Mechanisms of upconversion fluorescences in Er3+, Tm3+ codoped fluorozircoaluminate glasses

JOURNAL OF

ELSEVIER

Journal of Non-Crystalline Solids 181 (1995) 100-109

Mechanisms of upconversion fluorescences in Er 3÷, Tm 3+ codoped fluorozircoaluminate glasses Xuelu Zou *, Aki Shikida, Hiroaki Yanagita, Hisayoshi Toratani Materials Research Laboratory, Hoya Corporation, 3-3-1 Musashino, Akishima, Tokyo 196, Japan Received 5 August 1993; revised manuscript received 7 December 1993

Abstract

The upconversion of infrared radiation into green and red fluorescences has been investigated for Er 3+ and Tm3+ codoped fluorozircoaluminate glasses under 790 nm excitation. Introduction of Tm3+ into Er 3+ doped system preferentially quenches the green upconversion fluorescence from the 453/2 level of Er 3+ due to the efficient cross-relaxation of 4113/2 ~ 4115/2(Er) : 3H 6 --+ 3Ha(Tm)which can significantly reduce the upconversion efficiency from the 4113/2 level to the emitting 4S3/2 level. However, the Tm3+ behaves as a good sensitizer of the red upconversion fluorescence t~romthe 4F9/2 . . . • .level of Er 3 + which is mainly populated by . the cross-relaxation of 3 H a ....~ 3 H 6 (Tm): 4 111/2~ 4 F9/2(Er). The excitation energy stored in the 4111/2 level of Er 3÷ migrates through the glass with a diffusion-limited regime. Through this regime, only Er 3÷ ions which have diffused their energy into the vicinity of Tm3+ ions can transfer the energy to the 3H5 level of Tm3÷. At lower Er 3+ concentrations (< 3 mol%), this diffusion-limited energy migration is not so efficient as to enable energy transfer upconversion from the 4111/2 level to the 4F9/2 level of Er 3÷.

1. Introduction

The passive conversion of infrared light to visible light, termed 'frequency upconversion', has been extensively investigated in rare-earth-ion-doped glasses and crystals by many researchers [1-4]. The recent development of near-infrared semiconductor diode lasers which has made possible upconversion pumping of rare earth laser systems has stimulated interest. The upconversion process has been proven to be multiple excitation processes represented by excited state absorption and energy transfer between

* Corresponding author. Tel: + 81-425 2748? Telefax: + 81-425 46 2742. E-mail: [email protected].?

two rare earth ions in a solid. Consequently, to enhance the efficiency of upconversion fluorescence, a multi-ion-doped system utilizing energy transfers is probably useful. For example, high upconversion efficiency has been recently reported for BaF2-ThF4 heavy-metal fluoride glass codoped with Yb 3+-Er 3+ and y b 3 + - T m 3+ combinations [5-7]. The purpose of this work is to study the processes of the energy transfer between Er 3+ and Tm 3+ ions to understand why the addition of Tm 3÷ to Er3+-doped fluorozircoaluminate glass can enhance the red upconversion fluorescence from the 4F9/2 level and reduce the green upconversion fluorescence from the 4S3/2 level of Er 3+. A detailed study of the green, red and infrared emissions under the 488, 790 and 980 nm excitations is presented in this paper.

0022-3093/95/$09.50 © 1995 Elsevier Science B.V, All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 5 1 0 - 9

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

2. Experimental procedures The glasses used in this study were prepared from highly purified materials (purity: 99.99%). Fluorozircoaluminate (AZF) glasses of composition 25AlF 313ZrF4-(11 - x - y)YF3-46(MgF 2 + CaF2 + SrF2 + BaF2)-5NaF-xErFa-yTmF3 (mol%) were melted in glassy carbon crucibles at 950-1000°C for 1 h in an argon gas atmosphere. The liquid was rapidly cooled to the glass transition temperature, and then annealed. All samples for optical property measurements were cut and polished by the same process to the size of 25 × 25 X 5 mm 3. Absorption measurements were performed at room temperature with a computer-controlled spectrophotometer (Hitachi-330). The emission spectra were measured by exciting samples with light from diode lasers operating at 790 and 980 nm, an Ar-laser operating at 488 nm, and a Xe-lamp. The light was chopped at 80 Hz and focused on the 25 × 25 mm 2

101

face of the samples. A position of 1 mm from an edge was excited to minimize the reabsorption of emission. The average beam size of the light from the pumping lasers was about 0.5 mm 2. The emission from the sample was focused onto a monochromator and detected by a R-2228 photomultiptier tube, a Ge-detector and an InAs-detector. All detectors and the photomultiplier tube were cooled by liquid nitrogen. The signal was intensified with a lock-in amplifier and processed by a computer. Integrated fluorescence intensities were determined by numerical integration from the corresponding fluorescence bands. The errors in these measurements were estimated to be < +5%. The fluorescence lifetime measurements were performed by exciting the samples with YAG-laser pumped dye lasers operating at about 645 and 782 nm, respectively. The fluorescence was detected with an S-1 photomultiplier tube or the InAs-detector. The fluorescence decay curves were recorded and averaged with a

