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Effect of Si doping on near-infrared emission and energy transfer of Bismuth in silicate glasses
Journal of Non-Crystalline Solids 358 (2012) 261–264
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Effect of Si doping on near-infrared emission and energy transfer of Bismuth in silicate glasses Nengli Dai a, Huaixun Luan a, Bing Xu b, Lvyun Yang a, Yubang Sheng a, Zijun Liu a, Jinyan Li a,⁎ a b
Wuhan National Laboratory for Optoelectronics, School of Optoelectronics Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China Wuhan Mechanical Technology College, Wuhan 430075, PR China
a r t i c l e
i n f o
Article history: Received 25 June 2011 Received in revised form 19 September 2011 Available online 18 October 2011 Keywords: Optical material; Si; Low concentration Bismuth; Energy transfer; Near infrared emission
a b s t r a c t We have studied the effects of Si doping on the near infrared (NIR) luminescence observed in low Bi doped ( 0.1 mol% ) glasses and the energy transfer from Yb 3+ to Bi. The broadband near infrared can only be observed when Si is introduced in the Bi-doped glass. The origin of this fluorescence can be attributed to Bi ions at low valence. Efficient energy transfer from Yb 3+ to Bi NIR active ions is achieved by co-doping of Si. There is an increment of about ~ 29 times of the emission intensity from Bi-related active center as the Yb 3+ concentration varies from 0 to 2.0 mol% and the amount of Si is 0.05 mol% under 980 nm excitation. The possible mechanism of energy transfer from Yb 3+ to Bi is also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Fiber amplifiers with broad and efficient gain are playing a key role in the development of modern optical communication. In 2001, Fujimoto and his co-workers reported a novel NIR emission centered at 1250 nm with 800 nm excitation and obtained optical amplification at 1300 nm from Bismuth-doped silicate glasses, which have drawn much attention on its potential application in optical fiber amplifiers [1,2]. Since then, a large number of Bi-doped glass hosts have been explored, mainly including silicate, germinate, phosphate, borate, and chalcogenide glasses [3–12]. For practical applications, it is imperative to increase the stimulated emission cross-section in Bi-doped glasses. One probable approach is sensitizing Bi-related active centers by some sensitizers. Recent researches have revealed that the intensity of Bi-related NIR luminescence can be largely increased through efficient energy transfer associated with Yb 3+ ( 2F5/2– 2F7/2) transition in Yb–Bi co-doped glasses [13–16]. To the best of our knowledge, few attention were drawn on the study of energy transfer process between sensitizers and low concentration Bi (b0.5 mol%) in silicate glasses, which was more similar with the concentration of Bi ions in silica fibers. In this work, optical characteristics of low concentration Bismuth and energy transfer from Yb 3+ to Bi ions were studied. The effects of additional Si in glass hosts on the luminescence and possible mechanism of the energy transfer between Yb 3+ and low concentration Bi (0.1 mol%) were also discussed.
Glass samples with compositions listed in Table 1 were prepared by a conventional melting method. Analytical pure reagent commercial oxides (N99.5%) were selected as the raw materials. Si powder was introduced into the raw material as reducing agent. The mixed batch of 30 g were melted in alumina crucible at 1580 °C for 2 hours in reducing atmosphere achieved by the introduction of Si powder. The glass melts were poured on the pre-heated steel mold and then annealed near the glass transition temperature for 2 hours in the annealing furnace. The obtained glasses were cut into 15 × 15 × 2 mm 3 and polished for optical measurement. Optical absorption spectroscopic was measured on PerkinElmer-Lambda35 spectrophotometer in the range of 200–1100 nm. The luminescence spectra were obtain by ZOLIX SBP300 spectrophotometer, detected by InGaAs photo-detector with excitation of 808 nm and 980 nm LD, respectively. The fluorescence lifetimes were measured by TRIAX550 spectrofluorometer and detected by InGaAs photo-detector and recorded by a storage digital oscilloscope (Tektronix TDS3052). All the measurements were carried out at room temperature in air.
