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A new transparent oxyfluoride glass ceramic with improved luminescence
Journal of Non-Crystalline Solids 353 (2007) 405–409 www.elsevier.com/locate/jnoncrysol
A new transparent oxyfluoride glass ceramic with improved luminescence Yunlong Yu, Daqin Chen, Yuansheng Wang *, Feng Liu, En Ma State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Received 17 January 2006; received in revised form 17 October 2006
Abstract A new type of glass ceramic containing BaF2 nano-crystals was prepared by melt quenching. Differential scanning calorimetry, X-ray diffraction and transmission electron microscopy were used to study its thermal behaviors and structural characteristics. Based on Judd– Ofelt theory, the spectroscopic properties of the 4I13/2 ! 4I15/2 transition of Er3+ in glass ceramic were evaluated. Notably, it is found that the fluorescence lifetime in the present system is much longer than that in most other glasses and glass ceramics. A comparative study on luminescence performance suggests that the obtained glass ceramic is a promising material for Er3+ doped fiber amplifiers. 2007 Elsevier B.V. All rights reserved. PACS: 51.70.+f Keywords: Glass ceramics; Optical fibers; Optical spectroscopy; STEM/TEM; Nano-crystals; Luminescence; Silicates
1. Introduction Over the past decades, inspirited by the rapid development of optical devices, such as solid lasers and optical amplifiers, transparent host materials based on rare earth (RE) ions have been extensively investigated [1,2]. Among numerous host materials, transparent oxyfluoride glass ceramics, which combine the advantages of the excellent optical properties of fluoride and high chemical and thermal stability of oxide, have attracted a great deal of attentions [3,4]. So far, most efforts on transparent oxyfluoride glass ceramics have been focused on oxide glassy matrix containing Pb(Cd)F2, LaF3 or CaF2 fluoride crystals [4–6]. RE doped BaF2 single crystal, characterized by the low phonon energy and the large transfer coefficient between the RE ions, has been revealed to be a suitable host to achieve laser and up-conversion [7,8]. Some investigations on the material with BaF2 crystals in germanate glassy *
Corresponding author. Tel./fax: +86 591 8370 5402. E-mail address: [email protected] (Y. Wang).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.029
matrix have been reported [9,10]. Taking into account the system compatibility with the standard silica fiber, the study on the silicate glassy host containing Er:BaF2 crystals remains a topic of practical application interest. In this paper, we report a new transparent glass ceramic fabricated by melt quenching and subsequent heating, and the investigation on its thermal behavior and structural characteristics. Based on Judd–Ofelt theory [11,12], the luminescence characteristics, such as spontaneous emission probability, stimulated emission cross-section and radiative lifetime, of the glass ceramic were evaluated. Moreover, the comparative analyses on the optical performances of this material and some other hosts were also performed. 2. Experimental Glass with the composition of 68SiO2–15BaF2– 13K2CO3–3La2O3–1Sb2O3–0.5ErF3 (in mol%) was prepared from high purity reagents (>3 N). The mixed batch, about 15 g, was melted in a covered platinum crucible under air atmosphere at 1300 C for 1 h, and then cast into
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Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
a copper mold, followed by annealing at a temperature 50 C below the glass transition temperature (Tg = 495 C) determined by differential scanning calorimetry (DSC, NETZSCH-TA4) to release the stress. To prepare the glass ceramic, the glass sample was heat treated at 600 C for 8 h. The long heating duration of 8 hours might guaranty a complete BaF2 crystallization. For the precursor glass and glass ceramic, the refractive indices determined by an Abbe refractometer are 1.546 and 1.551, respectively, while the densities measured based on the Archimede’s principle are 3.217 and 3.239 g cm3, respectively. To study the thermal behavior, DTA analyses were carried out in air atmosphere at the heating rates of 5, 10, 15, 20, 25 C/min, respectively. In order to identify the precipitated crystalline phase, X-ray diffraction (XRD) measurement was performed on an X-ray powder diffractometer (DMAX2500) with Cu Ka1 radiation. The microstructure was further analyzed by transmission electron microscopy (TEM, JEM-2010). The absorption spectra were recorded in the range of 300–1650 nm by an UV-near-infrared spectrophotometer (Lamda900) with resolution of 1.0 nm. The emission spectra were acquired with 980 nm excitation light from 450 W Xenone lamp. By using an InP/InGaAs photomultiplier tubes (PMT) detector (R5509), the infrared luminescence signals through the emission monochromator (M300) were detected. The fluorescence decay curves at 1530 nm were recorded with a NIR PMT (R5509) when excited at 980 nm by a microsecond flash lamp (lF900). All the measurements were carried out at room temperature.
