Anomalous decays in Nd3+ doped LaAlO3 single crystal

Journal of Physics and Chemistry of Solids 85 (2015) 102–105

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Anomalous decays in Nd3 þ doped LaAlO3 single crystal A. Bednarkiewicz n, P.J. Dereń, K. Lemański Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 23 February 2015 Accepted 8 May 2015 Available online 12 May 2015

Luminescence decays of metastable Nd3 þ :4F3/2 level were measured and studied in wide of 10–300 K temperature range and compared with the radiative lifetimes derived from standard Judd–Ofelt theory. Nearly resonant cross-relaxation was found to non-radiatively depopulate the metastable level already in 10 K, whereas above 50 K increased reabsorption of radiation and thermalization of higher Stark component of the 4F3/2 level occur, which resulted in the Boltzmann population of this metastable Nd3 þ level and anomalous increase of luminescence decays with temperature increment. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Neodymium Luminescence decays Judd–Ofelt theory Optical properties Perovskites

1. Introduction Due to the practical importance for laser science, spectral properties of neodymium doped materials have been studied extensively for many years. Currently, the Nd3 þ ion is the most widely used activator in insulating crystals. Across many matrices doped with Nd3 þ ions, an optimal balance between doping level and amplification efficiency was established. It was proven, that while increasing Nd3 þ concentration the absorption rises, the photoluminescence intensity rises until optimum Nd3 þ content is reached and then falls down. Accordingly, the luminescence lifetimes is reduced due to parasitic concentration quenching resulted from the cross-relaxation (CR) processes (4F3/2; 4I9/2)-(4I15/2; 4I15/2) [1]. This in turn, prevents the efficient population inversion of the metastable level, causing less efficient laser action. In this paper, we have studied the spectral properties and more specifically temporal behavior of photoluminescence in Nd3 þ doped LaAlO3 single crystal. Until now, spectroscopic properties of LaAlO3 crystals doped with a few Rare Earth (RE3 þ ) ions were investigated. The Eu3 þ , Tm3 þ , Pr3 þ , Ho3 þ and Er3 þ doped LaAlO3 were investigated by Dereń and Krupa [2]; Dereń et al. [3,4] Gocalińska, Dereń et al. [5], Dereń et al. [6,7], Antic-Fidancev, Dereń and Mahiou [8,9] respectively. Spectral and laser properties of neodymium (Nd3 þ ) doped LaAlO3 were investigated by us recently [10]. Previously, the optical properties of similar neodymium doped LaXO3 (X¼ Y, Ga, Al) perovskites were investigated by Garcia-Rubio et al. [11] (Nd3 þ : LaGaO3), Dominiak-Dzik et al. [12] (Nd3 þ :LaGaO3), and by Orera et al. [13] (Nd3 þ :LaGaO3 and NdGaO3). It was shown, that LaGaO3

was able to accept large amount of rare earth ions, which however led to significant concentration quenching of the 4F3/2 luminescence level. Moreover it was shown, that depending on the ions content, Nd3 þ doped perovskites may exhibit a domination of 4 F3/2-4I9/2 transition (NdGaO3) [11] over 4F3/2-4I11/2 or oppositely the 4F3/2-4I11/2 transition dominates over the 4F3/2-4I9/2 transition (for instance in LaGdO3). Our previous studies revealed the second case for 1% Nd doped LaAlO3. Interestingly, Orera et al. estimated quantum efficiency to be close to unity, as long as the Nd3 þ concentration was kept below 1%, to minimize cross-relaxation based quenching. Cross-relaxation is a mechanism depopulating excited states, manifested by the shortening of luminescence lifetimes (LT). While cross relaxation is not directly temperature dependent, the phonon assisted cross relaxation is. Thus, one should expect the concentration quenching to be temperature dependent. One should therefore expect shorter LT at room temperature, where the phonon assisted processes become more significant than at lower temperatures. The experimental luminescence lifetimes vs. temperature behavior observed by us [10] and other authors [11] is however opposite to the expected one. Moreover, the physical mechanisms explaining the experimental results are not satisfying so far. Therefore, in this work we present abnormal luminescence decay times behavior of the Nd3 þ doped single LaAlO3 crystal in the 10–300 K temperature range. The experimental behavior has been quantified by the proposed phenomenological model. The possible explanation was provided together with discussion.