⁎ Corresponding author. Tel.: + 86 027 87559463; fax: + 86 027 87559463. E-mail addresses: [email protected], [email protected] (J. Li). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.09.023
3. Results Fig. 1 shows the absorption and NIR photoluminescence (inset) spectra of SACB and SACB-Si. For the SACB-Si sample, absorption bands centered at ~ 470 nm and 700 nm are observed obviously in Fig. 1, which should be attributed to Bi-related NIR active center, in accord with the results reported previously [1,5,7,12]. In the inset of Fig. 1, intense NIR emission at ~1300 nm can be measured from
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Table 1 Compositions of the Si–Bi–Yb doped silicate glasses. Glass sample
Composition (mol%) SiO2
Al2O3
CaO
Bi2O3
Yb2O3
Si
SACB SACB-Si SAC-Si SACYB-Si1 SACYB-Si2 SACYB-Si3 SACYB-Si4 SACYB-Si5 SACY-Si
65 65 65 65 65 65 65 65 65
10 10 10 10 10 10 10 10 10
25 25 25 25 25 25 25 25 25
0.1 0.1 – 0.1 0.1 0.1 0.1 0.1 –
– – – 0.5 1.0 1.5 2.0 2.5 2.0
– 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
SACB-Si under excitation of 808 nm LD. But as shown in Fig. 1, there are no apparent absorption and emission peaks in glass SACB. NIR luminescence cannot be observed from single Si doped silicate glass without Bismuth (sample SAC-Si), which is suggested that near-IR fluorescence centered at 1300 nm is derived from Bi ions rather than Si, and Si played a supporting role only for the near-IR emission of Bi. Despite the fact that several proposals have been made tentatively, assigning the role of emission center to Bi 5+, Bi +, Bi metal clusters or negatively charged Bismuth dimmers, the origin of NIR emission still remains highly debated [1,3,17–22]. According to Duffy's theory of optical basicity, the increasing basicity of the host glass will facilitate formation of higher valence states of Bi[23,24]. Based on our previous study, we tend to consider low valent Bi ions as NIR emission centers. In this paper, Bi-related NIR active center is named as Bi active center. Results presented above indicate that the addition of Si powder into glass component can strengthen the infrared emission from Bi greatly. The reducing condition introduced by Si powder promotes the formation of Bi active centers in Bi doped glass and emitting superior NIR luminescence. Reaction formula of this phenomenon in the melting process can be expressed as:
Fig. 2. Absorption spectra of glass samples SACYB-Si1, 2, 3, 4, 5.
In 2005, Russian researcher E.M.Dianov and Japanese scientist Haruna. T fabricated Bi-doped silica fiber by employing the MCVD method [25,26]. According to their reports, the concentration of Bi active ions in core glass measured by X-ray microanalysis was about 0.02% in most of the fibers [27,28]. But as we know, in most previous
researches on luminescence properties, optical application and energy transfer process between Yb 3+ and Bi-related active ions in glass hosts, the concentration of Bi was very high (N0.5 mol%), much higher than that in fiber cores (b0.02 mol%). We prepared glass samples SACYB-Si1, 2, 3, 4, 5 to explore the most efficient energy transfer project when Bi is in low concentration. Fig. 2 shows the absorption spectra of SACB-Si and SACYB-Si samples. Apparent absorption bands centered at ~470 nm and 700 nm originating from Bi active centers can be seen in the figure. The strong absorption peak observed at 980 nm in glass SACYB-Si should be ascribed to 2F7/2– 2F5/2 transition of Yb 3+. With the introduction of Yb 3+ and the increment of concentration, the absorption intensity at ~470 nm and 700 nm becomes weaker gradually. On the contrary, enhancement of absorption band at 980 nm attributed to Yb 3+ can be observed simultaneously. Based on above analysis of optical basicity, the reduction of absorption strength at Bi-related absorption bands may be attributed to the oxidbillity of Yb 3+[23,24]. Increasing oxidbillity with the introduction of Yb2O3 facilitates formation of higher valence states of Bi, which leads to the decrement of Bi active centers. Consequently, absorption bands at ~470 nm and 700 nm related to Bi active centers weaken gradually. Infrared emission properties of SACB-Si and SACYB-Si1, 2, 3, 4, 5 pumped by 980 nm LD are shown in Fig. 3. The amount of Bi and Si
Fig. 1. Absorption spectra and NIR emission spectra (inset) of SACB and SACB-Si.
