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Yb3+-codoped lead oxyfluorosilicate glass
Chemical Physics Letters 385 (2004) 263–267 www.elsevier.com/locate/cplett
Upconversion fluorescence spectroscopy of Er3þ /Yb3þ -codoped lead oxyfluorosilicate glass Shiqing Xu *, Zhongmin Yang, Junjie Zhang, Guonian Wang, Shixun Dai, Lili Hu, Zhonghong Jiang Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 390 Qinghe RD, Jiading, Shanghai 201800, PR China Received 29 October 2003; in final form 19 December 2003 Published online: 19 January 2004
Abstract Upconversion fluorescence properties of a new Er3þ /Yb3þ -codoped lead oxyfluorosilicate glass under 975 nm excitation are investigated. The blue, intense green and red emissions centered at 408, 529, 545, and 667 nm, corresponding to the 2 H9=2 ! 4 I15=2 , 2 H11=2 ! 4 I15=2 , 4 S3=2 ! 4 I15=2 , and 4 F9=2 ! 4 I15=2 transitions of Er3þ , respectively, were simultaneously observed at room temperature. The important role of Yb2 O3 in upconversion intensity is observed, and the influence of Yb2 O3 and PbF2 on blue, green and red emissions is compared and discussed. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are evaluated. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the upconversion of infrared light to visible light by rare earth ions doped glasses has been investigated extensively, due to the possibility of infrared-pumped visible lasers and the potential applications in areas such as color display, optical data storage, optoelectronics, medical diagnostics, sensor, and undersea optical communication [1–3]. Many trivalent rare-earth ions such as Er3þ , Tm3þ Ho3þ , Pr3þ and Nd3þ were introduced as absorption and emission centers in glass hosts. Among the rare-earth ions, Er3þ is the most popular as well as one of the most efficient ions [4–6]. So far, most efforts have been spent on fluoride glasses owing to their lower phonon energies. Unlike fluoride glasses, upconversion is seldom observed in oxide glasses with high phonon energies and can be limited to germanate [7], tellurite [8], and gallate [8] glasses that have comparatively low phonon energies. Unfortunately silicate glasses, which are the most chemically and mechanically stable and also are more easily fabricated into various shapes such as a rod and optical fiber [9], have *
Corresponding author. Fax: +86-21-59914516. E-mail address: [email protected] (S. Xu).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.104
very faint upconversion fluorescence due to their large phonon energies [10]. Therefore, the design of a new silicate glass host for Er3þ realizing intense upconversion fluorescence is a target at present. Since the spectral region of the 2 F7=2 ! 2 F5=2 transition of Yb3þ overlaps that of the 4 I15=2 ! 4 I11=2 transition of Er3þ , it is possible to achieve an effective Yb3þ to Er3þ transfer mechanism of the excitation energy [11], but in Er3þ /Yb3þ -codoped glass, there exists optimal Yb3þ doping content [12]. In the last few years, many authors have demonstrated pulsed, as well as continuous wave operation of Er3þ /Yb3þ -codoped glass lasers [13]. As is known, glasses based on mixed oxide–halide systems combine the good optical properties of halide glasses (a broad range of optical transmittance and low optical losses) with the better chemical and thermal stability of oxide glasses [14]. In this Letter, we report the upconversion emissions of a new Er3þ /Yb3þ codoped lead oxyfluorosilicate glass under 975 nm excitation at room temperature. 2. Experiments Er3þ /Yb3þ -codoped 50SiO2 –50PbF2 –5Yb2 O3 –1Er2 O3 (SPYE) glass (mol%) was prepared using the conven-
S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267 4
G11/2
25
2
H9/2
4
F3/2,5/2
-1
20
4
2
H11/2
S3/2
F9/2
15
F7/2
4
4
3
4
I9/2
2
10 F5/2
4
I11/2
2 0 F7/2
4
I15/2
3+
Er
Yb
545 nm
I13/2
5
667 nm
4
408 nm 529 nm
tional melting and quenching method described in [15]. Er3þ -singly doped 50SiO2 –50PbF2 –1Er2 O3 (SPE) glass (mol%) is also prepared for a comparison. One undoped 50SiO2 –50PbF2 (SP) glass is also prepared for measuring the Raman spectrum. UV/VIS/NIR absorption spectra were recorded between 300 and 1700 nm using a spectrophotometer on glass samples optically polished. The upconversion luminescence spectra were obtained with a TRIAX550 spectrofluorimeter upon excitation of 975 nm laser diode with a maximum power of 2 W. In order to compare the luminescence intensity of Er3þ in different samples as accurate as we can, the position and power (100 mW) of the pumping beam and the width (1 mm) of the slit to collect the luminescence signal were fixed under the same condition, and the samples was set at the same place in the experimental setup. In addition, the integrated intensities for the blue, green and red emissions were also calculated to illustrate the variations of the luminescence intensity. The lifetimes for 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 levels of Er3þ were measured using a modulated 975 nm LD and Tektronix TDS3052 digital oscilloscope controlled by a computer. Raman spectrum was recorded on a FT Raman spectrophotometer (Nicolet MODULE) within the range of 100– 1500 cm1 . Nd:YAG operating at 1064 nm was used as the excitation source, and the laser power level was 500 mW. All the measurements were taken at room temperature.
