Spectroscopic studies of Er3+ centers in KYF4

Journal of Alloys and Compounds 341 (2002) 362–365

L

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Spectroscopic studies of Er 31 centers in KYF 4 M. Yin b

a,b ,

*, V.N. Makhov c , N.M. Khaidukov d , J.C. Krupa a

a ´ , B.P. No 1, Orsay 91406, France Groupe de Radiochimie, Institut de Physique Nucleaire Department of Physics, University of Science and Technology of China, Hefei 230026, China c Lebedev Physical Institute, Moscow 117924, Russia d Kumakov Institute of General and Inorganic Chemistry, Moscow 117907, Russia

Abstract A systematic spectroscopic study of KYF 4 :Er 31 (5%) in the visible region is reported. By using site selective excitation (emission) at 12 K, emission (excitation) spectra originating in Er 31 in two different crystallographic sites are distinguished. Transient measurement shows that the 4 S 3 / 2 Er 31 level in the two sites decays exponentially with lifetimes of 0.53 ms for site A and 0.82 ms for site B, respectively. The phenomenon of green up-conversion emission under red excitation to 4 F 9 / 2 level is explained by 4 F 9 / 2 , 4 F 9 / 2 → 4 F 7 / 2 , 4 I 11 / 2 up-conversion excitation.  2002 Elsevier Science B.V. All rights reserved. Keywords: KYF 4 :Er 31 ; Luminescence; Up-conversion; Energy transfer

1. Introduction Fluoride compounds have attracted great interest in recent years. A highlight of such study is concerned with visible quantum cutting (or two-photon luminescence, photon-cascade emission) [1,2], mainly because they have potential use in non-mercury fluorescent tubes. By using energy transfer between two different lanthanide ions, quantum efficiency of near 200% was achieved for LiGdF 4 :Eu 31 [3,4]. Similar fluoride compounds KYF 4 :RE (RE5Er, Tm, Ho, Yb, Nd) are very attractive for developing VUV and up-conversion-pumped solid-state lasers [5– 7]. KYF 4 is a multisite crystal. The structure study shows 31 that Y ions occupy six crystallographic sites in the crystal, which fall into two types [8]. However, when rare earth ions are introduced into the crystal, they are not distributed among all six sites equally [9]. Bouffard and co-workers have studied site-selective up-conversion excitation of KYF 4 :Er 31 and found that the emission spectrum can only be interpreted as due to Er 31 in two sites [10]. We present here a detailed spectroscopic study of Er 31 centers in KYF 4 . To benefit from the narrowness of the

*Corresponding author. Tel.: 133-1-6915-7345; fax: 133-1-69156470. E-mail address: [email protected] (M. Yin).

inhomogeneous energy transitions and to reduce the phonon effect, all the experiments were conducted at 12 K. Through excitation and emission spectra at 12 K, we concluded that in the crystal KYF 4 :Er 31 (5%), Er 31 ions occupy two sites. By using site-selective excitation (emission) at 12 K, emission (excitation) spectra originating in two different crystallographic sites were separated clearly. The lifetime of the 4 S 3 / 2 levels of Er 31 in the two sites are 0.53 and 0.82 ms. The mechanism of green up-conversion luminescence under red excitation is also discussed.

2. Experiment The sample used here was grown by the hydrothermal synthesis technique with erbium concentration of 5% in mass. Laser-selective excitation and excitation spectra were recorded using radiations from dye lasers coumarin 480 and DCM 640 pumped by the third and the second harmonics of YAG:Nd 31 (20 W Quantel), respectively. The emission spectra were analyzed with a Jobin-Yvon HR-1000 monochrometer and detected by a Hamamatsu R374 photomultiplier. The output was analyzed by a Stanford SR-510 lock-in amplifier and stored in computer memory. The luminescent decay profile was recorded with a Lecroy 9350M oscilloscope (500 MHz) interfaced with a computer.

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00038-5

M. Yin et al. / Journal of Alloys and Compounds 341 (2002) 362 – 365

3. Results and discussion

3.1. Two sites It is well known that for an environment of noncubic symmetry and an odd number of f electrons, a free ion level with angular momentum J splits into J11 / 2 Stark components [11]. From the emission spectrum measured at 12 K under 355-nm laser excitation (the lowest one in Fig. 1(a)), one can deduce that there is not only one site in the crystal as the degenerated number of transition 4 S 3 / 2 → 4 I 15 / 2 is larger than expected (8) for a terminal level 4 I 15 / 2 . Using site-selective excitation, the spectrum can be separated into two spectra arising from the Er 31 in the two sites (named site A and site B, respectively), as shown in Fig. 1(a). The fact that the spectrum of site A plus that of site B form the spectrum obtained by nonselective 355-nm excitation suggests that there are only two sites in the crystal KYF 4 :Er 31 (5%). Fig. 1(a) also tells us that the peak at 561.86 nm only originates in site A and that at 547.76 nm, in site B. Monitoring at these two wavelengths provides excitation spectra for the two sites (Fig. 1(b)). As shown in the figure, the multiplet 4 F 7 / 2

