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Structure and spectroscopic properties of germanium dioxide doped phosphate glasses
Materials Science & Engineering B 240 (2019) 121–124
Contents lists available at ScienceDirect
Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Structure and spectroscopic properties of germanium dioxide doped phosphate glasses ⁎
Chao Wanga,b, Yajie Wanga,b, Liyan Zhanga, , Danping Chena, a b
T
⁎
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: GeO2 Phosphate glass Gain coefficient
Germanium dioxide (GeO2)-doped phosphate glasses with large gain coefficient and systematical factor (SFL) were prepared by high temperature melting for laser glass applications. Their physical parameters and optical properties were investigated such as glass transition temperature (Tg), density, Raman spectra, fluorescence emission spectra, decay cure and absorption spectra. Compared with the traditional phosphate glass, the introduction of GeO2 to phosphate glass results in a high fluorescent intensity and a long lifetime. All these advantages might mean that GeO2-doped phosphate glasses are a promising candidate as a gain medium for ultra-fast and high-power laser.
1. Introduction As a kind of gain medium of 1 μm lasers, Ytterbium (Yb3+)-doped laser materials [1] have attracted much attention for many applications, such as tunable and ultrafast lasers [2–6]. The Yb3+ ion has a simple energy level scheme, consisting of two manifolds, namely, the 2 F7/2 ground state and the 2F5/2 excited state. Owing to this unique level structure, Yb3+ lasers have great advantages such as low heat generation, efficient energy storage in the excited state [7], broad absorption and emission bandwidth, which lead to avoidance of unwanted processes [8] including excited absorption and concentration quenching. Furthermore, the intrinsic small quantum defect [9] in the Yb3+ laser is expected for operation at a high power level. Among the popular laser materials that were investigated for the Yb3+ dopant, phosphate glasses [10] are of immense interest because they exhibit facile glass-forming ability, good chemical durability, low optical dispersion, broad transition range from ultraviolet to infrared. Moreover, phosphate glass can be doped with 26 wt% without concentration quenching [11] because of the high solubility of lanthanum ions, which allows a high gain density to obtain a laser output at a relatively low pumped threshold condition. The luminescent property of Yb3+ in phosphate glasses is related with the local environment around it. Phosphate glasses can be made with a range of structures, from a cross-linked network of tetrahedra. The tetrahedra are classified using the Qi terminology, where i represents the number of bridging oxygen per tetrahedron [12]. The fraction of Ge-O-Ge and Ge-O-P
⁎
linkages in the mixed-network former glasses GeO2-NaPO3 [13] increases as the germanium dioxide increases, while the fraction of the PO-P linkages decreases. Furthermore, the luminescent property could be improved by chancing the network structures to enhance the asymmetry degree around the Yb3+ ligand. In this work, we report on a structural and spectroscopic study of the GeO2 doped P2O5-Al2O3-Yb2O3 glass system. As a network former, GeO2 is added into the glass system and linked with P tetrahedra to create a variation of the Yb3+ ligand’s local environment. In this paper, we develop a new kind of lasing phosphate glass with a good luminescent property for high-peak and high-average power based on this variation [14,15]. 2. Experiment The molar composition of glass was (85-x)P2O5-15Al2O3-xGeO2 (x = 0, 5, 15, which was simplified as PG0, PG1, and PG2, respectively). Yb2O3 with an active dopant level of 5 wt% was added into this glass. The glasses were prepared through the conventional meltquenching method, as shown in Fig. 1. The raw materials were first melted in a SiO2 crucible, and the glass liquid was bubbled with O2 for the OH− removal. The melt was then transferred to a Pt crucible. After the clarifying and stirring process, the glasses were cast into heated steel molds for annealing, then cut and polished to 0.5 mm for the spectroscopic tests. The refractive index (n) was measured by the V prism method. The
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (D. Chen).
https://doi.org/10.1016/j.mseb.2019.01.019 Received 31 December 2017; Received in revised form 1 December 2018; Accepted 22 January 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 240 (2019) 121–124
C. Wang et al.
Fig. 1. Flow chart of glass preparation.
glass transition temperature (Tg) was determined using a NetzschSTA449/C differential scanning calorimeter (DSC) at a heating rate of 10 °C/min. The absorption spectra were recorded with a PerkinElmer Lambda 900 UV/VIS/NIR spectrophotometer in the range of 900–1000 nm. The emission spectra were recorded by a FLSP920 spectrofluorimeter (Edinburg Co., UK) with an 896 nm pump. All data were obtained at room temperature.