Energy (10 cm )
264
3+
3+
Er
Fig. 1. Energy level diagram of Er3þ and Yb3þ , and upconversion mechanisms of SPYE glass under 975 nm excitation at room temperature. The solid lines stand for the absorption and emission transitions for Er3þ and Yb3þ . The dashed lines stand for energy transfer from Er3þ and Yb3þ . The curves represent the nonradiative relaxations.
Table 1 The ET efficiency ðgÞ and lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 levels of Er3þ in SPE and SPYE glasses Samples
g (%)
Lifetimes (ls) 2
SPE SPYE
0 81.2
2
H9=2
68 116
H11=2
169 255
4
4
S3=2
210 346
F9=2
174 270
3. Results and discussion
g¼1
sYb =s0Yb ;
ð1Þ
where sYb and s0Yb were the 2 F5=2 lifetimes of Yb3þ ions with and without Er3þ ions, respectively. The ET efficiency of SPYE glass was shown in Table 1. It is seen that the ET efficiency is very high. Fig. 2 shows the upconversion emission spectra of Er3þ in the range of 400 to 700 nm for SPE and SPYE glass samples under 975 nm excitation at room temperature, respectively. The upconversion emission bands centered at 408, 529, 545, and 667 nm are attributed to
4
4
I15/2
2
H11/2
4
I15/2
S3/2
I15/2
4
4
F9/2
H9/2
Intensity (arb.units)
400
4
I15/2
(b) (a)
2
Fig. 1 illustrates a simplified energy level diagram of the SPYE glass system pumped at 975 nm. The energy transfer (ET) from Yb3þ to Er3þ , Yb(2 F5=2 ) + Er(4 I15=2 ) ! Yb(2 F7=2 ) + Er (4 I11=2 ), act as indirect pumping of Er3þ . So the transfer efficiency will play an important role in Er3þ /Yb3þ system. Because of the large spectral overlap between Yb3þ emission (2 F5=2 ! 2 F7=2 ) and Er3þ absorption (4 I15=2 ! 4 I11=2 ) as well as the short lifetime of 4 I11=2 level (Er3þ ) in SPYE glass, it is predicable that the energy transfer (ET) from Yb3þ to Er3þ has a high efficiency in SPYE glass. The ET efficiency ðgÞ can be expressed as [16]
450
500 550 600 Wavelength (nm)
650
700
Fig. 2. The upconversion emission spectra of Er3þ in SPE: (a) SPYE; (b) glasses under 975 nm the excitation at room temperature.
the transitions from the excited states 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 to the ground state 4 I15=2 of Er3þ , respectively. The emission at 408 nm is much weaker than the other emissions. In addition, the upconversion emission intensity of SPE glass sample is much weaker than that of SPYE glass sample under the same exci-
S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267
Log [Intensity (a.u.)]