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splits into four sublevels for each site. The appearance of a small peak belonging to the other site shows that there is an energy transfer between the two sites and the transfer is forth and back. Compared with its analogue LiYE 4 :Er, the splitting of 4 F 7 / 2 is much larger [12]. The wavelength values, together with those of 4 S 3 / 2 → 4 I 15 / 2 for Er 31 in the two sites are listed in Table 1. Under red excitation into the 4 F 9 / 2 level, the sample shows bright green emission from the 4 S 3 / 2 state. The up-conversion excitation spectra of the green luminescence (detected at 561.86 nm for site A or 547.76 nm for site B) consist of the same six peaks as in the normal downconversion excitation spectrum (Fig. 2(a)). They can be divided apparently into two groups: three of them in the higher energy side are broad and strong, the other three in the low-energy side are very sharp and weak. We may suppose that the two groups of excitation peaks are corresponding to the 4 I 15 / 2 → 4 F 9 / 2 absorption transitions of 31 the two sites in KYF 4 :Er (5%). Here we call the lower energy one site A and the other one with higher energy, site B. The extra line in Fig. 2(a), denoted by an arrow, maybe originates from excited-state absorption 4 2 I 9 / 2 → G 7 / 2 . Fig. 2(a) shows that the up-conversion is mainly contributed by site B. It is interesting to note that in Fig. 2(c), the green emission from site A and site B of 4 S 3 / 2 level has nearly the same excitation spectra, suggesting that they have the same up-conversion channel. Comparing the three spectra in Fig. 2(a), one can find that the relative intensity of the two sites in normal downconversion and in up-conversion excitation spectra are quite different. In the former spectrum, they have comparative intensity, while in the latter one, the transition intensity in site A is much weaker. Fig. 2(b) is the emission spectrum in the region of 4 F 9 / 2 → 4 I 15 / 2 for KYF 4 :Er 31 (5%) at 12 K. The excitation wavelength used here is 644.14 nm (site B). Excitation to site A gives the same spectrum (not shown in the figure). Table 1 Wavelengths of the 4 I 15 / 2 → 4 F 7 / 2 and 4 S 3 / 2 → 4 I 15 / 2 transitions observed in site-selective excitation and emission spectra in KYF 4 :Er 31 (5%) at 12 K Transitions

Fig. 1. (a) Emission spectra of KYF 4 :Er 31 (5%) at 12 K under selective excitation to site A, site B and under 355-nm excitation. (b) Excitation spectra in the region of 4 I 15 / 2 → 4 F 7 / 2 at 12 K.

Site A (nm)

Site B (nm)

4

I 15 / 2 → 4 F 7 / 2

482.88 484.1 484.8 487.26

483.39 484.25 484.79 486.4

4

S 3 / 2 → 4 I 15 / 2

543.66 545.12 545.5 546.52 550.24

543.24 544.14 545.38 547.76 550.3 552.9 554.14 555.1

553.88 561.86

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M. Yin et al. / Journal of Alloys and Compounds 341 (2002) 362 – 365

Fig. 2. (a) Excitation spectra of KYF 4 :Er 31 (5%) at 12 K. The luminescence is detected at 682.06 nm for site A of 4 F 9 / 2 level, 561.86 nm for site A of 4 S 3 / 2 level and 547.76 nm for site B of 4 S 3 / 2 level. (b) Emission with lex 5644.14 nm. (c) Up-conversion emission spectra of KYF 4 :Er 31 (5%) at 12 K under selective excitation to the two sites in the 4 F 9 / 2 level. Please note the similarity of the two spectra in (c).

Both excitations giving the same spectrum implies that there is an interaction between the two sites. The dynamic study supports the above analysis. As shown in Fig. 3(b), under selective excitation, a very good one exponential fluorescence decay is obtained for Er 31 in site A with a lifetime of 0.23 ms. However, when site B is excited, the decay profile of the luminescence originating in site A rises first and then decays, showing clearly there is an energy transfer from site B (the higher energy site) to site A (Fig. 3(c)). Due to this interaction between the two sites, excitation to site A and site B of the 4 F 9 / 2 level for KYF 4 :Er 31 (5%) results in the same up-conversion luminescence, as shown in Fig. 2(c).

