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Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellurite glasses
Accepted Manuscript Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellrutie glasses Yanyan Guo, Xiuling Liu, Huisheng Duan, Yongshuo Yang, Guoying Zhao, Feifei Huang, Gongxun Bai, Junjie Zhang PII: DOI: Reference:
S1350-4495(18)30905-8 https://doi.org/10.1016/j.infrared.2019.03.025 INFPHY 2895
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
8 December 2018 15 March 2019 22 March 2019
Please cite this article as: Y. Guo, X. Liu, H. Duan, Y. Yang, G. Zhao, F. Huang, G. Bai, J. Zhang, Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellrutie glasses, Infrared Physics & Technology (2019), doi: https://doi.org/10.1016/j.infrared.2019.03.025
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Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellrutie glasses Yanyan Guo1, 2 *, Xiuling Liu1, 2 , Huisheng Duan1 , Yongshuo Yang 3 , Guoying Zhao4 , Feifei Huang 5 , Gongxun Bai 5 , Junjie Zhang 5 1 School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, PR China 2 Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun University of Science and Technology, Changchun 130022, PR China 3 School of Materials Science and Engineering, Tianjin University, Tianjin 300350, PR China
4 School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China 5 China Jiliang University, Zhejiang 310018, PR China *Corresponding author: [email protected]
With 980 nm laser diode (LD) pumping sources, 2.7 µm emission of Er3+ -doped tungsten-tellurite (TWL) glasses were investigated. Absorption spectra were measured and an irregularly increased absorption predicted the phenomena of concentration quenching or clustering at 4 mol% Er2O3 concentration. Emission properties of 2.7 µm, 1.5 µm and visible regions were tested to evaluate the energy transfer process of Er3+ ions. With the increment of Er3+ ions, the excited-state absorption (ESA) from Er3+: 4I13/2 level was becoming more apparent and the lifetime of 1.5 µm emission was decreased. At the same time, the up-conversion emissions at green and red regions were decreased. These phenomena gave the evidence of concentration quenching or clustering. Thus, the best concentration of Er 3+ ions in this tungsten-tellurite glass was about 3 mol%. Based on its advantages on high solubility of Er3+ ions, strong emission and good thermal stability, tungsten-tellurite glass is a good candidate for mid-infrared laser matrix.
Keywords: Erbium; 2.7 µm emission; highly concentration doped; tellurite glass 1. Introduction Since the researchers obtained 2.7 µm emissions in Er3+-doped crystalline in 1967, the development of solid state lasers operating at 2.7 µm regions has attracted many attentions [1, 2]. Due to the strong absorption of H2O in this spectral region, Er3+ doped lasers have been extensively studied for the military and bio-photonic applications [3, 4, 5]
. The 2.7 µm emission of Er3+ ion corresponding to the 4I11/2→4I13/2 transition is closely linked to above
applications. However, Er3+ ions suffer from self-terminating of the 4I11/2 level because of the much shorter life time of emitting level (4I11/2) compared with terminal laser level (4I13/2). Consequently, it requires some way to deplete the terminal level population and conserve emitting level population for efficient 2.7 µm emission. Energy transfer up-conversion: 4I13/2 + 4I13/2→ 4I15/2 + 4I9/2 is an efficient approach for population depletion of terminal level, which can be achieved by high doping concentration of Er3+ ions [6]. As to say, an efficient approach for enhanced 2.7 µm emissions is high concentration of erbium, which is the major work we expounded in present paper. Additionally, it is very important for host materials that low phonon energy generates low non-radiative relaxation rate. Among all host materials, glass is much more proper than others to device application [7].In the past few years, a lot of glass matrixes were investigated for mid-infrared fiber laser, such as fluoride glass, fluorophosphate glass, chalcogenide glass, tellurite glass, germinate glass and bismuthate glass [6-16]. Nowadays,
the highest output power achieved in Er3+ doped ZBLAN glass fiber reaches up to 24 W which is reported by S. Tokita in 2011 [9]. But since then it is difficult to achieve higher output in Er3+ doped ZBLAN glass fiber. Although mid-infrared laser has been reported in Pr3+-doped chalcogenide glass, a lot of efforts are still necessary to obtain high output power and practical application [17]. There are several advantages in the heavy metal oxide glasses, such as low phonon energy, wide transmission, broad emission bandwidth, and good thermal stability and chemical durability. Except for the cost of raw materials, the fabrication route of heavy metal oxide glasses is simpler than that of fluoride and chalcogenide glasses. For heavy metal oxide glasses, tellurite glass is an attractive host material because of the low phonon energy (~760 cm-1) and large refractive index. In tellurite glass, two main categories of tellurite glass are TeO2-ZnO-Na2O (TZN) based and TeO2-WO3-La2O3 (TWL) based system. Some researchers think that the W—O banding leads to high phonon energy (930 cm-1) and it is harmful to 2.7 µm emissions