tation conditions. The relative integral intensities of the blue (408 nm), green (529 and 545 nm) and red (667 nm) emissions in SPYE glass increase by a factor about 104.8, 156.9, 64.6, and 21.4, respectively, when compared with those in SPE glass. These results suggest that there is a very effective Yb3þ to Er3þ transfer mechanism of the excitation energy. It is also important to point out that the upconversion process in SPYE glass was so efficient that the green emission could readily be seen with the naked eye, at pump power as low as 50 mW. The pump power dependence of the aforementioned four upconversion emission bands was analyzed and the results are depicted in log–log plots of Fig. 3. According to Fig. 3, the results indicate that a two-photon upconversion process is assigned to the green (529 and 545 nm) and red (667 nm) emissions from 2 H11=2 , 4 S3=2 and 4 F9=2 levels, respectively, while a three-photon process is responsible for blue (667 nm) emission from 2 H9=2 level. According to the energy matching conditions and the dependence on excitation power, the possible upconversion mechanisms for the emission bands could be explained as illustrated in Fig. 1 [17]. For the green emission, first, the Er3þ ion was excited from ground state 4 I15=2 to the excited state 4 I11=2 by one of the three processes: ground state absorption, phonon-assisted energy transfer (PAET) from the Yb3þ2 F5=2 level, and ET from the 4 I11=2 level of adjacent Er3þ . Among the three processes, PAET from Yb3þ is the main one, since the Yb3þ has a larger absorption cross section than the Er3þ around 975 nm and the upconversion fluorescence in SPE glass under the same excitation conditions is very weak. Second, the populated 4 I11=2 level was excited to the 4 F7=2 level by the same three processes: excited state absorption (ESA), PAET from Yb3þ , and ET from the 4 I11=2 level of adjacent Er3þ . The populated Er3þ4 F7=2 level then
408 nm slope=2.56 529 nm slope=1.99 545 nm slope=1.91 667 nm slope=1.89
1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 Log [Excititation power (mW)] Fig. 3. Log–log plot of upconversion fluorescence emission intensity as a function of pump power at 975 nm.
265
relaxes rapidly and nonradiatively to the next lower levels 2 H11=2 and 4 S3=2 resulting from the small energy gap between them. Because the energy gap below the 2 H11=2 level is very smaller, for example, about 744 cm1 in our experiment, the most Er3þ relaxes nonradiatively to the 4 S3=2 level and the 4 S3=2 level is populated, while the 2 H11=2 level is not populated by means of above process. It is considered that the 2 H11=2 level is populated from 4 S3=2 by a fast thermal equilibrium between the levels [18,19]. The above processes then produces the two 2 H11=2 ! 4 I15=2 and 4 S3=2 ! 4 I15=2 green emissions centered at 529 and 545 nm, respectively. An analysis based on a simple three-level system comprised of 4 I15=2 (level 0), 4 S3=2 (level 3), and 2 H11=2 (level 4) predicts that thermalization of the 2 H11=2 level can be expressed by the following equation [20]: I4 p4r g4 hx4 E43 ¼ exp ; ð2Þ I3 p3r g3 hx3 kT where p3r and p4r are the total spontaneous emission rates, hx3 and hx4 are photon energies, and g3 and g4 are the degeneracies ð2J þ 1Þ of the 4 S3=2 and 2 H11=2 levels, respectively. The I terms represent the integrated intensity of a given transition at a temperature. The E43 term is the energy gap between the 2 H11=2 and 4 S3=2 levels and k is the BoltzmannÕs constant. In our experiments, the calculated value of I4 =I3 equal to 0.331. This value is in excellent agreement with the experimental value of 0.329, which indicates that the 2 H11=2 level is populated from 4 S3=2 by a fast thermal equilibrium between the levels. Therefore, the green upconversion fluorescence emitted through 4 S3=2 ! 4 I15=2 transition is much brighter than that of through 2 H11=2 ! 4 I15=2 transition. There exist two main possible pumping mechanisms for red emission in SPYE glass. The first pumping mechanism comprises the population of the 4 S3=2 level, by means of the process described previously, followed by a fast nonradiative decay through multiphonon interaction from the populated 4 S3=2 to 4 F9=2 and then to the 4 I15=2 ground state. In the other possible mechanism, the Er3þ populated 4 I11=2 level mostly relaxed nonradiatively to the long-living 4 I13=2 level. The populated 4 I13=2 level may be excited to the 4 F9=2 level by the same three processes: excited state absorption, phonon-assisted energy transfer from Yb3þ , and energy transfer from the 4 I11=2 level of adjacent Er3þ . The above processes then produces the 4 F9=2 ! 4 I15=2 red emission centered at 667 nm. For the blue emission, the energy gap equal to 24631 cm1 between 2 H9=2 and 4 I15=2 levels calculated by absorption spectrum data, and the maximum pump photon energy is 10 256 cm1 . Therefore, the direct step-wise two-photon excitation4 I15=2 ! 4 I11=2 ! 2 H9=2 is not the case. A third step excitation is necessary to reach the 2 H9=2 level. A possible mechanism is the 4 F9=2 ! 2 H9=2 transition by absorption of a third pump photon by thesame three processes: ESA, PAET
266
S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267
Intensity (a.u.)
