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Ho3+ co-doped Gd0.9La0.1VO4 crystal
ARTICLE IN PRESS
Journal of Crystal Growth 280 (2005) 212–216 www.elsevier.com/locate/jcrysgro
Growth and spectral properties of Yb3+/Ho3+ co-doped Gd0.9La0.1VO4 crystal Wei Xionga,, Peizhi Yanga, Jingying Liaoa, Shukun Linb a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,China b Department of Chemistry, University of Fuzhou, Fuzhou, Fujian 350002,China Received 19 November 2004; accepted 3 March 2005 Available online 18 April 2005 Communicated by M. Schieber
Abstract In this paper, crack-free and transparent Gd0.9La0.1VO4 single-crystal co-doped with Yb3+ and Ho3+ ions has been grown by Czochralski method. The absorption and emission spectra of the crystal were measured at room temperature. With the absorption spectrum, the intensity parameters Ol of Ho3+ ions were fitted based on Judd–Ofelt theory, which are O2 ¼ 7:87 1020 cm2 , O4 ¼ 7:41 1020 cm2 and O6 ¼ 1:99 1020 cm2 . The spectral parameters of Ho3+ ions were also calculated. The emission spectrum of Gd0.9La0.1VO4 crystal co-doped with Yb3+ and Ho3+ ions was detected when the sample was excited by LD at 980 nm and the intense up-conversion emissions were observed at 550, 660 and 750 nm. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60; 42.70.Hj; 81.10 Keywords: A1. Doping; A2. Czochralski method; A2. Single-crystal growth; B1. Vanadates; B2. Spectral properties
1. Introduction Ho3+-doped crystals have many important applications for their 13 channels range from 400 to 3000 nm. The up-conversion emission of Ho3+ is interesting when it was sensitized with other elements. Yb3+ is an important sensitizer, it could Corresponding author. Tel.: +86 21 52414237;
fax: +86 21 52413903. E-mail address: [email protected] (W. Xiong).
increase the pumping light absorption efficiency for its simple energy level scheme and it is favourable for laser-diode (LD) -pumped laser; a search for new crystals activated with Yb3+ is important [1,2]. It is known that Ho3+ can exhibit visible emission when it was sensitized by Yb3+, which is explained by Yb3+–Ho3+ stepwise upconversion mechanism [3,4]. Recently, rare-earth-doped vanadate crystals, such as Nd:YVO4 and Nd:GdVO4, are widely studied and used for LD-pumped solid state lasers,
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.052
ARTICLE IN PRESS W. Xiong et al. / Journal of Crystal Growth 280 (2005) 212–216
due to their good laser properties and good chemical and physical properties [5–7]. Ostroumov et al. reported a new laser crystal Nd: Gd0.5La0.5 VO4 by using La3+ ions to replace some Gd3+ ions in Nd:GdVO4 [8]. La–Gd disorder in the crystal makes the absorption and fluorescence peaks broader than in Nd:GdVO4 crystal. In order to make use of the advantages of the vanadate matrix, we grew a new laser crystal Yb3+/Ho3+: Gd0.9La0.1VO4. In this paper, we report on the material preparation and crystal growth of Yb3+/Ho3+ co-doped Gd0.9La0.1VO4. The spectroscopic properties of the crystal were studied, and the upconversion luminescence was observed when it was excited at 980 nm.
213
Fig. 1. As-grown Ho3+/Yb3+:Gd0.9La0.1VO4 crystal along aaxis.
2. Material preparation and crystal growth The preparation of REVO4 (RE ¼ Gd, La, Yb, Ho) was based on commercial starting materials of Gd2O3, La2O3, Yb2O3 and Ho2O3 with 99.995% purity. The raw materials were synthesized by liquid-phase method, according to the reaction: NH4 VO3 þ REðNO3 Þ3 þ 2NH3 H2 O ¼ REVO4 þ 3NH4 NO3 þ H2 O: The crystal was grown by Czochralski method in a RF furnace heating an iridium (Ir) crucible with N2 atmosphere. The polycrystalline mate rials were mixed in the crucible according to the stoichiometric composition Yb3+/Ho3+: Gd0.9 La0.1VO4. A rectangular a-axis GdVO4 single crystal was used as seed. The pulling rate was 1.0–1.5 mm/h and the rotating rate was 10–15 r/ min. The growth stage needs a dozen hours including seeding, broadening and keeping constant diameter. After growth, the crystal was cooled to room temperature slowly. The crystal boule which is orange and free from crack is shown in Fig. 1, and the size of the crystal is about 25 15 20 mm3. The Ho and Yb concentration were measured to be 1 and 3 at%, respectively, using inductively coupled plasma optical emission spectroscopy (ICP-OES).
