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Kinetics of transfer and backtransfer in Yb3+-Er3+ codoped fluoroindate glasses
JOURNAL
OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 72-74 (1997) 954-955
Kinetics of transfer and backtransfer in Yb3+-Er3+ fluoroindate glasses
codoped
I.R. Martin, V.D. Rodriguez *, V. Lavin, U.R. Rodriguez-Mendoza Dpto. Fisica Fundamental .I’ Experimentul, Universidad La Laguna. 38206 La Laguna, Tenerife, Spain
Abstract
The dynamics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb”+ and Er”+ have been studied using the fluorescence ‘transfer function’. From the fitting to the decay curves of the luminescence the critical radii for these processes have been obtained. These values are also calculated with the Dexter formula. Keywords:
Backtransfer;
Fluoroindate
glasses; Yb; Er
1. Introduction
The great efficiency of up-conversion process in the Yb3+-Er3+ system depends on the small energy mismatch between the levels 2FS,2(Yb3+) and 4111,z(Er3f). Thus, forward and backward processes are possible between these levels which are important for the up-conversion efficiency [ 11. However, only a few works deal with the backtransfer mechanism [2,3] and scarce attention has been paid to the Yb3+ -Er3 + couple. In order to analyze the dynamics of these energy transfer processes the optical properties of single and double doped fluoroindate glasses (0.1 mol% of Yb3+ and 0.25, 0.75 or 2.25 mol% of Er3+) have been studied. 2. Results and discussion NIR absorption spectra at RT of glasses with 2.25 mol% of Yb3+ and 2.5 mol% of Er3+ are *Corresponding vrodriguez@,ull.es.
author.
Fax: 34 22 25 69 73; e-mail:
presented in Fig. 1. Emission spectra of Yb3 + and Er3+are also shown in this figure. Remarkable overlaps are observed in the spectra of the two ions. In double doped samples it has been possible to excite only Yb3+ ions in 940 nm, however emission spectrum presents contrithe . . butlon of the transitions 2Fs,z + 2F,,2(Yb3+) and 4111,2 -+ 4115,2(Er3+), indicating that the energy transfer processes Yb3 + + Er3 + are very efficient. In order to analyze the fluorescence dynamics of the Yb3’ ions we have measured the decay curves of the luminescence by excitation at 940 nm and detecting at 1040 nm, where the emission from the Er3+ ions is negligible. In samples containing only 0.1 mol% of Yb3 +, the decay curves are exponential with a lifetime zD of 1.67 ms. In double doped samples, when the concentration on Er3+ is increased, a slow decay in long times becomes relevant (see Fig. 2), this is a consequence of the efficient backtransfer Er3+ + Yb3+ and the long lifetime of the 4111,,2level rA (7.4 ms) of the Er3+ ions. These decay curves with forward and backward energy transfer processes can be described
0022-2313/97/$17.00 ‘(,’ 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)003 12-2
I.R. Martin et al. 1 Journal ofluminescence
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72-74 119971 954-955
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Fig. 2. Luminescence decays at RT in fluoroindate glasses doped with 0.1 mot% of Yb3+ and 0.25 (a) or 2.25 mol”% (b) of Er3+. The solid lines are the fits to Eq. (1).
Fig. 1. (a) Absorption spectra at RT in fluoroindate glasses doped with 2.25 mol% of Yb’+ (-) and with 2.5 mol% of Er3+ (---). (b) Emission spectra of Yb3+ (-) and Er3+ (---).
using the fluorescence “transfer function” [3]. In this model the fluorescence of donors I(t) under flash excitation is expressed using the response functions of donors K,(t) and acceptors K,,(t), which can be obtained from the Yokota-Tanimoto model [4], however, we have used the InokutiHirayama formula [5] for K,(t) because the migration processes between Yb3+ ions for a concentration of 0.1 mol% of Yb3 ’ are negligible in fluoroindate glasses [S]. The expression for Z(t) in the “transfer function” model is given by
scopic data using the Dexter formula [7]. The relatively high value obtained for RCA,, from the fitting is probably due to the correlation between acceptor and donor is neglected in the ‘transfer function’ model. The decay of the Er3+ emission is also in good agreement with the behavior predicted by the “transfer function” model (data not shown).
Acknowledgements Supported in part by “Gobierno Autonomo de Canarias” under Contract 226-206/94.
References where Cn = s + l/tn, CA = s + l/zA and f and L-‘(f) are the direct and inverse Laplace transformation off(t), respectively. The fits to Eq. (1) are also included in Fig. 2. The obtained critical radii (for forward RCDAand backward RCAD energy transfer) do not depend appreciably on Er3+ concentration. The mean value? obtained from the fittings are 12.2 and 24.3 A for, RCoA and RCAD, respectively, and 11.2 and 14.9 A if these are calculated from spectro-
[l]
Ph. Goldner and F. Pelle, J. Lumin. 60 & 61 (1994) 651. [Z] D.L. Huber, D.S. Hamilton and B. Barnett, Phys. Rev. B. 16 (1977) 4642. [3] A. Brenier, G. Boulon, C. Madej and C. Pedrini, J. Lumin. 54 (1993) 271. [4] M. Yokota and 0. Tanimoto, J. Phys. Sot. Japan 22 (1967) 779. [S] M. Inokuti and F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [6] I.R. Martin, V.D. Rodriguez, M. Morales, U.R. RodriguezMendoza and V. Lavin, J. Appl. Spectrosc. 62 (1995) 78. [7] D.L. Dexter, J. Chem. Phys. 21 (1953) 836.
OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 72-74 (1997) 954-955
Kinetics of transfer and backtransfer in Yb3+-Er3+ fluoroindate glasses
codoped
I.R. Martin, V.D. Rodriguez *, V. Lavin, U.R. Rodriguez-Mendoza Dpto. Fisica Fundamental .I’ Experimentul, Universidad La Laguna. 38206 La Laguna, Tenerife, Spain
Abstract
The dynamics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb”+ and Er”+ have been studied using the fluorescence ‘transfer function’. From the fitting to the decay curves of the luminescence the critical radii for these processes have been obtained. These values are also calculated with the Dexter formula. Keywords:
Backtransfer;
Fluoroindate
glasses; Yb; Er
1. Introduction
The great efficiency of up-conversion process in the Yb3+-Er3+ system depends on the small energy mismatch between the levels 2FS,2(Yb3+) and 4111,z(Er3f). Thus, forward and backward processes are possible between these levels which are important for the up-conversion efficiency [ 11. However, only a few works deal with the backtransfer mechanism [2,3] and scarce attention has been paid to the Yb3+ -Er3 + couple. In order to analyze the dynamics of these energy transfer processes the optical properties of single and double doped fluoroindate glasses (0.1 mol% of Yb3+ and 0.25, 0.75 or 2.25 mol% of Er3+) have been studied. 2. Results and discussion NIR absorption spectra at RT of glasses with 2.25 mol% of Yb3+ and 2.5 mol% of Er3+ are *Corresponding vrodriguez@,ull.es.
author.
Fax: 34 22 25 69 73; e-mail:
presented in Fig. 1. Emission spectra of Yb3 + and Er3+are also shown in this figure. Remarkable overlaps are observed in the spectra of the two ions. In double doped samples it has been possible to excite only Yb3+ ions in 940 nm, however emission spectrum presents contrithe . . butlon of the transitions 2Fs,z + 2F,,2(Yb3+) and 4111,2 -+ 4115,2(Er3+), indicating that the energy transfer processes Yb3 + + Er3 + are very efficient. In order to analyze the fluorescence dynamics of the Yb3’ ions we have measured the decay curves of the luminescence by excitation at 940 nm and detecting at 1040 nm, where the emission from the Er3+ ions is negligible. In samples containing only 0.1 mol% of Yb3 +, the decay curves are exponential with a lifetime zD of 1.67 ms. In double doped samples, when the concentration on Er3+ is increased, a slow decay in long times becomes relevant (see Fig. 2), this is a consequence of the efficient backtransfer Er3+ + Yb3+ and the long lifetime of the 4111,,2level rA (7.4 ms) of the Er3+ ions. These decay curves with forward and backward energy transfer processes can be described
0022-2313/97/$17.00 ‘(,’ 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)003 12-2
I.R. Martin et al. 1 Journal ofluminescence
955
72-74 119971 954-955
0
3
6
9
12
15
18
Fig. 2. Luminescence decays at RT in fluoroindate glasses doped with 0.1 mot% of Yb3+ and 0.25 (a) or 2.25 mol”% (b) of Er3+. The solid lines are the fits to Eq. (1).
Fig. 1. (a) Absorption spectra at RT in fluoroindate glasses doped with 2.25 mol% of Yb’+ (-) and with 2.5 mol% of Er3+ (---). (b) Emission spectra of Yb3+ (-) and Er3+ (---).
using the fluorescence “transfer function” [3]. In this model the fluorescence of donors I(t) under flash excitation is expressed using the response functions of donors K,(t) and acceptors K,,(t), which can be obtained from the Yokota-Tanimoto model [4], however, we have used the InokutiHirayama formula [5] for K,(t) because the migration processes between Yb3+ ions for a concentration of 0.1 mol% of Yb3 ’ are negligible in fluoroindate glasses [S]. The expression for Z(t) in the “transfer function” model is given by
scopic data using the Dexter formula [7]. The relatively high value obtained for RCA,, from the fitting is probably due to the correlation between acceptor and donor is neglected in the ‘transfer function’ model. The decay of the Er3+ emission is also in good agreement with the behavior predicted by the “transfer function” model (data not shown).
Acknowledgements Supported in part by “Gobierno Autonomo de Canarias” under Contract 226-206/94.
References where Cn = s + l/tn, CA = s + l/zA and f and L-‘(f) are the direct and inverse Laplace transformation off(t), respectively. The fits to Eq. (1) are also included in Fig. 2. The obtained critical radii (for forward RCDAand backward RCAD energy transfer) do not depend appreciably on Er3+ concentration. The mean value? obtained from the fittings are 12.2 and 24.3 A for, RCoA and RCAD, respectively, and 11.2 and 14.9 A if these are calculated from spectro-
[l]
Ph. Goldner and F. Pelle, J. Lumin. 60 & 61 (1994) 651. [Z] D.L. Huber, D.S. Hamilton and B. Barnett, Phys. Rev. B. 16 (1977) 4642. [3] A. Brenier, G. Boulon, C. Madej and C. Pedrini, J. Lumin. 54 (1993) 271. [4] M. Yokota and 0. Tanimoto, J. Phys. Sot. Japan 22 (1967) 779. [S] M. Inokuti and F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [6] I.R. Martin, V.D. Rodriguez, M. Morales, U.R. RodriguezMendoza and V. Lavin, J. Appl. Spectrosc. 62 (1995) 78. [7] D.L. Dexter, J. Chem. Phys. 21 (1953) 836.