Table 1 Predicted spontaneous-emission probabilities of Er 3÷ and Tm 3+ in fluorozircoaluminate glass Er3+ Transition

Tm 3+ Average frequency

A M + Amd (s- 1)

Branching ratios

Transition

(~-1) 4113/2 ~ 4115/2 4111/2 _~ 4115/2 4113/2 419/2 ~ 4115/2 4113/2 4111/2 4F9/2 ~ 4115/2 4113/2 4111/2 419/2 453/2 ~ 4115/2 4113/2 4Ill/2 419/2 2Hll/2-* 4115/2 4F7/2 ~ 4115/2 4F5/2 ~ 4115/2 F3/2 "* 4115/2 H9/2 ~ 4115/2 --~ 4113/2 4Ill/2

~

6545 10277 3732 12531 5986 2254 15361 8816 5084 2830 18 484 11 939 8207 5953 19 194 20576 22272 22 676 24691 18 146 14414

Average frequency

Aed +Amd (s-1)

Branching ratios

112.7 175.8 3.0 593.6 102.6 15.1 1278.8 34.6 128.1 2.2 395.2 225.9 146.7 5.7

1.00 0.98 0.02 0.84 0.14 0.02 0.89 0.02 0.09 0.00 0.51 0.29 0.19 0.01

481.3 90.2 395.2 116.6 36.1 10.6

0.43 0.08 0.35 0.10 0.03 0.01

(cm-1) 118.5 117.3 15.6 93.2 34.2 1.2 972.5 52.4 45.7 1.5 894.6 357.2 27.8 43.2 3422.0 2089.0 994.0 847.0 941.5 1064.8 306.2

1.00 0.88 0.12 0.72 0.27 0.01 0.91 0.05 0.04 0.00 0.68 0.27 0.02 0.03 -

3H 4 ~ 3H 6 3H 5 ---*3H 6 --* 3H 4 3F4 ~ 3H 6 -* 3H 4 --* 3H 5 3F3 ~ 3H 6 ~ 3H 4 ~ 3H 5 ~ 3F4 3F 2 --~ 3H 6 ~ 3H 4 --* 3H 5 ~ 3F4 ~ 3F3 1G 4 --~ 3H 6 ~ 3H 4 "-') 3H 5 ~ 3F4 ~ 3F3 ~ 3F2

5956 8257 2301 12642 6686 4385 14620 8664 6363 1978 15 175 9219 6918 2533 555 21 459 15503 13 202 8817 6839 6284

102

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

computer-controlled transient digitizer. The errors in these measurements were estimated to be < _ 10%.

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3. Results and discussion

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3.1. Optical properties of Er 3 + and Tm 3 + doped fluorozircoaluminate glass

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The absorption spectra of glass samples containing Tm 3+ and Er 3+ ions with total concentrations of 0.1-12 mol% were measured at room temperature. From these absorption measurements, we estimated the radiative transition probabilities and the branching ratios for the transitions of Er 3+ and Tm 3÷ ions in fluorozircoaluminate glass using the Judd-Ofelt theory. Since details of the Judd-Ofelt theory have been well described earlier [8-11], only the calculated results will be presented here. The matrix elements U u), used in this study, were calculated by Weber [11] for Er 3÷ in LaF3 and by Carnall et al. [12] for Tm 3+ in LaF3. The intensity parameters were found to be 0~2) --- 2.91 x 10 -20 cm 2, 0 4 = 1.27 x 10 -20 cm 2 and I2~6) = 1.11 X 10 -20 cm ~ )for 2 mol% ErF 3 doped glass, and 12(2) = 1.90 X 10 -20 cm 2, O(4) = 1 . 6 5 X 1 0 -20 cm 2 and /2(6) = l . l l X 10 -20 cm 2 for 2 mol% TmF 3 doped glass. In order to evaluate the validity of the intensity parameters, I2(t), obtained by a least-squares fit to the values of measured oscillator strengths, the root mean square values (rms) were calculated by the equation rms = [E(Sm-Sc)2/~'.S2] 1/2 w h e r e S m and Sc are the measured and calculated line strengths, respectively, and the summation is taken over the bands used to calculate the OU) parameters. The rms deviations were found to be 3.4% for the Er 3÷ doped glass and 4.5% for the Tm 3÷ doped glass. Using these intensity parameters and matrix elements for emissions, the forced electric dipole transition probabilities, Acd, and the magnetic dipole transition probabilities, Amd, were calculated for the given transitions of Er 3+ and Tm 3+. The values of total spontaneous-emission probabilities (Acd +Amd) and branching ratios are listed in Table 1. It can be seen from Table 1 that the spontaneous-emission probability of the 3H 6 ---->3F4 transition of Tm 3+ is about six times larger than that of the 4115/2 ~ 419/2 transition of Er 3+. Introduction of Tm 3+ to an Er3+-doped system can be concluded