Fig. 2. X-ray powder diffraction pattern. Inset: the XRD rocking curves of Ho3+/Yb3+:Gd0.9La0.1VO4 crystal.
The phase analysis and lattice parameters measurement of the as-grown Yb3+/Ho3+: Gd0.9La0.1VO4 crystal was performed by X-ray diffraction (XRD). The result is shown in Fig. 2, and the lattice parameters were calculated as follows: a ¼ b ¼ 0:72261 nm, c ¼ 0:63582 nm, which are bigger than that of pure GdVO4 crystal (a ¼ 0:72126 nm, c ¼ 0:63483 nm) [9]. This can be attributed to the radius of La3+ ions, which is bigger than that of Gd3+ ions. XRD analysis indicates the Yb3+/Ho3+: Gd0.9La0.1VO4 crystal belongs to tetragonal system (space group I41/ amd), in which the Gd3+ lattice sites were replaced by Yb3+, Ho3+, La3+ ions. The top-right corner
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of Fig. 2 is the XRD rocking curves from (1 0 0)oriented polished slice of the crystal. It shows that the diffraction peak appears at y ¼ 12:5341, which is approximate to the pure GdVO4 (y ¼ 12:3341) [9]. This result demonstrates that the doped LaVO4 may have effect on the crystalline perfection of the GdVO4 crystal, for its monoclinic system (space group P21/m).
Table 1 The absorption cross section of Yb3+ and Ho3+ in Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal Ion
Transition
Spectrum band (nm)
FWHM (nm)
sa (l) (1020 cm2)
Ho3+
5
I8-5F5 I8-5S2,5F4 5 I8-5F3 5 I8-5F2,3K8 5 I8-5G6 5 I8-5G5
629–670 528–557 484–498 465–479 446–465 414–426
6 5 3 10 7 4
5.73 5.22 1.58 1.09 13.7 4.09
2F7/2-2F5/2
915–1035
40
2.01
5
3. Spectral properties Yb3+
3.1. Absorption spectrum A sample of Yb3+/Ho3+:Gd0.9La0.1VO4 with the dimension 7 8 1.5 mm3 was cut from the as-grown crystal. The absorption spectrum between 350 and 1100 nm was recorded using a Lambda 900 UV–VIS–NIR spectrophotometer at room temperature, as shown in Fig. 3. The six absorption bands from 400 to 800 nm belong to the 4f10–4f10 transitions of Ho3+ ions, one broad band around 980 nm corresponding to the 2F7/2-2F5/2 transition of Yb3+ ions, whose FWHM is about 40 nm. The absorption cross section sa can be calculated by using the formula 1 lnðI 0 ðlÞ=IðlÞÞ, NL where N is the ionic concentration of Ho3+ and Yb3+ ions (1.02 1020 cm3 and 3.43 1020 cm3, respectively), L is the sample thickness (1.5 mm),
I 0 ðlÞ is the incident light intensity and IðlÞ is the transmitted light intensity. The calculated absorption cross sections of Yb3+/Ho3+ ions are listed in Table 1. From Table 1, it can be seen that the radiation 5 I8-5F5, 5I8-5S2, 5F4 , 5I8-5G6 transitions of Ho3+ ions have larger absorption cross section, but their FWHM is small, this can lead the sample unfavourable to be pumped. Yb3+ ions have large absorption cross section and FWHM at 980 nm, which suggests that the Yb3+/Ho3+:Gd0.9La0.1VO4 crystal could be easily pumped by LD.
sa ¼
1.2 1.0
A
0.8 0.6 0.4 0.2 0.0 400
500
600 700 800 900 1000 1100 Wavelength (nm)
Fig. 3. Absorption spectrum of Yb3+/Ho3+:Gd0.9La0.1VO4 crystal.