Fig. 3. Luminescence spectra of silicate glasses SACYB-Si1, 2, 3, 4, 5 under excitation of 980 nm LD.
2Bi2 O3 þ 3Si → 3SiO2 þ 4Biactive−center :
ð1Þ
N. Dai et al. / Journal of Non-Crystalline Solids 358 (2012) 261–264
powder is constant(0.1 and 0.05 mol%, respectively), while the concentration of Yb is varied from 0 mol% to 2.5 mol%. Two characteristic emissions are observed in the spectra, which should be ascribed to the Yb 3+ ( 2F5/2– 2F7/2) transition at 1020 nm and Bi active centers at around 1220 nm. Bi active centers in the Yb–Bi co-doped glasses show stronger near-infrared emission than that in the single Bi doped glass. Inset of Fig.3 presents the magnified patterns of Fig. 3 at the range of 1100–1450 nm. With the increased Yb concentration, the emission intensity from Bi active centers increased at first and then decreased, strongest at ~2.0 mol%. The decrease in the luminescence intensity with Yb concentration of 2.5 mol% could be induced by concentration quenching between Yb 3+ ions duo to radiative reabsorption. Slight red-shift of central wavelength of Bi-related emission peak from 1180 nm to 1223 nm is observed, while reasons for this phenomenon are uncertain and need exploration in details. Fig. 4 shows NIR photoluminescence spectra of single Bi doped and Yb–Bi co-doped glasses (SACB-Si and SACYB-Si4)excited by 808 nm and 980 nm LDs, respectively. Under excitation of 980 nm LD, the intensity of NIR emission at 1200 nm from SACYB-Si4 is ~29 times higher than that of SACB-Si (in mol%). For further research, we compared the emission intensity of sample SACYB-Si4 excited by 980 nm LD and SACB-Si excited by 808 nm LD. The luminescent intensity under 980 nm excitation is about 3 times higher than that of the latter. 4. Discussion The lifetime is the most important data supporting the energy transfer from sensitizers to acceptors. Fluorescence decay curves near 1020 nm corresponding to the 2F5/2 energy lever of Yb 3+ from glass samples SACYB-Si4 and SACY-Si were measured and presented in Fig. 5. The mean lifetime (τm) of Yb 3+ at 1020 nm can be calculated by following formula [29]: ∞
=
∞
τm ¼ ∫t0 Iðt Þtdt ∫t0 Iðt Þdt:
ð2Þ
Where I(t) is the luminescence intensity as a function of time t. The calculated lifetimes are 500 μs for sample SACYB-Si4 and 1570 μs for SACY-Si. An obvious decrease of sensitizer's emission decay lifetime with the introduction of Bi can be observed, which is a characteristic feature of nonradiative energy transfer process. Therefore, the enhancement of NIR emission from Bi in Yb–Bi codoped glasses (SACYB-Si1, 2, 3, 4) could arise from an energy
263
Fig. 5. Decay curves of the Yb3+ (2F5/2 → 2F7/2, 1020 nm) transition in (a) SACY-Si and (b) SACYB-Si4 under excitation of 976 nm.
