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Infrared-to-visible upconversion fluorescence of Er3+-doped novel lithium–barium–lead–bismuth glass
Materials Science and Engineering A 394 (2005) 83–86
Infrared-to-visible upconversion fluorescence of Er3+-doped novel lithium–barium–lead–bismuth glass Hongtao Suna,b,∗ , Shiqing Xua,b , Liyan Zhanga , Jialu Wua , Junjie Zhanga , Lili Hua , Zhonghong Jianga a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 16 August 2004
Abstract Er3+ -doped lithium–barium–lead–bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd–Ofelt intensity parameters Ωt (t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd–Ofelt theory, were found to be Ω2 = 3.05 × 10−20 cm2 , Ω4 = 0.95 × 10−20 cm2 , and Ω6 = 0.39 × 10−20 cm2 . Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 , respectively, were observed at room temperature. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived 4 I11/2 level is supposed to serve as the intermediate state responsible for the intense upconversion processes. The intense upconversion luminescence of Er3+ -doped lithium–barium–lead–bismuth glass may be a potentially useful material for developing upconversion optical devices. © 2004 Elsevier B.V. All rights reserved. Keywords: Optical materials; Rare-earth doped glasses; Optical spectroscopy; Upconversion emission
1. Introduction Recently, with the commercialization of 980 and 800 nm solid state laser diodes (LD), the upconversion of infrared light to visible light by rare earth ions doped glasses and crystals has been investigated extensively, due to the possibility of infrared-pumped visible lasers and the potential applications in areas such as optical data storage, lasers, sensors, and optical displays [1–4]. Er3+ ion is the most studied among the rare earth ions, and the upconversion process of this ion in various kinds of host materials has been investigated [5–13]. Glass materials are attractive as hosts because planar waveguides and optical fibers can be fabricated easily compared to crystalline materials. Effective Er3+ upconversion flurescence has been observed in halide glasses, such as ZrF4 –BaF2 –LaF3 –AlF3 –NaF (ZBLAN) and ∗
Corresponding author. Tel.: +86 21 5991 0994; fax: +86 21 399 0393. E-mail address: [email protected] (H. Sun).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.043
InF3–ZnF2 –BaF2 –SrF2 (IZBS) glasses [14–16]. The maximum phonon energy of these glasses is small enough to reduce the non-radiative loss due to multiphonon relaxation. Reducing the multiphonon relaxation process would lead to higher emission efficiency. Although these halide glasses possess low cutoff phonon frequency, however, they are hygroscopic and are limited in applications. Because fluoride glasses are relatively more stable, most research works concentrated on this type of glasses, and upconversion lasers have been demonstrated [17,18]. Upconversion is difficult to generate in conventional oxide glasses due to their high-phonon energies, corresponding to the stretching vibrations of the oxide glass network former. However, oxide glasses have attractive properties, such as high chemical stability and ease of fabrication. Upconversion fluorescence has been reported in Er3+ doped K2 O–Bi2 O3 –Ga2 O3 , TeO2 –Na2 O and TeO2 –PbO glasses [19–21]. Great efforts are still attracted to study of other oxide glasses as suitable upconversion hosts. Glass with the
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molar composition (mol%) 10Li2 O–10BaO–20PbO–60Bi2 O3 (L10 B10 P20 B60 ) is characterized by good glass stability, wide transmission region (0.44–8.3 m), high density (7.59 g/cm3 ) and high refractive index (2.3). The long infrared cutoff edge indicated that the phonon energy of L10 B10 P20 B60 glass is small compared to other conventional oxide glasses. In this paper, we report on the fabrication and characterization of new Er3+ doped L10 B10 P20 B60 glass. The Judd–Ofelt intensity parameters Ωt (t = 2, 4, 6), were determined by using Judd–Ofelt theory and the experimental absorption spectrum. Intense upconversion fluorescence bands at around 525, 546, and 657 nm have been observed, and the possible upconversion mechanisms are discussed and estimated. Fig. 1. Absorption spectrum of 1 mol% Er3+ -doped L10 B10 P20 B60 glass.