3.2. Fluorescence spectrum The up-conversion emission spectrum of Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal was measured with the FLUOROLOG-3 spectrophotometer at room temperature. The sample was excited by InGaAs LD operating at 980 nm. There are three up-conversion emission peaks of Ho3+ ions as depicted in Fig. 4. The strongest one is located at 660 nm, which is attributed to the 5 F5-5I8 transition. The emission peaks around 550 and 753 nm are relatively weak, corresponding to the 5S2-5I8 and 5S2-5I7 transitions, respectively. The intensity of red emission is about 10 times more intense than that of green emission. However, there is no blue emission of Ho3+ ions in the sample, some oxy-fluoride materials codoped with Yb3+/Ho3+ can give the blue emission [8], this may be associated with the structure of matrix.
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Table 2 The intensity parameters Ol of Ho3+ ions in different crystals
24
Intensity (a.u.)
20 Crystal
O2 (1020 cm2)
O4 (1020 cm2)
O6 (1020 cm2)
Yb/Ho:Gd0.9La0.1VO4 Ho:YVO4 Ho:GGG Ho:LaF3 Ho:YAlO3
7.87 0.812 0.24 1.16 1.82
7.41 1.477 1.41 1.38 2.38
1.99 1.836 1.49 0.88 1.53
16 12 8 4 0 450
500
550
600 650 700 Wavelength (nm)
750
800
Fig. 4. Up-conversion emission spectrum of Yb3+/Ho3+: Gd0.9La0.1VO4 crystal excited at 980 nm.
3.3. Discussion According to the absorption spectrum, the intensity parameters Ol of Ho3+ ions were fitted based on Judd–Ofelt theory [10,11]. Compared with other crystals, such as YVO4 [12], GGG [13], LaF3 [14] and YAlO3 [14], the intensity parameters Ol of Ho3+ ions in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal is larger, as shown in Table 2. This indicates that Ho3+ ions may have large radiative oscillator strengths and integrated emission cross sections in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal. In order to investigate the optical transition probabilities of Ho3+ ions in Gd0.9La0.1VO4 crystal, we also applied the Judd–Ofelt theory to calculate spectral parameters including oscillator strengths, radiative transition probabilities, radiative lifetime, fluorescence branch ratio and integrated emission cross section [15]. The result is shown in Table 3. We can see that the 5F5-5I8, 5 S2-5I8, 5F4-5I8 and 5S2-5I7 transitions of Ho3+ ions have larger oscillator strength and integrated emission cross section than that in other Ho3+ single-doped crystals, such as Ho3+: YAB[17]. The up-conversion luminescence of Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal is the result of energy transfer from Yb3+ to Ho3+ ions. Owing to close matching of the levels 2F5/2(Yb3+) and 5I6(Ho3+), the ground state Ho3+ ions can be easily excited to the 5I6 level. With a second energy transfer, the same Ho3+ ions are excited to the 5S2 (5F4) level,
then it relax to the 5I8 level with green emission at 550 nm [3,16]. There are two possible ways for the production of red emission. One is the Ho3+ relax from the 5S2 (5F4) level to the 5F5 level by nonradiative processes, and then to the 5I8 level emitting red laser at 660 nm. The other is the Ho3+ ions in the 5I6 level relax to the 5I7 level, and they are excited to the 5F5 state with the energy transfer from Yb3+ (2F5/2) by phonon-assisted transition, then it relax to 5I8 level with red emission [1]. If Ho3+ ions in the populated 5I7 level relax to the 5I8 level directly, the near-IR emission at 753 nm can be observed. The energy transfer from Yb3+ to Ho3+ ions can be described with the help of the level diagram as illustrated in Fig. 5.
