Gain properties and concentration quenching of Er3+-doped niobium oxyfluorosilicate glasses for photonic applications

Optical Materials 36 (2014) 823–828

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Gain properties and concentration quenching of Er3+-doped niobium oxyfluorosilicate glasses for photonic applications D. Ramachari a, L. Rama Moorthy a,b,⇑, C.K. Jayasankar a a b

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Chadalawada Ramanamma Engineering College, Renigunta Road, Tirupati 517 506, India

a r t i c l e

i n f o

Article history: Received 3 August 2013 Received in revised form 11 October 2013 Accepted 4 December 2013 Available online 31 December 2013 Keywords: Niobium oxyfluorosilicate glasses McCumber’s theory Gain properties Concentration quenching Optical amplifiers

a b s t r a c t Structural and spectral properties of erbium-doped niobium oxyfluorosilicate glasses were characterized by the differential thermal analysis, Raman, infrared, photoluminescence and lifetime measurements. With the increase of Er3+ ions concentration, the lifetime of 4I13/2 level has been decreased due to the concentration quenching effect. The McCumber’s theory was adopted to predict the emission cross-section 4 4 4 4 (rM emi ) of I13/2 ? I15/2 transition from the absorption cross-section (rabs) of I15/2 ? I13/2 transition of Er3+ ions. The gain parameters such as optical gain, gain bandwidth and gain coefficient for amplification were evaluated for the 4I13/2 ? 4I15/2 emission transition (1.54 lm). The Er3+ -doped niobium oxyfluorosilicate glass covers both C (1530–1565 nm) and the L (1565–1625 nm) band regions in the optical communication window. From the results of these investigations, it is concluded that the Er3+-doped niobium oxyfluorosilicate glasses are more useful for photonic device applications. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Rapid development of telecommunication networks stimulates the demand for high bit rate optical transmission systems and integrated optical components. Recently, intensive research has been carried out towards the development of integrated optical amplifiers with wide bandwidth and flat gain which are used in the wavelength division multiplexing (WDM) systems [1]. Silica based erbium (Er3+)-doped fiber amplifier (EDFA) is one of the key devices in the optical communication window around 1.55 lm [2,3]. The near-infrared (NIR) emission of Er3+ ions at 1.55 lm, associated with the 4I13/2 ? 4I15/2 transition has drawn particular interest for use in the telecommunication applications, because of its location in the third optical telecommunication window [4,5]. Most of the EDFA devices are made with silicate-based glass fibers, which have relatively narrow bandwidth emission (<40 nm) resulting narrow gain that limits the transmission capacity of WDM systems [6]. The phonon energy of the host is strongly influenced by fluorides and heavy metal oxides (PbO, Bi2O3, WO3, Nb2O5) [7–9]. However, it is difficult to prepare fluoride glasses owing to their low chemical and mechanical stabilities. When compared with fluoride glasses, the oxide glasses generally have better chemical and mechanical stabilities. The incorporation of heavy metal ⇑ Corresponding author at: Department of Physics, Sri Venkateswara University, Tirupati 517 502, India. Tel.: +91 877 2289472; fax: +91 877 222521. E-mail address: [email protected] (L. Rama Moorthy). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.12.013

oxides such as Nb2O5 into oxyfluorosilicate glasses, enhances the chemical durability and mechanical stability with flexible optical properties and minimizes the phonon energy of the host glasses [10,11]. The present work reports the experimental research on the visible and infrared emissions along with the gain properties of Er3+-doped niobium oxyfluorosilicate (NKZLSEr) glasses. The differential thermal analysis (DTA), Raman and infrared (IR) absorption studies were carried out to investigate the glass transition temperature (Tg) and structure of the host glass. The decrease of 4 I13/2 level lifetime with the increase of Er3+ ions concentration has been analyzed based on the effect of concentration quenching. The stimulated emission cross-section (rM emi ) and the gain coefficient G(k) of 4I13/2 ? 4I15/2 emission transition were calculated from the absorption data using the McCumber’s theory [12]. 2. Experimental Er3+-doped glasses with molar composition of 20Nb2O5 + 20K2O + (20  x) ZnF2 + 10LiF + 30SiO2 + xEr2O3 were prepared by melt quenching technique (x = 0.1, 0.5, 1.0 & 2.0 (mol%)) and are named as NKZLSEr01, NKZLSEr05, NKZLSEr10 and NKZLSEr20 based on the concentrations of Er3+ ions. About 15 g batch compositions of analytical grade Nb2O5, K2O, ZnF2, LiF, SiO2 and Er2O3 were thoroughly grinded in an agate mortar and the homogeneous mixtures taken into platinum crucible were kept for melting in an electric furnace at 1230 °C for 1.30 h. The melts were air quenched by pouring onto pre-heated brass mould and annealed at 420 °C for 12 h to remove the thermal stress and strain.