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Wavelength (nm) Fig. 1. Emission spectra of Er3+-doped and Er 3+, Tm 3+ codoped fluoride glasses excited at 790 nm. Dotted and solid lines represent the emission spectra of 3 tool% Er 3+ doped and 3 mol% Er 3+, 1.5 mol% Tm 3+ codoped glasses, respectively.

to enhance the pumping efficiency for 790 nm diode laser excitation, and may increase in the upconversion fluorescence efficiencies. 3.2. Energy transfers between Er 3 + and Tm 3 + ions Fig. 1 shows the emission spectra of Er3+-doped and Er 3÷, Tm 3÷ codoped glasses measured at room temperature under 790 nm excitation within the 419/2 level of Er 3 + and the 3 F level of Tm 3 + It is well . . 4 4 " 4 known that the exotatlon from the 115/2 to the I9/2 levels of Er 3÷ gives rise to the g r e e n (453/2 __.>4 I15/2) and red (4F9/2 ---)4115/2) upconversion fluorescences in Er 3÷ doped fluoride glass due to the excited state absorption and the upconversion by energy transfer. In Fig. 1, the dotted line represents the upconversion fluorescence spectra of the Er 3+ singly doped glass, which shows that the intensity of the green fluorescence is much stronger than that of the red fluorescence. Only 1.5 mol% Tm 3+ ions are introduced into this system; however, the green fluorescence intensity decreases and nearly vanishes. By contrast, the red fluorescence intensity increases by 11 times as compared with that of the Er 3+ singly doped system as shown in Fig. 1. This behavior

X. Zou et al. /Journal of Non-Crystalline Solids 181 U995) 100-109

indicates the presence of efficient energy transfers between Er 3+ and Tm 3+ in this system. To understand the mechanisms of energy transfers in the Er 3+, Tm 3+ codoped system, we measured fluorescences originating from the 453/2, 4F9/2, 4111/2 and 4113/2 levels of Er 3+ in various concentration of Tm 3+ and 3 mol% Er 3+ codoped glass under 790 nm excitation. In Fig. 2 the intensity ratios of these fluorescences for the Er 3+, Tm 3+ codoped system to those for the Er 3+ singly doped system are plotted versus the Tm 3+ concentration. With increasing Tm 3+ concentration, the intensity ratio of the 4 I l l / 2 ~ 4115/2 a n d 4113/2 ---->4115/2 fluorescences decrease due to the energy transfer from Er 3+ to Tm 3+, in which the dominant processes are usually assigned to the cross relaxations: 4 I l l / 2 ---) 4 1 1 5 / 2 ( E r ) : 3 H 6 ---> 3Hs(Tm )

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Concentration of Tm 3÷ (tool%) Fig. 2. Dependences of the Er 3+ emission intensities on the Tm 3+ concentration for Er 3+ (3 tool%), Tm 3+ codoped samples excited at 790 nm. The emission intensities of Er3+-doped and Er 3+, Tm 3+ codoped samples are denoted by l(Er) and I (Er+ Tin), respectively. Lines are a guide to the eye.

Fig. 3. Dependences of the Er 3+ emission intensities on the Tm 3+ concentration for Er 3+ (3 tool%), Tm 3+ codoped samples excited at 488 nm. The emission intensities of Er3+-doped and Er 3+, Tm 3+ eodoped samples are denoted by I(Er) and l ( E r + Tin), respectively. Lines are a guide to the eye.

the order of two or three phonons of the host glass, the Tm 3+ can decay rapidly from the 3H 5 level to the 3H 4 level. This means that the 3H 4 level of Tm 3+ can be populated by both cross-relaxations. From Fig. 2 one can also find that, with increasing Tm 3+ concentration, the g r e e n (453/2 "-') 4115/2) upconversion fluorescence decreases strongly as well as the 4113/2 ~ 4115/2 Stokes fluorescence, but the red ( 4 F9/2 --->4 I15/2)- upconversion fluorescence increases strongly. When the Tm 3+ concentration is more than about 1.5 mol%, however, the green upconversion fluorescence becomes independent and nearly vanishes; at the same time, the red upconversion fluorescence decreases. The concentration dependence of t h e 4 5 3 / 2 ~ 4115/2 upconversion fluorescence is very similar to that of the 4113/2 ~ 4115/2 fluorescence. Fig. 3 presents the dependences of the intensity ratio of Er 3+ fluorescences on the Tm 3+ concentration under 488 nm excitation. When the Tm 3+ concentration increases, the intensity ratio of the fluorescences originating from the 4Ill/2, 4113/2 , 453/2 and 4F9/2 levels decrease monotonically. This behavior