580 C (Tp) corresponding to the crystallization of BaF2 confirmed by XRD analysis. Fig. 2 shows the XRD patterns of the precursor glass and the heat-treated sample. The diffuse hump for the precursor glass evidences its amorphous structure. However, several sharp crystalline peaks emerge, indicating the precipitation of BaF2 phase from the glass matrix, for the heat-treated sample. Using the XRD peak widths and Scherrer equation, the mean size of the crystallites was evaluated to be about 11 nm. The TEM bright field image of the glass ceramic, as presented in Fig. 3, demonstrates the homogeneous distribution of spherical BaF2 crystals with 8–12 nm in size among the glassy matrix. The inset of Fig. 3 is the selected area electron diffraction (SAED) pattern showing BaF2 polycrystalline rings. From the data of non-isothermal experiments, two most important kinetic parameters for BaF2 crystallization, i.e., the apparent activation energy (Ea) and the Avrami exponent (n), were determined by Ozawa equation
3. Results The precursor glass was visually transparent, appearing pink due to Er3+. The DSC trace is shown in Fig. 1, which exhibits a unique exothermal peak centered at about
Fig. 2. The XRD patterns of the precursor glass and glass ceramic.
Fig. 1. DSC trace of the precursor glass recorded at a heating rate of 10 K/min.
Fig. 3. TEM bright field image and the corresponding SAED pattern of the glass ceramic.
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
Fig. 4. Variation of lattice parameter of cubic BaF2 as a function of ErF3 content in the glass ceramic.
[13] to be 383 ± 20 kJ and 1.25 ± 0.05, respectively, suggesting that the crystallization mechanism is a diffusioncontrolled growth process with zero nucleation rate [14]. To study the influence of Er3+ doping level on the precipitated phase, glass ceramics with various content of ErF3 were prepared and examined by XRD. It is evident that, for BaF2 crystals, the mean size keeps almost the same, while the lattice parameter deduced from the position of XRD peaks varies with ErF3 content, as shown in Fig. 4. The doping of ErF3 results in the lattice contraction of BaF2 phase, which implies the incorporation of Er3+ into BaF2:Er3+ ions with radius of 0.114 nm substitute for the bigger Ba2+ with radius of 0.156 nm. The lattice contraction is remarkable when Er3+ content is low (<0.1%), with further increase of ErF3 content from 0.1% to 2.0%, the lattice parameter decreases slightly but monotonously. It is thus proposed that when Er3+ doping content is below 0.1%, lots of them incorporate into BaF2; however, Er:BaF2 soon approaches its solid solution limit, thus further increasing of ErF3 to the sample results in only minor supplementation of Er3+ to BaF2. The application of Judd–Ofelt theory to evaluate the spectroscopic properties of RE in oxyfluoride glass ceramics has been reported by several authors [15–17]. Although there are some doubts about this application on the heterogeneous composite system, the method seems to be still a best way to evaluate some spectroscopic parameters at present. Based on the data from the absorption experiments and Judd–Ofelt theory, the intensity parameters, Xt (t = 2, 4, 6), for the precursor glass and glass ceramic, determined from a least square fitting between the experi-
Table 1 Judd–Ofelt parameters, Xt (in unit of 1020 cm2), of Er3+ in the precursor glass and glass ceramic (rms: the root-mean-square errors) Sample
X2
X4
X6
Rms (%)
X4/X6
Glass Glass ceramic
4.88 3.81
0.95 0.57
0.37 0.18
1.6% 2.9%
2.57 3.17
407
Fig. 5. The emission spectrum of Er3+ 4I13/2 ! 4I15/2 transition in the glass and glass ceramic. The inset shows the absorption (dot line) and the stimulated (solid line) emission cross-sections determined from the absorption and emission spectra.
