Upconversion properties of Er3+, Yb3+ and Tm3+ codoped fluorophosphate glasses

Upconversion properties of Er3+, Yb3+ and Tm3+ codoped fluorophosphate glasses

Spectrochimica Acta Part A 68 (2007) 531–535 Upconversion properties of Er3+, Yb3+ and Tm3+ codoped fluorophosphate glasses Meisong Liao a,b,∗ , Lili...

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Spectrochimica Acta Part A 68 (2007) 531–535

Upconversion properties of Er3+, Yb3+ and Tm3+ codoped fluorophosphate glasses Meisong Liao a,b,∗ , Lili Hu a , Yongzheng Fang a , Junjie Zhang a , Hongtao Sun a , Shiqing Xu c , Liyan Zhang c a

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Granduate school of Chinese Academy of Sciences, Shanghai 201800, PR China c College of Information Engineering, China Jiliang University, Hangzhou, PR China Received 17 October 2006; received in revised form 9 December 2006; accepted 16 December 2006

Abstract Er3+ , Yb3+ and Tm3+ codoped fluorophosphate glasses emitting blue, green and red upconversion luminescence at 970 nm laser diode excitation were studied. It was shown that Tm3+ behaves as the sensitizer to Er3+ for the green upconversion luminescence through the energy transfer process: Tm3+ :3 H4 + Er3+ :4 I15/2 → Er3+ :4 I9/2 + Tm3+ :3 H6 , and for the red upconversion luminescence through the energy transfer process: Tm3+ :3 F4 + Er3+ :4 I11/2 → Tm3+ :3 H6 + Er3+ :4 F9/2 . Moreover, Er3+ acts as quenching center for the blue upconversion luminescence of Tm3+ . The sensitization of Tm3+ to Er3+ depends on the concentration of Yb3+ . The intensity of blue, green and red emissions can be changed by adjusting the concentrations of the three kinds of rare earth ions. This research may provide useful information for the development of high color and spatial resolution devices and white light simulation. © 2006 Elsevier B.V. All rights reserved. Keywords: Glasses; Rare earth; Upconversion properties

1. Introduction The visible upconversion luminescence of rare earth ions exhibits extensive applications in color display, high density optical data storage and reading, biomedical diagnostics and optical communications, etc. [1–6]. In previous researches, Tm3+ /Yb3+ codoped fluoride and tellurite glasses were investigated under 980 nm laser diode (LD) excitation and were shown to be effective blue luminescence materials through the Tm3+ :1 G4 → 3 H6 transition [7,8]. Er3+ /Yb3+ codoped germanate and tellurite glasses were studied under 980 nm LD pump for their comparably intensive green and red upconversion emissions which are due to the Er3+ :4 S3/2 → 4 I15/2 and Er3+ :4 F9/2 → 4 I15/2 transitions, respectively [9,10]. In these Tm3+ /Yb3+ or Er3+ /Yb3+ codoped glasses, Yb3+ ion acts as an effective sensitizer, absorbing the pump energy and transferring it to the Tm3+ or Er3+ ion. The Er3+ /Tm3+ codoped fluoride ∗ Corresponding author at: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China. Tel.: +86 21 59911204; fax: +86 21 39910393. E-mail address: [email protected] (M. Liao).

1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.12.023

glass was also studied under 790 nm excitation [11], and it has been shown that Tm3+ behaves as a sensitizer for the red upconversion luminescence and as a quenching center for the green upconversion luminescence. When the Tm3+ , Yb3+ and Er3+ ions are simultaneously introduced into the glass matrix, a material emitting blue, green and red upconversion luminescence can be obtained. This material can be used in image devices, high color and spatial resolution devices and white light simulation [12,13]. Spectroscopic properties of the three rare earth ions doped fluoride and tellurite glasses were reported in previous researches [12–14]. However, until now, reported works on Tm3+ , Yb3+ and Er3+ codoped glasses are far from systematic. The influence of Yb3+ , Tm3+ and Er3+ concentrations on the intensity of upconversion luminescence is not yet clarified in detail. In this work, the influence of the concentrations of the three rare earth ions on visible upconverison luminescence under 970 nm excitation was studied. The mechanisms of energy transfer among the rare earth ions were discussed as well. The matrix is a fluorophosphate glass containing limited metaphosphate. Rare earth doped fluorophosphate glasses, exhibiting promising prospect in laser technology and fiber amplification, are featured with low