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The DTA measurement was done using a Rigaku Thermoplus TG 8120 thermal analyzer in order to determine the characteristic glass transition (Tg) and peak crystallization (Tp) temperatures. The refractive index (n) of the NKZLSEr10 glass was measured by the Brewster’s angle method using the 650 nm diode laser. The density of the glass was determined by the Archimedes’s principle, using distilled water as an immersion liquid. The Raman spectrum was recorded using the 632.15 nm line of argon ion laser in the range of 400–1400 cm1 by the back scattering geometry using a Horiba Jobin Yvon Lab Ram HR 800 confocal Raman spectrometer. The infrared absorption spectrum of the precursor glass (NKZLS) was measured on a BRUCKER-Alpha spectrometer at a resolution of 2 cm1. Absorption spectrum in the region 350–1800 nm was recorded with a JASCO-V-570 UV–Visible-NIR spectrophotometer with a spectral resolution of 0.1 nm. Excitation and the visible emission spectra were recorded using Jobin Yvon Fluorolog-3 spectrofluorimeter using xenon arc lamp as an excitation source. The NIR emission spectrum was recorded using Dongwoo monochromator (Monora 511i) and the InGaAs detector. The decay curves were measured by exciting the samples with 980 nm radiation of diode laser by monitoring the emission at 1.5 lm and the signal was acquired by a digital oscilloscope (LeCroy 200 MHz Oscilloscope). 3. Results and discussion 3.1. Thermal properties Fig. 1 shows the DTA curve of precursor glass, showing the glass transition (Tg) and peak crystallization (Tp) temperatures at 516 °C and 682 °C, respectively. The temperature difference (DT = Tp  Tg) between the Tp and Tg is normally being used to estimate the glass formation ability as well as its stability. Since the method of fiber drawing is a reheating process, any crystallization during the process increases the scattering loss of the fiber and then degrades its optical properties. To draw the fiber from the bulk glass, it is desirable that the host glass has to posses higher DT values. The present niobium oxyfluorosilicate glass exhibits higher DT value of 166 °C, compared to the other reported glass hosts [13,14]. 3.2. Structural properties Fig. 2a presents the Raman spectrum of precursor glass which shows intense bands of niobium-oxygen units, compared to those

Fig. 2. (a) Raman and (b) infrared absorption spectrum of NKZLS glass.

of other network components [15–17]. The highest intense band at 687 cm1 is the phonon energy of the host glass corresponding to the vibrations of Nb–O–Nb bonds of NbO6 octahedrons and the band at 879 cm1 corresponds to the vibrations of the bonds in the chain Nb–O–Si(–O–K). The band at 1002 cm1 lies outside the range of niobium frequencies and hence, it has been attributed to the stretching vibrations of SiO4 tetrahedrons. Usually in most of the silicate glasses, the Si-O stretching mode is coupled with RE3+ ions even at low SiO2 composition. In the present niobium oxyfluorosilicate glass, the Er3+ ions are surrounded selectively by Nb2O5 rich phase and are not affected by Si–O bonds. The infrared (IR) absorption spectrum of precursor glass shown in Fig. 2b, exhibits the sharp distinctive and characteristic absorption bands in the region of 400–4000 cm1. The band at 536 cm1 is related to the stretching of the Nb–O bonds in the NbO6 octahedrons, while the other band at about 845 cm1 is related to the stretching of the Si–O bonds in the SiO4 tetrahedrons and the band at 2920 cm1 is attributed to asymmetric and symmetric stretching modes of interstitial H2O molecules [16,18]. 3.3. Absorption spectrum and Judd–Ofelt analysis

Fig. 1. Differential thermal analysis curve of NKZLS glass.