2. Materials and methods n

Corresponding author. E-mail address: [email protected] (A. Bednarkiewicz).

http://dx.doi.org/10.1016/j.jpcs.2015.05.005 0022-3697/& 2015 Elsevier Ltd. All rights reserved.

The LaAlO3:Nd3 þ single crystal was grown by the Czochralski

A. Bednarkiewicz et al. / Journal of Physics and Chemistry of Solids 85 (2015) 102–105

method by Oxide Carbide USA. Neodymium concentration was 1 wt%. The sample was cut of the bulk in a form of a cylinder (∅ ¼5 mm and 15 mm length). The refraction index varied in the range from n ¼2.06 to 2.22 starting from 2400 nm to 350 nm [14]. Howard et al. [15] described the structure of LaAlO3 as monoclinic with space group R-3c (No. 167) at room temperature (a ¼0.537, b¼ 0.537 and c ¼1.315 nm). The lanthanide ion occupies D3 site symmetry and is coordinated by 12 oxygen ions. The luminescence and absorption spectra (Fig. 3a, b) were recorded on Jobin Yvon THR1000 spectrophotometer with resolution better than 0.05 nm and Carry spectrophotometer. The Hamamatsu photomultiplier with R406 characteristics was used together with 1200 holographic grating. The spectra were corrected with spectral response of the system, by dividing the acquired data by a calibrating curve. The calibrating curve was obtained from blackbody emissivity taking into account Planck's law for specific blackbody temperature and parameters of the acquisition. The temperature dependent luminescence lower resolution spectra (on Fig. 3c) were measured with Ocean Optics SD 2000 CCD miniature spectrophotometer with resolution around 0.4 nm. All the luminescence spectra were excited with 514.5 nm of Ar þ laser line, whereas lifetimes (Figs. 1 and 2) were measured either with 20492 cm  1 (488 nm, excitation of 2G9/2 level) line of dye laser excited with XeCl excimer laser νexc ¼32 467 cm  1 (308 nm, 20 ns pulses) or with OPO νexc ¼13 715 cm  1 (729 nm, excitation of 4F7/2 multiplet) pumped by the 3rd harmonics of Nd:YAG laser (10 ns pulses). The emission of 4F3/2 level was monitored at 11583 cm  1 (863 nm). Low temperature measurements were carried with Criocooler APD Cryiogenics HC-2.

103

Fig. 2. Experimental luminescence decay times of the 4F3/2 level and bi-exponential model fitting. The experiment was repeated with two different excitation lines: 729 nm (empty triangles – experiment, thin lines-model) or 488 nm (full circles – experiment, thick line- model). Fitting with Eq. (1) – dotted line and with Eq. (2) – solid line.

3. Results and discussion We decided to evaluate the temperature dependence of luminescence decay behavior of 4F3/2 level of Nd3 þ doped LaAlO3 single crystal. For that purpose the luminescence lifetimes were measured in the 10–300 K temperature range. In order to achieve higher reliability of the results, the experiments were repeated twice in the 10–50 K temperature range. Fig. 1 shows representative 4F3/2-4I9/2 luminescence decay profiles for 11, 77 and 268 K temperatures. The decay profiles obtained at 11 (31/ 456 μs) and 268 K (56/349 μs) demonstrated bi-exponential behavior, while the decay measured at 77 K (344 μs) was a single exponent. The luminescence decay profiles shown on Fig. 1 were used to perform Inokuti-Hirayama modeling with a relationship I(t)/ I0 ¼exp[  t/τ  (C/C0)Γ(1  3/S)(t/τ)3/S], where S equals 6, 8, 10 for

Fig. 1. A comparison of representative 11 K (31/456 μs), 77 K (344 μs) and 268 K (56/349 μs) 4F3/2 luminescence decay profiles under 729 nm excitation.