4. Conclusion High-quality single crystal of Yb3+3+: Gd0.9La0.1VO4 crystal was grown by Czochralski method. Combined with the absorption spectrum of the crystal, the spectral parameters of Ho3+ ions were calculated based on Judd–Ofelt theory, some optical transitions of Ho3+ ions in Gd0.9La0.1VO4 crystal may be created for their large oscillator strength and integrated emission cross section. Under 980 nm diode laser pumping, the crystal revealed visible up-conversion emission due to the energy transferring from Yb3+ to Ho3+. The up-conversion red emission is very strong, which may realize the laser output. The spectroscopy experiment results indicate that Yb3+/Ho3+:Gd0.9La0.1VO4 crystal is a promising laser material for applications in LD pumping laser.
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Table 3 Spectral parameters of Ho3+ ions in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal A(J00 -J0 ) (s1)
trad (ms)
bJ0 (%)
0.053 0.713 1.863 1.599
27.22 137.5 116.0 19.81
3327.8
9.06 45.76 38.60 6.59
0.047 0.630 1.646 1.412
644 961 1448 2297 4348
14.31 7.687 2.919 0.543 0.026
10131 2444.4 408.8 30.21 0.4016
76.8
77.81 18.77 3.14 0.23 0.00
12.64 6.793 2.579 0.480 0.023
545 755 1027 1391 1947 3527
3.900 5.081 2.110 0.824 1.841 0.118
3856.0 2617.7 587.5 125.0 142.6 2.798
136.4
52.60 35.71 8.01 1.71 1.94 0.03
3.446 4.490 1.865 0.728 1.627 0.105
Radiation transition
Radiation wavelength (nm)
I4-5I8 I4-5I7 5 I4-5I6 5 I4-5I5
756 1234 2172 4868
F5-5I8 F5-5I7 5 F5-5I6 5 F5-5I5 5 F5-5I4 S2-5I8 S2-5I7 5 S2-5I6 5 S2-5I5 5 S2-5I4 5 S2-5F5
5 5
5 5
5 5
Pcal(J00 -J0 ) ( 106)
5G
6 5K ,5F 8 2 5
S2,5F4 490nm
5
F5
2F
5I
5/2
5
5
550nm 660nm
I6 753nm
2
F7/2
Yb3+
5I
7
5I
8
Ho3+
Fig. 5. Relevant energy levels of Ho3+ and Yb3+ ions.
Acknowledgements This work was supported by the natural science item fund (K2001009) of Fujian Educational Committee. The authors would like to thank Doctor Shiwei Wang and Miss Liqiong An for their experimental assistance.
S(J00 -J0 ) ( 1018 cm)
References [1] J. Li, J. Wang, H. Tan, et al., J. Crystal Growth 256 (2003) 324. [2] A.V. Kiryanov, V. Aboites, A.M. Belovolov, et al., J. Lumin. 102–103 (2003) 715. [3] I.R. Marin, V.D. Rodriguez, V. Lavin, et al., J. Alloys Compd. 275–277 (1998) 345. [4] L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. [5] H.-D. Jiang, H.-J. Zhang, J.-Y. Wang, et al., Opt. Commun. 198 (2001) 447. [6] C.Q. Wang, H.J. Zhang, Y.T. Chow, et al., Opt. Laser Technol. 33 (2001) 439. [7] H. Zhang, X. Meng, L. Zhu, et al., J. Crystal Growth 193 (1998) 370. [8] V.G. Ostroumov, G. Huber, A.I. Zagumennyi, et al., Opt. Commun. 124 (1996) 63. [9] JCPDS diffraction file:17–260. [10] B.R. Judd, Phys. Rev. 127 (1962) 750. [11] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [12] W. Yang, M. Wu, J. Chen, Spectrosc. Spectral Anal. 21 (2001) 28. [13] Q. Wang, Chin. J. Lasers 17 (1990) 31. [14] M.J. Weber, J. Chem. Phys. 57 (1972) 562. [15] W. Xiong, S. Lin, Y. Xie, J. Crystal Growth 263 (2004) 353. [16] Th. Rothacher, W. Luthy, H.P. Weber, Opt. Commun. 155 (1998) 68. [17] F. Xiaofeng, L. Baosheng, W. Pu, et al., Optronics Lasers 6 (1995) 33.