Fig. 3. Raman spectra of the three PG samples.
intensity of the two bands decreased, (i.e., one at 728–761 cm−1 (marked 3) and another at 1154–1172 cm−1 (marked 4)), which could be assigned to the vibration modes of symmetric stretching of P-O-P in P1 units and the symmetric stretching of a non-bridging oxygen on a P2 tetrahedron, respectively. This result could reflect the continuous change from P-O-P to Ge-O-P and Ge-O-Ge connectivity.
3. Results and discussion 3.1. Glass transition temperature and density Fig. 2 shows the glass transition temperature and density of the series PG glasses. Tg and ρ sharply increased with the increasing GeO2 content. This result could be attributable to the formation of the Ge-O network [16,17]. The single-network former glass become a doublenetwork former glass because of the GeO2 addition. Furthermore, the glass network structure could be compact.
3.3. Absorption and emission cross-section spectra The performance of Yb3+-glass can be accessed from the spectroscopic properties, such as absorption and emission cross-section, which can be obtained from the absorption spectra. The absorption crosssection is given as follows [21]:
3.2. Raman
2.303lg σabs (λ ) =
The Raman spectra of samples PG0, PG1, and PG2 are shown in Fig. 3 to investigate the variation of glass structure. In reference [13], the band at 390–410 cm−1 for phosphate glass was attributed to the OP-O bending modes. The band near 620 cm−1 was assigned to the symmetric stretching of P-O-P linkages. For the germanate glass, a fourfold coordinated germanium produced the Raman band at 400 cm−1, which was attributed to the bending vibrations of GeO4 [18]. The 558–665 cm−1 region bands were assigned to symmetric stretching modes of Ge-O-Ge [19]. In this work, the intensity of bands at 410 cm−1 (marked 1) and 620 cm−1 (marked 2) increased upon GeO2 addition to the phosphate glass because of the formation of Ge-OP and Ge-O-Ge [13,20] in the phosphate glass network. Meanwhile, the
( ) I0 I
(1)
Nl
where σabs is the absorption cross-section; l is the sample thickness; I lg I0 is the absorbance; and N is the Yb3+ ion concentration (ions/ 3 cm ), given as:
( )
N=2×
ρ × wt % × NA M
(2)
where ρ is the density; M is the relative molecular mass of Yb2O3; and wt% is the weight percent of Yb2O3; NA is Avogadro number. The McCumber method was used to calculate the emission cross-section of Yb3+ ions [22]:
σemi (λ ) = σabs (λ )
Ezl − hcλ−1 ⎞ Zl hc exp ⎛ (λp−1 − λ−1)⎤ = 1.33σabs (λ ) exp ⎡ Zu kT ⎦ ⎣ kT ⎝ ⎠ ⎜
⎟
(3) where Zl/ Zu is the partition function about 4/3; T is the room temperature about 298 K; Ezl is the zero line energy, defined as the energy separation between the lowest components of the upper (2F5/2) and lower states (2F7/2), λp is wavelength of main absorption peak about 974 nm; and k, h, and c are the Boltzmann constant, the Plank constant, and the velocity of light, respectively. Fig. 4 shows the absorption and emission cross-section spectra of the samples for different GeO2 concentrations. As for the important parameter of the laser performance, the values of emission cross-section of PG0, PG1, and PG2 at 1003 nm were 1.29 × 10−20 cm2, 1.18 × 10−20 cm2, 1.09 × 10−20 cm2, respectively. The absorption cross-section was a negative correction with density; hence, the emission cross-section was reduced with density increasing.