transfer from Yb 3+ to Bi active centers and this energy transfer should be a resonant and nonradiative process, which can be investigated using the Forster–Dexter theory based on the electrostatic interaction. The energy transfer efficiency (ηET) is defined as the ratio of donors that are depopulated by energy transfer to the acceptors over the total number of donors being excited. The experimental energy transfer efficiency can be calculated by equation as follows [29]: ηET ¼ 1− τm =τ0
ð3Þ
Where τm, τ0 are measured emission decay lifetime of Yb 3+ in samples SACYB-Si4 and SACY-Si, which are calculated by formula (2). The ηET of sample SACYB-Si4 calculated by Eq. (3) is about 68.2%. Fig. 6 shows the possible mechanism of energy transfer from Yb 3+ to Bi active centers in our work. Yb 3+ ions absorb 980 nm emission and are excited from ground state to excited state ( 2F7/2– 2F5/2), then some energy is released in the form of near infrared emission at 1020 nm, as well as other energy will be transferred to Bi NIR active ions in terms of nonradiative process. The energy transfer process can enhance broadband NIR luminescence from Bi ions significantly when excited by 980 nm LD. Fluorescence decay curves of Yb 3+ in varying Yb 3+ and constant Bi doped glasses (SACYB-Si1, 2, 3, 4) are measured to investigate the effects of Yb 3+ concentration on the ηET and find the optimal
1.0
0.5
Excited State
2
0
Ground State
F7/2 Yb3+
Fig. 4. The emission properties of (a) SACYB-Si excited by 980 nm LD. (b) SACB-Si excited by 808 nm LD and (c) SACB-Si excited by 980 nm LD.
Energy Transfer
NIR
F5/2
1020 nm
2
980nm
Energy ( 104cm-1)
1.5
Bi active center
Fig. 6. Energy level diagram of Yb2O3–Bi2O3 co-doped glasses which exhibits Yb3+ → Bi active centers energy transfer.
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transfer process can be analyzed by means of Dexter's theory. Since the concentration of Bi is as low as 0.1 mol%, comparable to that in fiber cores, the law of energy transfer may be helpful to subsequent researches of Yb–Bi co-doped fibers. Acknowledgments This work was partly financially supported by the National High-Technology Research and Development Program of China (2011AA030201). The author would like to appreciate Huazhong University of Science &Technology Analytical and Testing Center for the spectroscopic measurement. References
Fig. 7. Decay curves of the Yb under excitation of 976 nm.
3+
2
2
( F5/2– F7/2, 1020 nm) transition in SACYB-Si1, 2, 3, 4
ratio of Yb/Bi. As shown in Fig. 7, the lifetime decrease gradually when the concentration of Yb is varied from 0.5 mol% to 2.0 mol% with constant Bi, and the calculated lifetimes are 890 μs, 770 μs, 680 μs, 500 μs, respectively. Since the content of Bi is constant, the decreasement of lifetime of Yb 3+ indicates more efficient energy transfer behavior. Consequently, when the concentration of Yb is 2.0 mol%, energy transfer from Yb 3+ to Bi active centers is most efficient with ηET of 68.2%. The above results imply that the law of energy transfer when the concentration of Bi is very low is approximately identical with the conclusions reported previously, despite that the optimal ratio of Yb/Bi is profoundly different, about 20 in our study and 1–3.5 in previous reports [13–16]. This phenomenon can be illustrated with above results in Fig. 1. Owing to the addition of Si powder, more Bi active centers will form, which means that more Yb 3+ ions will be required to work as sensitizers. 5. Conclusion In conclusion, Bi doped and Yb–Bi co-doped silicate glasses were successfully prepared with the introduction of Si powder in the melting process. The effect of Si powder on NIR emission was investigated. It is suggested that the addition of reducing agent (such as Si powder) plays an important role in facilitating the NIR luminescence from active Bi ions. Absorption, luminescence spectra and fluorescence decay curves of Yb single doped and Yb–Bi co-doped glasses were measured and studied. Efficient energy transfer from Yb 3+ to Bi active centers with efficiency of 68.2% can be observed in our work, which is a resonant and nonradiative process. The mechanism of this energy