104

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

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Fig. 4. Tm3÷ concentrationdependencesof the 453/2 and 4F9/2 lifetimes of Er3+ for Er3+ (3 mol%), Tin3+ codopedsamples. The lifetimesof Er3+-doped and Er3+, Tm3+ codopedsamples are denotedby ~'(Er3÷ ) and ~"(Er +Tin), respectively.Linesare a guide to the eye. is consistent with that in the case of 790 nm excitation for the 4111/2 --> 4115/2 and 4113/2 --> 4115/2 fluorescences since the emitting levels are quenched by almost the same energy transfer processes for both the 488 nm and 790 nm excitations.However, the 453/2 "~ 4115/2 and 4F9/2 ~ 4115/2 fluorescences exhibit different behaviors for the 790 nm and 488 nm excitation. Under the 488 nm excitation, quenching of the 453/2 ~ 4115/2 and 4F9/2 --* 4115/2 fluorescences is caused by the energy transfers from Er 3+ to Tm 3+, i.e., cross-relaxations [15]:

453/2 ~ 419/2(Er): 3H 6 ~ 3H4(Tm ) 453/2 ~ 4Ii1/2(Er): 3H 6 -~ 3Hs(Tm ) 4F9/2 ~ 4115/2(Er ) : a n 6 ~ 3F2,3(Tm ). These cross-relaxations have also been confirmed by our measurements of the 453/2 and 4F9/2 lifetimes as shown in Fig. 4. The lifetime decrease strongly with increasing the Tm 3+ concentration due to the above cross-relaxation. Under the 790 nm excitation, however, these cross-relaxations are expected to be rather insignificant compared with the other energy transfer processes such as

4113/2 --> 4115/2(Er): 3H 6 --> 3Hg(Tm ) 3H 4 ~ 3H6(Tm): 4111/2 -~ 4F9/E(Er )

because (1) codoping with Tm 3+ can enhance the red (4F9/2 ~ 4115/2) fluorescence and (2) quenching of the green (453/2 --* 4115/2) fluorescence is more marked. Indeed, in the case of 790 nm excitation, the upconversion fluorescences originating from the 453/2 and 4F9/2 levels are strongly influenced by the populations of the intermediate levels from which upward excitations take place. This is because the emitting levels are only populated by the excited state absorption and energy transfer from these intermediate levels. That is, for the 790 nm excitation, the green and red upconversion fluorescences are due only to cooperative and/or successive transitions via 419/2 and successive transitions from 4Ill/2 and 4113/2 multiplets. The latter two transitions are possible but the former one is not possible since the reported multiphonon relaxation rate of the 419/2 is more than 3 × l0 s s-1 [16] which is far greater than the spontaneous emission rate of 117.3 s -1. However, in the case of 488 nm excitation, the 453/2 and 4F9/2 levels are only populated by the multiphonon relaxation from the excited level of 4F7/2 due to the extremely large multiphonon relaxation rate of > 107 s- 1 for the multiplet [16]. In other words, the mechanisms of the green and red fluorescences for the 488 nm excitation is readily distinguishable from that for the 790 nm excitation, which causes the mechanisms and rates of quenching of the emitting levels to be different. Fig. 5 shows variation of the upconversion fluorescences from the 453/2 and 4F9/2 levels of Er 3+ under 980 nm excitation. The Tm 3+ concentration dependence of the green (453/2 ~ 4115/2) upconversion fluorescence is different from that in the case of 790 nm excitation as shown in Figs. 2 and 5. The quenching of the green fluorescence is less under the 980 nm excitation than under the 790 nm excitation. This is because the emitting 453/2 level is mainly populated by upconversion from the 4113/2 level for the 790 nm excitation, but the 4 1 1 1 / 2 level for the 980 nm excitation [17]. By contrast, the Tm 3+ concentration dependence of the red (4F9/2 upconversion fluorescence for the 980 nm excitation is similar to that in the case of 790 nm excitation as shown in Figs. 2 and 5. This indicates that the emitting 4F9/2 level is populated by the same energy transfer processes for both the 790 nm and 980 nm excitation.

--'~4115/2)

X.. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

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Fig. 5. Tm 3+ concentration dependences of the 453/2 --'~ 4115/2 and 4F9/2 -->4115/2 emission intensities for Er a+ (3 mol%), Tm 3÷ codoped samples excited at 980 nm. The emission intensities of Era+-doped and Er a+, Tm 3+ codoped samples are denoted by l(Er) and l(Er+Tm), respectively. Lines are a guide to the eye.