mental oscillator strengths and the calculated oscillator strengths, are listed in Table 1. The emission spectra of Er3+ 4I13/2 ! 4I15/2 transition in the precursor glass and glass ceramic are presented in Fig. 5, from which the emission peak wavelength kem and the width of the emission band Dkeff are obtained. Based on the Judd–Ofelt intensity parameters, the spontaneous emission probability Atotal, the radiative lifetime scal and the stimulated emission cross-section rem are evaluated. The absorption cross-section ra, determined from the absorption spectrum, and the stimulated emission crosssection are presented in the inset of Fig. 5. By fitting the fluorescence decay curve, the lifetime sexp of 4I13/2 level was estimated, which was then used to deduce the quantum efficiency (g = sexp/srad) of this level. All the acquired results are listed in Table 2. 4. Discussion According to Judd–Ofelt theory, X2 is sensitive to the local structure surrounding RE ions, and it will decrease with the change of RE ion from a covalent bond with the oxide ligand to a more ionic (less covalent) bond with the fluoride [18,19]. Therefore, the decrease of X2 during the crystallization of the precursor glass indicates the incorporation of RE ions into the fluoride nano-crystals, which is in accordance with the result deduced from the variation of BaF2 lattice parameter with ErF3 content stated above. On the other hand, the value of X6, which is proportional to the rigidity of the host [15], also decreases in the transition from glass to glass ceramic. X4/X6 is the spectroscopic quality factor, which is usually used to characterize the intensity of the major laser transitions of Er3+ [20]. It was described by Sardar et al. [21] that the larger the X4/ X6 value the more intense the laser transitions are. The spectroscopic quality factor for Er3+ in the precursor glass
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Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
Table 2 Some spectroscopic parameters of Er3+ 4I13/2 ! 4I15/2 transition in the glass ceramic Atotal (s1)
scal (ms)
sexp (ms)
g (%)
kem (nm)
Dkeff (nm)
rem (ke) (1020 cm2)
53.56
18.67
13.39
72
1537
37.8
0.44
and glass ceramic is 2.57 and 3.17, respectively, within the range of 0.126–3.372 for Er3+ in various hosts [22]. These values, especially that for the glass ceramic, are much larger than those found in most other glasses, glass ceramics and Er3+:YAG single-crystal (X4/X6 = 0.32) reported previously [21,23], indicating that this material might be a desirable candidate for stimulated emission. The lifetime of Er3+ 4I13/2 level is an important parameter for the optical amplifier. A critical factor in the success of Er3+ doped fiber amplifier in optical communication is the long lifetime of the metastable state that permits the required high population inversions to be obtained under steady-state conditions using modest pump powers [24]. As shown in Table 3, compared to that in other host materials, the measured lifetime (sexp = 13.39 ms) of 4I13/2 for Er3+ in the present glass ceramic is remarkably longer, which is about 3.5 times longer than that of Er3+:YAG transparent ceramic (sexp = 3.1 ms) [2]. The long lifetime of Er3+ in the present glass ceramic could be explained as follows: firstly, since part of Er3+ ions incorporate into BaF2 nano-crystal, the low phonon energy (346 cm1 [25]) of BaF2 host depresses the non-radiative relaxation rates; secondly, in glass matrix, the strong columbic repulsion between RE ions and residual Ba2+ ions at non-bridging sites cumber the clustering of Er–Er ions, which result in the weakening of the concentration quenching effect [26]. For an amplifier device, the figure-of-merit (FOM) for gain is defined as the product of lifetime and emission
cross-section: s · rem [26]. Since the gain bandwidth of optical amplifier is mainly determined by the emission width and the stimulated emission cross-section, FOM for bandwidth is defined as Dk · rem [24,26]. As shown in Table 4, although the value of FOM for bandwidth of the present glass ceramic is smaller than those of the glass ceramics containing CaF2 and PbF2 crystals report previously, it is still larger than that of the commercial Al silica fiber. However, the FOM for gain of the present system is preferable to those of the other glass ceramics, and equivalent to that of the Al silica fiber, which indicates the present glass ceramic a promising host material for Er3+ doped fiber amplifiers. 5. Conclusion A new type of Er3+ doped transparent oxyfluoride glassy ceramic with BaF2 nano-crystals homogenously dispersed among glassy matrix was prepared. The crystallization mechanism of BaF2 was revealed to be a diffusioncontrolled growth process with zero nucleation rate. The spectroscopic quality factor for Er3+ in the obtained glass ceramic is 10 times as larger as that in the Er3+:YAG crystal. The fluorescence lifetime of Er3+ 4I13/2 ! 4I15/2 transition in this material is remarkably longer than that in other glasses and glass ceramics reported previously. These results imply that this glass ceramic might be a promising material for Er3+ doped fiber amplifiers.