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effect of Si doping on near-infrared emission and energy transfer of Bismuth in silicate glasses Nengli Dai a, Huaixun Luan a, Bing Xu b, Lvyun Yang a, Yubang Sheng a, Zijun Liu a, Jinyan Li a,⁎ a b
Wuhan National Laboratory for Optoelectronics, School of Optoelectronics Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China Wuhan Mechanical Technology College, Wuhan 430075, PR China
a r t i c l e
i n f o
Article history: Received 25 June 2011 Received in revised form 19 September 2011 Available online 18 October 2011 Keywords: Optical material; Si; Low concentration Bismuth; Energy transfer; Near infrared emission
a b s t r a c t We have studied the effects of Si doping on the near infrared (NIR) luminescence observed in low Bi doped ( 0.1 mol% ) glasses and the energy transfer from Yb 3+ to Bi. The broadband near infrared can only be observed when Si is introduced in the Bi-doped glass. The origin of this fluorescence can be attributed to Bi ions at low valence. Efficient energy transfer from Yb 3+ to Bi NIR active ions is achieved by co-doping of Si. There is an increment of about ~ 29 times of the emission intensity from Bi-related active center as the Yb 3+ concentration varies from 0 to 2.0 mol% and the amount of Si is 0.05 mol% under 980 nm excitation. The possible mechanism of energy transfer from Yb 3+ to Bi is also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Fiber amplifiers with broad and efficient gain are playing a key role in the development of modern optical communication. In 2001, Fujimoto and his co-workers reported a novel NIR emission centered at 1250 nm with 800 nm excitation and obtained optical amplification at 1300 nm from Bismuth-doped silicate glasses, which have drawn much attention on its potential application in optical fiber amplifiers [1,2]. Since then, a large number of Bi-doped glass hosts have been explored, mainly including silicate, germinate, phosphate, borate, and chalcogenide glasses [3–12]. For practical applications, it is imperative to increase the stimulated emission cross-section in Bi-doped glasses. One probable approach is sensitizing Bi-related active centers by some sensitizers. Recent researches have revealed that the intensity of Bi-related NIR luminescence can be largely increased through efficient energy transfer associated with Yb 3+ ( 2F5/2– 2F7/2) transition in Yb–Bi co-doped glasses [13–16]. To the best of our knowledge, few attention were drawn on the study of energy transfer process between sensitizers and low concentration Bi (b0.5 mol%) in silicate glasses, which was more similar with the concentration of Bi ions in silica fibers. In this work, optical characteristics of low concentration Bismuth and energy transfer from Yb 3+ to Bi ions were studied. The effects of additional Si in glass hosts on the luminescence and possible mechanism of the energy transfer between Yb 3+ and low concentration Bi (0.1 mol%) were also discussed.
Glass samples with compositions listed in Table 1 were prepared by a conventional melting method. Analytical pure reagent commercial oxides (N99.5%) were selected as the raw materials. Si powder was introduced into the raw material as reducing agent. The mixed batch of 30 g were melted in alumina crucible at 1580 °C for 2 hours in reducing atmosphere achieved by the introduction of Si powder. The glass melts were poured on the pre-heated steel mold and then annealed near the glass transition temperature for 2 hours in the annealing furnace. The obtained glasses were cut into 15 × 15 × 2 mm 3 and polished for optical measurement. Optical absorption spectroscopic was measured on PerkinElmer-Lambda35 spectrophotometer in the range of 200–1100 nm. The luminescence spectra were obtain by ZOLIX SBP300 spectrophotometer, detected by InGaAs photo-detector with excitation of 808 nm and 980 nm LD, respectively. The fluorescence lifetimes were measured by TRIAX550 spectrofluorometer and detected by InGaAs photo-detector and recorded by a storage digital oscilloscope (Tektronix TDS3052). All the measurements were carried out at room temperature in air.