[18, 19]
. However, the TWL system is quite
beneficial for fiber drawing because of good thermal stability (no obvious crystallization temperature Tx) and low coefficient of thermal expansion. Even more, researchers (SIOM) have already obtained laser output at 2.0 µm in Tm3+ doped TWL glass [20]. In this paper, Er3+ ions singly doped TWL glass was prepared. Judd-Ofelt intensity parameters are calculated according to the absorption and which determined the calculated radiative transition probability and radiative lifetime of excited states. The 2.7 µm, 1.5 µm emissions and up-conversion are tested to explain the energy transfer mechanism. Additionally, the rate equation analysis is used to calculate 4I13/2 state and 4I11/2 state energy transfer up-conversion rates as a quantitative explanation for energy transfer mechanism. 2. Experimental details Glasses with molar composition of 70TeO2― 20WO3― (10-x) La2O3― xEr2O3 (x=0, 0.5, 1, 2, 3, and 4, signed as TWL and Er0.5-4, respectively) were prepared by conventional melting and quenching method in alumina crucibles at 900 ℃ for 40 minutes. After that, the refined melts were casted on a preheated stainless steel mould and annealed below the glass transition temperature for several hours before they were cooled to room temperature with a rate of 20 ℃/h. Finally, the samples were cut and polished into a shape of 10×10×0.5 mm for refractive index test and a shape of 10×10×1 mm for optical measurements. Glass transition (Tg) and crystallization onset (Tx) temperatures were measured at a heating rate of 10 ℃ per minute via DSC measurements using an apparatus from NETZSCH STA 409PC. Glass density indexes were measured by the Archimedes method using distilled water as an immersion liquid with maximum error of ±2%. The FTIR transmittance spectra of TWL sample were respectively measured by a Perkin-Elmer-Lambda 900 UV/VIS/NIR spectrophotometer (400 to 2500 nm) and a Perkin-Elmer 1600 series Fourier transform infrared spectrometer FTIR (2.5 to 10 μm) with a resolution of 0.1 nm. Absorption spectra were tested by a Perkin-Elmer-Lambda 900 UV/VIS/NIR spectrophotometer (400 to 1700 nm) with 1 nm steps. Emission spectra in the range of 2500 to 2850 nm, 1400 to 1700 nm and 500 to 750 nm were determined by a combined fluorescence lifetime steady state spectrometer TRIAX 320 type spectrometer using a 980 nm LD source. The fluorescence lifetime of 4I13/2 state were measured with light pulse of 980 nm LD. For the sake of obtaining comparable results, all the experimental conditions were consistently maintained and carried out at room temperature. 3. Results and discussions.
Figure 1 DSC curve of TWL glass with a heating rate of 10 ℃/min (left) and Raman spectra of TWL glass (right)
Figure 1 (left) shows the DSC curve of tungsten-telluriteTWL glass with the characteristic temperature (Tg and Tx). The value of ΔT (Tx―Tg) is about 104 ℃ which is much higher than some of fluoride and chalcogenide glasses [5, 17]. The glass transition temperature Tg is around 468 ℃. No obvious crystallization peaks are observed from this curve which matches the results reported in ref [21]. Thus, it is possible to obtain high laser damage threshold and large working range for drawing fiber. Figure 1 (right) shows the Raman spectrum of TWL glass with characteristic vibration in TWL glass. It is classified that Te‒O and W‒O vibrations can be distinguished clearly as reported in other papers [22,
23
]. Differently, the intensity W‒O vibration in our study is much lower than
that of Te‒O vibration. So, we can expect a probablely higher 2.7 μm emission in this TWL glass. Figure 2 shows the absorption spectra of Er3+-doped tungsten - tellurite glasses in the spectra range from 400 to 1700 nm. The peak positions and shape of each Er3+ transition in our sample are similar to those in other reports [13]
. Considering the strong absorption intensity of 980 nm and 808 nm and their commercialized purchase, 980 nm
pumping is a more appropriate utilization of pumping energy than 808 nm pumping. The inset figure A shows the transmission and the photo of our samples with the thickness of 5 mm. The approximate number of transmission is about 80%. Considering the high refractive index of TWL glass (~2.06), the total Fresnel reflection loss of the two surfaces of TWL glass is in close proximity to 21% [21]. That is to say, the theoretical IR transmittance of TWL glass is about 80%. This value matches our experimental result (Fig. 2 A). In other words, the 2.7 μm emission can be expected in this tellurite glass. Figure 2 (C/D/E) shows the absorption spectra of 1530 nm, 980 nm and 808 nm and figure 2 B shows the tendency of above spectra. It is clear that the growth has a slower trend until the molar concentration of Erbium ions is 3%. As to say, the linearity growth was destroyed. It pretends that the emissions of erbium will not growth linearly with 4% erbium ions. Additionally, the high solubility of Er2O3 ascribes to the high content of La2O3 in TWL glass and there is no serious distortion of glass networks [21, does not mean intense emission of Er2O3.