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Fig. 1. Upconversion spectra of Er3+ , Yb3+ and Tm3+ codoped fluorophosphate glasses under 970 nm LD excitation.

non-liner refractive index and high Abbe index [15,16]. In previous research [17], we found fluorophosphate glass can be good matrix for upconverison luminescence, if only the phosphate content in the composition is limited at low level. At the same time this fluorophosphate glass has good solution ability to rare earth, and is easy to be prepared without crystallization. 2. Experimental Glasses with compositions (mol%) of (54–x–y–z)RF2 – 37AlF3 –3Al(PO3 )3 –6KF–xErF3 –yYbF3 –zTm2 O3 were prepared by the conventional melting and quenching method. RF2 represents alkaline earth fluorides including MgF2 , CaF2 , SrF2 and BaF2 . Tm2 O3 was introduced instead of TmF3 , since it is cheaper. Because its content is very limited, the influence on composition is insignificant. Samples were named according to the x, y and z values as follows: 0.5Er–1Yb–0.1Tm, 0.5Er–3Yb–0.1Tm, 0.5Er–5Yb–0.1Tm, 0.5Er–5Yb–0.05Tm, 1.0Er–5Yb–0.1Tm, 1.0Er–0.1Tm, 0.5Er–3Yb, 5Yb–0.1Tm, 5Yb–0.05Tm. All starting materials are of analytical grade. The glass samples were prepared by melting required amounts of metaphosphate and the fluoride compounds in a platinum crucible at the temperature of 950–1050 ◦ C. After melting, heating was continued for 20 min to homogenize the liquid. Then, the liquid was cast into a graphite mould. To remove strain, the glass samples were annealed for 4 h in an oven at 420 ◦ C. Finally, the glass samples were cut and polished. Polishing is performed with cerium oxide. The size of the glass is 1 mm × 10 mm × 20 mm. UV/vis/NIR absorption spectra were recorded by using a spectrophotometer to determine the energy levels of Er3+ and Tm3+ , respectively. The upconversion luminescence spectra

were obtained at room temperature with a TRIAX550 spectrofluorimeter upon excitation of 970 nm LD with a maximum power of 2 W. In order to compare the luminescence intensity in different samples as accurate as we can, the position and power of the pumping beam and the width of the slit of the spectroscopy to collect the luminescence signal were fixed at the same condition, and the sample was set at the same place in the experiment setup. The fluorescence lifetime of transition Er3+ :4 I13/2 → 4 I15/2 was measured with pulsed signal of 970 nm laser diode and a HP546800B 100-MHz oscilloscope. 3. Results Blue, green and red upconversion spectra of Er3+ , Yb3+ and Tm3+ codoped samples are shown in Fig. 1. With the incre-

Fig. 2. Upconversion spectra of Yb3+ /Tm3+ codoped fluorophosphate glasses under 970 nm LD excitation.

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Fig. 3. Upconversion spectra of Er3+ /Tm3+ and Er3+ /Yb3+ codoped fluorophosphate glasses under 970 nm excitation.