Fig. 3 shows the absorption spectrum of NKZLSEr10 glass in the 350–1800 nm region. A total of eight absorption bands centered at 1537, 976, 798, 654, 548, 524, 490 and 448 nm are attributed to the optical transitions of Er3+ ions from the ground state 4I15/2 to the 4 I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2 and 4F5/2 excited states, respectively [19]. The present work oscillator strength (fexp = 4.32 R  109 e(t)dt) are determined from the absorption bands using R the integrated absorption coefficient ( e(t)dt) [20]. According to

D. Ramachari et al. / Optical Materials 36 (2014) 823–828

Fig. 3. Absorption spectrum of NKZLSEr10 glass in the region 400–1800 nm .

the Judd–Ofelt (JO) theory [21,22], the calculated oscillator strength (fcal) of an electric dipole absorption transition from an initial state wJ to a final state w’J’ depends on three JO intensity parameters Xk (k = 2, 4, 6), which is given by the relation 2

fcal ðWJ ! w0 J 0 Þ ¼

8p2 mct ðn2 þ 2Þ 3hð2J þ 1Þ 9n

X

Xk ðWJkU k kW0 J0 Þ

2

ð1Þ

k¼2;4;6

where m is the mass of electron, c is the velocity of light, t is the energy (cm1) of the transition (wJ ? w0 J0 ), h is the Planck’s constant, J is the quantum number of angular momentum of the initial state and n is the refractive index of the medium. The JO intensity parameters (Xk) originate from a static crystal field and have physical significance relevant to the fundamental properties of the host matrix. By employing the least-squares fit to the experimental (fexp) oscillator strengths, the JO intensity parameters of X2 = 5.82  1020 cm2, X4 = 2.35  1020 cm2 and X6 = 2.37  1020 cm2 along with calculated oscillator strengths (fcal) are obtained for the NKZLSEr10 glass. The small root mean square deviation (drms) of ±0.39  106, indicates the good fit between the fexp and fcal values. The evaluated JO intensity parameters follow the order as X2 > X6 > X4 and are compared with those of other reported Er3+-doped systems [19,23–26] in Table 1. It can be seen that, the X2 parameter for the NKZLSEr10 glass is larger than those of silicate, tellurite and phosphate glasses, but nearly close to that of LBTAF glass. The larger X2 parameter in the NKZLSEr10 glass indicates the higher degree of covalence between the Er3+ and O2 ions. On the otherhand, the X4 and X6 parameters are less sensitive to the local environment and hence strongly influenced by the vibrational levels associated with the Er3+ ions bound to the ligand atoms.

825

exhibits a total of six bands centered at 367, 380, 408, 445, 453, and 490 nm from 4I15/2 ground state to the 4G9/2, 4G11/2, 4F9/2, 4F3/2, 4 F5/2, and 4F7/2 states, respectively. Among all the excitation bands, the band at 380 nm is the most intense band and hence used as excitation wavelength to measure the visible luminescence spectrum in the region 450–720 nm as shown in Fig. 4b. The emission spectrum exhibits an intense green band at 551 nm associated with a weak green band at 529 nm corresponding to the 4S3/2 ? 4 I15/2 and 2H11/2 ? 4I15/2 transitions, respectively. In addition to the two green bands, a week red emission band at 663 nm has been observed corresponding to the 4F9/2 ? 4I15/2 transition. The near infrared (NIR) emission spectrum of NKZLSEr10 glass shown in Fig. 5 reveals a strong emission band (kp) at 1.54 lm corresponding to the 4I13/2 ? 4I15/2 transition under 980 nm excitation. The full width at half maximum (FWHM) of the emission band is 93 nm, which is relatively wider compared to those obtained for other hosts as compared in Table 2 [6,9,24,27,28]. The large bandwidth, which is useful for WDM applications [29] is due to the superposition of Stark splitting of the excited and ground states, in addition to the inhomogeneous broadening by the site-to site variation. 3.5. Absorption and emission cross-sections From the absorption band data of 4I15/2 ? 4I13/2 transition at 1.54 lm (Fig. 3), the absorption cross-section (rabs) has been determined by using Beer–Lambert’s equation [30].