Fig. 2. Dependence of the glass transition temperature and density on the GeO2 content. 122
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Fig. 4. Absorption cross-section (a) and emission cross-section (b) of Yb3+ in the series PG samples.
3.4. Emission spectra The emission spectra of the PG samples pumped at 896 nm in the 925 nm–1100 nm wavelength range were measured to investigate the GeO2 contents dependence of the fluorescence at 1 μm. Fig. 5 presents the results. The luminescent intensity of Yb3+ clearly increased and the shapes of the fluorescence spectra were not much difference between them as the GeO2 contents increase. The intensity of sample PG1 was clearly higher than that of sample PG0, whereas the intensity of samples PG1 and PG2 had little distinction. This result can be explained by the local environment of Yb3+ ligand being changed because of the formation of Ge-O-P and Ge-O-Ge in phosphate glass network. Therefore, the high asymmetry degree of the Yb3+ ligand and the Yb3+ ions dispersion promoted the enhancement of luminescence properties. 3.5. Fluorescence lifetime The measured fluorescence lifetime (τm ) of Yb3+ was illustrated in Fig. 6 and listed in Table 1. Fig. 6 shows that the measured lifetime enhanced with the increasing GeO2 content. The improvement of the Yb3+ ions dispersion was responsible for the increase of fluorescence lifetime. The radiative lifetime (τc ) was calculated by Eq. (4) based on the integrated absorption cross-section in Table 1.
τc =
3λ4 32πcn2Σabs
Fig. 6. Fluorescence decay curve of samples. Table 1 Integrated absorption cross section (Σabs ), radiative lifetime (τc ) and fluorescence lifetime (τm ) in the series PG samples.
(4)
where λ is the main absorption peak wavelength (974 nm); n is the refractive index (approximately 1.5); c is the speed of light in a vacuum
Samples
Σabs (10−20cm2*nm)
τc (μs)
τm (μs)
PG0 PG1 PG2
75.5 70.8 62.3
527 562 634
456 532 657
and Σabs is the integrated absorption cross-section. Seen from Table 1, the measured fluorescence lifetime was lower than radiative lifetime in samples. The fluorescence trapping effect of Yb3+ ions did not exist in the PG samples.
3.6. Laser performance parameters The figure of merits of the Yb3+ doped laser material was given by Imin , defined as a measure of the ease of pumping the laser material. This parameter is important in evaluating the laser performance. Imin shows the minimum pumping intensity. Meanwhile, Imin is calculated as follows [23]:
Imin = βmin × Isat
(5)
where Fig. 5. Emission spectra of the Yb3+-doped PG glasses with different GeO2 contents.
βmin =
123
σabs (λlaser ) σabs (λlaser ) + σemi (λlaser )
(6)
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Table 2 Spectroscopic and laser performance parameters of the PG samples. Sample
σabs ( λlaser ) (10−20cm2)
σabs ( λp ) (10−20cm2)
G = σemi *τm (10−20cm2*ms)
βmin
Isat (kW/cm2)
Imin (kW/cm2)
SFL
PG0 PG1 PG2 LSY [25] SBB15 [26] QX [27]
0.23 0.21 0.19
2.94 2.99 2.39
0.59 0.63 0.72 0.37 0.7 1.4
0.1513 0.1510 0.1484 0.0785 0.109 0.171
15.2 12.7 12.9 22.23 17.24 10.79
2.300 1.918 1.914 1.75 1.88 1.82
0.26 0.33 0.38 0.211 0.372 0.769
Isat =
hc λp σabs (λp) τm
glasses had large gain coefficient and SFL parameters, compared with the traditional phosphate glass. Thus, PG glass is a competitive gain medium that can be applied to the high-peak power and high-average power for next-generation nuclear fusion.