Table 3 Lifetime of Er3+ 4I13/2 level in the different hosts GC sexp (ms) Reference
Containing BaF2
Containing CaF2
Containing PbF2
Containing LaF3
YAG ceramic
13.39 Present work
7.56 [16]
4.0 [17]
7.2 [27]
3.1 [2]
ZBLAN
Silicate
Phosphate
Tellurite
Bismuth
10.3 [28]
7 [29]
5.4 [30]
7.5 [31]
2.7 [32]
G sexp (ms) Reference
(GC: oxyfluoride glass ceramic, G: glass).
Table 4 Comparison of FOM for gain and for bandwidth of Er3+ in different oxyfluoride glass ceramics (GC) and the commercial Al silica fiber used for device fabrication Host materials
FOM for gain (s · rem) (1020 ms cm2)
FOM for bandwidth (Dk · rem) (1020 nm cm2)
Reference
GC containing BaF2 GC containing CaF2 GC containing PbF2 Al silica fiber
5.7 4.4 2.1 5.7
16.7 40.6 32.3 13.4
Present work [16] [17,33] [26]
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 50672098) and the project of Nano-molecular Functional Materials of Fujian Province China (2005HZ01-1). References [1] B.N. Samson, P.A. Tick, N.F. Borrelli, Opt. Lett. 26 (2001) 145. [2] D.K. Sardar, C.C. Russell, J.B. Gruber, T.H. Allik, J. Appl. Phys. 97 (2005) 123501. [3] M. Mortier, P. Goldner, C. Chateau, M. Genotelle, J. Alloys Comp. 323&324 (2001) 245. [4] Y. Wang, J. Ohwaki, Appl. Phys. Lett. 63 (1993) 3268. [5] M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. [6] J. Fu, J.M. Parker, P.S. Flower, R.M. Browm, MRS Bull. 37 (2002) 1843. [7] M.P. Miller, J.C. Wright, J. Chem. Phys. 68 (1978) 1548. [8] V. Grover, S.N. Achary, S.J. Patwe, A.K. Tyagi, MRS Bull. 38 (2003) 1413. [9] G.S. Qin, W.P. Qin, B.J. Chen, C.F. Wu, S.L. E, J.L. Lin, D. Zhao, H.Q. Liu, S.H. Huang, Chinese J. Lumi. 22 (2001) 397. [10] G.S. Qin, W.P. Qin, B.J. Chen, S.L. E, Z.J. Ge, X.G. Ren, S.H. Huang. Spec. Spec. Anal. 22 (2002) 705. [11] B.R. Judd, Phys. Rev. 127 (1962) 750. [12] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [13] T. Ozawa, Polymer 12 (1971) 150. [14] J.W. Christian, The Theory of Transformations in Metals and Alloys – Part I, second ed., Pergamon, Oxford, 1975. [15] M. Abril, J. Me´ndez-Ramos, I.R. Martı´n, U.R. Rodrı´guez-Mendoza, V. Lanı´n, P. Nu´n˜ez, A.D. Lozano-Gorrı´n, J. Appl. Phys. 95 (2004) 5271. [16] D. Chen, Y. Wang, Y. Yu, E. Ma, Z. Hu, J. Phys. Condens. Matter 17 (2005) 6545.