24, 25
]. However, the high solubility of Er2O3
n
Figure 2 Absorption spectra of Er3+-doped TWL glasses (left). Inset A presents the transmission of TWL glass and the picture of TWL glass without erbium ions. Inset B presents the tendency of absorption intensity at 4I13/2, 4I11/2 and 4I9/2. Inset C, D and E present the magnification of absorption at 4I13/2 (C), 4I11/2 (D), 4I9/2 (E) levels, respectively.
Judd-Ofelt (J-O) theory is an effective way to investigate the local structure and foreboding the vicinity surrounding rare-earth ions
[26, 27]
. The J-O parameters were calculated from the adaptive least-square training
algorithm of measured and experimental oscillator strengths [24]. Table 1 shows the J-O parameters, the predicted spontaneous transition probability and radiative lifetime of Er3+ ions in this TWL glass and other host matrices [28, 29]
. We identify with that Ω2 is generally determined by the degree of asymmetry of local structure and the
covalency bond between rare earth ions and negative ions [30, 31]. Analysis on the values listed in Table 1, we can find out that the asymmetry of local structure in TWL glass is intermediate between fluoride and bismuthate-germanate glasses. We also identify with that Ω6 is inversely proportional to the covalency bond between rare earth ions and negative ions [32]. While the value of Ω6 in TWL glass is smallest and is close to fluoride. It indicates a higher covalence associated with rare-earth ions. The branching ratio for 4I11/2 → 4I13/2 transition is 20.29%, which is just lower than fluoride glass. Additionally, the calculated lifetime of 4I11/2 is 4.66 ms, which is just lower than fluoride glass. Based above parameters, we can expect an intense 2.7 μm in this TWL glass. Table 1 Judd-Ofelt parameters, spontaneous transition probability, branching ratio and radiative lifetime of Er3+ ions in different glasses
Parameters
Fluoride[28]
Tellurite[29]
Bismuth-germanate[29]
This work
Ω2
2.73
5.97
7.23
4.78
Ω4
0.7
1.43
2.75
0.91
0.49
0.8
1.17
0.43
A( I11/2→ I13/2) (s )
15.94
44.28
65.26
43.57
β(%)
23.48
17.35
16.01
20.29
4
14.73
3.92
2.98
4.66
Ω6 4
4
τ( I11/2) (ms)
-1
τ(4I13/2) (ms)
13.89
4.62
2.45
5.08
Figure 3 and 5 show the near- and mid-infrared emission spectra and upconversion of Er3+-doped TWL glasses respectively. The intensity of near- and mid-infrared emission spectra exhibits an increase tendency with the increment of Er3+ concentration until 4%. While the upconversion exhibits a different tendency compared with near- and mid-infrared emission.
Fig. 3 the near- (A) and mid-infrared (B) emission spectra of Er3+-doped TWL glasses. Figure C presents the tendency of near- and mid- infrared emission with Er3+ concentration. Figure D depicts the measured lifetime of 4I13/2 level.
Fig. 4 the energy level schemes of Er3+ ions
Based on above emission spectra, the energy transfer process can be predicted combined with energy level schemes of Er3+ ions (shown in fig. 4). Firstly, the 4I13/2 level is the lower level of 2.7 μm emission as well as the upper level of 1.5 μm emission. So, the intensity and lifetime of 1.5 μm emission is an important factor for 2.7 µm emission [33]. With the increment of Er3+ ions, the intensity of 1.5 µm emission rises in the first and then decreases. While the lifetime of 1.5 µm emission decreases from 3.61 ms to 0.97 ms. The increased intensity and decreased lifetime of 1.5 µm emission promise an increased 2.7 µm emission, which is verified by measured 2.7 µm emission spectra. Secondly, the 4I13/2 level is the lower of 2.7 μm emission as well as the lower level of excited state absorption (ESA1: 4I13/2 + a photo→4F9/2), which is shown in Fig. 4. The 4F9/2 level is the upper level for upconversion (650 nm). Increased intensity of 650 nm means increased consume of 4I13/2 level, which is helpful for 2.7 µm emission and is related with decreased lifetime of 1.5 µm emission. Additionally, the 4I11/2 level is the upper level of 2.7 μm emission as well as the lower level of excited state absorption (ESA2: 4I11/2 + a photo → 4F7/2) and excited transfer upconversion (ETU1: 4I11/2 + 4I11/2 → 4F7/2) [21]. Both ESA2 and ETU1 process are harmful to 2.7 μm emission. The 4F7/2 level is very important for upconversion (534 nm 2H11/2 and 553 nm 4S3/2). Fortunately, the intensity of upconversion (534 nm and 553 nm) decreases till 1% Er3+ ions. With the increment of Er3+ ions, the upconversion (534 nm and 553 nm) drastically changes not only the intensity but also the shape. The changed spectra shape indicates the aggregation of 4S3/2 level. With the increase of Er3+ ions, interionic distance of excited Er3+ ion decreases and the interactions among Er3+ ions is more intense34. However, it is not encourage for upconversion. Especially the upconversion is vanished almostly with 4% Er3+ ions.