As shown in Figs• 2 and 3, introduction of Tm 3+ into the Er3+-doped system can significantly reduce 4 • the population of the 4Ill/2 and 113/2 intermediate levels, especially that of the 4113/2 level for both the 488 nm and 790 nm excitations. Moreover, the Tm 3÷ • 4 4 concentraUon dependence of the I13/3 --> 115/2 fluorescence is very similar to that of the $3/2 ~ 4115/2 fluorescence for the 790 nm excitation, but is distinguishable from that of the 453/2 --->4115/2 fluorescence for the 488 nm excitation. These characteristics indicate that the g r e e n ( 4 5 3 / 2 -----> 4115/2) fluorescence is strongly influenced by the excitation population of the 4113/2 level for the 790 nm excitation, but independent of the population of the 4113/2 level for the 488 nm excitation. With introduction of Tm 3+ into the Er3+-doped system, by reason that the population of the 4113/2 intermediate level is reduced due to the cross-relaxation of 4113/2 "'> 4115/2(Er):3H 6 --> 3H4(Tm), the contribution of upconversion from this level to the green upconversion fluorescence is greatly reduced. Taking

105

into account all the possible energy transfers as mentioned above, a decrease in the green fluorescence intensity can be attributed to the following processes of energy transfers, i.e., two cross-relaxation channels of 453/2 --->419/2(Er) : 3H 6 --->3Ha(Tm) and 453/2 "'> 4111/2([~r) : 3H 6 --->3H5(Tm) for both the 488 nm and 790 nm excitation, and the cross-relaxation of 4113/2 --> 4115/2(Er) : 3H 6 ---->3H4(Tm) only for the 790 nm excitation. The latter is the dominant process as compared with the former two processes for quenching the population of emitting 453/2 level in the case of 790 nm excitation, since it can significantly reduce the upconversion efficiency from the 4113/2 intermediate level. However, the red (4F9/2 ---->4115/2) upconversion fluorescence is distinguishable from the green (453/2 --->4115/2) upconversion fluorescence under both the 790 nm and 980 nm excitations as shown in Figs. 2 and 5. Its intensity increases up to about 1.5 mol% Tm 3+ and then decreases due to the cross-relaxation of 4F9/2 -"->4115/2(Er) : 3H 6 --->3F2.3(Tm). Although the populations of intermediate levels such as 4Ill/2 and 4113/2 decrease with increasing Tm 3+ concentration, the 4F9/2 --> 4115/2 fluorescence in the Er 3+, Tm 3+ codoped system is more intense than that in the Er 3÷ singly doped system for both the 790 nm and 980 nm excitation. To explain this effect, we propose possible energy transfer and upconversion processes in Fig. 6. When both the 419/2 level of Er 3÷ and the 3F4 level of Tm 3+ are excited simultaneously, the excitation energy stored in the 3F4 level of Tm 3÷ transfers non-resonantly to the 4113/2 level of Er 3÷ via the cross-relaxation of 3F4 "->3H 4(Tm): 4115/2 -> 4113/2(Er) for which the 3F4 lifetime decreases with an increase of the Er 3÷ concentration as shown in Fig. 7. At the same time, the excitation energy stored in the 419/2 level relaxes non-radiatively by multiphonon relaxation to the 4Ill/2 and 4113/2 levels due to the extremely large multiphonon relaxation rate of > 3 × 105 s -1 [16]. Then the non-resonant energy transfer: 4113/2 ----> 4115/32(Er): 3H 6 --->3H4(Tm) occurs, thereby causing the H 4 level to be populated. Finally, the 4F9/2 level is populated by the non-resonant energy transfer: 3H 4 -'-> 3H6(Tm): 4 I l l / 2 --->4F9/2(Er). This energy transfer process may cause the increase in the 3H 4 ---->3H 6 fluorescence intensity to be much less than expected from the efficient Er3+(4113/2 )--->

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

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Fig. 8. The 3H 4 ~ 3H 6 emission intensities of Tm 3+ versus both the concentrations of Tm 3+ and Er 3+ in the samples excited at 790 nm. Lines are a guide to the eye. ( T m / E r ) : the emission intensities of Er 3+, Tm 3+ codoped samples; (Tin): the emission intensities of Tma+-doped samples.

Fig. 6. Mechanism of the sensitization effect of Tm 3+ ions on the red upconversion fluorescence of Er a+ ions in fluoride glass.

Tm3+(3H4) energy transfer as shown in Fig. 8. Fig. 8 shows t h e 3 H 4 ---> 3H 6 fluorescence intensities versus the concentrations of Tm 3+ and Er 3+ in Tm 3+ singly and (12 - x) Er 3+ and x T m 3+ doubly doped systems. T h e 3 H 4 ---> 3H 6 fluorescence intensity for the Er 3+, Tm 3÷ codoped system is less than that for the Tm 3+ singly doped system. The reason for this behavior is due to a storage of a major fraction of the excitation energy in the 3H 4 level by the energy transfers from Er 3+ to Tm 3+ which can be trans-

fered back to the Er 3+ via the cross-relaxation of 3 H 4 ~ 3 n 6 ( T m ) : 4 I n / 2 --~ 4 F 9 / 2 ( E r ) . Suppose w e can reduce the efficiency of the cross-relaxation of 3 H 4 "-~ 3 H 6 ( T m 3 + ) :

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4.0

I ....