(7)
where βmin is the minimum fraction of the Yb3+ ions that must be excited to balance the gain exactly with the ground-state absorption at the laser wavelength; Isat is the pumping saturation intensity; λlaser is 1003 nm; and λp is 974 nm. A systematical factor (SFL) was generally evaluated and defined as follows to assess the laser performance of Yb3+ doped laser glasses [24]:
SFL =
Acknowledgment This work was funded by the National Natural Science Foundation of China (Grant No. 51872308).
σemi × τm Imin
References [1] S. Cui, L. Zhang, H. Jiang, Y. Feng, Chin. Opt. Lett. 15 (2017) 041402. [2] D. Ehrt, Curr. Opin. Solid State Mater. Sci. 7 (2003) 135–141. [3] B.I. Galagan, I.N. Glushchenko, B.I. Denker, V.E. Kisel, S.V. Kuril'chik, N.V. Kuleshov, S.E. Sverchkov, IEEE J. Quantum Elect. 39 (2009) 891. [4] C. Hönninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees, A. Braun, G.A. Mourou, I. Johannsen, A. Giesen, W. Seeber, U. Keller, Appl. Phys. B 69 (1999) 3–17. [5] R. Lan, L. Pan, I. Utkin, Q. Ren, H. Zhang, Z. Wang, R. Fedosejevs, Opt. Express 18 (2010) 4000–4005. [6] V. Petrov, U. Griebner, D. Ehrt, W. Seeber, Opt. Lett. 22 (1997) 408–410. [7] K. Krishnaiah, C. Jayasankar, V. Venkatramu, S. Leόn-Luis, V. Lavín, S. Chaurasia, L. Dhareshwar, Appl. Phys. B 113 (2013) 527–535. [8] D. Ehrt, Phys. Chem. Glasses B 56 (2015) 217–234. [9] U. Griebner, R. Koch, H. Schönnagel, S. Jiang, M. Myers, D. Rhonehouse, S. Hamlin, W. Clarkson, D. Hanna, Adv. Solid State Lasers Optical Soc. Am. (1996) YL7. [10] D. He, S. Kang, L. Zhang, L. Chen, Y. Ding, Q. Yin, L. Hu, High Power Laser Sci. Eng. 5 (2017) e1. [11] J. Zhu, P. Zhou, X. Wang, X. Xu, Z. Liu, IEEE J. Quantum Elect. 48 (2012) 480–484. [12] R.K. Brow, J. Non-Cryst. Solids 263–264 (2000) 1–28. [13] J. Ren, H. Eckert, J. Phys. Chem. C 116 (2012) 12747–12763. [14] X. Chen, J. Wang, X. Zhao, G. Bai, W. Gong, K. Liu, C. Zhao, X. Li, H. Pi, J. Li, Y. Yang, B. He, J. Zhou, Chin. Opt. Lett. 15 (2017) 071407. [15] M. Jiang, P. Ma, L. Huang, J. Xu, P. Zhou, X. Gu, High Power Laser Sci. Eng. 5 (2017) e30. [16] G.S. Henderson, D.R. Neuville, B. Cochain, L. Cormier, J. Non-Cryst. Solids 355 (2009) 468–474. [17] L. Peng, J.F. Stebbins, J. Non-Cryst. Solids 353 (2007) 4732–4742. [18] V.N. Sigaev, I. Gregora, P. Pernice, B. Champagnon, E.N. Smelyanskaya, A. Aronne, P.D. Sarkisov, J. Non-Cryst. Solids 279 (2001) 136–144. [19] M.R. Sahar, A.W.M.A. Hussein, R. Hussin, J. Non-Cryst. Solids 353 (2007) 1134–1140. [20] S. Kumar, S. Murugavel, K. Rao, J. Phys. Chem. B 105 (2001) 5862–5873. [21] S. Dai, J. Yang, L. Wen, L. Hu, Z. Jiang, J. Lumin. 104 (2003) 55–63. [22] D. McCumber, Phys. Rev. 136 (1964) A954. [23] X. Zou, H. Toratani, Phys. Rev. B 52 (1995) 15889. [24] P. Wang, B. Peng, W. Li, C.Q. Hou, J. She, H. Guo, M. Lu, Solid State Sci. 14 (2012) 550–553. [25] Z. Xuelu, T. Hisyoshi, Phys. Rev. B 52 (1995) 15889. [26] N. Dai, L. Hu, J. Yang, S. Dai, A. Lin, J. Alloy Compd. 363 (2004) 1–5. [27] D.L. Deloach, A.S. Payne, K.L. Smith, W.L. Kway, J. Opt. Soc. Am. B 11 (1994) 269.
