Composition dependent structural and optical properties of Sm3+ doped boro-tellurite glasses

Composition dependent structural and optical properties of Sm3+ doped boro-tellurite glasses

Journal of Luminescence 131 (2011) 2746–2753 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/lo...

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Journal of Luminescence 131 (2011) 2746–2753

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Composition dependent structural and optical properties of Sm3 þ doped boro-tellurite glasses K. Maheshvaran a, K. Linganna b, K. Marimuthu a,n a b

Department of Physics, Gandhigram Rural University, Gandhigram 624302, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 February 2011 Received in revised form 29 May 2011 Accepted 25 June 2011 Available online 6 July 2011

Boro-tellurite glasses with the composition (69–x)H3BO3 þ xTeO2 þ 15MgCO3 þ 15K2CO3 þ 1Sm2O3 (where x ¼ 0, 10, 20, 30 and 40 wt%) doped with trivalent samarium have been prepared and their structural and spectroscopic behavior were studied and reported. The FTIR spectra reveal the presence of BO3 and BO4 non-bridging oxygen as well as strong OH  bonds in the prepared glasses. Through the optical absorption spectra, Judd–Ofelt intensity parameters (Ol, l ¼ 2, 4 and 6) have been evaluated and the same is in turn used to predict radiative properties such as radiative transition probability (A), stimulated emission cross-section (sEP ) and branching ratios (bR) for the excited levels of Sm3 þ ions corresponding to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2-6H11/2 transitions. Structural and spectral dependence of the Sm3 þ ions due to the compositional changes have been examined and reported. The lifetime of the 4G5/2 level is found to be non-exponential for all the prepared glasses indicating a cross-relaxation among the Sm3 þ ions. The structural and spectroscopic results corresponding to compositional changes have been compared with the similar studies and reported. & 2011 Elsevier B.V. All rights reserved.

Keywords: Samarium ion Photoluminescence Judd–Ofelt analysis Emission cross-section Radiative lifetime

1. Introduction The optical studies of rare earth (RE) doped glasses draw much attention due to their wide applications in areas such as solid state lasers, solar concentrators, optical detectors, waveguide lasers, optical fibers, sensors, display monitor, optical data storage and undersea optical communications [1–3]. Of all the glasses, borate glass has importance due to its special physical properties like high transparency, low melting point, high thermal stability and good rare earth ions solubility [3]. Among the oxide glasses, tellurite glasses possess encouraging properties such as good mechanical strength, chemical durability, low process temperature, high dielectric constant and excellent transmission in the visible and IR wavelength regions. They also have lower phonon energy and larger refractive index, compared to other oxide glasses [4]. The origin of the extraordinary nonlinear optical properties of TeO2 based glasses is attributed to the high hyper polarity of a lone electron pair related to the 5s orbital of the tellurium atom [5]. The tellurite based glasses possess weaker Te– O bonds, which can be easily broken and this is advantageous for accommodating rare earth ions and heavy metal oxides [6]. The pure TeO2 is a conditional glass former and requires fast quenching to form glass. The presence of TeO2 in the alkali borate glass

n

Corresponding author. Tel.: þ91 451 2452371; fax: þ91 451 2454466. E-mail address: [email protected] (K. Marimuthu).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.06.047

matrix decreases its hygroscopic nature, improves the glass quality and enhances the IR transmission [7]. Absorption and emission analysis of lithium boro-tellurite glasses were studied and reported by Sooraj Hussain et al. [8]. Rada et al. [9] reported the structure of the TeO2–B2O3 glasses through IR and DFT studies. Jamalaiah et al. [10] explored and reported that Sm3 þ ion with 4f5 electronic configuration exhibit a strong orange red fluorescence in the visible region. Supriya and Buddhudu [11] reported the luminescence studies of Sm3 þ , Dy3 þ :TeO2–B2O3– P2O5–Li2O glasses. This paper reports the structural and optical studies on Sm3 þ doped boro-tellurite glasses. Through the XRD and FTIR measurements the structure of boro-tellurite glasses have been studied and reported. Through the absorption, luminescence and lifetime measurements, optical behavior of the prepared glasses have been discussed and compared with the similar studies. The Judd–Ofelt theory has been used to explore the radiative properties of the Sm3 þ doped boro-tellurite glasses and the results were compared with similar reported systems.

2. Experimental Sm3 þ doped boro-tellurite (BnTS, where n refers 0, 1, 2, 3 and 4) glasses were prepared by the following conventional melt quenching technique. The chemicals used are H3BO3, TeO2, MgCO3, K2CO3 and Sm2O3 of high purity (99.99%) analytical grade.