5.0

2.0 I0.0

4.0 8.0

6.0 6.0

r

8.0 4.0

l l , , , l , l l

I0.0 12.0 T m 3+ 2.0 0.0 E r 3+

C o n c e n t r a t i o n of Era+ a n d Tma+ (tool%) 6.0

C o n c e n t r a t i o n of Era+ (mol%) Fig. 7. The 3F4 lifetime of Tm 3+ versus the Er 3+ concentration for Tm 3+ (2 tool%), Er 3+ codoped samples. Lines are a guide to the eye.

Fig. 9. The 3H 4 --~ 3H 6 emission intensities of Tin 3+ versus both the concentrations of Tm 3+ and Er 3+ in the samples excited by a Xe-lamp. Lines are a guide to the eye. ( T m / E r ) : the emission intensities of Er 3+, Tm 3+ eodoped samples; (Tin): the emission intensities of Tm3+-doped samples.

107

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109

of the energy levels higher than the 419/2 and 3F4 levels are excited and cause the population of the 4F9/2 level to be higher than that of the 4111/2 level, • for which the cross-relaxation of 3H 4 _..>3 H 4(Tm) : 4111/2 ~ 4F9/2(Er) becomes difficult. The 3H 4 "-> 3H 6 fluorescence intensity for the Er 3÷ and Tm 3+ doubly doped system is therefore about four times higher than that for the Tm 3÷ singly doped system in the given range of concentrations ( < 4 mol% Tm 3÷ ) as shown in Fig. 9. Comparing the results in Figs. 8 and 9, it is obvious that the behavior of the 3H 4 ---->3H 6 fluorescence in the case of 790 nm excitation is distinguishable from that in the case of Xe-lamp excitation. This fact indicates that the efficient cross-relaxation of 3H 4 --->3H 6( T m ) : 4 I l l / 2 ~ 4F9/2(Er) exists in the Er 3+, Tm 3+ codoped system for the 790 nm excitation.

'° L 0.80

g

0.80

I

~'i

......... 4111rl-4IistimlasiiioaofF_J+ 3H6-~H~rams/lionof Tin~+

.......... .r7~........... ~. . . . . . . . .

.......

,0

i........ o.8o

iil

i

i

i-4-" .......

!. . . . . . .

i .........

5 0.80

............. ..............'--/ ........:-I ...........'.......... 02

0.0 900

0

1000

~

1100

0.20

1200

1300

0.0

Wavelength (nm) 4

Fig. 11. Spectral overlap of IllZ2 ~ 4115/2 emission of Er a+ and 3H6 --~ 3H 5 absorption of Trn3+ in the samples.

3.3. Dynamics of the 4111/2excited state 4

4

Decays of the 111/2 --> I15/2 fluorescence around 980 nm were measured for Er 3+ singly doped and Er 3+, Tm 3+ doubly doped glass samples. The decay is single exponential and the time constants are nearly the same with increasing Er a+ concentration in Er a÷ singly doped glass as shown in Fig. 10, which indicates that no concentration quenching occurs for this level. However, by introducing about 2 tool% Tm 3+ ions into the Era+-doped system, the time constants strongly decrease and this decreasing rate increases with increasing Er 3+ concentration.

.... o---_o

.....

°

5.,5

~

2.7 ~C]...._..._cT ~ 0.0

i

0.0

I

i

I

t

,

I

I

I

~ mol% I

I

2.0 4.0 6.0 8.0 i0.0 Concentration of Er a+ (tool%)

,

12.0

Fig. 10. Concentrational dependence of the 4111/2 lifetime of Er 3+ in the samples with and without Tin3+. Lines are a guide to the eye.

This behavior can only be explained by the Er 3+---~ Tm 3+ energy transfers in which the dominant processes is assigned to the cross relaxation of 4Ili/2 --* 4115/2(Er) : 3H 6 ~ 3H5(Tm) as mentioned above. It is interesting to discuss here the possible regimes of Er a+ donor decay which can occur in the Er 3+, Tm 3÷ codoped glass. Energy migration between donor ions can be described either as a diffusion process or as a random walk (namely hopping model) and it was shown that these two models lead to similar results [18,19]. The reason why the 4Ill/2 lifetime in the presence of Tm 3+ decreases rapidly with increasing Er 3+ concentration can be explained as follows. When the Tm 3+ ions are introduced into the Er3÷-doped system, owing to the small spectral overlap of the 4 I l l / 2 ~ 4115/2 fluorescence and the 3H 6 ~ 3H 5 absorption as shown in Fig. 11, only a small fraction of the excited donor Er 3÷ ions in the 4111/2 are within the critical transfer distance of an acceptor Tm 3÷ ion. Their energy can be transferred to the 3H 5 level of Tm 3+ with 2020 cm -1 of energy released to the lattice phonons. Those more distant Er 3+ ions should first diffuse the energy into the vicinity of a Tm 3÷ ion before the cross relaxation of 4111/2 ~ 4115/2(Er): 3H 6 -* 3Hs(Tm) occurs. Consequently, an increase in the Er 3+ concentration enhances the rate of diffusion-limited energy migration