3.4. Visible and NIR emission spectra Fig. 4a shows the excitation spectrum of NKZLSEr10 glass by monitoring the emission at 551 nm. The excitation spectrum

Table 1 Comparison of JO parameters (Xk  1020 cm2) of NKZLSEr10 glass with other Er3+doped systems. Glass material

X2

X4

X6

Trend

NKZLSEr10 (Present work) LBTAF [19] SiO2 + Na2O [23] TNE [24] SAL [25] KTFP [26]

5.82 5.89 4.23 5.18 5.59 5.09

2.35 1.10 1.04 2.02 1.42 0.69

2.37 1.47 0.61 0.40 0.87 1.45

X2 > X6 > X4 X2 > X6 > X4 X2 > X4 > X6 X2 > X4 > X6 X2 > X4 > X6 X2 > X6 > X4

Fig. 4. (a) Excitation (kem = 551 nm) and (b) visible emission (kex = 380 nm) spectrum of NKZLSEr10 glass.

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Fig. 6. Absorption (rabs) and emission (rM emi ) cross-sections of transitions in NKZLSEr10 glass. Fig. 5. NIR emision spectrum corresponding to NKZLSEr10 glass.

rabs ¼

2:303 log Nl

I15/2 M 4I13/2

4

I13/2 ? 4I15/2 transition of

I0  I

ð2Þ

where I0 is the incident optical intensity, I is the output optical intensity through the sample, N is the Er3+ ion concentration (ions/cc) and l is the sample thickness. Using the McCumber’s the4 4 ory [12], the emission cross-section (rM emi ) of I13/2 ? I15/2 transition at 1.54 lm has been predicted from absorption cross-section (rabs) by the following relation

rMemi ¼ rabs

4

   Zl hc 1 1 exp  kT kZL k Zu

ð3Þ

where Zl and Zu are the partition functions for the lower (4I15/2) and the upper (4I13/2) levels, respectively and k is the Boltzmann constant, T is the temperature, kZL is the wavelength of the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets (zero-phonon line). The absorption (rabs) and emission (rM emi , McCumber’s theory) cross-sections of 4I15/2 M 4I13/2 transitions in the wavelength region 1400–1625 nm for the NKZLSEr10 glass are shown in Fig. 6. The evaluated maxima of absorption (rabs) and emission (rM emi ) crosssections are found to be 5.92  1021 cm2 and 6.97  1021 cm2, respectively for the NKZLSEr10 glass. The stimulated emission cross-section (remi ) of 7.46  1021 cm2 determined for the 4 I13/2 ? 4I15/2 transition from the emission spectrum (Fig. 5) is in good agreement with that of rM emi , obtained from McCumber’s theory (Fig. 6). From Table 2, it observed that the gain bandwidth (remi  FWHM) of 4I13/2 ? 4I15/2 transition in the NKZLSEr10 glass is larger than those of silicate, germinate, tellurite, and phosphate

glasses [6,9,24,27,28]. The higher values of remi (7.46  1021 cm2), remi  FWHM (694  1028 cm3) and optical gain (remi  sexp, 15.82  1024 cm2 s) obtained for NKZLSEr10 glass pertaining to the 4I13/2 ? 4I15/2 emission transition are more useful for the development of near infrared optical amplifiers at 1.54 lm [31]. 3.6. Gain properties On the basis of absorption (rabs) and emission (rM emi ) cross-sections, one can calculate the wavelength dependence of gain coefficient (G(k)) as a function of population inversion (b) between the upper (4I13/2) and ground (4I15/2) levels to determine the gain property quantitatively. Assuming that the Er3+ ions are either in the ground state or in the upper state, the gain coefficient G(k), can be calculated by the relation [32]