A large SFL value meant a good luminescence performance of the laser glasses. Table 2 lists the parameters of the PG samples (e.g., laser gain coefficient G and SFL). For laser glasses σemi *τm is generally desirable for to be as large as possible to provide high gain, for Imin to be as small as possible to minimize the pump losses. Therefore, SFL is very useful to evaluate both the pump losses and the product of σemi and τm . The emission crosssection σemi had an important effect on laser properties of the Yb3+ ions because a larger σemi indicated a higher laser gain. In spite of the lower σemi , the series PG glasses exhibited higher gain coefficient values (G = σemi *τm ) because of the longer τm . Table 2 shows that SFL values increased with the increasing GeO2 content in the series PG glasses. The value of G and SFL for PG2 are 0.72 and 0.38, respectively, which are higher than those of LSY glass and near to SBB15 glass, but lower than commercial QX glass. The larger the SFL, the better general laser properties the laser glasses had. Therefore, we believe that the PG glasses were a promising laser glass for high-peak power and highaverage power. 4. Conclusion In summary, we prepared a novel GeO2 modified Yb: phosphate glass with large fluorescent emission intensity and gain coefficient. The glass transition temperature and density sharply increased with the increasing GeO2 content because the single-network former glass became a double-network former glass. Moreover, the Raman spectra demonstrated that the glass network structure could be compact. As an important parameter of the laser performance, the values of emission cross-section of PG0, PG1, and PG2 at 1003 nm were 1.29 × 10−20 cm2, 1.18 × 10−20 cm2, 1.09 × 10−20 cm2, respectively. However, the series PG glasses had a long lifetime because of the improvement of Yb3+ ions dispersion. The results showed that GeO2 doped phosphate
124
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Structure and spectroscopic properties of germanium dioxide doped phosphate glasses ⁎
Chao Wanga,b, Yajie Wanga,b, Liyan Zhanga, , Danping Chena, a b
T
⁎
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: GeO2 Phosphate glass Gain coefficient
Germanium dioxide (GeO2)-doped phosphate glasses with large gain coefficient and systematical factor (SFL) were prepared by high temperature melting for laser glass applications. Their physical parameters and optical properties were investigated such as glass transition temperature (Tg), density, Raman spectra, fluorescence emission spectra, decay cure and absorption spectra. Compared with the traditional phosphate glass, the introduction of GeO2 to phosphate glass results in a high fluorescent intensity and a long lifetime. All these advantages might mean that GeO2-doped phosphate glasses are a promising candidate as a gain medium for ultra-fast and high-power laser.