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A new transparent oxyfluoride glass ceramic with improved luminescence Yunlong Yu, Daqin Chen, Yuansheng Wang *, Feng Liu, En Ma State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Received 17 January 2006; received in revised form 17 October 2006
Abstract A new type of glass ceramic containing BaF2 nano-crystals was prepared by melt quenching. Differential scanning calorimetry, X-ray diffraction and transmission electron microscopy were used to study its thermal behaviors and structural characteristics. Based on Judd– Ofelt theory, the spectroscopic properties of the 4I13/2 ! 4I15/2 transition of Er3+ in glass ceramic were evaluated. Notably, it is found that the fluorescence lifetime in the present system is much longer than that in most other glasses and glass ceramics. A comparative study on luminescence performance suggests that the obtained glass ceramic is a promising material for Er3+ doped fiber amplifiers. 2007 Elsevier B.V. All rights reserved. PACS: 51.70.+f Keywords: Glass ceramics; Optical fibers; Optical spectroscopy; STEM/TEM; Nano-crystals; Luminescence; Silicates
1. Introduction Over the past decades, inspirited by the rapid development of optical devices, such as solid lasers and optical amplifiers, transparent host materials based on rare earth (RE) ions have been extensively investigated [1,2]. Among numerous host materials, transparent oxyfluoride glass ceramics, which combine the advantages of the excellent optical properties of fluoride and high chemical and thermal stability of oxide, have attracted a great deal of attentions [3,4]. So far, most efforts on transparent oxyfluoride glass ceramics have been focused on oxide glassy matrix containing Pb(Cd)F2, LaF3 or CaF2 fluoride crystals [4–6]. RE doped BaF2 single crystal, characterized by the low phonon energy and the large transfer coefficient between the RE ions, has been revealed to be a suitable host to achieve laser and up-conversion [7,8]. Some investigations on the material with BaF2 crystals in germanate glassy *
Corresponding author. Tel./fax: +86 591 8370 5402. E-mail address: [email protected] (Y. Wang).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.029
matrix have been reported [9,10]. Taking into account the system compatibility with the standard silica fiber, the study on the silicate glassy host containing Er:BaF2 crystals remains a topic of practical application interest. In this paper, we report a new transparent glass ceramic fabricated by melt quenching and subsequent heating, and the investigation on its thermal behavior and structural characteristics. Based on Judd–Ofelt theory [11,12], the luminescence characteristics, such as spontaneous emission probability, stimulated emission cross-section and radiative lifetime, of the glass ceramic were evaluated. Moreover, the comparative analyses on the optical performances of this material and some other hosts were also performed. 2. Experimental Glass with the composition of 68SiO2–15BaF2– 13K2CO3–3La2O3–1Sb2O3–0.5ErF3 (in mol%) was prepared from high purity reagents (>3 N). The mixed batch, about 15 g, was melted in a covered platinum crucible under air atmosphere at 1300 C for 1 h, and then cast into
406
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
a copper mold, followed by annealing at a temperature 50 C below the glass transition temperature (Tg = 495 C) determined by differential scanning calorimetry (DSC, NETZSCH-TA4) to release the stress. To prepare the glass ceramic, the glass sample was heat treated at 600 C for 8 h. The long heating duration of 8 hours might guaranty a complete BaF2 crystallization. For the precursor glass and glass ceramic, the refractive indices determined by an Abbe refractometer are 1.546 and 1.551, respectively, while the densities measured based on the Archimede’s principle are 3.217 and 3.239 g cm3, respectively. To study the thermal behavior, DTA analyses were carried out in air atmosphere at the heating rates of 5, 10, 15, 20, 25 C/min, respectively. In order to identify the precipitated crystalline phase, X-ray diffraction (XRD) measurement was performed on an X-ray powder diffractometer (DMAX2500) with Cu Ka1 radiation. The microstructure was further analyzed by transmission electron microscopy (TEM, JEM-2010). The absorption spectra were recorded in the range of 300–1650 nm by an UV-near-infrared spectrophotometer (Lamda900) with resolution of 1.0 nm. The emission spectra were acquired with 980 nm excitation light from 450 W Xenone lamp. By using an InP/InGaAs photomultiplier tubes (PMT) detector (R5509), the infrared luminescence signals through the emission monochromator (M300) were detected. The fluorescence decay curves at 1530 nm were recorded with a NIR PMT (R5509) when excited at 980 nm by a microsecond flash lamp (lF900). All the measurements were carried out at room temperature.
580 C (Tp) corresponding to the crystallization of BaF2 confirmed by XRD analysis. Fig. 2 shows the XRD patterns of the precursor glass and the heat-treated sample. The diffuse hump for the precursor glass evidences its amorphous structure. However, several sharp crystalline peaks emerge, indicating the precipitation of BaF2 phase from the glass matrix, for the heat-treated sample. Using the XRD peak widths and Scherrer equation, the mean size of the crystallites was evaluated to be about 11 nm. The TEM bright field image of the glass ceramic, as presented in Fig. 3, demonstrates the homogeneous distribution of spherical BaF2 crystals with 8–12 nm in size among the glassy matrix. The inset of Fig. 3 is the selected area electron diffraction (SAED) pattern showing BaF2 polycrystalline rings. From the data of non-isothermal experiments, two most important kinetic parameters for BaF2 crystallization, i.e., the apparent activation energy (Ea) and the Avrami exponent (n), were determined by Ozawa equation
3. Results The precursor glass was visually transparent, appearing pink due to Er3+. The DSC trace is shown in Fig. 1, which exhibits a unique exothermal peak centered at about
Fig. 2. The XRD patterns of the precursor glass and glass ceramic.