GðkÞ ¼ NEr ½brM emi ðkÞ  ð1  bÞrabs ðkÞ

ð4Þ 3+

where NEr stands for the 1.0 mol% of Er ions concentration (3.72  1020 ions/cm3) and b represents the population inversion, defined as b = (N1/N1 + N2) in which N1 and N2 are the number of ions in the ground (4I15/2) and excited (4I13/2) states, respectively [33]. Fig. 7 shows the wavelength dependence of the gain coefficient for the 4I13/2 ? 4I15/2 emission transition calculated for the NKZLSEr10 glass by varying the b value from 0.0 to 1.0, with an increment of 0.2. It is worth noting that the gain coefficient is positive, when b is >0.4. The bandwidth at b = 0.4 is about 60 nm, which is larger compared to the values found in the conventional silicatebased erbium doped fiber amplifiers (EDFA). Thus, it is concluded that for population inversion (b) > 40%, the NKZLSEr10 glass exhibits a flat gain in the 1440–1620 nm region. This region covers the

Table 2 Comparison of full width at half maximum (FWHM), stimulated emission cross-section (remi) of the 4I13/2 ? 4I15/2 transition along with the gain bandwidth (remi  FWHM) and experimental lifetime (sexp) of the 4I13/2 level of Er3+-doped NKZLS glass with other Er3+-doped glasses. Glass material

FWHM (nm)

remi (1021 cm2)

remi  FWHM (1028 cm3)

sexp (ms)

NKZLSEr10 (Present work) Reported Silicate [6] GaBPGe [9] Tellurite [24] GaBP1 [27] Phosphate [28]

93

7.46

694

2.12

40 58 59 62 56

5.50 10.3 6.88 10.3 6.77

220 597 406 638 379

5.00 3.20 2.50 5.00 7.36

D. Ramachari et al. / Optical Materials 36 (2014) 823–828

Fig. 7. Gain spectra of NKZLSEr10 glass for different values of the population inversion factor of b, ranging from 0.0 to 1.0.

both C (1530–1565 nm) and the L (1565–1625 nm) band regions in the optical communication window and provide more channels in the wavelength division multiplex (WDM) networks. For population inversion of b = 1.0, the achieved gain coefficient of 2.58 cm1 (Fig. 7) is higher than the gain coefficient 2.05 cm1 obtained for SNZE1.0 glass [33]. 3.7. Decay measurements and concentration quenching Fig. 8 shows the decay profiles of the 4I13/2 ? 4I15/2 emission transition at 1.54 lm for different concentrations of Er3+ ions in NKZLSEr glasses recorded under 980 nm excitation. All the decay profiles are well fitted to single exponential function and the experimental decay times (sexp) of 4I13/2 level are found to be 4.25, 2.90, 2.12 and 1.45 ms for NKZLSEr01, NKZLSEr05, NKZLSEr10 and NKZLSEr20 glasses, respectively. This clearly indicates that the quenching of 4I13/2 level lifetime (sexp) with the increase of Er3+ ions concentration is due to the phenomenon of concentration quenching. The lifetime of an excited level (sexp) can be written as

1

sexp

¼

1

sR

Fig. 9. Variation of 4I13/2 excited state lifetime on the Er3+ ions concentration.

non-radiative loss rate refered to as concentration quenching. The contribution to the non-radiative loss (Wnr) due to multiphonon relaxation is negligible, because of the large energy gap (6500 cm1) between the 4I13/2 and 4I15/2 states and also due to the low phonon energy (687 cm1) of the host. The concentration quenching rate (WQ) involves the transfer of the excited-state energy between Er3+ ions that terminates, when the energy traped by other chemical elements impurity defects or OH groups [30]. In order to confirm that the observed quenching is due to OH groups, the authors measured the infrared (IR) absorption spectrum for the host glass as shown in Fig. 2b. No evidence is observed related to the stretching vibrations of free OH groups. This indicates that the decrease of 4I13/2 lifetime with the increase of Er3+ ions is due to the concentration quenching. According to earlier reports [34,35], the decrease in the lifetime (sexp) of 4I13/2 excited state with increasing Er3+ concentration can be described by the following empirical equation:

s0

sexp ¼ 1þ

þ W nr þ W Q

ð5Þ

where sR is the radiative lifetime (4.89 ms) for the NKZLSEr10 glass obtained from the JO theory, Wnr is the non-radiative decay rate due to the multiphonon relaxation and WQ represents an additional