904
200
400
600 800 1000 1200 Raman shift (cm-1)
1400
Fig. 4. Raman spectrum of undoped SP glass at room temperature.
from Yb3þ , and ET from the 4 I11=2 level of adjacent Er3þ . The branching ratio calculated by Judd–Ofelt theory [21,22] for the transitions 2 H9=2 ! 4 I15=2 is 10.6%, and from Table 1 it is seen that the lifetimes of 2 H9=2 level of Er3þ is very low. Therefore, the radiative transition probability involved in the above processes can be small and the blue emission observed is very weak. According to Table 1, it can be seen that the ET efficiency is very high, and the lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 level of Er3þ in SPYE glass sample are much larger than that of in SPE glass sample. The results indicate that the ET rate is dominant to increasing the upconversion luminescence for blue, green, and red emissions in SPYE glass sample. Fig. 4 shows the Raman spectrum of undoped SP glass. For SP glass, the dominated features are the broad mode at about 904 cm1 assigned to bending vibrations of O–Si–O linkages. The spectrum indicates the highest frequency phonon band is at about 904 cm1 , smaller than that of pure SiO2 glass which has a maximum phonon band at 1100 cm1 [23]. The result indicates that the lead fluoride added enters the glass lower the maximum phonon energy of silicate glasses, and thus it reduces the nonradiative loss due to the mutiphonon relaxation, and enhances radiative transition and upconversion luminescence intensity of Er3þ in SP glass. From the above results it can deduce that fluoride ions in the glass network also have an important influence on the upconversion fluorescence of Er3þ .
4. Conclusion We have experimentally investigated upconversion fluorescence properties and mechanism in a new SPYE glass. The emissions of four visible fluorescence bands centered at 408, 529, 545, and 667 nm, corresponding to
the 2 H9=2 ! 4 I15=2 , 2 H11=2 ! 4 I15=2 , 4 S3=2 ! 4 I15=2 , and 4 F9=2 ! 4 I15=2 transition of Er3þ , respectively, were simultaneously observed under 975 nm excitation at room temperature. In SPYE glass, the ET efficiency is very high, and the lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 level of Er3þ are much larger than that of in SPE glass. The results indicate that the ET rate is dominant to increasing the upconversion luminescence for blue, green, and red emissions in SPYE glass sample. The upconversion processes involve a sequential two-photon absorption for the green (529 and 545 nm) and red (667 nm) emissions, while a sequential three-photon absorption for the blue emission (408 nm). Raman spectrum indicates that fluoride ions in the oxyfluorosilicate glasses network have an important influence on the efficiency of the upconversion process owing to lower the maximum phonon energy of glass host. The intense upconversion fluorescence of SPYE glass may be a potentially useful material for developing upconversion fiber optical devices.
Acknowledgements This work was financially supported by Shanghai Science and Technology Foundation (022261046) and Chinese National Natural Science Foundation (60207006).
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S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267 [17] H. Lin, G. Meredith, S. Jiang, X. Peng, T. Luo, N. Peyghambarian, E.Y.B. Pun, J. Appl. Phys. 93 (2003) 186. [18] E. Maurice, G. Monnom, B. Dussardier, A. Saissy, D.B. Ostrowsky, Opt. Lett. 19 (1994) 990. [19] H. Berthou, C.K. Jorgensen, Opt. Lett. 15 (1990) 1100.