K. Maheshvaran et al. / Journal of Luminescence 131 (2011) 2746–2753

The batch composition (in wt%) of the Sm3 þ doped boro-tellurite glasses and their codes are as follows:

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3. Results and discussion 3.1. XRD and FTIR spectral analysis

69B2O3 þ 0TeO2 þ15MgO þ15K2Oþ 1Sm2O3 ————————————— B0TS

The XRD pattern of the Sm3 þ :B0TS glass shown in Fig. 1 exhibits broad scattering at lower angles, which is the characteristic long range structural disorder and confirms the amorphous nature of the prepared glasses. The FTIR spectra of the Sm3 þ doped boro-tellurite glasses shown in Fig. 2 contain several peaks broad or moderate in bandwidth specifying the local structure and their peak assignments are shown in Table 2. The bands found in the region 3375–3560 cm  1 belong to hydroxyl groups due to OH stretching vibrations. The peaks observed between 2800 and 2900 cm  1 are due to the characteristic of hydrogen bond in the glasses. The boron atoms are connected to the three oxygen atoms (BO3 units) and the band around 1658 cm  1 reveal the stretching vibration of borate triangles [12–14]. All the spectra exhibit peaks around 1463 cm  1, which are due to B–O  vibrations attached to the large segments of borate network. The bands around 1020 cm  1 is attributed to B– O bond stretching vibrations in BO4 tetrahedra from tri-, tetra- and penta-borate groups [9,14,15]. The band around 839 cm  1 is attributed to B–O bond stretching vibration in BO4 tetrahedra from diborate groups. The band around 717 cm  1 is attributed to Te–O bending vibration in TeO3 and TeO6 units. The peak positions around 450 cm  1 reveal the Te–O–Te or O–Te–O linkage bending vibrations [16,17] and all these assignments are in good agreement with the reported literature [9,14,16].

59B2O3 þ 10TeO2 þ 15MgOþ15K2Oþ1Sm2O3 —————————————B1TS 49B2O3 þ 20TeO2 þ 15MgOþ15K2Oþ1Sm2O3 —————————————B2TS 39B2O3 þ 30TeO2 þ 15MgOþ15K2Oþ1Sm2O3 —————————————B3TS 29B2O3 þ 40TeO2 þ 15MgOþ15K2Oþ1Sm2O3 —————————————B4TS

60

50

Counts

The selected composition of about 20 gm was thoroughly mixed and ground in an agate mortar to obtain homogeneous mixture. The mixture is taken into a porcelain crucible and heated at 850 1C in an electrical furnace for 45 min. The melt was poured on to a preheated brass plate and pressed by another brass plate to obtain uniform thickness of about 1.5 mm. In order to remove the strain and improve the mechanical strength, the glass samples were annealed for 7 h in another furnace maintained at 350 1C and allowed to reach the room temperature. The prepared glasses were well polished on both sides to obtain optical quality glasses. The amorphous nature of the prepared glasses was confirmed through X-ray diffraction studies using JEOL 8530C X-ray diffractometer employing CuKa radiation. The FTIR spectra of the glasses were recorded using Perkin-Elmer Paragon 500 spectrometer with a spectral resolution of 4 cm  1 following the KBr pellet technique. Optical absorption spectra were recorded in the wavelength region 350–1750 nm using CARY 500 spectrophotometer with a spectral resolution of 70.1 nm. The luminescence spectra of the prepared glasses were measured between the wavelength 500–750 nm using Perkin-Elmer LS55 spectrometer with a spectral resolution of 71.0 nm. The lifetime measurements were carried out using a Sciencetech modular spectrophotometer using a xenon flash lamp as an excitation source. All these measurements were carried out at room temperature only. The refractive indices of the prepared glasses were measured through Abbe refractometer at sodium wavelength using mono bromonaphthalin as a contact liquid. The density of the prepared glasses was measured following Archimedes method using xylene as an immersion liquid. The physical properties of the Sm3 þ doped boro-tellurite glasses are presented in Table 1.

40

30

20

10 10

20

30

40

50

60

70

80

2θ Fig. 1. XRD Pattern of the Sm3 þ :B0TS glass.

Table 1 Physical properties of the Sm3 þ -doped boro-tellurite glasses. Sl. No.

Physical properties

B0TS

B1TS

B2TS

B3TS

B4TS

1 2 3 4 5 6 7 8 9 10 11

Density, r (g/cm3) Sample thickness (mm) Refractive index, nd (589.3 nm) Rare earth ion concentration, N (1020 ions/cm3) Polaron radius, rp (A1) Inter ionic distance, ri (A1) Field strength, F (1014 cm  2) Electronic polarizability, ae (10  22 cm3) Molar refractivity, Rm (cm3) Dielectric constant (e) Reflection losses, R (%)