ment of YbF3 , blue and red luminescence increases gradually for the samples with 0.5 mol% ErF3 and 0.1 mol% Tm2 O3 . Green luminescence increases evidently when YbF3 increases from 1 to 3 mol%. With the increment of YbF3 content, the ratio of the intensity of blue luminescence to that of green or red luminescence increases; the ratios of the intensity of green luminescence to that of blue and red luminescence decrease; the ratio of the intensity of red luminescence to that of blue luminescence decreases and to that of green luminescence increases. It can be found that the proportions between the emissions can be adjusted by changing the concentration of YbF3 . By the comparison between the upconversion spectra of 0.5Er–5Yb–0.05Tm and 0.5Er–5Yb–0.1Tm samples in Fig. 1, it can be found that with the decrease of Tm2 O3 concentration from 0.1 to 0.05 mol%, the intensity of blue luminescence increases. In Fig. 2, for the Tm3+ /Yb3+ codoped samples, with the decrease of Tm2 O3 concentration from 0.1 to 0.05 mol%, the intensity of blue luminescence decreases. Consequently, the quenching concentration for Tm3+ in Yb3+ /Tm3+ codoped glasses is higher than in Er3+ , Yb3+ and Tm3+ codoped glasses. In Fig. 1, through the comparison of the upconversion spectra between 0.5Er–5Yb–0.1Tm and 1.0Er–5Yb–0.1Tm, it can be found that with the increment of Er3+ concentration, blue luminescence intensity decreases and green and red luminescence intensity increases. The upcoversion spectra of Er3+ /Yb3+ and Er3+ /Tm3+ codoped samples are shown in Fig. 3. Both of them show comparatively weak upconversion emissions. When the spectrum of 0.5Er–3Yb glass in Fig. 3 is compared with that of 0.5Er–3Yb–0.1Tm glass in Fig. 1, it can be found that the intensity of luminescence increases due to the introduction of Tm3+ . Namely, Tm3+ is the sensitizer for Er3+ . However, the sensitizing effect of Tm3+ depends strongly on the concentration of Yb3+ , which can be shown by the comparison between the spectra of 1.0Er–0.1Tm and 1.0Er–5Yb–0.1Tm glasses in Fig. 1. Upconversion spectra of 0.5Er–5Yb–0.1Tm and 0.5Er–3Yb samples under different pump power were measured, and the results of 0.5Er–5Yb–0.1Tm glass are depicted in log–log plots of as shown in Fig. 4. As can be seen, the 476 nm emission presents nearly cubic and the 653 and 794 nm emissions present quadratic dependence on the excitation power. The 522 and 543 nm emissions present dependence between square and cube on the excitation power. For the 0.5Er–3Yb sample, the slope of

Fig. 4. Log–log plot of upconversion luminescence intensity as a function of pump power in 0.5Er–5Yb–0.1Tm glass.

the log–log plot corresponding to 543 nm is 2.05 and to 653 nm is 2.03, and it shows a typical two-photon process. 4. Discussion Schematic diagram of the energy levels of Er3+ , Yb3+ and Tm3+ ions and the energy transfer among them are depicted in Fig. 5. The blue emission is ascribed to the Tm3+ :1 G4 →

Fig. 5. Schematic diagram of the energy levels of Er3+ , Yb3+ and Tm3+ ions with the main transitions for blue, green and red upconversion luminescence.

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Table 1 Energy levels of rare earth ions Er3+ and Tm3+ in the fluorophosphate glass Rare earth ions Er3+ Tm3+