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[20] D.C. Yeh, W.A. Sibley, M. Suscavage, M.G. Drexhage, J. Appl. Phys. 62 (1987) 266. [21] B.R. Judd, Phys. Rev. 127 (1962) 750. [22] S. Ofelt, J. Chem. Phys. 37 (1962) 511. [23] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187.
Upconversion fluorescence spectroscopy of Er3þ /Yb3þ -codoped lead oxyfluorosilicate glass Shiqing Xu *, Zhongmin Yang, Junjie Zhang, Guonian Wang, Shixun Dai, Lili Hu, Zhonghong Jiang Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 390 Qinghe RD, Jiading, Shanghai 201800, PR China Received 29 October 2003; in final form 19 December 2003 Published online: 19 January 2004
Abstract Upconversion fluorescence properties of a new Er3þ /Yb3þ -codoped lead oxyfluorosilicate glass under 975 nm excitation are investigated. The blue, intense green and red emissions centered at 408, 529, 545, and 667 nm, corresponding to the 2 H9=2 ! 4 I15=2 , 2 H11=2 ! 4 I15=2 , 4 S3=2 ! 4 I15=2 , and 4 F9=2 ! 4 I15=2 transitions of Er3þ , respectively, were simultaneously observed at room temperature. The important role of Yb2 O3 in upconversion intensity is observed, and the influence of Yb2 O3 and PbF2 on blue, green and red emissions is compared and discussed. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are evaluated. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the upconversion of infrared light to visible light by rare earth ions doped glasses has been investigated extensively, due to the possibility of infrared-pumped visible lasers and the potential applications in areas such as color display, optical data storage, optoelectronics, medical diagnostics, sensor, and undersea optical communication [1–3]. Many trivalent rare-earth ions such as Er3þ , Tm3þ Ho3þ , Pr3þ and Nd3þ were introduced as absorption and emission centers in glass hosts. Among the rare-earth ions, Er3þ is the most popular as well as one of the most efficient ions [4–6]. So far, most efforts have been spent on fluoride glasses owing to their lower phonon energies. Unlike fluoride glasses, upconversion is seldom observed in oxide glasses with high phonon energies and can be limited to germanate [7], tellurite [8], and gallate [8] glasses that have comparatively low phonon energies. Unfortunately silicate glasses, which are the most chemically and mechanically stable and also are more easily fabricated into various shapes such as a rod and optical fiber [9], have *
Corresponding author. Fax: +86-21-59914516. E-mail address: [email protected] (S. Xu).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.104
very faint upconversion fluorescence due to their large phonon energies [10]. Therefore, the design of a new silicate glass host for Er3þ realizing intense upconversion fluorescence is a target at present. Since the spectral region of the 2 F7=2 ! 2 F5=2 transition of Yb3þ overlaps that of the 4 I15=2 ! 4 I11=2 transition of Er3þ , it is possible to achieve an effective Yb3þ to Er3þ transfer mechanism of the excitation energy [11], but in Er3þ /Yb3þ -codoped glass, there exists optimal Yb3þ doping content [12]. In the last few years, many authors have demonstrated pulsed, as well as continuous wave operation of Er3þ /Yb3þ -codoped glass lasers [13]. As is known, glasses based on mixed oxide–halide systems combine the good optical properties of halide glasses (a broad range of optical transmittance and low optical losses) with the better chemical and thermal stability of oxide glasses [14]. In this Letter, we report the upconversion emissions of a new Er3þ /Yb3þ codoped lead oxyfluorosilicate glass under 975 nm excitation at room temperature. 2. Experiments Er3þ /Yb3þ -codoped 50SiO2 –50PbF2 –5Yb2 O3 –1Er2 O3 (SPYE) glass (mol%) was prepared using the conven-
S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267 4
G11/2
25
2
H9/2
4
F3/2,5/2
-1
20
4
2
H11/2
S3/2
F9/2
15
F7/2
4
4
3
4
I9/2
2
10 F5/2
4
I11/2
2 0 F7/2
4
I15/2
3+
Er
Yb