3.2. Up-conversion mechanism Transient measurement shows that under site-selective excitation at 12 K, the 4 S 3 / 2 level of the two sites decays exponentially with lifetimes of 0.53 ms (site A) and 0.82 ms (site B), respectively. One example is given in Fig.

Fig. 3. Luminescence decay after resonance excitation to the 4 S 3 / 2 and 4 F 9 / 2 levels in the crystal of KYF 4 :Er 31 (5%) at 12 K. (a) Site B of 4 S 3 / 2 level with lex 5486.4 nm and lem 5545.38 nm; (b) site A of 4 F 9 / 2 level with lex 5655.9 nm and lem 5682.06 nm; (c) decay curve of site A under excitation to site B of 4 F 9 / 2 level with lex 5652.4 nm and lem 5682.06 nm; (d) decay curve of green emission under red excitation to 4 F 9 / 2 level (dashed line). The solid line is the fitting result with Eq. (2). The excitation pulse used has a width of 10 ns and occurs at t50.

3(a). The values were obtained by a linear fitting process. We would like to point out that the above lifetimes are somewhat different from the result reported in Ref. [10] where after a pulsed excitation into 4 F 9 / 2 (pulse width 500 ms) at 77 K, the green emission from 4 S 3 / 2 level decays with constants of 0.82 ms (site A) and 0.91 ms (site B). To understand the mechanism of the up-conversion, the fluorescence decay curve of the green emission is measured (Fig. 3(d), dashed line). Considering the energy level structure of Er 31 ions [13], together with the fact that during the up-conversion measurement the emission from the 2 H 9 / 2 level is not detected, it is reasonable to assume that the following up-conversion excitation is responsible for the phenomenon (Fig. 4) 4

F 9 / 2 , 4 F 9 / 2 → 4 F 7 / 2 , 4 I 11 / 2

According to the model, we can write the following kinetics equations

M. Yin et al. / Journal of Alloys and Compounds 341 (2002) 362 – 365

365

N 20 3 k c n S (t) 5 ]]] [exp(2k S t) 2 exp(22k F t)] 2k F 2 k S 5 A[exp(2t /tS ) 2 exp(22t /tF )]

(2)

in which A 5 (N 20 3 k c ) /(2k F 2 k S ), tS 5 1 /k S and tF 5 1 / k F . The best fit (Fig. 3(d), solid line) of the experimental results with Eq. (2) gives parameters tF 5 0.25 ms and tS 5 1.0 ms. Within the experimental error, the former is 4 just consistent with the lifetime of site A in the F 9 / 2 level (Fig. 3(b)) and the latter one is corresponding to the lifetime of site B in the 4 S 3 / 2 level (Fig. 3(a)). The quite good agreement between experiment and theory supports the up-conversion model depicted in Fig. 4. The small misfit in the latter part of the curve predicts that other interactions, for example the energy transfer between the two sites in 4 F 9 / 2 level, may also be involved in the process.

4. Conclusion We have presented evidence that there are only two crystallographic sites for Er 31 in the crystal of KYF 4 :Er 31 (5%). The two sites have different emission spectra in the region of 4 S 3 / 2 → 4 I 15 / 2 , while in the region 4 F 9 / 2 → 4 I 15 / 2 , their emission spectra are the same due to the existence of an energy transfer process. The 4 F 9 / 2 , 4 F 9 / 2 → 4 F 7 / 2 , 4 I 11 / 2 up-conversion excitation results in the green emission from the 4 S 3 / 2 level. Fig. 4. Up-conversion model. The thick lines represent transitions induced by the laser, thin lines represent fluorescence transitions and the dashed lines represent up-conversion excitation.

dn ]S 5 2 k S n S 1 k c n F 3 n F dt dn F ] 5 2 kFnF 2 kcnF 3 nF dt

(1)

Here n S and n F are the ion numbers in levels 4 S 3 / 2 and 4 F 9 / 2 , k S and k F are the decay rates of the two levels, respectively and k c is the transfer rate. However, the experimental result (Fig. 3(b)) shows that the decay of the 4 F 9 / 2 level is a pure one exponential process. It implies that dn F / dt | n F is a linear relation. In other words, compared with the first term, the second term in the above second equation can be neglected. Therefore, under this satisfactory approximation, the second equation can be simplified as dn ]F 5 2 k F n F dt with solution n F (t) 5 N0 exp(2k F t). Then Eq. (1) can be solved as

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