827



N Er Q

p

ð6Þ

where s0 is the ideal lifetime in the limit of zero Er3+ ions concentration, Q is the quenching concentration and p is a phenomenological parameter characterizing the steepness of the luminescence decay. In the absence of multiphonon relaxation, s0 should coincide with srad in the limit NEr ? 0. By using Eq. (6) and considering the value of s0 = 4.94 ms, the parameters of Q (2.71  1020 ions/cm3) and p (0.89) are obtained by fitting the experimental data as shown in Fig. 9. 4. Conclusions

Fig. 8. Decay curves for the 4I13/2 ? 4I15/2 transition for different concentrations of Er3+ ions in NKZLS glasses.

Differential thermal analysis, Raman and infrared studies were carried out to determine the glass transition temperature and structure of precursor glass, respectively. Er3+-doped NKZLSEr glasses were investigated by using the UV–Visible-NIR absorption, infrared emission and luminescence decay measurements applying the JO and McCumber’s theories. The decrease of 4I13/2 level lifetime with the increase of Er3+ ions concentration is due to the effect of concentration quenching. The evaluated radiative parameters such as remi (7.46  1021 cm2), remi  sexp (15.82  1024 cm2 s), remi  FWHM (694  1028 cm3) and G(k) (2.58 cm1) confirm that the NKZLSEr glasses could be potentially useful for the development of wide-band optical fiber amplifiers, eye-safe lasers, frequency up-conversions, sensors and waveguide lasers.

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Acknowledgements The corresponding author, LRM would like to thank the DST, New Delhi for the sanction of major research project Lt. No. SR/S2/LOP-10/2009. Also one of the authors (CKJ) is grateful to DAE-BRNS, Govt. of India for the sanction of major research project (No. 2009/34/36/BRNS/3174) under MoU between S.V. University, Tirupati and BARC, Mumbai. References [1] A.Q. Le Quang, V.G. Truong, A.M. Jurdyc, B. Jacquier, J. Zyss, I. Ledoux, J. Appl. Phys. 101 (2007) 023110–23117. [2] E. Desurvire, Erbium-doped Fiber Amplifiers: Principles and Applications, Wiley, New York, 1994. [3] J.S. Wilkinson, M. Hempstead, Solid State Mater. Sci. 2 (1997) 194–199. [4] F. Rivera-Lopez, P. Babu, L. Jyothi, U.R. Rodriguez-Mendoza, I.R. Martin, C.K. Jayasankar, V. Lavin, Opt. Mater. 34 (2012) 1235–1240. [5] J.D.B. Bradley, M. Pollnau, Lasers Photon Rev. 5 (2011) 368–403. [6] X.L. Zou, T. Izumitani, J. Non-Cryst. Solids 162 (1993) 68–80. [7] R. Badla, L.M. Lacha, J. Fernandez, J.M. Fernandez-Navarro, Opt. Mater. 21 (2003) 1771–1775. [8] H. Ticha, L. Tichy, Optoelectron. Adv. Mater. Rapid Commun. 5 (2011) 1277– 1281. [9] Y. Gangfeng, L. Tao, J. Rare Earths 26 (2008) 924–927. [10] S. Shen, A. Jha, Opt. Mater. 25 (2004) 321–333. [11] J. Wang, H. Song, X. Kong, H. Peng, B. Sun, B. Chen, J. Zhang, W. Xu, J. Appl. Phys. 93 (2003) 1482–1486. [12] D.E. McCumber, Phys. Rev. A 134 (1964) 299–306. [13] L.F. Santos, L. Wondraczek, J. Deubener, R.M. Almeida, J. Non-Cryst. Solids 353 (2007) 1875–1881.

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