⁎ Corresponding author. Tel.: + 86 027 87559463; fax: + 86 027 87559463. E-mail addresses: [email protected], [email protected] (J. Li). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.09.023
3. Results Fig. 1 shows the absorption and NIR photoluminescence (inset) spectra of SACB and SACB-Si. For the SACB-Si sample, absorption bands centered at ~ 470 nm and 700 nm are observed obviously in Fig. 1, which should be attributed to Bi-related NIR active center, in accord with the results reported previously [1,5,7,12]. In the inset of Fig. 1, intense NIR emission at ~1300 nm can be measured from
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Table 1 Compositions of the Si–Bi–Yb doped silicate glasses. Glass sample
Composition (mol%) SiO2
Al2O3
CaO
Bi2O3
Yb2O3
Si
SACB SACB-Si SAC-Si SACYB-Si1 SACYB-Si2 SACYB-Si3 SACYB-Si4 SACYB-Si5 SACY-Si
65 65 65 65 65 65 65 65 65
10 10 10 10 10 10 10 10 10
25 25 25 25 25 25 25 25 25
0.1 0.1 – 0.1 0.1 0.1 0.1 0.1 –
– – – 0.5 1.0 1.5 2.0 2.5 2.0
– 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
SACB-Si under excitation of 808 nm LD. But as shown in Fig. 1, there are no apparent absorption and emission peaks in glass SACB. NIR luminescence cannot be observed from single Si doped silicate glass without Bismuth (sample SAC-Si), which is suggested that near-IR fluorescence centered at 1300 nm is derived from Bi ions rather than Si, and Si played a supporting role only for the near-IR emission of Bi. Despite the fact that several proposals have been made tentatively, assigning the role of emission center to Bi 5+, Bi +, Bi metal clusters or negatively charged Bismuth dimmers, the origin of NIR emission still remains highly debated [1,3,17–22]. According to Duffy's theory of optical basicity, the increasing basicity of the host glass will facilitate formation of higher valence states of Bi[23,24]. Based on our previous study, we tend to consider low valent Bi ions as NIR emission centers. In this paper, Bi-related NIR active center is named as Bi active center. Results presented above indicate that the addition of Si powder into glass component can strengthen the infrared emission from Bi greatly. The reducing condition introduced by Si powder promotes the formation of Bi active centers in Bi doped glass and emitting superior NIR luminescence. Reaction formula of this phenomenon in the melting process can be expressed as:
Fig. 2. Absorption spectra of glass samples SACYB-Si1, 2, 3, 4, 5.
In 2005, Russian researcher E.M.Dianov and Japanese scientist Haruna. T fabricated Bi-doped silica fiber by employing the MCVD method [25,26]. According to their reports, the concentration of Bi active ions in core glass measured by X-ray microanalysis was about 0.02% in most of the fibers [27,28]. But as we know, in most previous
researches on luminescence properties, optical application and energy transfer process between Yb 3+ and Bi-related active ions in glass hosts, the concentration of Bi was very high (N0.5 mol%), much higher than that in fiber cores (b0.02 mol%). We prepared glass samples SACYB-Si1, 2, 3, 4, 5 to explore the most efficient energy transfer project when Bi is in low concentration. Fig. 2 shows the absorption spectra of SACB-Si and SACYB-Si samples. Apparent absorption bands centered at ~470 nm and 700 nm originating from Bi active centers can be seen in the figure. The strong absorption peak observed at 980 nm in glass SACYB-Si should be ascribed to 2F7/2– 2F5/2 transition of Yb 3+. With the introduction of Yb 3+ and the increment of concentration, the absorption intensity at ~470 nm and 700 nm becomes weaker gradually. On the contrary, enhancement of absorption band at 980 nm attributed to Yb 3+ can be observed simultaneously. Based on above analysis of optical basicity, the reduction of absorption strength at Bi-related absorption bands may be attributed to the oxidbillity of Yb 3+[23,24]. Increasing oxidbillity with the introduction of Yb2O3 facilitates formation of higher valence states of Bi, which leads to the decrement of Bi active centers. Consequently, absorption bands at ~470 nm and 700 nm related to Bi active centers weaken gradually. Infrared emission properties of SACB-Si and SACYB-Si1, 2, 3, 4, 5 pumped by 980 nm LD are shown in Fig. 3. The amount of Bi and Si
Fig. 1. Absorption spectra and NIR emission spectra (inset) of SACB and SACB-Si.