545 nm
I13/2
5
667 nm
4
408 nm 529 nm
tional melting and quenching method described in [15]. Er3þ -singly doped 50SiO2 –50PbF2 –1Er2 O3 (SPE) glass (mol%) is also prepared for a comparison. One undoped 50SiO2 –50PbF2 (SP) glass is also prepared for measuring the Raman spectrum. UV/VIS/NIR absorption spectra were recorded between 300 and 1700 nm using a spectrophotometer on glass samples optically polished. The upconversion luminescence spectra were obtained with a TRIAX550 spectrofluorimeter upon excitation of 975 nm laser diode with a maximum power of 2 W. In order to compare the luminescence intensity of Er3þ in different samples as accurate as we can, the position and power (100 mW) of the pumping beam and the width (1 mm) of the slit to collect the luminescence signal were fixed under the same condition, and the samples was set at the same place in the experimental setup. In addition, the integrated intensities for the blue, green and red emissions were also calculated to illustrate the variations of the luminescence intensity. The lifetimes for 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 levels of Er3þ were measured using a modulated 975 nm LD and Tektronix TDS3052 digital oscilloscope controlled by a computer. Raman spectrum was recorded on a FT Raman spectrophotometer (Nicolet MODULE) within the range of 100– 1500 cm1 . Nd:YAG operating at 1064 nm was used as the excitation source, and the laser power level was 500 mW. All the measurements were taken at room temperature.
Energy (10 cm )
264
3+
3+
Er
Fig. 1. Energy level diagram of Er3þ and Yb3þ , and upconversion mechanisms of SPYE glass under 975 nm excitation at room temperature. The solid lines stand for the absorption and emission transitions for Er3þ and Yb3þ . The dashed lines stand for energy transfer from Er3þ and Yb3þ . The curves represent the nonradiative relaxations.
Table 1 The ET efficiency ðgÞ and lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 levels of Er3þ in SPE and SPYE glasses Samples
g (%)
Lifetimes (ls) 2
SPE SPYE
0 81.2
2
H9=2
68 116
H11=2
169 255
4
4
S3=2
210 346
F9=2
174 270
3. Results and discussion
g¼1
sYb =s0Yb ;
ð1Þ
where sYb and s0Yb were the 2 F5=2 lifetimes of Yb3þ ions with and without Er3þ ions, respectively. The ET efficiency of SPYE glass was shown in Table 1. It is seen that the ET efficiency is very high. Fig. 2 shows the upconversion emission spectra of Er3þ in the range of 400 to 700 nm for SPE and SPYE glass samples under 975 nm excitation at room temperature, respectively. The upconversion emission bands centered at 408, 529, 545, and 667 nm are attributed to
4
4
I15/2
2
H11/2
4
I15/2
S3/2
I15/2
4
4
F9/2
H9/2
Intensity (arb.units)
400
4
I15/2
(b) (a)
2
Fig. 1 illustrates a simplified energy level diagram of the SPYE glass system pumped at 975 nm. The energy transfer (ET) from Yb3þ to Er3þ , Yb(2 F5=2 ) + Er(4 I15=2 ) ! Yb(2 F7=2 ) + Er (4 I11=2 ), act as indirect pumping of Er3þ . So the transfer efficiency will play an important role in Er3þ /Yb3þ system. Because of the large spectral overlap between Yb3þ emission (2 F5=2 ! 2 F7=2 ) and Er3þ absorption (4 I15=2 ! 4 I11=2 ) as well as the short lifetime of 4 I11=2 level (Er3þ ) in SPYE glass, it is predicable that the energy transfer (ET) from Yb3þ to Er3þ has a high efficiency in SPYE glass. The ET efficiency ðgÞ can be expressed as [16]
450
500 550 600 Wavelength (nm)
650
700
Fig. 2. The upconversion emission spectra of Er3þ in SPE: (a) SPYE; (b) glasses under 975 nm the excitation at room temperature.
the transitions from the excited states 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 to the ground state 4 I15=2 of Er3þ , respectively. The emission at 408 nm is much weaker than the other emissions. In addition, the upconversion emission intensity of SPE glass sample is much weaker than that of SPYE glass sample under the same exci-
S. Xu et al. / Chemical Physics Letters 385 (2004) 263–267
Log [Intensity (a.u.)]