Journal of Crystal Growth 280 (2005) 212–216 www.elsevier.com/locate/jcrysgro
Growth and spectral properties of Yb3+/Ho3+ co-doped Gd0.9La0.1VO4 crystal Wei Xionga,, Peizhi Yanga, Jingying Liaoa, Shukun Linb a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,China b Department of Chemistry, University of Fuzhou, Fuzhou, Fujian 350002,China Received 19 November 2004; accepted 3 March 2005 Available online 18 April 2005 Communicated by M. Schieber
Abstract In this paper, crack-free and transparent Gd0.9La0.1VO4 single-crystal co-doped with Yb3+ and Ho3+ ions has been grown by Czochralski method. The absorption and emission spectra of the crystal were measured at room temperature. With the absorption spectrum, the intensity parameters Ol of Ho3+ ions were fitted based on Judd–Ofelt theory, which are O2 ¼ 7:87 1020 cm2 , O4 ¼ 7:41 1020 cm2 and O6 ¼ 1:99 1020 cm2 . The spectral parameters of Ho3+ ions were also calculated. The emission spectrum of Gd0.9La0.1VO4 crystal co-doped with Yb3+ and Ho3+ ions was detected when the sample was excited by LD at 980 nm and the intense up-conversion emissions were observed at 550, 660 and 750 nm. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60; 42.70.Hj; 81.10 Keywords: A1. Doping; A2. Czochralski method; A2. Single-crystal growth; B1. Vanadates; B2. Spectral properties
1. Introduction Ho3+-doped crystals have many important applications for their 13 channels range from 400 to 3000 nm. The up-conversion emission of Ho3+ is interesting when it was sensitized with other elements. Yb3+ is an important sensitizer, it could Corresponding author. Tel.: +86 21 52414237;
fax: +86 21 52413903. E-mail address: [email protected] (W. Xiong).
increase the pumping light absorption efficiency for its simple energy level scheme and it is favourable for laser-diode (LD) -pumped laser; a search for new crystals activated with Yb3+ is important [1,2]. It is known that Ho3+ can exhibit visible emission when it was sensitized by Yb3+, which is explained by Yb3+–Ho3+ stepwise upconversion mechanism [3,4]. Recently, rare-earth-doped vanadate crystals, such as Nd:YVO4 and Nd:GdVO4, are widely studied and used for LD-pumped solid state lasers,
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.052
ARTICLE IN PRESS W. Xiong et al. / Journal of Crystal Growth 280 (2005) 212–216
due to their good laser properties and good chemical and physical properties [5–7]. Ostroumov et al. reported a new laser crystal Nd: Gd0.5La0.5 VO4 by using La3+ ions to replace some Gd3+ ions in Nd:GdVO4 [8]. La–Gd disorder in the crystal makes the absorption and fluorescence peaks broader than in Nd:GdVO4 crystal. In order to make use of the advantages of the vanadate matrix, we grew a new laser crystal Yb3+/Ho3+: Gd0.9La0.1VO4. In this paper, we report on the material preparation and crystal growth of Yb3+/Ho3+ co-doped Gd0.9La0.1VO4. The spectroscopic properties of the crystal were studied, and the upconversion luminescence was observed when it was excited at 980 nm.
213
Fig. 1. As-grown Ho3+/Yb3+:Gd0.9La0.1VO4 crystal along aaxis.
2. Material preparation and crystal growth The preparation of REVO4 (RE ¼ Gd, La, Yb, Ho) was based on commercial starting materials of Gd2O3, La2O3, Yb2O3 and Ho2O3 with 99.995% purity. The raw materials were synthesized by liquid-phase method, according to the reaction: NH4 VO3 þ REðNO3 Þ3 þ 2NH3 H2 O ¼ REVO4 þ 3NH4 NO3 þ H2 O: The crystal was grown by Czochralski method in a RF furnace heating an iridium (Ir) crucible with N2 atmosphere. The polycrystalline mate rials were mixed in the crucible according to the stoichiometric composition Yb3+/Ho3+: Gd0.9 La0.1VO4. A rectangular a-axis GdVO4 single crystal was used as seed. The pulling rate was 1.0–1.5 mm/h and the rotating rate was 10–15 r/ min. The growth stage needs a dozen hours including seeding, broadening and keeping constant diameter. After growth, the crystal was cooled to room temperature slowly. The crystal boule which is orange and free from crack is shown in Fig. 1, and the size of the crystal is about 25 15 20 mm3. The Ho and Yb concentration were measured to be 1 and 3 at%, respectively, using inductively coupled plasma optical emission spectroscopy (ICP-OES).