X. Zou et aL /Journal of Non-Crystalline Solids 181 (1995) 100-109

108

and thus promotes the efficiency of the Er3+(4111/2) Tm 3+ (3H 5) energy transfer. In the diffusion-limited model, Yokota and Tanimoto [20] and then Weber [21] demonstrated in the case of dipole-dipole interaction that at long times the fluorescence of the donors decays exponentially with a time constant, z F, given by [21,22] 1

1

1

"/'F

TO

TD

(1)

1

1 .

TF

.

.

KDCDCA,

(2)

TO

with K D = 6.777r (U3DUDA)1/4. The regime of donor decay described by Eq. (2) is well known as diffusion-limited decay. However, an increase in the concentration of donors produces a faster migration of energy and it was shown that for very fast diffusion the decay of the donor fluorescence is purely exponential, and then [23] 1 1 . . . . TF

VDCD,

4--

/0

(3)

TO

with VD a constant depending on the type of interaction• As can be seen from Fig. 12, the straight line for the plot of the experimental data has a slope of 1.03 which agrees well with the prediction from Eq. (2). This fact indicates that the energy transfer from the 4111/2 level of Er 3+ to the 3H 5 level of Tm 3+ can be expressed in the framework of the diffusionlimited regime. Also, the migration among Er a+ ions in the 4111/2 level can be ascribed not to the fast diffusion but to the diffusion-limited mechanism since Eq. (3) is not valid for the experimental data in the figure. Indeed, the distinction between the diffusion-limited regime and fast diffusion is in relation

/

o/o

6.5

v

o/S1op¢=1.03

1

5 4.5

where % is the purely radiative lifetime of the donors (Era+); 1 / z D is the decay rate due to the diffusion given by l / r o = 2.72~tCAD(UDA/D) 1/4, where CA is the concentration of acceptors, UDA is the donor-acceptor transfer constant and D is the diffusion constant. If the donor concentration is denoted by C o and the donor-donor transfer constant is denoted by UDD, the diffusion constant can be written as D = 3.375UDDCD4/3. Finally one obtains .

7.5

3.s L

o.o

/O

,io ,'., 2'.o

;.o ;., ,.o

Ln(CACo)(tool%) Fig. 12. Experimental data of Ln(1/zF-1/'r o) versus ln(CDCA). The data are fitted to a straight line with slope of 1.03.

to the strength of the cross-relaxation with respect to the diffusion one, the efficient cross-relaxation of 4 I l l / 2 "~ 4115/2(Er): 3H 6 --->3Hs(Tm) occurring in our glass systems contributes to limiting the diffusion among Er 3+ ions. Note that this efficient crossrelaxation does not necessarily lead to a rapid energy transfer from Er 3+ to Tm 3+, because the excited donor Er 3+ ions in the 4111/2 transfer their energy to the acceptor Tm 3÷ ions with the diffusion-limited regime. That is, the E r 3 + ( 4 I l l / 2 ) ~ Tm3+(3Hs) energy transfer rate is mainly dominated by the rate of the diffusion-limited energy migration among the donor ions, but is not determined by the rate of the cross-relaxation o f 4 I l l / 2 '--->4115/2(Er) : 3 H 6 ~ 3 H 5(Tm). In fact, at lower dopant concentrations of Er 3+ or Tm 3÷, the energy diffusion among the donor Er 3+ ions becomes so slow that a great part of the excited Er 3+ ions in the 4111/2 can emit directly to the ground state and/or convert to the upper emitting levels such as 4F9/2 before they diffuse the excited energy into the vicinity of a Tm 3+ ion. It is therefore expected that the lower the Er 3+ concentration, the longer the time of energy diffusion will be, which is more advantageous to the energy transfer upconversion from the excited 4111/2 level to the emitting 4F9/2 level. This point has been confirmed by our experiments as shown in Fig. 13. In Fig. 13 • • • 4 we present the intensity ratio of the F9/2 --> 4115/2 fluorescence in Er 3+ and 2 mol% Tm 3+ codoped system to that in the Er 3+ singly doped system

X. Zou et al. /Journal of Non-Crystalline Solids 181 (1995) 100-109 160

I

I

I

I

I

140 40

120 100 E

80

\\

~A E I.420 u.I v

[]

60 E

U.I

40

x

20

o 0.0

I 2.0

i

i

4.0

6.0

I ~O"-T-"