tation conditions. The relative integral intensities of the blue (408 nm), green (529 and 545 nm) and red (667 nm) emissions in SPYE glass increase by a factor about 104.8, 156.9, 64.6, and 21.4, respectively, when compared with those in SPE glass. These results suggest that there is a very effective Yb3þ to Er3þ transfer mechanism of the excitation energy. It is also important to point out that the upconversion process in SPYE glass was so efficient that the green emission could readily be seen with the naked eye, at pump power as low as 50 mW. The pump power dependence of the aforementioned four upconversion emission bands was analyzed and the results are depicted in log–log plots of Fig. 3. According to Fig. 3, the results indicate that a two-photon upconversion process is assigned to the green (529 and 545 nm) and red (667 nm) emissions from 2 H11=2 , 4 S3=2 and 4 F9=2 levels, respectively, while a three-photon process is responsible for blue (667 nm) emission from 2 H9=2 level. According to the energy matching conditions and the dependence on excitation power, the possible upconversion mechanisms for the emission bands could be explained as illustrated in Fig. 1 [17]. For the green emission, first, the Er3þ ion was excited from ground state 4 I15=2 to the excited state 4 I11=2 by one of the three processes: ground state absorption, phonon-assisted energy transfer (PAET) from the Yb3þ2 F5=2 level, and ET from the 4 I11=2 level of adjacent Er3þ . Among the three processes, PAET from Yb3þ is the main one, since the Yb3þ has a larger absorption cross section than the Er3þ around 975 nm and the upconversion fluorescence in SPE glass under the same excitation conditions is very weak. Second, the populated 4 I11=2 level was excited to the 4 F7=2 level by the same three processes: excited state absorption (ESA), PAET from Yb3þ , and ET from the 4 I11=2 level of adjacent Er3þ . The populated Er3þ4 F7=2 level then
408 nm slope=2.56 529 nm slope=1.99 545 nm slope=1.91 667 nm slope=1.89
1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 Log [Excititation power (mW)] Fig. 3. Log–log plot of upconversion fluorescence emission intensity as a function of pump power at 975 nm.
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relaxes rapidly and nonradiatively to the next lower levels 2 H11=2 and 4 S3=2 resulting from the small energy gap between them. Because the energy gap below the 2 H11=2 level is very smaller, for example, about 744 cm1 in our experiment, the most Er3þ relaxes nonradiatively to the 4 S3=2 level and the 4 S3=2 level is populated, while the 2 H11=2 level is not populated by means of above process. It is considered that the 2 H11=2 level is populated from 4 S3=2 by a fast thermal equilibrium between the levels [18,19]. The above processes then produces the two 2 H11=2 ! 4 I15=2 and 4 S3=2 ! 4 I15=2 green emissions centered at 529 and 545 nm, respectively. An analysis based on a simple three-level system comprised of 4 I15=2 (level 0), 4 S3=2 (level 3), and 2 H11=2 (level 4) predicts that thermalization of the 2 H11=2 level can be expressed by the following equation [20]: I4 p4r g4 hx4 E43 ¼ exp ; ð2Þ I3 p3r g3 hx3 kT where p3r and p4r are the total spontaneous emission rates, hx3 and hx4 are photon energies, and g3 and g4 are the degeneracies ð2J þ 1Þ of the 4 S3=2 and 2 H11=2 levels, respectively. The I terms represent the integrated intensity of a given transition at a temperature. The E43 term is the energy gap between the 2 H11=2 and 4 S3=2 levels and k is the BoltzmannÕs constant. In our experiments, the calculated value of I4 =I3 equal to 0.331. This value is in excellent agreement with the experimental value of 0.329, which indicates that the 2 H11=2 level is populated from 4 S3=2 by a fast thermal equilibrium between the levels. Therefore, the green upconversion fluorescence emitted through 4 S3=2 ! 4 I15=2 transition is much brighter than that of through 2 H11=2 ! 