Fig. 2. X-ray powder diffraction pattern. Inset: the XRD rocking curves of Ho3+/Yb3+:Gd0.9La0.1VO4 crystal.
The phase analysis and lattice parameters measurement of the as-grown Yb3+/Ho3+: Gd0.9La0.1VO4 crystal was performed by X-ray diffraction (XRD). The result is shown in Fig. 2, and the lattice parameters were calculated as follows: a ¼ b ¼ 0:72261 nm, c ¼ 0:63582 nm, which are bigger than that of pure GdVO4 crystal (a ¼ 0:72126 nm, c ¼ 0:63483 nm) [9]. This can be attributed to the radius of La3+ ions, which is bigger than that of Gd3+ ions. XRD analysis indicates the Yb3+/Ho3+: Gd0.9La0.1VO4 crystal belongs to tetragonal system (space group I41/ amd), in which the Gd3+ lattice sites were replaced by Yb3+, Ho3+, La3+ ions. The top-right corner
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of Fig. 2 is the XRD rocking curves from (1 0 0)oriented polished slice of the crystal. It shows that the diffraction peak appears at y ¼ 12:5341, which is approximate to the pure GdVO4 (y ¼ 12:3341) [9]. This result demonstrates that the doped LaVO4 may have effect on the crystalline perfection of the GdVO4 crystal, for its monoclinic system (space group P21/m).
Table 1 The absorption cross section of Yb3+ and Ho3+ in Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal Ion
Transition
Spectrum band (nm)
FWHM (nm)
sa (l) (1020 cm2)
Ho3+
5
I8-5F5 I8-5S2,5F4 5 I8-5F3 5 I8-5F2,3K8 5 I8-5G6 5 I8-5G5
629–670 528–557 484–498 465–479 446–465 414–426
6 5 3 10 7 4
5.73 5.22 1.58 1.09 13.7 4.09
2F7/2-2F5/2
915–1035
40
2.01
5
3. Spectral properties Yb3+
3.1. Absorption spectrum A sample of Yb3+/Ho3+:Gd0.9La0.1VO4 with the dimension 7 8 1.5 mm3 was cut from the as-grown crystal. The absorption spectrum between 350 and 1100 nm was recorded using a Lambda 900 UV–VIS–NIR spectrophotometer at room temperature, as shown in Fig. 3. The six absorption bands from 400 to 800 nm belong to the 4f10–4f10 transitions of Ho3+ ions, one broad band around 980 nm corresponding to the 2F7/2-2F5/2 transition of Yb3+ ions, whose FWHM is about 40 nm. The absorption cross section sa can be calculated by using the formula 1 lnðI 0 ðlÞ=IðlÞÞ, NL where N is the ionic concentration of Ho3+ and Yb3+ ions (1.02 1020 cm3 and 3.43 1020 cm3, respectively), L is the sample thickness (1.5 mm),
I 0 ðlÞ is the incident light intensity and IðlÞ is the transmitted light intensity. The calculated absorption cross sections of Yb3+/Ho3+ ions are listed in Table 1. From Table 1, it can be seen that the radiation 5 I8-5F5, 5I8-5S2, 5F4 , 5I8-5G6 transitions of Ho3+ ions have larger absorption cross section, but their FWHM is small, this can lead the sample unfavourable to be pumped. Yb3+ ions have large absorption cross section and FWHM at 980 nm, which suggests that the Yb3+/Ho3+:Gd0.9La0.1VO4 crystal could be easily pumped by LD.
sa ¼
1.2 1.0
A
0.8 0.6 0.4 0.2 0.0 400
500
600 700 800 900 1000 1100 Wavelength (nm)
Fig. 3. Absorption spectrum of Yb3+/Ho3+:Gd0.9La0.1VO4 crystal.