2. Experimental details The glasses used in this work were synthesized by conventional melting and quenching method. The starting materials are reagent-grade Li2 O, BaO, PbO and Bi2 O3 . The batch composition of the host glass is 10Li2 O–10BaO–20PbO–60Bi2 O3 (mol%) (L10 B10 P20 B60 ). The Er3+ doping concentration in the glass were 0.5, 1, 2, 3, 4 mol%, which were introduced as Er2 O3 with 99.99% purity. To compare the upconversion emission intensity with conventional oxide glasses, 1 mol% Er3+ -doped 60GeO2 –40PbO (mol%) (GP) glass was also prepared. About 20 g batches of the well-mixed raw materials were melted at 1000 ◦ C for 20–30 min in covered aluminium oxide crucibles in an electronic furnace with O2 atmosphere. When the melting was completed, the liquid was cast into preheated stainless steel plate. The obtained glasses were annealed for several hours at the glass transition temperatures before cooling them to room temperature at a rate of 15 ◦ C/h, and then were cut and polished carefully in order to meet the requirements for optical measurements. UV–vis/NIR absorption spectra were recorded between 400 and 1600 nm by a Perkin Elmer Lambda 900 spectrophotometer. The upconversion luminescence spectra were obtained with a TRIAX550 spectrofluorimeter upon excitation of 975 nm LD with a maximum power of 2 W. The lifetimes for the 4 I11/2 level of Er3+ were measured using a modulated 800 nm LD and a Tektronix TDS3052 digital oscilloscope controlled by a computer. The lifetimes were calculated by fitting the exponential function to the dacay data. All the measurements were performed at room temperature.
sity parameters Ωt (t = 2, 4, 6), determined based on the experimental absorption spectrum and Judd–Ofelt theory, were found to be Ω2 = 3.05 × 10−20 cm2 , Ω4 = 0.95 × 10−20 cm2 , and Ω6 = 0.39 × 10−20 cm2 . The Judd–Ofelt intensity parameters are important for investigations of local structure and bonding in the vicinity of rare-earth ions. According to previous studies [22], Ω2 is related with asymmetry of the glass hosts. The larger the Ω2 , the more asymmetric of the glass host. It can be seen that Ω2 in L10 B10 P20 B60 glass is larger than those in fluorophosphate, fluoride and oxyfluoride germanate glasses, but smaller than those in germanate, silicate, aluminate, and phosphate glasses [23]. These differences indicate that the asymmetry of the L10 B10 P20 B60 glass host is higher than those in fluorophosphate, fluoride and oxyfluoride germanate glasses, and lower than those in germanate, silicate, aluminate, and phosphate glasses. Fig. 2 shows the room temperature upconversion spectra of 1 mol% Er3+ -doped L10 B10 P20 B60 and GP glasses under 975 nm excitation. The observed emissions correspond to transitions of Er3+ ions from excited states to ground state. Intense green and red emission bands at around 525, 546, and 657 nm are attributed to the 4 H11/2 → 4 I15/2 , 4S 4 4 4 3/2 → I15/2 , and F9/2 → I15/2 transitions, respectively. The intensity of the green and red emissions in 1 mol% Er3+ -
3. Results and discussion Fig. 1 shows the absorption spectrum consists of seven absorption bands centered at 1532, 977, 800, 653, 547, 521, 488 nm, corresponding to the absorptions from the ground state 4 I15/2 to the excited states 4 I13/2 , 4 I11/2 , 4 I9/2 , 4 F9/2 , 4S , 4H 4 3/2 11/2 and F7/2 , respectively. The Judd–Ofelt inten-
Fig. 2. Upconversion fluorescence spectra of 1 mol% Er3+ -doped L10 B10 P20 B60 and GP glasses under 975 nm excitation.