3H

Energy levels in the fluorophosphate glasses (cm−1 ) 4I 13/2 6545

4I 11/2

4I 9/2

4F 9/2

4H 11/2

4F 7/2

10277

12531

15385

19268

20576

3F 4

3H 5

3H 4

3F 3

1G 4

6039

8264

12642

14641

21598

1G 4 energy level gets population 6 transition. The through the following process: Tm3+ :3 H6 + Yb3+ :2 F5/2 → Tm3+ :3 H5 + Yb3+ :2 F7/2 ; Tm3+ in 3 H5 relaxes to 3 F4 ; Tm3+ :3 F4 + Yb3+ :2 F5/2 → Tm3+ :3 F2,3 + Yb3+ :2 F7/2 ; Tm3+ in 3 F2,3 relaxes to 3 H ; Tm3+ :3 H + Yb3+ :2 F 3+ 1 3+ 2 4 4 5/2 → Tm : G4 + Yb : F7/2 . This is a process that involved three photons. At the same time, a few Tm3+ ions can be sensitized by coupled cluster state Yb3+ :2 F5/2 Yb3+ :2 F5/2 to 1 G4 . This is a two-photon process [18]. So the 476 nm emission presents nearly a cubic dependence on the excitation power. Though the phonon energy of the matrix is about 1000 cm−1 , the blue luminescence of Tm3+ /Yb3+ codoped samples can be seen by naked eye under excitation power as low as 70 mW. It might be ascribed to the fact that the relaxations of 3 H5 to 3 F4 and 3 F2,3 to 3 H4 need the assistance of multiphonon relaxation, and the density of high energy phonon states is comparatively low in this glass. For the Er3+ /Yb3+ codoped sample, the 522 nm and 543 nm green luminescence derives from 2 H11/2 to 4 S3/2 , respectively. They involve the following process: Er3+ in 4 I15/2 is excited to 4 I11/2 by ground state absorption (GSA) or energy transfer (ET) from Yb3+ in 2 F5/2 ; Er3+ in 4 I11/2 is excited to 4F 7/2 by excited state absorption (ESA), ET or cross-relaxation (CR) Er3+ :4 I11/2 + Er3+ :4 I11/2 → Er3+ :4 F7/2 + Er3+ :4 I15/2 ; Er3+ ions in 4 F7/2 relax to 2 H11/2 and 4 S3/2 . It is a two-photon process, so for 0.5Er–3Yb sample, the green luminescence is a typically two-photon process. For the Er3+ , Yb3+ and Tm3+ doped samples, Tm3+ behaved as a sensitizer: Tm3+ :3 H4 + Er3+ :4 I15/2 → Er3+ :4 I9/2 + Tm3+ :3 H6 , Er3+ in 4 I9/2 relaxes to 4 I11/2 . Then, Er3+ in 4 I11/2 is excited to 4 F7/2 through the aforementioned process. This is a three-photon process, so for 0.5Er–5Yb–0.1Tm sample in Fig. 4, the 522 and 543 nm emissions present log–log plot dependence on the excitation power between square and cube. It is indicated in Table 1 that the 3 H4 energy level of Tm3+ matches the 4 I9/2 energy level of Er3+ very well. The 653 nm red luminescence derives from 4 F9/2 of 3+ Er . For the Er3+ /Yb3+ codoped sample, 653 nm emission involves the following process: Er3+ in 4 I15/2 is excited to 4 I11/2 by GSA or ET; Er3+ ion in 4 I11/2 relaxes to 4I 3+ ion in 4 I 4F 13/2 ; Er 13/2 is excited to 9/2 by ESA, 3+ 4 3+ 4 3+ 4 ET or CR: Er : I13/2 + Er : I11/2 → Er : F9/2 + Er3+ :4 I15/2 . It is a two-photon process. For the Er3+ , Yb3+ and Tm3+ codoped samples, Tm3+ behaves as the sensitizer: Tm3+ :3 F4 + Er3+ :4 I11/2 → Tm3+ :3 H6 + Er3+ :4 F9/2 . It is shown in Table 1 that the energy gaps between 3 F4 and 3 H6 and that between 4 F9/2 and 4 I11/2 are a close match. The measured lifetimes of 1529 nm fluorescence of 0.5Er–3Yb and

0.5Er–3Yb–0.1Tm samples were 8.0 and 2.2 ms, respectively. It suggests that for sample 0.5Er–3Yb, more Er3+ ions in 4 I13/2 emit 1529 nm photons through 4 I13/2 → 4 I15/2 transition, which weakens the red upcoversion emission. The sensitizations of Tm3+ to Er3+ for green and red emissions depend on the populations of 3 H4 and 3 F4 , which need the sensitization of Yb3+ in 2 F5/2 first. Yb3+ in 2 F5/2 also sensitizes the Er3+ directly, so in Fig. 3, the intensity of upconversion luminescence of 1.0Er–0.1Tm glass is comparatively weak. The sensitization of Tm3+ to Er3+ depopulates 3 H4 and 3 F energy level and weakens the 476 nm emission. There4 fore, with the increment of Er3+ content, blue emission of the 0.5Er–5Yb–0.1Tm and 1.0Er–5Yb–0.1Tm samples decreases. The green and red emissions increase with the increment of Er3+ content because of the CR process between Er3+ ions. For the Yb3+ /Tm3+ codoped glass, the 476 nm blue luminescence can be quenched by the following process: Tm3+ :1 G4 + Tm3+ :3 H6 → Tm3+ :3 H4 + Tm3+ :3 H5 . However, when the concentration of Tm3+ ions is not too high, this quenching process is not notable because the 3 H4 and 3 H5 energy levels are still the interim states for the blue upconversion emission. Tm3+ ions in 3 F4 or 3 H4 can be sensitized again to 1 G4 . It is just like a cycle. However, when Er3+ ions is introduced, 3 F4 and 3 H4 states are depopulated by the energy transfer to Er3+ ions, and the cycle breaks. Therefore, the blue luminescence quenching occurs at a relative lower concentration of Tm3+ ions in Er3+ , Yb3+ and Tm3+ codoped glasses than in Yb3+ /Tm3+ codoped glasses.