Fig. 5 the upconversion of Er3+-doped TWL glasses. The inset depicts the variation tendency with concentration of Er3+ ions.
According to the Fuchbauer-Ladenburg theory, the stimulated emission cross-section (σe) for 2.7 µm emission (transition 4I11/2-4I13/2) is calculated from the measured emission spectra [11]. Because the upper and lower levels of Er3+: 4I11/2-4I13/2 transition belongs to excited levels, the absorption cross section (σa) is evaluated from the σe using the McCumber equation [7]:
a e Zl Zu exp Ezl hc1 k BT
eq. 1
Here Zl and Zu are partition functions of the lower and upper manifolds respectively. When the matrix of Er3+ is certain, the value of σe is mainly determined by
I
I d . Figure 5 shows σe (peak value is 6.91×10-21
cm2) and σa (peak value is 6.74×10-21cm2) of 1% Er3+ doped TWL glass (Sample Er1) and table 2 lists the σe value of each samples. Firstly, the σe value is higher than σa value, although the σe value of TWL glass is lower than other matrix [7, 13, 21]. It promises a high possibility for positive gain. Secondly, the change of Er3+ ions’ absorption in the range of 2708nm to 2718nm in present sample is not apparent. Thirdly, the sample Er3 expresses the highest emission intensity as shown in Figure 3. After that, the sample Er4 expresses a decreased 2.7 μm emission. However, its calculated σe value is 6.79×10-21 cm2 which is slightly lower than σe of sample Er1. It provides a further proof that the value of σe is mainly determined by
I
I d .
Fig. 6 the 2.7 µm emission and absorption cross-section of Er3+ doped TWL glass
According to the stimulated emission cross section and absorption cross section, the gain coefficient G (λ) and the gain performance of Er3+ ion in TWL glass can be evaluated. The gain of Er3+ ion 2.7 µm can be calculated in the ambient temperature by following equation [7]:
G N P e 1 P a
eq. 2
Here N is the concentration of Er3+ ion in TWL glass (3.60 × 1020cm-3 in 1 mol% Er3+). Figure 6 shows the gain coefficient of Er3+ ion in TWL glass and the maximum G value is 2.49 cm-1. While the concentration of Er3+ ions increase, the value of N increase as well as G. Table 2 shows the maximum G value of each sample. It is merited that the highest concentration of Er3+ gets the highest gain. Table 2 the maximum G value of each Er3+-doped TWL glass
Sample -21
2
Maximum σe(×10 cm ) -1
Maximum G cm
Er0.5
Er1
Er2
Er3
Er4
6.69
6.91
6.80
7.13
6.79
1.20
2.49
4.90
7.70
9.77
Figure 7 the gain coefficient of 1% Er3+ in TWL glasses
4. Conclusions In present paper, Er3+-doped tungsten-tellrutie glasses were successfully prepared by conventional melting and quenching method. The DSC curve shows a potential for optical fiber application. The absorption spectra predict the decreased emission until 4 mol% Er2O3 concentration which can ascribe to the phenomena of concentration quenching or clustering. Additionally, with the increment of Er 3+ ions the excited-state absorption (ESA) from Er 3+: 4I13/2 level was becoming more apparent as well as the lifetime of 1.5 µm emission was decreased, which gives a powerful push on 2.7 µm emission until 4% Er3+ ions doped in TWL glass. These phenomena give the evidence of concentration quenching or clustering. Thus, the best concentration of Er3+ ions in this tungsten-tellurite glass is about 3 mol%. The σe value of TWL glass is lightly higher than σa value, which ensures the positive gain. High solubility of Er 3+ ions, strong emission and good thermal stability makes this tungsten-tellurite glass a good candidate for mid-infrared laser matrix.
Conflict of interest statement The authors declared that they have no conflicts of interest to this work.
Acknowledgement
This work was supported by National Natural Science Foundation of China [No. 51502022, 61605115], Jilin Province Department of Education [2015-55] and Changchun University of Science and Technology [XQNJJ-2014-12].
Reference
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Highlights
1. A strong 2.7μm emission is obtained in highly Er3+ doped tungsten-tellrutie glasses. 2. The excited-state absorption (ESA) from Er3+: 4I13/2 level is beneficial for 2.7μm emission. 3. The lifetime of Er3+: 4I13/2 level is decreased with the concentration of Er 3+ ions.
4. The best concentration of Er3+ ions for 2.7μm emission in TWL glass is 3mol%.
Graphical abstract
The 3% Er3+ dope tungsten-tellrutie glass exhibits an intense 2.7 μm emission as well as a short lifetime of 1.5 μm emission , which is beneficial for mid-infrared laser.