H. Sun et al. / Materials Science and Engineering A 394 (2005) 83–86
Fig. 3. Upconversion luminescence intensities as a function of Er3+ concentration.
doped L10 B10 P20 B60 glass is about 10 times stronger than that in 1 mol% Er3+ -doped GP glass. It is also important to point out that the green emission is bright enough to be observed by the naked eye at excitation power as low as 70 mW for 1 mol% Er3+ -doped L10 B10 P20 B60 glass. The emission intensities are normalized with respect to the peak intensity at 547 nm. Fig. 3 illustrates the dependence of upconversion luminescence intensity on Er3+ concentrations. With increasing Er3+ concentration, the intensities of green emission at 525 nm increase slightly, while those of green emission at 546 nm and red emission at 657 nm increase largely. Three possible mechanisms have been proposed to explain the frequency upconversion phenomena of rare earth ions, namely excited state absorption (ESA), energy transfer upconversion (ET), and photon avalanche upconversion processes. It was suggested that the dominant mechanisms are ESA and ET for green and red emission under 975 nm excitation [23]. In frequency upconversion process, the upconversion emission intensity Iup increases in proportion to the nth power of infrared (IR) excitation intensity IIR , that n , where n is the number of IR photons absorbed is, Iup ∝ IIR per visible photon emitted. A plot of log Iup versus log IIR yields a straight line with slope n. Fig. 4 shows such a plot for the 525, 546, and 657 nm emissions in Er3+ doped
Fig. 4. Dependence of upconversion fluorescence intensity on excitation power under 975 nm excitation for 1 mol% Er3+ -doped L10 B10 P20 B60 glass.
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Fig. 5. Simplified energy level diagram of Er3+ and possible transition pathways in Er3+ -doped L10 B10 P20 B60 glass.
L10 B10 P20 B60 glass under 975 nm excitation. Values of 1.87, 1.89 and 1.99 were obtained for n corresponding to the 525, 546, and 657 nm emission bands, respectively. The results indicate that a two-photon process populates the 4 S3/2 , the 2H 4 11/2 and the F9/2 levels. According to the energy matching and quadratic dependence on excitation power, the possible upconversion mechanisms for the emission bands are discussed based on the simplified energy levels of Er3+ presented in Fig. 5. For the green emissions, in the first step, the 4 I11/2 level is directly excited at 975 nm. The measured lifetime of 4 I11/2 level for 1 mol% Er3+ -doped L10 B10 P20 B60 glass is 230 us, which are much longer than those in other oxide glasses [24]. Thus, the second step involves the excitation processes based on the long-lived 4 I11/2 level as follows: ET, 4 I11/2 (Er3+ ) + 4 I11/2 (Er3+ ) → 4 F7/2 (Er3+ ) + 4 I15/2 (Er3+ ); ESA, 4 I11/2 (Er3+ ) + a photon → 4 F7/2 (Er3+ ). The populated Er3+ 4 F7/2 level then relaxes rapidly and nonradiatively to the next lower levels 2 H11/2 and 4 S3/2 resulting from the small energy gap between them. Er3+ ion at the 2 H11/2 state can also decay to the 4 S3/2 state due to multiphonon relaxation process. The estimated energy gap between the 2 H11/2 state and the next lower state 4 S3/2 is ∼800 cm−1 [19]. Thus, multiphonon relaxation rate is very large and the 525 nm emission intensity is reduced. With increasing Er3+ concentration, the energy transfer rate increases, and thus, the green emission increases. The above processes then produces the two 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 green emissions centered at 525 and 546 nm, respectively. The red emission at 657 nm is originated from the 4 F9/2 → 4 I15/2 transition and the population of 4 F9/2 is based on the processes as follows: 4 I13/2 +4 I11/2 → 4 I15/2 +4 F9/2 (ET); 4 I13/2 + a photon → 4 F9/2 (ESA). The 4 I13/2 level is populated owing to the non-radiative relaxation from the upper 4 I11/2 level. Besides, the nonradiative process from 4 S3/2 state, which is populated by means of the process described previously, to 4 F9/2 level also contributes to the red emission. With increasing Er3+ concentration, the energy transfer rate between excited ions increases, and so the red emission increases. The ET process mainly depends on the Er3+ ion concentrations in the
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glasses. The red emission intensities increase more rapidly than green emission intensities with increasing Er3+ concentrations, and then the ET plays an important role in the red emissions.