3.2. Fluorescence spectrum The up-conversion emission spectrum of Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal was measured with the FLUOROLOG-3 spectrophotometer at room temperature. The sample was excited by InGaAs LD operating at 980 nm. There are three up-conversion emission peaks of Ho3+ ions as depicted in Fig. 4. The strongest one is located at 660 nm, which is attributed to the 5 F5-5I8 transition. The emission peaks around 550 and 753 nm are relatively weak, corresponding to the 5S2-5I8 and 5S2-5I7 transitions, respectively. The intensity of red emission is about 10 times more intense than that of green emission. However, there is no blue emission of Ho3+ ions in the sample, some oxy-fluoride materials codoped with Yb3+/Ho3+ can give the blue emission [8], this may be associated with the structure of matrix.
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Table 2 The intensity parameters Ol of Ho3+ ions in different crystals
24
Intensity (a.u.)
20 Crystal
O2 (1020 cm2)
O4 (1020 cm2)
O6 (1020 cm2)
Yb/Ho:Gd0.9La0.1VO4 Ho:YVO4 Ho:GGG Ho:LaF3 Ho:YAlO3
7.87 0.812 0.24 1.16 1.82
7.41 1.477 1.41 1.38 2.38
1.99 1.836 1.49 0.88 1.53
16 12 8 4 0 450
500
550
600 650 700 Wavelength (nm)
750
800
Fig. 4. Up-conversion emission spectrum of Yb3+/Ho3+: Gd0.9La0.1VO4 crystal excited at 980 nm.
3.3. Discussion According to the absorption spectrum, the intensity parameters Ol of Ho3+ ions were fitted based on Judd–Ofelt theory [10,11]. Compared with other crystals, such as YVO4 [12], GGG [13], LaF3 [14] and YAlO3 [14], the intensity parameters Ol of Ho3+ ions in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal is larger, as shown in Table 2. This indicates that Ho3+ ions may have large radiative oscillator strengths and integrated emission cross sections in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal. In order to investigate the optical transition probabilities of Ho3+ ions in Gd0.9La0.1VO4 crystal, we also applied the Judd–Ofelt theory to calculate spectral parameters including oscillator strengths, radiative transition probabilities, radiative lifetime, fluorescence branch ratio and integrated emission cross section [15]. The result is shown in Table 3. We can see that the 5F5-5I8, 5 S2-5I8, 5F4-5I8 and 5S2-5I7 transitions of Ho3+ ions have larger oscillator strength and integrated emission cross section than that in other Ho3+ single-doped crystals, such as Ho3+: YAB[17]. The up-conversion luminescence of Yb3+/ Ho3+:Gd0.9La0.1VO4 crystal is the result of energy transfer from Yb3+ to Ho3+ ions. Owing to close matching of the levels 2F5/2(Yb3+) and 5I6(Ho3+), the ground state Ho3+ ions can be easily excited to the 5I6 level. With a second energy transfer, the same Ho3+ ions are excited to the 5S2 (5F4) level,
then it relax to the 5I8 level with green emission at 550 nm [3,16]. There are two possible ways for the production of red emission. One is the Ho3+ relax from the 5S2 (5F4) level to the 5F5 level by nonradiative processes, and then to the 5I8 level emitting red laser at 660 nm. The other is the Ho3+ ions in the 5I6 level relax to the 5I7 level, and they are excited to the 5F5 state with the energy transfer from Yb3+ (2F5/2) by phonon-assisted transition, then it relax to 5I8 level with red emission [1]. If Ho3+ ions in the populated 5I7 level relax to the 5I8 level directly, the near-IR emission at 753 nm can be observed. The energy transfer from Yb3+ to Ho3+ ions can be described with the help of the level diagram as illustrated in Fig. 5.