S1350-4495(18)30905-8 https://doi.org/10.1016/j.infrared.2019.03.025 INFPHY 2895
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
8 December 2018 15 March 2019 22 March 2019
Please cite this article as: Y. Guo, X. Liu, H. Duan, Y. Yang, G. Zhao, F. Huang, G. Bai, J. Zhang, Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellrutie glasses, Infrared Physics & Technology (2019), doi: https://doi.org/10.1016/j.infrared.2019.03.025
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Optimization by energy transfer process of 2.7 µm emission in highly Er3+-doped tungsten-tellrutie glasses Yanyan Guo1, 2 *, Xiuling Liu1, 2 , Huisheng Duan1 , Yongshuo Yang 3 , Guoying Zhao4 , Feifei Huang 5 , Gongxun Bai 5 , Junjie Zhang 5 1 School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, PR China 2 Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun University of Science and Technology, Changchun 130022, PR China 3 School of Materials Science and Engineering, Tianjin University, Tianjin 300350, PR China
4 School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China 5 China Jiliang University, Zhejiang 310018, PR China *Corresponding author: [email protected]
With 980 nm laser diode (LD) pumping sources, 2.7 µm emission of Er3+ -doped tungsten-tellurite (TWL) glasses were investigated. Absorption spectra were measured and an irregularly increased absorption predicted the phenomena of concentration quenching or clustering at 4 mol% Er2O3 concentration. Emission properties of 2.7 µm, 1.5 µm and visible regions were tested to evaluate the energy transfer process of Er3+ ions. With the increment of Er3+ ions, the excited-state absorption (ESA) from Er3+: 4I13/2 level was becoming more apparent and the lifetime of 1.5 µm emission was decreased. At the same time, the up-conversion emissions at green and red regions were decreased. These phenomena gave the evidence of concentration quenching or clustering. Thus, the best concentration of Er 3+ ions in this tungsten-tellurite glass was about 3 mol%. Based on its advantages on high solubility of Er3+ ions, strong emission and good thermal stability, tungsten-tellurite glass is a good candidate for mid-infrared laser matrix.
Keywords: Erbium; 2.7 µm emission; highly concentration doped; tellurite glass 1. Introduction Since the researchers obtained 2.7 µm emissions in Er3+-doped crystalline in 1967, the development of solid state lasers operating at 2.7 µm regions has attracted many attentions [1, 2]. Due to the strong absorption of H2O in this spectral region, Er3+ doped lasers have been extensively studied for the military and bio-photonic applications [3, 4, 5]
. The 2.7 µm emission of Er3+ ion corresponding to the 4I11/2→4I13/2 transition is closely linked to above
applications. However, Er3+ ions suffer from self-terminating of the 4I11/2 level because of the much shorter life time of emitting level (4I11/2) compared with terminal laser level (4I13/2). Consequently, it requires some way to deplete the terminal level population and conserve emitting level population for efficient 2.7 µm emission. Energy transfer up-conversion: 4I13/2 + 4I13/2→ 4I15/2 + 4I9/2 is an efficient approach for population depletion of terminal level, which can be achieved by high doping concentration of Er3+ ions [6]. As to say, an efficient approach for enhanced 2.7 µm emissions is high concentration of erbium, which is the major work we expounded in present paper. Additionally, it is very important for host materials that low phonon energy generates low non-radiative relaxation rate. Among all host materials, glass is much more proper than others to device application [7].In the past few years, a lot of glass matrixes were investigated for mid-infrared fiber laser, such as fluoride glass, fluorophosphate glass, chalcogenide glass, tellurite glass, germinate glass and bismuthate glass [6-16]. Nowadays,
the highest output power achieved in Er3+ doped ZBLAN glass fiber reaches up to 24 W which is reported by S. Tokita in 2011 [9]. But since then it is difficult to achieve higher output in Er3+ doped ZBLAN glass fiber. Although mid-infrared laser has been reported in Pr3+-doped chalcogenide glass, a lot of efforts are still necessary to obtain high output power and practical application [17]. There are several advantages in the heavy metal oxide glasses, such as low phonon energy, wide transmission, broad emission bandwidth, and good thermal stability and chemical durability. Except for the cost of raw materials, the fabrication route of heavy metal oxide glasses is simpler than that of fluoride and chalcogenide glasses. For heavy metal oxide glasses, tellurite glass is an attractive host material because of the low phonon energy (~760 cm-1) and large refractive index. In tellurite glass, two main categories of tellurite glass are TeO2-ZnO-Na2O (TZN) based and TeO2-WO3-La2O3 (TWL) based system. Some researchers think that the W—O banding leads to high phonon energy (930 cm-1) and it is harmful to 2.7 µm emissions
[18, 19]
. However, the TWL system is quite