Fig. 3. Luminescence spectra of silicate glasses SACYB-Si1, 2, 3, 4, 5 under excitation of 980 nm LD.
2Bi2 O3 þ 3Si → 3SiO2 þ 4Biactive−center :
ð1Þ
N. Dai et al. / Journal of Non-Crystalline Solids 358 (2012) 261–264
powder is constant(0.1 and 0.05 mol%, respectively), while the concentration of Yb is varied from 0 mol% to 2.5 mol%. Two characteristic emissions are observed in the spectra, which should be ascribed to the Yb 3+ ( 2F5/2– 2F7/2) transition at 1020 nm and Bi active centers at around 1220 nm. Bi active centers in the Yb–Bi co-doped glasses show stronger near-infrared emission than that in the single Bi doped glass. Inset of Fig.3 presents the magnified patterns of Fig. 3 at the range of 1100–1450 nm. With the increased Yb concentration, the emission intensity from Bi active centers increased at first and then decreased, strongest at ~2.0 mol%. The decrease in the luminescence intensity with Yb concentration of 2.5 mol% could be induced by concentration quenching between Yb 3+ ions duo to radiative reabsorption. Slight red-shift of central wavelength of Bi-related emission peak from 1180 nm to 1223 nm is observed, while reasons for this phenomenon are uncertain and need exploration in details. Fig. 4 shows NIR photoluminescence spectra of single Bi doped and Yb–Bi co-doped glasses (SACB-Si and SACYB-Si4)excited by 808 nm and 980 nm LDs, respectively. Under excitation of 980 nm LD, the intensity of NIR emission at 1200 nm from SACYB-Si4 is ~29 times higher than that of SACB-Si (in mol%). For further research, we compared the emission intensity of sample SACYB-Si4 excited by 980 nm LD and SACB-Si excited by 808 nm LD. The luminescent intensity under 980 nm excitation is about 3 times higher than that of the latter. 4. Discussion The lifetime is the most important data supporting the energy transfer from sensitizers to acceptors. Fluorescence decay curves near 1020 nm corresponding to the 2F5/2 energy lever of Yb 3+ from glass samples SACYB-Si4 and SACY-Si were measured and presented in Fig. 5. The mean lifetime (τm) of Yb 3+ at 1020 nm can be calculated by following formula [29]: ∞
=
∞
τm ¼ ∫t0 Iðt Þtdt ∫t0 Iðt Þdt:
ð2Þ
Where I(t) is the luminescence intensity as a function of time t. The calculated lifetimes are 500 μs for sample SACYB-Si4 and 1570 μs for SACY-Si. An obvious decrease of sensitizer's emission decay lifetime with the introduction of Bi can be observed, which is a characteristic feature of nonradiative energy transfer process. Therefore, the enhancement of NIR emission from Bi in Yb–Bi codoped glasses (SACYB-Si1, 2, 3, 4) could arise from an energy
263
Fig. 5. Decay curves of the Yb3+ (2F5/2 → 2F7/2, 1020 nm) transition in (a) SACY-Si and (b) SACYB-Si4 under excitation of 976 nm.
