Energy transfer luminescence in (Eu3+,Nd3+) : tellurite glass

Optical Materials 13 (2000) 381±388

Energy transfer luminescence in (Eu3‡,Nd3‡ ) : tellurite glass K. Annapurna a, R.N. Dwivedi a, S. Buddhudu a

b,*

Glass Technology Section, Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Calcutta 700 032, India b Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Received 12 February 1999; accepted 22 April 1999

Abstract Here, we bring out an infrared transmitting new optical glass based on TeO2 added with AlF3 and LiF, containing dual rare earth ions (Eu3‡ ,Nd3‡ ) as the dopants with a purpose to examine their luminescence and also the decay times pertaining to a prominent transition of Eu3‡ (5 D0 ® 7 F2 at 615 nm) as a function of temperature both in the presence and absence of Nd3‡ ions. The energy transfer rates …Wtr †, critical distances …R0 † and transfer eciencies …gtr † have been evaluated based on the measured lifetime data of this glass. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Energy transfer; Luminescence; Tellurite glass

1. Introduction In the past several years, tellurium based glasses have been the subject of investigations with all greater interest as these are found to be the IRtransmitting (15 lm) materials for their use as the optical components in the forms of windows, prisms, laser glasses and also in the area of ®bre optic communications [1±5]. Earlier, some preliminary research works were carried out on optical spectra of Nd3‡ : TeO2 ±B2 O3 ±RiF (R ˆ Li, Na, K) glasses [6]. It has been a well known fact that the TeO2 chemical by itself does not form a glass, however it has been considered that with an addition of an intermediate (AlF3 ) and a modi®er (LiF), the very TeO2 would form a good glass with high strength and transparency. The present paper aims in reporting yet another newly developed

*

Corresponding author. Fax: +91-08574-27499/25211

(Eu3‡ ,Nd3‡ ): TeO2 ±AlF3 ±LiF optical glass and to analyse its energy transfer from the donor (Eu3‡ ) ion to the acceptor (Nd3‡ ) ion by the measurement of the temperature dependent (10±300 K) luminescence spectra and also the lifetimes of a prominent emission transition (5 D0 ® 7 F2 ) of Eu3‡ ions. 2. Experimental For the present work, three kinds of glasses such as (i) reference (host) glass, (ii) Eu3‡ -glass and (iii) (Eu3‡ + Nd3‡ )-glass, have been developed and their compositions (in mol%) are given below: (i) Host glass: 79TeO2 + 6AlF3 + 15LiF, (ii) Eu3‡ -glass: 78TeO2 + 6AlF3 + 15LiF, (iii) (Eu3‡ + Nd3‡ )-glass: 77TeO2 + 6AlF3 + 15LiF. While making the Eu3‡ and (Eu3‡ ,Nd3‡ )glasses, only the TeO2 content was changed by keeping the contents of both the glass intermediate

0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 0 8 8 - 9

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(AlF3 ) and the modi®er (LiF) in appropriate quantities ®xed uniformly in all three glasses. The dopant ion concentrations were ®xed at 1 mol% each. The Te-glasses have been prepared by using analar grade TeO2 , AlF3 , LiF and rare earth oxides (Eu2 O3 , Nd2 O3 ) of 99.99% purity. The batches of 50 g each thoroughly mixed were melted in platinum crucible ®rst at 300°C for 10 min. and later at 700°C for 30 min. to ensure the homogeneity. The melt was then poured on to a steel plate and quenched with the other. The resulted glass samples were annealed at 300°C and then cooled slowly to room temperature. For the reference glass only the physical properties (d and nd ) were determined by adopting the conventional procedures and the thermal properties (Tg and Tm ) were measured on a Shimadzu Di€raction Thermal

Analysis (DTA) system. This instrument is sensitive for the ready evaluation of the thermal properties in the range of 300±1600 K. The glass nature of the material studied was con®rmed by an X-ray phase analysis (TUR M62 Di€ractometer Cu Ka radiation with Ni ®lter). The infrared transmission ability was also measured and the glass infrared cut-o€ wavelength is found to 15 lm from the spectral pro®le recorded on Shimadzu spectrophotometer. The physical and thermal property data for the host glass. Density (d, g/cm3 ) Refractive index (nd ) Glass transition temperature (Tg , °C) Glass softening point (Tm , °C)

Fig. 1. Luminescence spectra of Eu3‡ -doped tellurite glass at 300 and 10 K.

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The total luminescence spectra of Eu3‡ and (Eu3‡ ,Nd3‡ ) : tellurite glass at di€erent temperatures (10±300 K) were measured on a Spex-1401 Double Spectrometer with a PMT (Hamamatsu Model C-3350) and with a lockin ampli®er of the model Sr-400 two channel gated photon counter by using a N2 -laser (337.1 nm) as the source of excitation. The decay times of the prominent ¯uorescence transition (5 D0 ® 7 F2 ) of the Eu3‡ in both Eu3‡ and (Eu3‡ ,Nd3‡ ) doped tellurite glass were measured at di€erent temperatures by using a Tektronix TDS-420 four channel digital oscilloscope. The low temperature cryogenic system was a He-Unit from Displex Air Products ®tted with a temperature controller of the model (9600-1 Silicon diode from Scienti®c Inst., Tokyo).