Fig. 3. A comparison of 4F3/2-4I9/2 emission (a) and 4I9/2- 4F3/2 absorption (b) spectra of Nd3 þ :LaAlO3 at 11 K. The temperature dependence of 4F3/2-4I9/2 emission (0′,1′21,2,3,4) was measured with lower spectral resolution and is shown (c) to explain the re-absorption increase vs. the increasing temperature (as arrows  860 and 880 nm indicate). The spectra at (c) were normalized to the 0′,1′-4 transition (  910 nm).

dipole–acceptor, dipole–quadrupole and quadrupole–quadrupole interaction respectively. The τ is the radiative lifetime and Γ(1  3/ S) is an Euler gamma function. The ln[  ln(I/I0)  t/τ] was plotted against the 1/S ln[t/τ]. The S parameter as well as R ¼C/C0 were then found from the linear fit. From the R and the acceptor concentration C ¼2.17  1020 ion/cm3 the critical distance was derived according to Rc ¼ (3R/4πC)1/3 formula. It was found, that the Rc increases from 7.1, through the 8.2–9.0 Å when increasing temperature from 11, 77 to 268 K. Pure dipole–dipole interaction was found for these temperatures since the S was estimated to be 5.43, 2.97 and 6.58 respectively. For 77 K purely single exponential decay, the S¼ 2.97 cancels the (t/τ)3/S non-exponential component in Inokuti-Hirayama model. The bi-single exponential luminescence decays could be explained by two possible mechanisms i.e. (i) ion–ion interaction (e.g. concentration quenching) and (ii) quenching due to defects and impurities, which both may severely influence the measured luminescence lifetimes. The concentration quenching was found to

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A. Bednarkiewicz et al. / Journal of Physics and Chemistry of Solids 85 (2015) 102–105

Table 1 Examples of possible cross relaxation processes possible in the system. The figures under the multiplets names describe Stark levels involved of the respective multiplet and their energy in [cm  1]. For the 4F5/2 starting multiplet, only a few possible CR’s were shown. (4F3/2

4

0 0 0 1 1 1 1 1

4 3 4 0 0 0 0 4

(11583) (11583) (11583) (11619) (11619) (11619) (11619) (11619)

4

4

( F5/2 0 0 0 0 0

(12 (12 (12 (12 (12

547) 547) 547) 547) 547)

I9/2) (608) (231) (608)

(608)

I9/2)

0 2 2 2 2

-

(135) (135) (135) (135)

-

(4I15/2

4

3 4 5 0 1 2 3 6

4 0 2 3 2 1 0 1

(6015) (6188) (6319) (5618) (5740) (5895) (6015) (6491)

I15/2)

(4I15/2

4

2 3 4 6 7

7 7 6 4 3

(5895) (6015) (6188) (6491) (6667)

(6188) (5618) (5895) (6015) (5895) (5740) (5618) (5740)

I15/2) (6667) (6667) (6491) (6188) (6015)

ΔE [cm  1] –12 8 23 –15 –17 –17 –15 –5 ΔE [cm  1] –15 0 3 3 0

Table 1. The energy mismatch between corresponding energy levels is low, which makes the depopulation non-radiative CR mechanism very efficient. It is also interesting to note an existence of local minimum observed for the long decay components and located at temperatures around 40-50 K. For temperatures higher than 50 K one may note slight increase of luminescence decays from 280 at 50 K to 340 μs at 80 K. It is worth to note, the energy gap between the Stark components in 4F3/2 multiplet i.e. 36 cm  1 corresponds to h⋅c 52 K, (this dependence was calculated from the E = λ and