beneficial for fiber drawing because of good thermal stability (no obvious crystallization temperature Tx) and low coefficient of thermal expansion. Even more, researchers (SIOM) have already obtained laser output at 2.0 µm in Tm3+ doped TWL glass [20]. In this paper, Er3+ ions singly doped TWL glass was prepared. Judd-Ofelt intensity parameters are calculated according to the absorption and which determined the calculated radiative transition probability and radiative lifetime of excited states. The 2.7 µm, 1.5 µm emissions and up-conversion are tested to explain the energy transfer mechanism. Additionally, the rate equation analysis is used to calculate 4I13/2 state and 4I11/2 state energy transfer up-conversion rates as a quantitative explanation for energy transfer mechanism. 2. Experimental details Glasses with molar composition of 70TeO2― 20WO3― (10-x) La2O3― xEr2O3 (x=0, 0.5, 1, 2, 3, and 4, signed as TWL and Er0.5-4, respectively) were prepared by conventional melting and quenching method in alumina crucibles at 900 ℃ for 40 minutes. After that, the refined melts were casted on a preheated stainless steel mould and annealed below the glass transition temperature for several hours before they were cooled to room temperature with a rate of 20 ℃/h. Finally, the samples were cut and polished into a shape of 10×10×0.5 mm for refractive index test and a shape of 10×10×1 mm for optical measurements. Glass transition (Tg) and crystallization onset (Tx) temperatures were measured at a heating rate of 10 ℃ per minute via DSC measurements using an apparatus from NETZSCH STA 409PC. Glass density indexes were measured by the Archimedes method using distilled water as an immersion liquid with maximum error of ±2%. The FTIR transmittance spectra of TWL sample were respectively measured by a Perkin-Elmer-Lambda 900 UV/VIS/NIR spectrophotometer (400 to 2500 nm) and a Perkin-Elmer 1600 series Fourier transform infrared spectrometer FTIR (2.5 to 10 μm) with a resolution of 0.1 nm. Absorption spectra were tested by a Perkin-Elmer-Lambda 900 UV/VIS/NIR spectrophotometer (400 to 1700 nm) with 1 nm steps. Emission spectra in the range of 2500 to 2850 nm, 1400 to 1700 nm and 500 to 750 nm were determined by a combined fluorescence lifetime steady state spectrometer TRIAX 320 type spectrometer using a 980 nm LD source. The fluorescence lifetime of 4I13/2 state were measured with light pulse of 980 nm LD. For the sake of obtaining comparable results, all the experimental conditions were consistently maintained and carried out at room temperature. 3. Results and discussions.
Figure 1 DSC curve of TWL glass with a heating rate of 10 ℃/min (left) and Raman spectra of TWL glass (right)
Figure 1 (left) shows the DSC curve of tungsten-telluriteTWL glass with the characteristic temperature (Tg and Tx). The value of ΔT (Tx―Tg) is about 104 ℃ which is much higher than some of fluoride and chalcogenide glasses [5, 17]. The glass transition temperature Tg is around 468 ℃. No obvious crystallization peaks are observed from this curve which matches the results reported in ref [21]. Thus, it is possible to obtain high laser damage threshold and large working range for drawing fiber. Figure 1 (right) shows the Raman spectrum of TWL glass with characteristic vibration in TWL glass. It is classified that Te‒O and W‒O vibrations can be distinguished clearly as reported in other papers [22,
23
]. Differently, the intensity W‒O vibration in our study is much lower than
that of Te‒O vibration. So, we can expect a probablely higher 2.7 μm emission in this TWL glass. Figure 2 shows the absorption spectra of Er3+-doped tungsten - tellurite glasses in the spectra range from 400 to 1700 nm. The peak positions and shape of each Er3+ transition in our sample are similar to those in other reports [13]
. Considering the strong absorption intensity of 980 nm and 808 nm and their commercialized purchase, 980 nm
pumping is a more appropriate utilization of pumping energy than 808 nm pumping. The inset figure A shows the transmission and the photo of our samples with the thickness of 5 mm. The approximate number of transmission is about 80%. Considering the high refractive index of TWL glass (~2.06), the total Fresnel reflection loss of the two surfaces of TWL glass is in close proximity to 21% [21]. That is to say, the theoretical IR transmittance of TWL glass is about 80%. This value matches our experimental result (Fig. 2 A). In other words, the 2.7 μm emission can be expected in this tellurite glass. Figure 2 (C/D/E) shows the absorption spectra of 1530 nm, 980 nm and 808 nm and figure 2 B shows the tendency of above spectra. It is clear that the growth has a slower trend until the molar concentration of Erbium ions is 3%. As to say, the linearity growth was destroyed. It pretends that the emissions of erbium will not growth linearly with 4% erbium ions. Additionally, the high solubility of Er2O3 ascribes to the high content of La2O3 in TWL glass and there is no serious distortion of glass networks [21, does not mean intense emission of Er2O3.