1. Introduction As a kind of gain medium of 1 μm lasers, Ytterbium (Yb3+)-doped laser materials [1] have attracted much attention for many applications, such as tunable and ultrafast lasers [2–6]. The Yb3+ ion has a simple energy level scheme, consisting of two manifolds, namely, the 2 F7/2 ground state and the 2F5/2 excited state. Owing to this unique level structure, Yb3+ lasers have great advantages such as low heat generation, efficient energy storage in the excited state [7], broad absorption and emission bandwidth, which lead to avoidance of unwanted processes [8] including excited absorption and concentration quenching. Furthermore, the intrinsic small quantum defect [9] in the Yb3+ laser is expected for operation at a high power level. Among the popular laser materials that were investigated for the Yb3+ dopant, phosphate glasses [10] are of immense interest because they exhibit facile glass-forming ability, good chemical durability, low optical dispersion, broad transition range from ultraviolet to infrared. Moreover, phosphate glass can be doped with 26 wt% without concentration quenching [11] because of the high solubility of lanthanum ions, which allows a high gain density to obtain a laser output at a relatively low pumped threshold condition. The luminescent property of Yb3+ in phosphate glasses is related with the local environment around it. Phosphate glasses can be made with a range of structures, from a cross-linked network of tetrahedra. The tetrahedra are classified using the Qi terminology, where i represents the number of bridging oxygen per tetrahedron [12]. The fraction of Ge-O-Ge and Ge-O-P
⁎
linkages in the mixed-network former glasses GeO2-NaPO3 [13] increases as the germanium dioxide increases, while the fraction of the PO-P linkages decreases. Furthermore, the luminescent property could be improved by chancing the network structures to enhance the asymmetry degree around the Yb3+ ligand. In this work, we report on a structural and spectroscopic study of the GeO2 doped P2O5-Al2O3-Yb2O3 glass system. As a network former, GeO2 is added into the glass system and linked with P tetrahedra to create a variation of the Yb3+ ligand’s local environment. In this paper, we develop a new kind of lasing phosphate glass with a good luminescent property for high-peak and high-average power based on this variation [14,15]. 2. Experiment The molar composition of glass was (85-x)P2O5-15Al2O3-xGeO2 (x = 0, 5, 15, which was simplified as PG0, PG1, and PG2, respectively). Yb2O3 with an active dopant level of 5 wt% was added into this glass. The glasses were prepared through the conventional meltquenching method, as shown in Fig. 1. The raw materials were first melted in a SiO2 crucible, and the glass liquid was bubbled with O2 for the OH− removal. The melt was then transferred to a Pt crucible. After the clarifying and stirring process, the glasses were cast into heated steel molds for annealing, then cut and polished to 0.5 mm for the spectroscopic tests. The refractive index (n) was measured by the V prism method. The
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (D. Chen).
https://doi.org/10.1016/j.mseb.2019.01.019 Received 31 December 2017; Received in revised form 1 December 2018; Accepted 22 January 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 240 (2019) 121–124
C. Wang et al.
Fig. 1. Flow chart of glass preparation.
glass transition temperature (Tg) was determined using a NetzschSTA449/C differential scanning calorimeter (DSC) at a heating rate of 10 °C/min. The absorption spectra were recorded with a PerkinElmer Lambda 900 UV/VIS/NIR spectrophotometer in the range of 900–1000 nm. The emission spectra were recorded by a FLSP920 spectrofluorimeter (Edinburg Co., UK) with an 896 nm pump. All data were obtained at room temperature.
Fig. 3. Raman spectra of the three PG samples.
intensity of the two bands decreased, (i.e., one at 728–761 cm−1 (marked 3) and another at 1154–1172 cm−1 (marked 4)), which could be assigned to the vibration modes of symmetric stretching of P-O-P in P1 units and the symmetric stretching of a non-bridging oxygen on a P2 tetrahedron, respectively. This result could reflect the continuous change from P-O-P to Ge-O-P and Ge-O-Ge connectivity.
3. Results and discussion 3.1. Glass transition temperature and density Fig. 2 shows the glass transition temperature and density of the series PG glasses. Tg and ρ sharply increased with the increasing GeO2 content. This result could be attributable to the formation of the Ge-O network [16,17]. The single-network former glass become a doublenetwork former glass because of the GeO2 addition. Furthermore, the glass network structure could be compact.