Fig. 1. DSC trace of the precursor glass recorded at a heating rate of 10 K/min.
Fig. 3. TEM bright field image and the corresponding SAED pattern of the glass ceramic.
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
Fig. 4. Variation of lattice parameter of cubic BaF2 as a function of ErF3 content in the glass ceramic.
[13] to be 383 ± 20 kJ and 1.25 ± 0.05, respectively, suggesting that the crystallization mechanism is a diffusioncontrolled growth process with zero nucleation rate [14]. To study the influence of Er3+ doping level on the precipitated phase, glass ceramics with various content of ErF3 were prepared and examined by XRD. It is evident that, for BaF2 crystals, the mean size keeps almost the same, while the lattice parameter deduced from the position of XRD peaks varies with ErF3 content, as shown in Fig. 4. The doping of ErF3 results in the lattice contraction of BaF2 phase, which implies the incorporation of Er3+ into BaF2:Er3+ ions with radius of 0.114 nm substitute for the bigger Ba2+ with radius of 0.156 nm. The lattice contraction is remarkable when Er3+ content is low (<0.1%), with further increase of ErF3 content from 0.1% to 2.0%, the lattice parameter decreases slightly but monotonously. It is thus proposed that when Er3+ doping content is below 0.1%, lots of them incorporate into BaF2; however, Er:BaF2 soon approaches its solid solution limit, thus further increasing of ErF3 to the sample results in only minor supplementation of Er3+ to BaF2. The application of Judd–Ofelt theory to evaluate the spectroscopic properties of RE in oxyfluoride glass ceramics has been reported by several authors [15–17]. Although there are some doubts about this application on the heterogeneous composite system, the method seems to be still a best way to evaluate some spectroscopic parameters at present. Based on the data from the absorption experiments and Judd–Ofelt theory, the intensity parameters, Xt (t = 2, 4, 6), for the precursor glass and glass ceramic, determined from a least square fitting between the experi-
Table 1 Judd–Ofelt parameters, Xt (in unit of 1020 cm2), of Er3+ in the precursor glass and glass ceramic (rms: the root-mean-square errors) Sample
X2
X4
X6
Rms (%)
X4/X6
Glass Glass ceramic
4.88 3.81
0.95 0.57
0.37 0.18
1.6% 2.9%
2.57 3.17
407
Fig. 5. The emission spectrum of Er3+ 4I13/2 ! 4I15/2 transition in the glass and glass ceramic. The inset shows the absorption (dot line) and the stimulated (solid line) emission cross-sections determined from the absorption and emission spectra.
mental oscillator strengths and the calculated oscillator strengths, are listed in Table 1. The emission spectra of Er3+ 4I13/2 ! 4I15/2 transition in the precursor glass and glass ceramic are presented in Fig. 5, from which the emission peak wavelength kem and the width of the emission band Dkeff are obtained. Based on the Judd–Ofelt intensity parameters, the spontaneous emission probability Atotal, the radiative lifetime scal and the stimulated emission cross-section rem are evaluated. The absorption cross-section ra, determined from the absorption spectrum, and the stimulated emission crosssection are presented in the inset of Fig. 5. By fitting the fluorescence decay curve, the lifetime sexp of 4I13/2 level was estimated, which was then used to deduce the quantum efficiency (g = sexp/srad) of this level. All the acquired results are listed in Table 2. 4. Discussion According to Judd–Ofelt theory, X2 is sensitive to the local structure surrounding RE ions, and it will decrease with the change of RE ion from a covalent bond with the oxide ligand to a more ionic (less covalent) bond with the fluoride [18,19]. Therefore, the decrease of X2 during the crystallization of the precursor glass indicates the incorporation of RE ions into the fluoride nano-crystals, which is in accordance with the result deduced from the variation of BaF2 lattice parameter with ErF3 content stated above. On the other hand, the value of X6, which is proportional to the rigidity of the host [15], also decreases in the transition from glass to glass ceramic. X4/X6 is the spectroscopic quality factor, which is usually used to characterize the intensity of the major laser transitions of Er3+ [20]. It was described by Sardar et al. [21] that the larger the X4/ X6 value the more intense the laser transitions are. The spectroscopic quality factor for Er3+ in the precursor glass