24, 25
]. However, the high solubility of Er2O3
n
Figure 2 Absorption spectra of Er3+-doped TWL glasses (left). Inset A presents the transmission of TWL glass and the picture of TWL glass without erbium ions. Inset B presents the tendency of absorption intensity at 4I13/2, 4I11/2 and 4I9/2. Inset C, D and E present the magnification of absorption at 4I13/2 (C), 4I11/2 (D), 4I9/2 (E) levels, respectively.
Judd-Ofelt (J-O) theory is an effective way to investigate the local structure and foreboding the vicinity surrounding rare-earth ions
[26, 27]
. The J-O parameters were calculated from the adaptive least-square training
algorithm of measured and experimental oscillator strengths [24]. Table 1 shows the J-O parameters, the predicted spontaneous transition probability and radiative lifetime of Er3+ ions in this TWL glass and other host matrices [28, 29]
. We identify with that Ω2 is generally determined by the degree of asymmetry of local structure and the
covalency bond between rare earth ions and negative ions [30, 31]. Analysis on the values listed in Table 1, we can find out that the asymmetry of local structure in TWL glass is intermediate between fluoride and bismuthate-germanate glasses. We also identify with that Ω6 is inversely proportional to the covalency bond between rare earth ions and negative ions [32]. While the value of Ω6 in TWL glass is smallest and is close to fluoride. It indicates a higher covalence associated with rare-earth ions. The branching ratio for 4I11/2 → 4I13/2 transition is 20.29%, which is just lower than fluoride glass. Additionally, the calculated lifetime of 4I11/2 is 4.66 ms, which is just lower than fluoride glass. Based above parameters, we can expect an intense 2.7 μm in this TWL glass. Table 1 Judd-Ofelt parameters, spontaneous transition probability, branching ratio and radiative lifetime of Er3+ ions in different glasses
Parameters
Fluoride[28]
Tellurite[29]
Bismuth-germanate[29]
This work
Ω2
2.73
5.97
7.23
4.78
Ω4
0.7
1.43
2.75
0.91
0.49
0.8
1.17
0.43
A( I11/2→ I13/2) (s )
15.94
44.28
65.26
43.57
β(%)
23.48
17.35
16.01
20.29
4
14.73
3.92
2.98
4.66
Ω6 4
4
τ( I11/2) (ms)
-1
τ(4I13/2) (ms)
13.89
4.62
2.45
5.08
Figure 3 and 5 show the near- and mid-infrared emission spectra and upconversion of Er3+-doped TWL glasses respectively. The intensity of near- and mid-infrared emission spectra exhibits an increase tendency with the increment of Er3+ concentration until 4%. While the upconversion exhibits a different tendency compared with near- and mid-infrared emission.
Fig. 3 the near- (A) and mid-infrared (B) emission spectra of Er3+-doped TWL glasses. Figure C presents the tendency of near- and mid- infrared emission with Er3+ concentration. Figure D depicts the measured lifetime of 4I13/2 level.
Fig. 4 the energy level schemes of Er3+ ions
Based on above emission spectra, the energy transfer process can be predicted combined with energy level schemes of Er3+ ions (shown in fig. 4). Firstly, the 4I13/2 level is the lower level of 2.7 μm emission as well as the upper level of 1.5 μm emission. So, the intensity and lifetime of 1.5 μm emission is an important factor for 2.7 µm emission [33]. With the increment of Er3+ ions, the intensity of 1.5 µm emission rises in the first and then decreases. While the lifetime of 1.5 µm emission decreases from 3.61 ms to 0.97 ms. The increased intensity and decreased lifetime of 1.5 µm emission promise an increased 2.7 µm emission, which is verified by measured 2.7 µm emission spectra. Secondly, the 4I13/2 level is the lower of 2.7 μm emission as well as the lower level of excited state absorption (ESA1: 4I13/2 + a photo→4F9/2), which is shown in Fig. 4. The 4F9/2 level is the upper level for upconversion (650 nm). Increased intensity of 650 nm means increased consume of 4I13/2 level, which is helpful for 2.7 µm emission and is related with decreased lifetime of 1.5 µm emission. Additionally, the 4I11/2 level is the upper level of 2.7 μm emission as well as the lower level of excited state absorption (ESA2: 4I11/2 + a photo → 4F7/2) and excited transfer upconversion (ETU1: 4I11/2 + 4I11/2 → 4F7/2) [21]. Both ESA2 and ETU1 process are harmful to 2.7 μm emission. The 4F7/2 level is very important for upconversion (534 nm 2H11/2 and 553 nm 4S3/2). Fortunately, the intensity of upconversion (534 nm and 553 nm) decreases till 1% Er3+ ions. With the increment of Er3+ ions, the upconversion (534 nm and 553 nm) drastically changes not only the intensity but also the shape. The changed spectra shape indicates the aggregation of 4S3/2 level. With the increase of Er3+ ions, interionic distance of excited Er3+ ion decreases and the interactions among Er3+ ions is more intense34. However, it is not encourage for upconversion. Especially the upconversion is vanished almostly with 4% Er3+ ions.