transfer from Yb 3+ to Bi active centers and this energy transfer should be a resonant and nonradiative process, which can be investigated using the Forster–Dexter theory based on the electrostatic interaction. The energy transfer efficiency (ηET) is defined as the ratio of donors that are depopulated by energy transfer to the acceptors over the total number of donors being excited. The experimental energy transfer efficiency can be calculated by equation as follows [29]: ηET ¼ 1− τm =τ0
ð3Þ
Where τm, τ0 are measured emission decay lifetime of Yb 3+ in samples SACYB-Si4 and SACY-Si, which are calculated by formula (2). The ηET of sample SACYB-Si4 calculated by Eq. (3) is about 68.2%. Fig. 6 shows the possible mechanism of energy transfer from Yb 3+ to Bi active centers in our work. Yb 3+ ions absorb 980 nm emission and are excited from ground state to excited state ( 2F7/2– 2F5/2), then some energy is released in the form of near infrared emission at 1020 nm, as well as other energy will be transferred to Bi NIR active ions in terms of nonradiative process. The energy transfer process can enhance broadband NIR luminescence from Bi ions significantly when excited by 980 nm LD. Fluorescence decay curves of Yb 3+ in varying Yb 3+ and constant Bi doped glasses (SACYB-Si1, 2, 3, 4) are measured to investigate the effects of Yb 3+ concentration on the ηET and find the optimal
1.0
0.5
Excited State
2
0
Ground State
F7/2 Yb3+
Fig. 4. The emission properties of (a) SACYB-Si excited by 980 nm LD. (b) SACB-Si excited by 808 nm LD and (c) SACB-Si excited by 980 nm LD.
Energy Transfer
NIR
F5/2
1020 nm
2
980nm
Energy ( 104cm-1)
1.5
Bi active center
Fig. 6. Energy level diagram of Yb2O3–Bi2O3 co-doped glasses which exhibits Yb3+ → Bi active centers energy transfer.
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transfer process can be analyzed by means of Dexter's theory. Since the concentration of Bi is as low as 0.1 mol%, comparable to that in fiber cores, the law of energy transfer may be helpful to subsequent researches of Yb–Bi co-doped fibers. Acknowledgments This work was partly financially supported by the National High-Technology Research and Development Program of China (2011AA030201). The author would like to appreciate Huazhong University of Science &Technology Analytical and Testing Center for the spectroscopic measurement. References
Fig. 7. Decay curves of the Yb under excitation of 976 nm.
3+
2
2
( F5/2– F7/2, 1020 nm) transition in SACYB-Si1, 2, 3, 4
ratio of Yb/Bi. As shown in Fig. 7, the lifetime decrease gradually when the concentration of Yb is varied from 0.5 mol% to 2.0 mol% with constant Bi, and the calculated lifetimes are 890 μs, 770 μs, 680 μs, 500 μs, respectively. Since the content of Bi is constant, the decreasement of lifetime of Yb 3+ indicates more efficient energy transfer behavior. Consequently, when the concentration of Yb is 2.0 mol%, energy transfer from Yb 3+ to Bi active centers is most efficient with ηET of 68.2%. The above results imply that the law of energy transfer when the concentration of Bi is very low is approximately identical with the conclusions reported previously, despite that the optimal ratio of Yb/Bi is profoundly different, about 20 in our study and 1–3.5 in previous reports [13–16]. This phenomenon can be illustrated with above results in Fig. 1. Owing to the addition of Si powder, more Bi active centers will form, which means that more Yb 3+ ions will be required to work as sensitizers. 5. Conclusion In conclusion, Bi doped and Yb–Bi co-doped silicate glasses were successfully prepared with the introduction of Si powder in the melting process. The effect of Si powder on NIR emission was investigated. It is suggested that the addition of reducing agent (such as Si powder) plays an important role in facilitating the NIR luminescence from active Bi ions. Absorption, luminescence spectra and fluorescence decay curves of Yb single doped and Yb–Bi co-doped glasses were measured and studied. Efficient energy transfer from Yb 3+ to Bi active centers with efficiency of 68.2% can be observed in our work, which is a resonant and nonradiative process. The mechanism of this energy
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