang, J. Appl. Phys. 103 (2008) 07D307. [21] J. Liu, F. Guo, B. Zhao, N. Zhuang, Y. Chen, Z. Gao, J. Chen, J. Cryst. Growth 310 (2008) 2613–2616. [22] D.J. Dentz, R.C. Puttbach, R.R. Belt, AIP Conf. Proc. 18 (1974) 954. [23] M.Y.A. Raja, D. Allen, W. Sisk, Appl. Phys. Lett. 67 (1995) 2123. [24] M. Huang, Z.-C. Xu, Thin Solid Films 450 (2004) 324.