4. Conclusions In Er3+ -doped L10 B10 P20 B60 glass, the Judd–Ofelt intensity parameters t , (t = 2, 4, 6) were calculated based on the experimental absorption spectrum and Judd-Ofelt theory. Under 975 nm excitation, frequency upconversion of Er3+ in L10 B10 P20 B60 glass has also been investigated. The intense 525, 546, and 657 nm upconversion fluorescence emissions are due to two-photon absorption processes, and the dominant mechanisms are ET and ESA for the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions. ErJ+ -doped L10 B10 P20 B60 glass with efficient upconversion fluorescence can be used as potential host material for upconversion lasers.
Acknowledgements This work was financially supported by the ‘Qimingxing’ Project of Shanghai Municipal Science and Technology Commission (04QNX1448) and Chinese National Science Foundation (60207006).
References [1] D. Lande, S.S. Orlov, A. Akella, L. Hesselink, R.R. Neurgaonkar, Opt. Lett. 22 (1997) 1722. [2] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014.
[3] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.M. Palmer, J. Appl. Phys. 84 (1998) 4649. [4] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Sci. 273 (1996) 1185. [5] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 65 (1994) 1874. [6] K. Kojima, S. Yoshida, H. Shiraishi, A. Maegawa, Appl. Phys. Lett. 67 (1995) 3423. [7] J.J. Ju, T.Y. Kwon, S.I. Yun, M. Cha, H.J. Seo, Appl. Phys. Lett. 69 (1996) 1358. [8] M. Takahashi, M. Shojiya, R. Kanno, Y. Kawamoto, K. Kadono, T. Ohtsuki, N. Peyghambarian, J. Appl. Phys. 81 (1997) 2940. [9] A.S. Oliveira, M.T. de Araujo, A.S. Gouveia-Neto, A.S.B. Sombra, et al., J. Appl. Phys. 83 (1998) 604. [10] M. Takahashi, M. Izuki, R. Kanno, Y. Kawamoto, J. Appl. Phys. 83 (1998) 3920. [11] M. Tsuda, K. Sousa, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. [12] D.F. de Sousa, L.F.C. Zonetti, M.J.V. Bell, R. Lebullenger, A.C. Hernandes, L.A.O. Nunes, J. Appl. Phys. 85 (1999) 2502. [13] P.V. dos Santos, E.A. Gouveia, M.T. de Araujo, A.S. Gouveia-Neto, A.S.B. Sombra, J.A. Medeiros Neto, Appl. Phys. Lett. 74 (1999) 3607. [14] A.S. Oliveira, M.T. de Araujo, A.S. Gouveia-Neto, J.A. Medeiros Neto, A.S.B. Sombra, Appl. Phys. Lett. 72 (1998) 753. [15] Cid B. de Araujo, L.S. Menezes, G.S. Maciel, L.H. Acioli, A.S.L. Gomes, Y. Messaddeq, A. Florez, M.A. Aegerter, Appl. Phys. Lett. 68 (1996) 602. [16] M. Shojiya, M. Takahashi, R. Kanno, Y. Kawamoto, K. Kadono, Appl. Phys. Lett. 67 (1995) 2453. [17] S. Tanable, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [18] T.J. Whitley, C.A. Millar, R. Wyatt, M.C. Brierley, D. Szebesta, Electron. Lett. 27 (1991) 1307. [19] S.Q. Man, E.Y.B. Pun, P.S. Chung, Appl. Phys. Lett. 77 (2000) 483. [20] J.Y. Allain, M. Monerie, H. Poignant, Electron. Lett. 28 (1992) 111. [21] C. Zhu, X. Lu, Z. Zhang, J. Non-Cryst. Solids 144 (1992) 89. [22] S. Tanable, J. Non-Cryst. Solids 259 (1999) 1. [23] S.Q. Xu, Z.M. Yang, S.X. Dai, G.N. Wang, L.L. Hu, Z.H. Jiang, Mater. Lett. 58 (2004) 1027. [24] Y.G. Choi, D.S. Lim, K.H. Kim, D.H. Cho, Electron. Lett. 35 (1999) 1765.