Fig. 5 the upconversion of Er3+-doped TWL glasses. The inset depicts the variation tendency with concentration of Er3+ ions.
According to the Fuchbauer-Ladenburg theory, the stimulated emission cross-section (σe) for 2.7 µm emission (transition 4I11/2-4I13/2) is calculated from the measured emission spectra [11]. Because the upper and lower levels of Er3+: 4I11/2-4I13/2 transition belongs to excited levels, the absorption cross section (σa) is evaluated from the σe using the McCumber equation [7]:
a e Zl Zu exp Ezl hc1 k BT
eq. 1
Here Zl and Zu are partition functions of the lower and upper manifolds respectively. When the matrix of Er3+ is certain, the value of σe is mainly determined by
I
I d . Figure 5 shows σe (peak value is 6.91×10-21
cm2) and σa (peak value is 6.74×10-21cm2) of 1% Er3+ doped TWL glass (Sample Er1) and table 2 lists the σe value of each samples. Firstly, the σe value is higher than σa value, although the σe value of TWL glass is lower than other matrix [7, 13, 21]. It promises a high possibility for positive gain. Secondly, the change of Er3+ ions’ absorption in the range of 2708nm to 2718nm in present sample is not apparent. Thirdly, the sample Er3 expresses the highest emission intensity as shown in Figure 3. After that, the sample Er4 expresses a decreased 2.7 μm emission. However, its calculated σe value is 6.79×10-21 cm2 which is slightly lower than σe of sample Er1. It provides a further proof that the value of σe is mainly determined by
I
I d .
Fig. 6 the 2.7 µm emission and absorption cross-section of Er3+ doped TWL glass
According to the stimulated emission cross section and absorption cross section, the gain coefficient G (λ) and the gain performance of Er3+ ion in TWL glass can be evaluated. The gain of Er3+ ion 2.7 µm can be calculated in the ambient temperature by following equation [7]:
G N P e 1 P a
eq. 2
Here N is the concentration of Er3+ ion in TWL glass (3.60 × 1020cm-3 in 1 mol% Er3+). Figure 6 shows the gain coefficient of Er3+ ion in TWL glass and the maximum G value is 2.49 cm-1. While the concentration of Er3+ ions increase, the value of N increase as well as G. Table 2 shows the maximum G value of each sample. It is merited that the highest concentration of Er3+ gets the highest gain. Table 2 the maximum G value of each Er3+-doped TWL glass
Sample -21
2
Maximum σe(×10 cm ) -1
Maximum G cm
Er0.5
Er1
Er2
Er3
Er4
6.69
6.91
6.80
7.13
6.79
1.20
2.49
4.90
7.70
9.77
Figure 7 the gain coefficient of 1% Er3+ in TWL glasses
4. Conclusions In present paper, Er3+-doped tungsten-tellrutie glasses were successfully prepared by conventional melting and quenching method. The DSC curve shows a potential for optical fiber application. The absorption spectra predict the decreased emission until 4 mol% Er2O3 concentration which can ascribe to the phenomena of concentration quenching or clustering. Additionally, with the increment of Er 3+ ions the excited-state absorption (ESA) from Er 3+: 4I13/2 level was becoming more apparent as well as the lifetime of 1.5 µm emission was decreased, which gives a powerful push on 2.7 µm emission until 4% Er3+ ions doped in TWL glass. These phenomena give the evidence of concentration quenching or clustering. Thus, the best concentration of Er3+ ions in this tungsten-tellurite glass is about 3 mol%. The σe value of TWL glass is lightly higher than σa value, which ensures the positive gain. High solubility of Er 3+ ions, strong emission and good thermal stability makes this tungsten-tellurite glass a good candidate for mid-infrared laser matrix.
Conflict of interest statement The authors declared that they have no conflicts of interest to this work.
Acknowledgement
This work was supported by National Natural Science Foundation of China [No. 51502022, 61605115], Jilin Province Department of Education [2015-55] and Changchun University of Science and Technology [XQNJJ-2014-12].
Reference
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Highlights
1. A strong 2.7μm emission is obtained in highly Er3+ doped tungsten-tellrutie glasses. 2. The excited-state absorption (ESA) from Er3+: 4I13/2 level is beneficial for 2.7μm emission. 3. The lifetime of Er3+: 4I13/2 level is decreased with the concentration of Er 3+ ions.
4. The best concentration of Er3+ ions for 2.7μm emission in TWL glass is 3mol%.
Graphical abstract
The 3% Er3+ dope tungsten-tellrutie glass exhibits an intense 2.7 μm emission as well as a short lifetime of 1.5 μm emission , which is beneficial for mid-infrared laser.