5. Conclusions Blue, green and red upconversion luminescence of Er3+ , Yb3+ and Tm3+ codoped fluorophosphate glasses at 970 nm LD excitation was studied. For the Er3+ , Yb3+ and Tm3+ codoped glasses, the blue luminescence is nearly a three-photon process, the green luminescence is a process between two-photon and three-photon, and the red luminescence is a two-photon process. Tm3+ behaves as the sensitizer to Er3+ for the green upconversion luminescence through the energy transfer process: Tm3+ :3 H4 + Er3+ :4 I15/2 → Er3+ :4 I9/2 + Tm3+ :3 H6 , and for the red upconversion luminescence through the energy transfer process: Tm3+ :3 F4 + Er3+ :4 I11/2 → Tm3+ :3 H6 + Er3+ :4 F9/2 . Er3+ acts as quenching center for the blue upconversion luminescence of Tm3+ . The blue luminescence quenching occurs at a lower concentration of Tm3+ ions in Er3+ , Yb3+ and Tm3+ codoped glasses than in Yb3+ /Tm3+ codoped glasses.

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The intensity of the blue, green and red upconversion emissions can be changed by adjusting the concentrations of the Er3+ , Yb3+ and Tm3+ rare earth ions in the fluorophosphate glasses. This research may provide useful information for the development of image devices. Acknowledgements The author would like to thank Ms. Ying Zhao for her help in the experiment. This work is financially supported by Chinese National Natural Science Foundation (no. 60508014, no. 50572110 and no. 50502030). References [1] S.Q. Xu, Z.M. Yang, J.J. Zhang, G.N. Wang, S.X. Dai, L.L. Hu, Z.H. Jiang, Chem. Phys. Lett. 385 (2004) 263. [2] W.S. Tsang, W.M. Yu, C.L. Mak, W.L. Tsui, K.H. Wong, H.K. Hui, J. Appl. Phys. 91 (2002) 1871. [3] G.S. Qin, W.P. Qin, C.F. Wu, S.H. Huang, J.S. Zhang, S.Z. Lu, D. Zhao, H.Q. Liu, J. Appl. Phys. 93 (2003) 4328. [4] L.Y. Wang, R.X. Yan, Z.Y. Huo, L. Wang, J.H. Zeng, J. Bao, X. Wang, Q. Peng, Y.D. Li, Angew. Chem. Int. Ed. 44 (2005) 6054.

535

[5] L.Y. Wang, Y.D. Li, Chem. Commun. 24 (2006) 2557. [6] V. Karunakaran, J. Luis P´erez Lustres, L. Zhao, N.P. Ernsting, O. Seitz, J. Am. Chem. Soc. 128 (2006) 2954. [7] X.B. Chen, W.M. Du, N. Sawanobori, G.Y. Zhang, Z.F. Song, Opt. Commun. 181 (2000) 171. [8] S.Q. Xu, J.J. Zhang, G.N. Wang, S.X. Dai, L.L. Hu, Z.H. Jiang, Chin. Phys. Lett. 21 (2004) 927. [9] Z.M. Yang, S.Q. Xu, L.L. Hu, Z.H. Jiang, J. Alloys Compd. 370 (2004) 94. [10] J.H. Yang, N.L. Dai, S.X. Dai, L. Wen, L.L. Hu, Z.H. Jiang, Chem. Phys. Lett. 376 (2003) 671. [11] X.L. Zou, A. Shikida, H. Yanagita, H. Toratani, J. Non-cryst. Solids 181 (1995) 100. [12] J.E.C. da Silva, G.F. de S´a, P.A. Santa-Cruz, J. Alloys Compd. 323–324 (2001) 336. [13] J.E.C. da Silva, G.F. de S´a, P.A. Santa-Cruz, J. Alloys Compd. 344 (2002) 260. [14] S.Q. Xu, H.P. Ma, D.W. Fang, Z.X. Zhang, Z.H. Jiang, Mater. Lett. 59 (2005) 3066. [15] M.S. Liao, H.T. Sun, L. Wen, Y.Z. Fang, L.L. Hu, Mater. Chem. Phys. 98 (2006) 154. [16] L.Y. Zhang, H.T. Sun, S.Q. Xu, K.F. Li, L.L. Hu, J. Lumin. 117 (2006) 46. [17] M.S. Liao, S.G. Li, H.T. Sun, Y.Z. Fang, L.L. Hu, J.J. Zhang, Mater. Lett. 60 (2006) 1783. [18] N. Eiichiro, S. Shigeo, Phys. Rev. Lett. 25 (1970) 1710.