Infrared-to-visible upconversion fluorescence of Er3+-doped novel lithium–barium–lead–bismuth glass Hongtao Suna,b,∗ , Shiqing Xua,b , Liyan Zhanga , Jialu Wua , Junjie Zhanga , Lili Hua , Zhonghong Jianga a
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 16 August 2004
Abstract Er3+ -doped lithium–barium–lead–bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd–Ofelt intensity parameters Ωt (t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd–Ofelt theory, were found to be Ω2 = 3.05 × 10−20 cm2 , Ω4 = 0.95 × 10−20 cm2 , and Ω6 = 0.39 × 10−20 cm2 . Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 , respectively, were observed at room temperature. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived 4 I11/2 level is supposed to serve as the intermediate state responsible for the intense upconversion processes. The intense upconversion luminescence of Er3+ -doped lithium–barium–lead–bismuth glass may be a potentially useful material for developing upconversion optical devices. © 2004 Elsevier B.V. All rights reserved. Keywords: Optical materials; Rare-earth doped glasses; Optical spectroscopy; Upconversion emission
1. Introduction Recently, with the commercialization of 980 and 800 nm solid state laser diodes (LD), the upconversion of infrared light to visible light by rare earth ions doped glasses and crystals has been investigated extensively, due to the possibility of infrared-pumped visible lasers and the potential applications in areas such as optical data storage, lasers, sensors, and optical displays [1–4]. Er3+ ion is the most studied among the rare earth ions, and the upconversion process of this ion in various kinds of host materials has been investigated [5–13]. Glass materials are attractive as hosts because planar waveguides and optical fibers can be fabricated easily compared to crystalline materials. Effective Er3+ upconversion flurescence has been observed in halide glasses, such as ZrF4 –BaF2 –LaF3 –AlF3 –NaF (ZBLAN) and ∗
Corresponding author. Tel.: +86 21 5991 0994; fax: +86 21 399 0393. E-mail address: [email protected] (H. Sun).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.11.043
InF3–ZnF2 –BaF2 –SrF2 (IZBS) glasses [14–16]. The maximum phonon energy of these glasses is small enough to reduce the non-radiative loss due to multiphonon relaxation. Reducing the multiphonon relaxation process would lead to higher emission efficiency. Although these halide glasses possess low cutoff phonon frequency, however, they are hygroscopic and are limited in applications. Because fluoride glasses are relatively more stable, most research works concentrated on this type of glasses, and upconversion lasers have been demonstrated [17,18]. Upconversion is difficult to generate in conventional oxide glasses due to their high-phonon energies, corresponding to the stretching vibrations of the oxide glass network former. However, oxide glasses have attractive properties, such as high chemical stability and ease of fabrication. Upconversion fluorescence has been reported in Er3+ doped K2 O–Bi2 O3 –Ga2 O3 , TeO2 –Na2 O and TeO2 –PbO glasses [19–21]. Great efforts are still attracted to study of other oxide glasses as suitable upconversion hosts. Glass with the
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molar composition (mol%) 10Li2 O–10BaO–20PbO–60Bi2 O3 (L10 B10 P20 B60 ) is characterized by good glass stability, wide transmission region (0.44–8.3 m), high density (7.59 g/cm3 ) and high refractive index (2.3). The long infrared cutoff edge indicated that the phonon energy of L10 B10 P20 B60 glass is small compared to other conventional oxide glasses. In this paper, we report on the fabrication and characterization of new Er3+ doped L10 B10 P20 B60 glass. The Judd–Ofelt intensity parameters Ωt (t = 2, 4, 6), were determined by using Judd–Ofelt theory and the experimental absorption spectrum. Intense upconversion fluorescence bands at around 525, 546, and 657 nm have been observed, and the possible upconversion mechanisms are discussed and estimated. Fig. 1. Absorption spectrum of 1 mol% Er3+ -doped L10 B10 P20 B60 glass.