4. Conclusion High-quality single crystal of Yb3+3+: Gd0.9La0.1VO4 crystal was grown by Czochralski method. Combined with the absorption spectrum of the crystal, the spectral parameters of Ho3+ ions were calculated based on Judd–Ofelt theory, some optical transitions of Ho3+ ions in Gd0.9La0.1VO4 crystal may be created for their large oscillator strength and integrated emission cross section. Under 980 nm diode laser pumping, the crystal revealed visible up-conversion emission due to the energy transferring from Yb3+ to Ho3+. The up-conversion red emission is very strong, which may realize the laser output. The spectroscopy experiment results indicate that Yb3+/Ho3+:Gd0.9La0.1VO4 crystal is a promising laser material for applications in LD pumping laser.
ARTICLE IN PRESS W. Xiong et al. / Journal of Crystal Growth 280 (2005) 212–216
216
Table 3 Spectral parameters of Ho3+ ions in Yb3+/Ho3+:Gd0.9La0.1VO4 crystal A(J00 -J0 ) (s1)
trad (ms)
bJ0 (%)
0.053 0.713 1.863 1.599
27.22 137.5 116.0 19.81
3327.8
9.06 45.76 38.60 6.59
0.047 0.630 1.646 1.412
644 961 1448 2297 4348
14.31 7.687 2.919 0.543 0.026
10131 2444.4 408.8 30.21 0.4016
76.8
77.81 18.77 3.14 0.23 0.00
12.64 6.793 2.579 0.480 0.023
545 755 1027 1391 1947 3527
3.900 5.081 2.110 0.824 1.841 0.118
3856.0 2617.7 587.5 125.0 142.6 2.798
136.4
52.60 35.71 8.01 1.71 1.94 0.03
3.446 4.490 1.865 0.728 1.627 0.105
Radiation transition
Radiation wavelength (nm)
I4-5I8 I4-5I7 5 I4-5I6 5 I4-5I5
756 1234 2172 4868
F5-5I8 F5-5I7 5 F5-5I6 5 F5-5I5 5 F5-5I4 S2-5I8 S2-5I7 5 S2-5I6 5 S2-5I5 5 S2-5I4 5 S2-5F5
5 5
5 5
5 5
Pcal(J00 -J0 ) ( 106)
5G
6 5K ,5F 8 2 5
S2,5F4 490nm
5
F5
2F
5I
5/2
5
5
550nm 660nm
I6 753nm
2
F7/2
Yb3+
5I
7
5I
8
Ho3+
Fig. 5. Relevant energy levels of Ho3+ and Yb3+ ions.
Acknowledgements This work was supported by the natural science item fund (K2001009) of Fujian Educational Committee. The authors would like to thank Doctor Shiwei Wang and Miss Liqiong An for their experimental assistance.
S(J00 -J0 ) ( 1018 cm)
References [1] J. Li, J. Wang, H. Tan, et al., J. Crystal Growth 256 (2003) 324. [2] A.V. Kiryanov, V. Aboites, A.M. Belovolov, et al., J. Lumin. 102–103 (2003) 715. [3] I.R. Marin, V.D. Rodriguez, V. Lavin, et al., J. Alloys Compd. 275–277 (1998) 345. [4] L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. [5] H.-D. Jiang, H.-J. Zhang, J.-Y. Wang, et al., Opt. Commun. 198 (2001) 447. [6] C.Q. Wang, H.J. Zhang, Y.T. Chow, et al., Opt. Laser Technol. 33 (2001) 439. [7] H. Zhang, X. Meng, L. Zhu, et al., J. Crystal Growth 193 (1998) 370. [8] V.G. Ostroumov, G. Huber, A.I. Zagumennyi, et al., Opt. Commun. 124 (1996) 63. [9] JCPDS diffraction file:17–260. [10] B.R. Judd, Phys. Rev. 127 (1962) 750. [11] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [12] W. Yang, M. Wu, J. Chen, Spectrosc. Spectral Anal. 21 (2001) 28. [13] Q. Wang, Chin. J. Lasers 17 (1990) 31. [14] M.J. Weber, J. Chem. Phys. 57 (1972) 562. [15] W. Xiong, S. Lin, Y. Xie, J. Crystal Growth 263 (2004) 353. [16] Th. Rothacher, W. Luthy, H.P. Weber, Opt. Commun. 155 (1998) 68. [17] F. Xiaofeng, L. Baosheng, W. Pu, et al., Optronics Lasers 6 (1995) 33.