8,0

10.0

o

12,o

109

level to the emitting 453/2 level. By contrast, an increase in the red upconversion fluorescence from the 4F9/2 level is due to the efficient energy transfer process o f 3H 4 --> 3 H 6 ( T m ) : 4 I l l / 2 ---->4F9/2(Er). T h e

energy stored in the 4Ill/2 level of Er 3+ migrates through the glass with a diffusion-limited regime. Through this regime, only Er 3+ ions which have diffused their energy into the vicinity of Tm 3÷ ions can transfer the energy to the 3H 5 level of Tm 3+. At lower Er 3÷ concentrations (~<3 mol% Er 3÷) this diffusion-limited energy migration is not so efficient as to enable the energy transfer upconversion from the 4Ill/2 level to the emitting 4F9/2 level of Er 3÷.

Concentration of E ra* (mol%)

Fig. 13. Concentrational dependences of the 4F9/2 ~ 4115/2 emission intensities of Er 3+ in the samples with and without Tm3+. The emission intensities of the Er3+-doped and Er 3+, Tm3+ (2 tool%) codoped samples are denoted by l(Er) and l(Er +Tin), respectively. Lines are a guide to the eye.

versus the Er 3÷ concentration. Under the simultaneous excitation of the 419~2 level and the 3F4 level, the intensity ratio of the F9/2 ~ 4115/2 fluorescence increases monotonically because the diffusion rate among Er 3+ ions becomes lower with decreasing Er a+ concentration. The 4F9/2 ~ 4115/2 fluorescence intensity, which is also presented in Fig. 13, increases up to about 3 mol% Er a÷ ions and above 3 mol% it decreases because the energy migration among Er 3+ ions in the 4111/2 level becomes so efficient as to build the population of this level and the efficiency of upconversion from this level to the emitting 4F9/2 level decrease.

4. Conclusions We have shown that Tm 3 + behaves as a quenching centre of the 453/2 -"> 4115/2 , 4Ill/2 ~ 4115/2 , and 4113/2 -'-> 4115/2 fluorescences, but behaves as a sensitizer of the fluorescence for the 4F9/2 ~ 4115/2 transition of Er 3÷ under the 790 nm excitation. The quenching of the green fluorescence from the 4S3/: level is attributable to the cross-relaxation of 4113/2 "-> 4115/2 (Er): 3H6 ~ 3H4(Tm), which significantly reduces the upconversion efficiency from the 4113/2

References [1] F.E. Auzel, Phys. Rev. B13 (1976) 2809. [2] J.C. Wright, in: Topics in Applied Physics, Vol. 15 (Springer, Berlin, 1976) p. 239. [3] A. Pollack, D.B. Chang and N.L. Moise, J. Appl. Phys. 60 (1986) 4077. [4] A.J. Silversmith, W. Lenth and R.M. Macfarlane, Appl. Phys. Lett. 51 (1987) 1977. [5] D.C. Yeh, W.A. Sibley, M.J. Suscavage and M.G. Drexhage, J. Appl. Phys. 62 (1987) 266. [6] D.C. Yeh, W.A. Sibley and M.J. Suscavage, J. Appl. Phys. 63 (1988) 4644. [7] R.S. Quimby, M.G. Drexhage and M.J. Suscavage, Electron. Lett. 23 (1987) 32. [8] B.R. Judd, Phys. Rev. 127 (1962) 750. [9] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [10] W.F. Krupke, Phys. Rev. 145 (1966) 325. [11] M.J. Weber, Phys. Rev. 157 (1967) 262. [12] W.T. Camall, P.R. Fields and K. Rajank, J. Chem. Phys. 42 (1965) 3797. [13] A. Brenier, R. Moncorge and C. Pedrini, IEEE J. Quantum Electron. QE-26 (1990) 967. [14] D.C. Yeh, R.R. Petrin, W.A. Sibley, V. Madigou and J.L. Adam, Phys. Rev. B39 (1989) 80. [15] J.P. Jouart, J. Lumin. 21 (1980) 153. [16] M.J. Weber, Phys. Rev. B8 (1972) 54. [17] X. Zou and T. Izumitani, J. Non-Cryst. Solids 162 (1993) 68. [18] R.K. Watts, Optical Properties of Ions in Solids, ed. B. Di Bartolo (Plenum, New York, 1975) p. 307. [19] B. Di Bartolo, Energy Transfer Processes in Condensed Matter, ed. B. Di Bartolo (Plenum, New York, 1984) p. 103. [20] M. Yokota and O. Tanimoto, Jpn. Phys. Soc. Jpn. 22 (1967) 779. [21] M.J. Weber, Phys. Rev. B4 (1971) 2932. [22] A. Brenier, C. Pedrini, B. Moine, J.L. Adam and C. Pledel, Phys. Rev. B41 (1990) 5364. [23] R.K. Watts and H.J. Richter, Phys Rev. B6 (1972) 1584.