3.3. Absorption and emission cross-section spectra The performance of Yb3+-glass can be accessed from the spectroscopic properties, such as absorption and emission cross-section, which can be obtained from the absorption spectra. The absorption crosssection is given as follows [21]:
3.2. Raman
2.303lg σabs (λ ) =
The Raman spectra of samples PG0, PG1, and PG2 are shown in Fig. 3 to investigate the variation of glass structure. In reference [13], the band at 390–410 cm−1 for phosphate glass was attributed to the OP-O bending modes. The band near 620 cm−1 was assigned to the symmetric stretching of P-O-P linkages. For the germanate glass, a fourfold coordinated germanium produced the Raman band at 400 cm−1, which was attributed to the bending vibrations of GeO4 [18]. The 558–665 cm−1 region bands were assigned to symmetric stretching modes of Ge-O-Ge [19]. In this work, the intensity of bands at 410 cm−1 (marked 1) and 620 cm−1 (marked 2) increased upon GeO2 addition to the phosphate glass because of the formation of Ge-OP and Ge-O-Ge [13,20] in the phosphate glass network. Meanwhile, the
( ) I0 I
(1)
Nl
where σabs is the absorption cross-section; l is the sample thickness; I lg I0 is the absorbance; and N is the Yb3+ ion concentration (ions/ 3 cm ), given as:
( )
N=2×
ρ × wt % × NA M
(2)
where ρ is the density; M is the relative molecular mass of Yb2O3; and wt% is the weight percent of Yb2O3; NA is Avogadro number. The McCumber method was used to calculate the emission cross-section of Yb3+ ions [22]:
σemi (λ ) = σabs (λ )
Ezl − hcλ−1 ⎞ Zl hc exp ⎛ (λp−1 − λ−1)⎤ = 1.33σabs (λ ) exp ⎡ Zu kT ⎦ ⎣ kT ⎝ ⎠ ⎜
⎟
(3) where Zl/ Zu is the partition function about 4/3; T is the room temperature about 298 K; Ezl is the zero line energy, defined as the energy separation between the lowest components of the upper (2F5/2) and lower states (2F7/2), λp is wavelength of main absorption peak about 974 nm; and k, h, and c are the Boltzmann constant, the Plank constant, and the velocity of light, respectively. Fig. 4 shows the absorption and emission cross-section spectra of the samples for different GeO2 concentrations. As for the important parameter of the laser performance, the values of emission cross-section of PG0, PG1, and PG2 at 1003 nm were 1.29 × 10−20 cm2, 1.18 × 10−20 cm2, 1.09 × 10−20 cm2, respectively. The absorption cross-section was a negative correction with density; hence, the emission cross-section was reduced with density increasing.
Fig. 2. Dependence of the glass transition temperature and density on the GeO2 content. 122
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Fig. 4. Absorption cross-section (a) and emission cross-section (b) of Yb3+ in the series PG samples.
3.4. Emission spectra The emission spectra of the PG samples pumped at 896 nm in the 925 nm–1100 nm wavelength range were measured to investigate the GeO2 contents dependence of the fluorescence at 1 μm. Fig. 5 presents the results. The luminescent intensity of Yb3+ clearly increased and the shapes of the fluorescence spectra were not much difference between them as the GeO2 contents increase. The intensity of sample PG1 was clearly higher than that of sample PG0, whereas the intensity of samples PG1 and PG2 had little distinction. This result can be explained by the local environment of Yb3+ ligand being changed because of the formation of Ge-O-P and Ge-O-Ge in phosphate glass network. Therefore, the high asymmetry degree of the Yb3+ ligand and the Yb3+ ions dispersion promoted the enhancement of luminescence properties. 3.5. Fluorescence lifetime The measured fluorescence lifetime (τm ) of Yb3+ was illustrated in Fig. 6 and listed in Table 1. Fig. 6 shows that the measured lifetime enhanced with the increasing GeO2 content. The improvement of the Yb3+ ions dispersion was responsible for the increase of fluorescence lifetime. The radiative lifetime (τc ) was calculated by Eq. (4) based on the integrated absorption cross-section in Table 1.
τc =
3λ4 32πcn2Σabs
Fig. 6. Fluorescence decay curve of samples. Table 1 Integrated absorption cross section (Σabs ), radiative lifetime (τc ) and fluorescence lifetime (τm ) in the series PG samples.
(4)
where λ is the main absorption peak wavelength (974 nm); n is the refractive index (approximately 1.5); c is the speed of light in a vacuum
Samples
Σabs (10−20cm2*nm)
τc (μs)
τm (μs)
PG0 PG1 PG2
75.5 70.8 62.3
527 562 634
456 532 657
and Σabs is the integrated absorption cross-section. Seen from Table 1, the measured fluorescence lifetime was lower than radiative lifetime in samples. The fluorescence trapping effect of Yb3+ ions did not exist in the PG samples.