408
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
Table 2 Some spectroscopic parameters of Er3+ 4I13/2 ! 4I15/2 transition in the glass ceramic Atotal (s1)
scal (ms)
sexp (ms)
g (%)
kem (nm)
Dkeff (nm)
rem (ke) (1020 cm2)
53.56
18.67
13.39
72
1537
37.8
0.44
and glass ceramic is 2.57 and 3.17, respectively, within the range of 0.126–3.372 for Er3+ in various hosts [22]. These values, especially that for the glass ceramic, are much larger than those found in most other glasses, glass ceramics and Er3+:YAG single-crystal (X4/X6 = 0.32) reported previously [21,23], indicating that this material might be a desirable candidate for stimulated emission. The lifetime of Er3+ 4I13/2 level is an important parameter for the optical amplifier. A critical factor in the success of Er3+ doped fiber amplifier in optical communication is the long lifetime of the metastable state that permits the required high population inversions to be obtained under steady-state conditions using modest pump powers [24]. As shown in Table 3, compared to that in other host materials, the measured lifetime (sexp = 13.39 ms) of 4I13/2 for Er3+ in the present glass ceramic is remarkably longer, which is about 3.5 times longer than that of Er3+:YAG transparent ceramic (sexp = 3.1 ms) [2]. The long lifetime of Er3+ in the present glass ceramic could be explained as follows: firstly, since part of Er3+ ions incorporate into BaF2 nano-crystal, the low phonon energy (346 cm1 [25]) of BaF2 host depresses the non-radiative relaxation rates; secondly, in glass matrix, the strong columbic repulsion between RE ions and residual Ba2+ ions at non-bridging sites cumber the clustering of Er–Er ions, which result in the weakening of the concentration quenching effect [26]. For an amplifier device, the figure-of-merit (FOM) for gain is defined as the product of lifetime and emission
cross-section: s · rem [26]. Since the gain bandwidth of optical amplifier is mainly determined by the emission width and the stimulated emission cross-section, FOM for bandwidth is defined as Dk · rem [24,26]. As shown in Table 4, although the value of FOM for bandwidth of the present glass ceramic is smaller than those of the glass ceramics containing CaF2 and PbF2 crystals report previously, it is still larger than that of the commercial Al silica fiber. However, the FOM for gain of the present system is preferable to those of the other glass ceramics, and equivalent to that of the Al silica fiber, which indicates the present glass ceramic a promising host material for Er3+ doped fiber amplifiers. 5. Conclusion A new type of Er3+ doped transparent oxyfluoride glassy ceramic with BaF2 nano-crystals homogenously dispersed among glassy matrix was prepared. The crystallization mechanism of BaF2 was revealed to be a diffusioncontrolled growth process with zero nucleation rate. The spectroscopic quality factor for Er3+ in the obtained glass ceramic is 10 times as larger as that in the Er3+:YAG crystal. The fluorescence lifetime of Er3+ 4I13/2 ! 4I15/2 transition in this material is remarkably longer than that in other glasses and glass ceramics reported previously. These results imply that this glass ceramic might be a promising material for Er3+ doped fiber amplifiers.
Table 3 Lifetime of Er3+ 4I13/2 level in the different hosts GC sexp (ms) Reference
Containing BaF2
Containing CaF2
Containing PbF2
Containing LaF3
YAG ceramic
13.39 Present work
7.56 [16]
4.0 [17]
7.2 [27]
3.1 [2]
ZBLAN
Silicate
Phosphate
Tellurite
Bismuth
10.3 [28]
7 [29]
5.4 [30]
7.5 [31]
2.7 [32]
G sexp (ms) Reference
(GC: oxyfluoride glass ceramic, G: glass).
Table 4 Comparison of FOM for gain and for bandwidth of Er3+ in different oxyfluoride glass ceramics (GC) and the commercial Al silica fiber used for device fabrication Host materials
FOM for gain (s · rem) (1020 ms cm2)
FOM for bandwidth (Dk · rem) (1020 nm cm2)
Reference
GC containing BaF2 GC containing CaF2 GC containing PbF2 Al silica fiber
5.7 4.4 2.1 5.7
16.7 40.6 32.3 13.4
Present work [16] [17,33] [26]
Y. Yu et al. / Journal of Non-Crystalline Solids 353 (2007) 405–409
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