383

3. Results and discussion Photoluminescence spectra measured at 300 K and 10 K for Eu3‡ and (Eu3‡ ,Nd3‡ ) : TeO2 ±AlF3 ± LiF glasses are shown in Figs. 1 and 2, respectively. Fig. 1 reveals ®ve emission transitions (5 D0 ® 7 F0;1;2;3;4 ) of Eu3‡ doped glass and in Fig. 2 for (Eu3‡ ,Nd3‡ ): glass only four emission transitions are observed excepting that of a forbidden transition (5 D0 ® 7 F0 ) which is suppressed due to the Nd3‡ availability as the codopant with the Eu3‡ ions in the glass matrix investigated. It is also interesting to notice that the more prominent transition (5 D0 ® 7 F2 at 615 nm) has got its ¯uorescence intensity signi®cantly brightened at 10 K compared at 300 K as we see in Figs. 1 and 2. Comparison of these two pro®les indicates that Eu3‡ ¯uorescence eciency has drastically been

Fig. 2. Luminescence spectra of (Eu3‡ ,Nd3‡ )-doped tellurite glass at 300 and 10 K.

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Fig. 3. Comparison of relative ¯uorescence intensity of the prominent transition …5 D0 !7 F2 † of Eu3‡ in Eu3‡ and (Eu3‡ ,Nd3‡ )-doped tellurite glass at di€erent temperatures (10±300 K).

reduced particularly for the transition 5 D0 ® 7 F2 and however, the magnetic dipole transition 5 D0 ® 7 F1 remains unchanged due to codopant presence along with the Eu3‡ in the glass. Thus, Fig. 3 presents a comparison of the relative ¯uorescence intensity of the prominent transition 5 D0 ® 7 F2 for Eu3‡ and (Eu3‡ ,Nd3‡ ): glass at di€erent temperatures. This decrease in the Eu3‡ emission intensity with the presence of codopant may be attributed to the direct energy transfer from Eu3‡ to Nd3‡ ions. In support of the spectral features in Figs. 1 and 2 and a graphical representation in Fig. 3, the measured decay curves (Figs. 4 and 5) of the prominent transition (5 D0 ® 7 F2 ) at 300 K and 10 K show the lifetimes of this transition as follows: Eu3‡ : glass …Eu3‡ ; Nd3‡ †: glass

s300 K ˆ 252:65 ls; s10 K ˆ 411:67 ls; s300 K ˆ 209:50 ls; s10 K ˆ 333:80 ls:

The ¯uorescence intensity I(t) has been approximated by taking the sum of the two exponential decay components as given below [7]: I…t† ˆ A1 exp …ÿt=s1 † ‡ A2 exp …ÿt=s2 †; where s1 and s2 are the short and long decay components, where as A1 and A2 are computer ®tting constants. By using the above equation, the average lifetime hsi or s is evaluated as follows: sˆ

A1 s21 ‡ A2 s22 : A 1 s 1 ‡ A2 s 2

Due to the introduction of weight factors (A1 and A2 ), a reasonable estimation of average lifetime values in the case of complicated decay pro®le has been achieved [8]. Temperature dependent decay times of 5 D0 ® 7 F2 for both Eu3‡ and (Eu3‡ ,Nd3‡ ): glasses are shown in Fig. 6 and the trends in this ®gure con®rm the reduction in the decay times as the temperature increased. The energy transfer in rare earth ions doped glasses is considered to be the

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Fig. 4. (a) Luminescence decay curve of the Eu3‡ transition (5 D0 !7 F2 ) in Eu3‡ -doped tellurite glass at 300 K. (b) Luminescence decay curve of the Eu3‡ transition (5 D0 !7 F2 ) in Eu3‡ -doped tellurite glass at 10 K.

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Fig. 5. (a) Luminescence decay curve of the Eu3‡ transition …5 D0 !7 F2 ) in (Eu3‡ ,Nd3‡ )-doped tellurite glass at 300 K. (b) Luminescence decay curve of the Eu3‡ transition (5 D0 !7 F2 ) in (Eu3‡ ,Nd3‡ )-doped tellurite glass at 10 K.

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Fig. 6. Comparison of the decay times of the prominent transition (5 D0 !7 F2 ) of Eu3‡ in Eu3‡ and (Eu3‡ ,Nd3‡ )-doped tellurite glass at di€erent temperatures (10±300 K).

resonant energy transfer from donor to the acceptor energy levels. The energy transfer probability, Wtr is evaluated from the following expression:

Another important energy transfer parameter namely energy transfer eciency gtr is calculated from

Wtr ˆ …1=s0 ÿ 1=s†;

The data related to Wtr ; R0 and gtr for (Eu3‡ ,Nd3‡ ): tellurite glass investigated is presented in Table 1 as a function of temperature (10±300 K). From Table 1 it is clear that the energy transfer between the donor and acceptor at lower temperatures is larger than at room temperature. In essence, it could be stated that, we have prepared IR-transmitting new optical glass based on TeO2 , added with AlF3 and LiF as glass network intermediates and modi®er respectively and have studied energy transfer from Eu3‡ ions to Nd3‡ ions. Luminescence spectra and decay curves of donor ion Eu3‡ showed pronounced changes on the addition of the acceptor Nd3‡ to the glass matrix.

5

7

3‡

where s0 is the lifetime of D0 ® F2 of Eu ions and s is the lifetime of the same transition in the presence of Nd3‡ ions. For the electric dipole±dipole mechanism, the critical distance, R0 is computed from ÿ
1=6 R0 ˆ 9Wtr s0 C ÿ2 †=…16p2 : This parameter is a measure of the energy transfer at which intrinsic emission probability of donor and energy transfer from donor to acceptor are coinciding to each other.

gtr ˆ …1 ÿ s=s0 †:

Table 1 Function of temperature Temp. (K)

Wtr (sÿ1 )

R0 (nm)

gtr

10 100 200 300

548.4 490.2 467.0 455.8

1.23 1.17 1.12 1.08

0.183 0.142 0.116 0.103

Acknowledgements We are gratefully thankful to Dr. H.S. Maiti, Director of the CGCRI, Calcutta for his kind cooperation and encouragement in the present work.

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