E = kBT equations, where h and kB are Planck and Boltzmann constants, respectively). Indeed, at around 50 K, we observe the change in the trend, i.e. decays grow for growing temperature (Fig. 2) and the population of higher Stark component of 4F3/2 multiplet becomes noticeable (Fig. 3). Next, the luminescence decays stays approximately constant at around 300 μs, up to room temperature. The temperature dependence of Nd3 þ luminescence lifetimes is usually described with thermally mixed states lifetime (TML): TML τrad (T ) =

be meaningful for Nd3 þ concentrations higher than 1 at% in YAG. Moreover two local site symmetries may exist in these perovskites due to presence of possible defects (like cracks or twins [16]), presence of other lanthanides (e.g. Er3 þ ) impurities and color centers. In the same time, non-radiative multiphonon relaxation, which typically leads to luminescence decay decrease, should be negligible. This is because the 4F3/2–4I15/2 energy separation (4916 cm  1) is over eight times larger than the cut off frequency (  570 cm  1) for optical phonons found in the matrix. The decay profiles exhibit Forster static quenching in the initial stage, which is manifested by a short non-exponential part accompanying the single exponent profile. Such a behavior is typical for cross-relaxation being faster than the energy migration. Our hypothesis is also supported by the presence of phonons [17,18] close to 608, 231 and 135 cm  1, which almost perfectly match the Stark positions of Nd3 þ in LaAlO3. Additionally, a very small energy mismatch between the 4F3/2-4I15/2 and the 4I9/2-4I15/2 transitions exists, which makes the phonon-assisted cross-relaxation quenching very probable. Table 1 demonstrates a few examples of possible cross relaxations emphasizing the negligible energy mismatch of these energy transfer processes. The luminescence decay curves I(t) were analyzed by fitting the experimental decay curves with double exponential model I(t)¼ A1 exp(  t/τ1)þA2 exp(  t/t2)þ constant (Fig. 2). The JO theory predicts radiative lifetime to be around 168 μs for the 4F3/2 level of Nd3 þ doped LaAlO3 [10]. Therefore, at room temperature we could confirm earlier results presenting low nonradiative rates and quantum efficiency close to unity [11] for perovskites. Similar behavior was observed by Garcia-Rubio et al. [11], who found also non-exponential decays for heavier doped samples. Luminescence decays measured at low temperatures present completely different behavior. At 10 K the decay time approaches 360 μs. Such a long luminescence decay times of the 4F3/2 level were already observed in crystals (e.g. Sr1  xNdxAl12  -xMgxO19 monoexponential decays 430 and 280 μs for x ¼0.03 and x ¼0.15 were found [19]) and glasses (e.g. calculated for fluorophosphate glasses [20] to be 350–570 μs depending on the composition). For temperatures between 10 and 50 K the luminescence decays drops rapidly. The depopulation mechanism appearing slightly over 10 K could result from a very small energy mismatch between the 4 F3/2-4I15/2 and the 4I9/2-4I15/2 transitions leading to efficient phonon-assisted cross-relaxation quenching. The other possible CR combinations (based on Stark levels evaluated in ref. [10]) that require assistance of low energy ( o20 cm  1), are presented in

1 + e−ΔE / kT 1/τA + 1/τBe−ΔE / kT

(1)

where ΔE is energy split between thermally mixed Stark levels of the 4F3/2 electron level, the k and T are Boltzmann constant and absolute temperature. The τ1 and τ2 are the lifetimes of the two 4 F3/2 Stark levels. The outcome of TML analysis is not always very satisfying [11]. Also in our case the thermally mixed lifetime does not properly fit the experimental data, anyway the thermalization of the higher Stark level of the 4F3/2 state should be taken into account since the splitting of the state is small (  36 cm  1). When the fitting procedure is restricted to the 10–50 K range, one can find fairly good agreement between theory and experimental data. The Fig. 2 presents the fitting curves extrapolated and plotted up to 300 K (dotted line). At temperatures above 50 K the agreement is not valid anymore and luminescence lifetime increases with the increase of temperature. We suppose the observed behavior could be explained by the energy transfers, that may lead to apparent luminescence lifetimes increase. Similar behavior is known for Yb3 þ doped materials [21]. Since the process is enhanced with increasing temperature, the Boltzmann factor would be therefore necessary to describe the observed behavior. As one may notice on Fig. 3 the intensity of 0′, 1′- 0 Stark–Stark transitions increase essentially just above 45 K. We have, therefore, modified Eq. (1) by adding Boltzmann part (TB):