2. Experimental details The glasses used in this work were synthesized by conventional melting and quenching method. The starting materials are reagent-grade Li2 O, BaO, PbO and Bi2 O3 . The batch composition of the host glass is 10Li2 O–10BaO–20PbO–60Bi2 O3 (mol%) (L10 B10 P20 B60 ). The Er3+ doping concentration in the glass were 0.5, 1, 2, 3, 4 mol%, which were introduced as Er2 O3 with 99.99% purity. To compare the upconversion emission intensity with conventional oxide glasses, 1 mol% Er3+ -doped 60GeO2 –40PbO (mol%) (GP) glass was also prepared. About 20 g batches of the well-mixed raw materials were melted at 1000 ◦ C for 20–30 min in covered aluminium oxide crucibles in an electronic furnace with O2 atmosphere. When the melting was completed, the liquid was cast into preheated stainless steel plate. The obtained glasses were annealed for several hours at the glass transition temperatures before cooling them to room temperature at a rate of 15 ◦ C/h, and then were cut and polished carefully in order to meet the requirements for optical measurements. UV–vis/NIR absorption spectra were recorded between 400 and 1600 nm by a Perkin Elmer Lambda 900 spectrophotometer. The upconversion luminescence spectra were obtained with a TRIAX550 spectrofluorimeter upon excitation of 975 nm LD with a maximum power of 2 W. The lifetimes for the 4 I11/2 level of Er3+ were measured using a modulated 800 nm LD and a Tektronix TDS3052 digital oscilloscope controlled by a computer. The lifetimes were calculated by fitting the exponential function to the dacay data. All the measurements were performed at room temperature.
sity parameters Ωt (t = 2, 4, 6), determined based on the experimental absorption spectrum and Judd–Ofelt theory, were found to be Ω2 = 3.05 × 10−20 cm2 , Ω4 = 0.95 × 10−20 cm2 , and Ω6 = 0.39 × 10−20 cm2 . The Judd–Ofelt intensity parameters are important for investigations of local structure and bonding in the vicinity of rare-earth ions. According to previous studies [22], Ω2 is related with asymmetry of the glass hosts. The larger the Ω2 , the more asymmetric of the glass host. It can be seen that Ω2 in L10 B10 P20 B60 glass is larger than those in fluorophosphate, fluoride and oxyfluoride germanate glasses, but smaller than those in germanate, silicate, aluminate, and phosphate glasses [23]. These differences indicate that the asymmetry of the L10 B10 P20 B60 glass host is higher than those in fluorophosphate, fluoride and oxyfluoride germanate glasses, and lower than those in germanate, silicate, aluminate, and phosphate glasses. Fig. 2 shows the room temperature upconversion spectra of 1 mol% Er3+ -doped L10 B10 P20 B60 and GP glasses under 975 nm excitation. The observed emissions correspond to transitions of Er3+ ions from excited states to ground state. Intense green and red emission bands at around 525, 546, and 657 nm are attributed to the 4 H11/2 → 4 I15/2 , 4S 4 4 4 3/2 → I15/2 , and F9/2 → I15/2 transitions, respectively. The intensity of the green and red emissions in 1 mol% Er3+ -
3. Results and discussion Fig. 1 shows the absorption spectrum consists of seven absorption bands centered at 1532, 977, 800, 653, 547, 521, 488 nm, corresponding to the absorptions from the ground state 4 I15/2 to the excited states 4 I13/2 , 4 I11/2 , 4 I9/2 , 4 F9/2 , 4S , 4H 4 3/2 11/2 and F7/2 , respectively. The Judd–Ofelt inten-
Fig. 2. Upconversion fluorescence spectra of 1 mol% Er3+ -doped L10 B10 P20 B60 and GP glasses under 975 nm excitation.
H. Sun et al. / Materials Science and Engineering A 394 (2005) 83–86
Fig. 3. Upconversion luminescence intensities as a function of Er3+ concentration.
doped L10 B10 P20 B60 glass is about 10 times stronger than that in 1 mol% Er3+ -doped GP glass. It is also important to point out that the green emission is bright enough to be observed by the naked eye at excitation power as low as 70 mW for 1 mol% Er3+ -doped L10 B10 P20 B60 glass. The emission intensities are normalized with respect to the peak intensity at 547 nm. Fig. 3 illustrates the dependence of upconversion luminescence intensity on Er3+ concentrations. With increasing Er3+ concentration, the intensities of green emission at 525 nm increase slightly, while those of green emission at 546 nm and red emission at 657 nm increase largely. Three possible mechanisms have been proposed to explain the frequency upconversion phenomena of rare earth ions, namely excited state absorption (ESA), energy transfer upconversion (ET), and photon avalanche upconversion processes. It was suggested that the dominant mechanisms are ESA and ET for green and red emission under 975 nm excitation [23]. In frequency upconversion process, the upconversion emission intensity Iup increases in proportion to the nth power of infrared (IR) excitation intensity IIR , that n , where n is the number of IR photons absorbed is, Iup ∝ IIR per visible photon emitted. A plot of log Iup versus log IIR yields a straight line with slope n. Fig. 4 shows such a plot for the 525, 546, and 657 nm emissions in Er3+ doped
Fig. 4. Dependence of upconversion fluorescence intensity on excitation power under 975 nm excitation for 1 mol% Er3+ -doped L10 B10 P20 B60 glass.