3.6. Laser performance parameters The figure of merits of the Yb3+ doped laser material was given by Imin , defined as a measure of the ease of pumping the laser material. This parameter is important in evaluating the laser performance. Imin shows the minimum pumping intensity. Meanwhile, Imin is calculated as follows [23]:
Imin = βmin × Isat
(5)
where Fig. 5. Emission spectra of the Yb3+-doped PG glasses with different GeO2 contents.
βmin =
123
σabs (λlaser ) σabs (λlaser ) + σemi (λlaser )
(6)
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Table 2 Spectroscopic and laser performance parameters of the PG samples. Sample
σabs ( λlaser ) (10−20cm2)
σabs ( λp ) (10−20cm2)
G = σemi *τm (10−20cm2*ms)
βmin
Isat (kW/cm2)
Imin (kW/cm2)
SFL
PG0 PG1 PG2 LSY [25] SBB15 [26] QX [27]
0.23 0.21 0.19
2.94 2.99 2.39
0.59 0.63 0.72 0.37 0.7 1.4
0.1513 0.1510 0.1484 0.0785 0.109 0.171
15.2 12.7 12.9 22.23 17.24 10.79
2.300 1.918 1.914 1.75 1.88 1.82
0.26 0.33 0.38 0.211 0.372 0.769
Isat =
hc λp σabs (λp) τm
glasses had large gain coefficient and SFL parameters, compared with the traditional phosphate glass. Thus, PG glass is a competitive gain medium that can be applied to the high-peak power and high-average power for next-generation nuclear fusion.
(7)
where βmin is the minimum fraction of the Yb3+ ions that must be excited to balance the gain exactly with the ground-state absorption at the laser wavelength; Isat is the pumping saturation intensity; λlaser is 1003 nm; and λp is 974 nm. A systematical factor (SFL) was generally evaluated and defined as follows to assess the laser performance of Yb3+ doped laser glasses [24]:
SFL =
Acknowledgment This work was funded by the National Natural Science Foundation of China (Grant No. 51872308).
σemi × τm Imin
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A large SFL value meant a good luminescence performance of the laser glasses. Table 2 lists the parameters of the PG samples (e.g., laser gain coefficient G and SFL). For laser glasses σemi *τm is generally desirable for to be as large as possible to provide high gain, for Imin to be as small as possible to minimize the pump losses. Therefore, SFL is very useful to evaluate both the pump losses and the product of σemi and τm . The emission crosssection σemi had an important effect on laser properties of the Yb3+ ions because a larger σemi indicated a higher laser gain. In spite of the lower σemi , the series PG glasses exhibited higher gain coefficient values (G = σemi *τm ) because of the longer τm . Table 2 shows that SFL values increased with the increasing GeO2 content in the series PG glasses. The value of G and SFL for PG2 are 0.72 and 0.38, respectively, which are higher than those of LSY glass and near to SBB15 glass, but lower than commercial QX glass. The larger the SFL, the better general laser properties the laser glasses had. Therefore, we believe that the PG glasses were a promising laser glass for high-peak power and highaverage power. 4. Conclusion In summary, we prepared a novel GeO2 modified Yb: phosphate glass with large fluorescent emission intensity and gain coefficient. The glass transition temperature and density sharply increased with the increasing GeO2 content because the single-network former glass became a double-network former glass. Moreover, the Raman spectra demonstrated that the glass network structure could be compact. As an important parameter of the laser performance, the values of emission cross-section of PG0, PG1, and PG2 at 1003 nm were 1.29 × 10−20 cm2, 1.18 × 10−20 cm2, 1.09 × 10−20 cm2, respectively. However, the series PG glasses had a long lifetime because of the improvement of Yb3+ ions dispersion. The results showed that GeO2 doped phosphate
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