τrad(T ) =

1 + e−ΔE1/ kT 1/τA + 1/τBe−ΔE1/ kT

+ A⋅e−ΔE2/ kT

(2)

The second component of the Eq. (2) is a Boltzmann factor scaled by an A parameter, describing additional population of the higher Stark components within a multiplet, and τΑ and τΒ are lifetimes of the two Stark components of the 4F3/2 level (separated by ΔE1 ¼36 cm  1). The A parameter has the [s] unit and describes the inverse of reabsorption rate. Based on Eq. (2), one may estimate A value to reach 257, thus the reabsorption rate would be WR ¼0.0039 [s  1] (see Fig. 4). The exponential factor is directly proportional to the measured lifetime, because the value of thermalization energy has a minus sign. Beside the reabsorption, also thermalization of the higher 4F3/2 Stark component has influence on the measured luminescence lifetime. Such modified lifetime has been obtained from the Eq. (2). The assumption was that at 11 K only lowest 4F3/2 Stark energy level is populated and the measured lifetime is equal to τ1. The increasing temperature increases population of the higher 4F3/2 Stark component and

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105

other side on the influence of the cross-relaxation processes, which also become more efficient at higher temperatures.

4. Conclusions The temperature dependence of 4F3/2 level lifetime and luminescence of Nd3 þ doped LaAlO3 were measured and analyzed in terms of three phenomena. Below 50 K thermal mixture of the two Stark components of the 4F3/2 level being non-radiatively depopulated by resonance cross-relaxation processes, which dominate here, while above 50 K reabsorption of radiation as well as the thermalization of the higher 4F3/2 energy level becomes pronounced, leading to abnormal lifetime increase vs. the temperature. Fig. 4. The energy level diagram of Nd3 þ ions in LaAlO3.

Acknowledgments Table 2 A comparison of fitting parameters obtained for the two experimental datasets (λexc ¼729 nm Δ on Fig. 2 and λexc ¼488 nm  on Fig. 2). The asterisks mark the fitting parameters.

Thermally mixed states model

Temperature range [K]

Parameter

10–50

τ1n τ2n ΔE1

Boltzman

50–300

ΔE2n

Thermally mixed states model and Boltzman

10–300

τ2n ΔE1 ΔE2n

Unit

[μs] [μs] [cm  1] 1

λexc ¼ 729 nm Δ



This work was supported by the Polish Committee for Scientific Research (KBN) within the Project number: N N507 372335, which is gratefully acknowledged. The authors want to thank Prof. J.C. Krupa for the sample, Ph.D. P. Solarz for technical assistance with the lifetimes measurements and professor W. Ryba-Romanowski for discussion.

101.2 24.7 36

560.4 179.9 36

References

λexc ¼ 488 nm

[cm

]

5.4

11.8

[μs] [cm  1] [cm  1]