85
Fig. 5. Simplified energy level diagram of Er3+ and possible transition pathways in Er3+ -doped L10 B10 P20 B60 glass.
L10 B10 P20 B60 glass under 975 nm excitation. Values of 1.87, 1.89 and 1.99 were obtained for n corresponding to the 525, 546, and 657 nm emission bands, respectively. The results indicate that a two-photon process populates the 4 S3/2 , the 2H 4 11/2 and the F9/2 levels. According to the energy matching and quadratic dependence on excitation power, the possible upconversion mechanisms for the emission bands are discussed based on the simplified energy levels of Er3+ presented in Fig. 5. For the green emissions, in the first step, the 4 I11/2 level is directly excited at 975 nm. The measured lifetime of 4 I11/2 level for 1 mol% Er3+ -doped L10 B10 P20 B60 glass is 230 us, which are much longer than those in other oxide glasses [24]. Thus, the second step involves the excitation processes based on the long-lived 4 I11/2 level as follows: ET, 4 I11/2 (Er3+ ) + 4 I11/2 (Er3+ ) → 4 F7/2 (Er3+ ) + 4 I15/2 (Er3+ ); ESA, 4 I11/2 (Er3+ ) + a photon → 4 F7/2 (Er3+ ). The populated Er3+ 4 F7/2 level then relaxes rapidly and nonradiatively to the next lower levels 2 H11/2 and 4 S3/2 resulting from the small energy gap between them. Er3+ ion at the 2 H11/2 state can also decay to the 4 S3/2 state due to multiphonon relaxation process. The estimated energy gap between the 2 H11/2 state and the next lower state 4 S3/2 is ∼800 cm−1 [19]. Thus, multiphonon relaxation rate is very large and the 525 nm emission intensity is reduced. With increasing Er3+ concentration, the energy transfer rate increases, and thus, the green emission increases. The above processes then produces the two 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 green emissions centered at 525 and 546 nm, respectively. The red emission at 657 nm is originated from the 4 F9/2 → 4 I15/2 transition and the population of 4 F9/2 is based on the processes as follows: 4 I13/2 +4 I11/2 → 4 I15/2 +4 F9/2 (ET); 4 I13/2 + a photon → 4 F9/2 (ESA). The 4 I13/2 level is populated owing to the non-radiative relaxation from the upper 4 I11/2 level. Besides, the nonradiative process from 4 S3/2 state, which is populated by means of the process described previously, to 4 F9/2 level also contributes to the red emission. With increasing Er3+ concentration, the energy transfer rate between excited ions increases, and so the red emission increases. The ET process mainly depends on the Er3+ ion concentrations in the
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glasses. The red emission intensities increase more rapidly than green emission intensities with increasing Er3+ concentrations, and then the ET plays an important role in the red emissions.
4. Conclusions In Er3+ -doped L10 B10 P20 B60 glass, the Judd–Ofelt intensity parameters t , (t = 2, 4, 6) were calculated based on the experimental absorption spectrum and Judd-Ofelt theory. Under 975 nm excitation, frequency upconversion of Er3+ in L10 B10 P20 B60 glass has also been investigated. The intense 525, 546, and 657 nm upconversion fluorescence emissions are due to two-photon absorption processes, and the dominant mechanisms are ET and ESA for the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions. ErJ+ -doped L10 B10 P20 B60 glass with efficient upconversion fluorescence can be used as potential host material for upconversion lasers.
Acknowledgements This work was financially supported by the ‘Qimingxing’ Project of Shanghai Municipal Science and Technology Commission (04QNX1448) and Chinese National Science Foundation (60207006).
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