4 I15=2 transition. There exist two main possible pumping mechanisms for red emission in SPYE glass. The first pumping mechanism comprises the population of the 4 S3=2 level, by means of the process described previously, followed by a fast nonradiative decay through multiphonon interaction from the populated 4 S3=2 to 4 F9=2 and then to the 4 I15=2 ground state. In the other possible mechanism, the Er3þ populated 4 I11=2 level mostly relaxed nonradiatively to the long-living 4 I13=2 level. The populated 4 I13=2 level may be excited to the 4 F9=2 level by the same three processes: excited state absorption, phonon-assisted energy transfer from Yb3þ , and energy transfer from the 4 I11=2 level of adjacent Er3þ . The above processes then produces the 4 F9=2 ! 4 I15=2 red emission centered at 667 nm. For the blue emission, the energy gap equal to 24631 cm1 between 2 H9=2 and 4 I15=2 levels calculated by absorption spectrum data, and the maximum pump photon energy is 10 256 cm1 . Therefore, the direct step-wise two-photon excitation4 I15=2 ! 4 I11=2 ! 2 H9=2 is not the case. A third step excitation is necessary to reach the 2 H9=2 level. A possible mechanism is the 4 F9=2 ! 2 H9=2 transition by absorption of a third pump photon by thesame three processes: ESA, PAET
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904
200
400
600 800 1000 1200 Raman shift (cm-1)
1400
Fig. 4. Raman spectrum of undoped SP glass at room temperature.
from Yb3þ , and ET from the 4 I11=2 level of adjacent Er3þ . The branching ratio calculated by Judd–Ofelt theory [21,22] for the transitions 2 H9=2 ! 4 I15=2 is 10.6%, and from Table 1 it is seen that the lifetimes of 2 H9=2 level of Er3þ is very low. Therefore, the radiative transition probability involved in the above processes can be small and the blue emission observed is very weak. According to Table 1, it can be seen that the ET efficiency is very high, and the lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 level of Er3þ in SPYE glass sample are much larger than that of in SPE glass sample. The results indicate that the ET rate is dominant to increasing the upconversion luminescence for blue, green, and red emissions in SPYE glass sample. Fig. 4 shows the Raman spectrum of undoped SP glass. For SP glass, the dominated features are the broad mode at about 904 cm1 assigned to bending vibrations of O–Si–O linkages. The spectrum indicates the highest frequency phonon band is at about 904 cm1 , smaller than that of pure SiO2 glass which has a maximum phonon band at 1100 cm1 [23]. The result indicates that the lead fluoride added enters the glass lower the maximum phonon energy of silicate glasses, and thus it reduces the nonradiative loss due to the mutiphonon relaxation, and enhances radiative transition and upconversion luminescence intensity of Er3þ in SP glass. From the above results it can deduce that fluoride ions in the glass network also have an important influence on the upconversion fluorescence of Er3þ .
4. Conclusion We have experimentally investigated upconversion fluorescence properties and mechanism in a new SPYE glass. The emissions of four visible fluorescence bands centered at 408, 529, 545, and 667 nm, corresponding to
the 2 H9=2 ! 4 I15=2 , 2 H11=2 ! 4 I15=2 , 4 S3=2 ! 4 I15=2 , and 4 F9=2 ! 4 I15=2 transition of Er3þ , respectively, were simultaneously observed under 975 nm excitation at room temperature. In SPYE glass, the ET efficiency is very high, and the lifetimes of 2 H9=2 , 2 H11=2 , 4 S3=2 , and 4 F9=2 level of Er3þ are much larger than that of in SPE glass. The results indicate that the ET rate is dominant to increasing the upconversion luminescence for blue, green, and red emissions in SPYE glass sample. The upconversion processes involve a sequential two-photon absorption for the green (529 and 545 nm) and red (667 nm) emissions, while a sequential three-photon absorption for the blue emission (408 nm). Raman spectrum indicates that fluoride ions in the oxyfluorosilicate glasses network have an important influence on the efficiency of the upconversion process owing to lower the maximum phonon energy of glass host. The intense upconversion fluorescence of SPYE glass may be a potentially useful material for developing upconversion fiber optical devices.
Acknowledgements This work was financially supported by Shanghai Science and Technology Foundation (022261046) and Chinese National Natural Science Foundation (60207006).
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