15 36 8.8

118 36 59

modifies an effective luminescence lifetime. For higher temperatures the measured lifetime comes from τ1 as well as from τ2 which balances the non-radiative CR processes. For higher temperatures, the lifetime of the thermalized level becomes saturated, as a result of the population equilibrium of the two closely spaced Stark levels. Similar models, using Boltzmann thermalization equation were described before by other authors, but for different samples and RE ions [22–24] From one hand, energy mismatch between corresponding energy levels is low, making the depopulation non-radiative CR mechanism very efficient. From the other hand, the spectral overlap between respective Stark components (especially the 020′, 1′) increases vs. temperature just above  50 K (Fig. 4). The average difference ΔE2 between emitted and absorbed energy within 4F3/224I9/2 transition (020′, 1′) is low, which was confirmed by data fitting (see Table 2). The radiative and non-radiative energy transfer mechanisms are presented on the Fig. 4. The energy levels were taken from our previous results of LaAlO3:Nd3 þ monocrystal [9]. At the temperature above 50 K, the thermalization of higher Stark 3F3/2 energy level become much more efficient, which results in lengthening the lifetime of 3F3/2-4I11/2 emission from the two Stark components. At about 80 K the higher Stark level is saturated, and the resultant lifetime is balanced on one side by the influence of reabsorption and thermalization processes, from the

[1] A. Bednarkiewicz, D. Wawrzynczyk, M. Nyk, W. Strek, Appl. Phys. B 103 (2010) 847. [2] P.J. Deren, J.C. Krupa, J. Lumin. 102 (2003) 386. [3] P.J. Deren, J. Lumin. 122 (2007) 40. [4] P.J. Deren, P. Goldner, O. Guillot-Noel, J. Alloy. Compd. 461 (2008) 58. [5] A. Gocalinska, P.J. Deren, P. Gluchowski, P. Goldner, O. Guillot-Noel, Opt. Mater. 30 (2008) 680. [6] P. Deren, P. Goldner, O. Guillot-Noel, J. Lumin. 119 (2006) 38. [7] P.J. Deren, J.C. Krupa, J. Alloy. Compd. 380 (2004) 362. [8] E. Antic-Fidancev, P.J. Deren, J.C. Krupa, J. Alloy. Compd. 380 (2004) 376. [9] P.J. Deren, R. Mahiou, Opt. Mater. 29 (2007) 766. [10] P.J. Deren, A. Bednarkiewicz, P. Goldner, O. Guillot-Noel, J. Appl. Phys. 103 (2008) 043102. [11] I. Garcia-Rubio, J.A. Pardo, R.I. Merino, R. Cases, V.M. Orera, J. Lumin. 86 (2000) 147. [12] G. Dominiak-Dzik, W. Ryba-Romanowski, S. Golab, M. Berkowski, Spectrochim. Acta A 54 (1998) 2051. [13] V.M. Orera, L.E. Trinkler, R.I. Merino, A. Larrea, J. Phys. Condens. Mater. 7 (1995) 9657. [14] L. Merker, K.D. Herrington, Appl. Opt. 3 (1964) 1311. [15] C.J. Howard, B.J. Kennedy, B.C. Chakoumakos, J. Phys. Condens. Mater. 12 (2000) 349. [16] G. Koren, A. Gupta, E.A. Giess, A. Segmuller, R.B. Laibowitz, Appl. Phys. Lett. 54 (1989) 1054. [17] T. Inagaki, K. Miura, H. Yoshida, J. Fujita, M. Nishimura, Solid State Ion. 118 (1999) 265. [18] P.X. Zhang, S.J. Huang, H.U. Habermeier, G.M. Zhao, Physica C 364 (2001) 647. [19] H.R. Verdun, D.E. Wortman, C.A. Morrison, J.L. Bradshaw, Opt. Mater. 7 (1997) 117. [20] K. Binnemans, R. Van Deun, C. Gorller-Walrand, J.L. Adam, J. Alloy. Compd. 275 (1998) 455. [21] D.S. Sumida, T.Y. Fan, Opt. Lett. 19 (1994) 1343. [22] G.K. Liu, X.Y. Chen, H.Z. Zhuang, S. Li, R.S. Niedbala, J. Solid State. Chem. 171 (2003) 123. [23] G.K. Liu, H.Z. Zhuang, X.Y. Chen, Nano Lett. 2 (2002) 535. [24] L.Q. Liu, E. Ma, R.F. Li, G.K. Liu, X.Y. Chen, Nanotechnology 18 (2007) 015403.