2.93 1.50 1.594 4.243 5.359 13.302 1.695 1.909 8.376 2.541 5.244

3.42 1.58 1.626 4.431 5.284 13.116 1.743 1.908 7.726 2.644 5.683

4.17 1.52 1.682 4.890 5.114 12.692 1.862 2.041 6.664 2.829 6.466

4.77 1.54 1.701 5.107 5.040 12.510 1.916 1.809 5.887 2.893 6.736

5.59 1.55 1.725 5.506 4.916 12.200 2.015 1.722 5.201 2.976 7.079

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B3TS

3.2. Absorption spectra

B4TS

The absorption spectra of the Sm3 þ -doped boro-tellurite glasses were recorded at room temperature in the wavelength range 350 1750 nm and as a representative case the absorption spectrum of the Sm3 þ :B0TS glass is shown in Fig. 3. The spectrum exhibit several in-homogeneously broadened bands due to f–f transitions from the ground 6H5/2 state to various excited states. The absorption spectra of the Sm3 þ :BnTS glasses are similar to other reported Sm3 þ -doped glasses [18–20] and aquo-ion [21]. The absorption bands of the Sm3 þ ions are classified into two groups. The first one is lower energy group, which lies between 850 2000 nm and the second one is higher energy group, which lies between 350 600 nm. It is observed from Fig. 3 that, the more intense transitions of Sm3 þ ions are found in the near infrared (NIR) region and the assignment in the UV–vis region is not easy due to the overlap of different 2S þ 1LJ levels. The transition from the ground 6H5/2 state to 6H and 6F terms are spin allowed (DS ¼0) and hence, the transition lying in the NIR region is intense. The spin allowed 6H5/2-6P3/2 transition in the

Transmission %

B2TS

B0TS B1TS

2000

1000

4000

3000

Wavenumber

(cm-1)

Fig. 2. Infrared spectra of the Sm3 þ -doped boro-tellurite glasses.

Table 2 Band positions (in cm  1) of FTIR spectra of the Sm3 þ -doped boro-tellurite glasses. Sl.No.

B0TS

B1TS

B2TS

B3TS

B4TS

Assignments

1 2 3 4 5 6 7 8 9 10

– 3419 2923 2848 – 1411 1021 – – –

– 3411 2925 2854 – 1456 1020 – – –

3556 3371 2925 2854 1635 1463 1017 840 710 438

3557 3374 2925 2858 1631 1436 1009 839 709 454

3555 3371 2925 2858 1658 1432 1005 837 706 426

Fundamental stretching of OH group O–H stretching vibrations Hydrogen bonding Hydrogen bonding Stretching vibration of borate triangles B–O  vibrations of the units attached to the large segment of borate network B–O bond stretching vibrations in BO4 tetrahedra from tri-, tetra- and penta-borate groups B–O bond stretching vibration in BO4 tetrahedra from diborate groups Te–O bending vibration in [TeO3] and [TeO6] units Te–O–Te or O–Te–O linkage bending vibrations

4

L13/2+4F7/2+6P3/2

4

D3/2

6

Absorption coefficient (cm-1)

F7/2

6

P7/2 6 4

4I

11/2 +

19/2 4I

5/2

6F

3/2

6H 4M

15/2 +

15/2 +

4I

9/2

13/2

6P

350

F9/2

G9/2 + 4M17/2 + 4F5/2

4M

4L 15/2

6F

6F

5/2

400

450

11/2

500 1000 Wavelength (nm)

1200

1400

Fig. 3. Absorption spectrum of the Sm3 þ :B0TS glass in the UV–VIS–NIR region.

1600

6F

1/2

K. Maheshvaran et al. / Journal of Luminescence 131 (2011) 2746–2753

UV–vis region is also found to be more intense than the other transitions. According to Jorgensen and Judd [22], the position and intensity of some of the electric dipole transitions of the RE ions are found to be sensitive to the environment around the RE ions and such transitions are termed as hypersensitive transitions. In the present work 6H5/2-6F1/2 and 6H5/2-6F3/2 are the hypersensitive transitions and studied as a function of glass composition to know the nature of the R–O bond. The absorption bands at 6553, 6780, 7294, 8157, 9285, 10638, 21142, 21739, 22831, 23641, 23981, 24817, 25707, 26667 and 27778 cm  1 arises corresponding to the 6H5/2-6H15/2 þ 6F1/2, 6F3/2, 6F5/2, 6F7/2, 6F9/2, 6F11/2, 4I11/2 þ 4M15/2 þ 4I9/2, 4I13/2, 4G9/2 þ 4M17/2 þ 4F5/2, 6P5/2, 4M19/2, 4L13/2 þ 4F7/2 þ 6P3/2, 4L15/2, 6P7/2 and 4D3/2 transitions, respectively. Nephelauxetic ratios and bonding parameters (b and d) have been calculated for the Sm3 þ :BnTS glasses using the following relation: [23]

b ¼ nc =na

ð1Þ 1

where nc is the wavenumber (in cm ) of a particular transition of the RE ion under investigation and na is the wavenumber (in cm  1) for the corresponding transition of an aquo-ion [21]. From the average values of b (referred as b) the bonding parameter d is calculated using the following formula:[23]



1b

b

100

ð2Þ

The bonding will be covalent or ionic depending on the positive or negative sign of d. The bonding parameters (b and d) of the prepared Sm3 þ doped boro-tellurite glasses are presented in Table 3. It is observed from Table 3 that, the d value of the prepared glasses possess ionic nature and the ionic nature gradually decreases with the increase in TeO2 content. Similar ionic nature have been observed in other reported Sm3 þ -doped glasses [18,24–26]. 3.3. Oscillator strengths, Judd Ofelt analysis and radiative properties The radiative transitions associated within the 4f5 configuration of the Sm3 þ ion have been analyzed using the Judd–Ofelt (JO) theory [27,28]. According to JO theory the calculated oscillator

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strengths (fcal), induced electric-dipole transitions from initial state (CJ) to the final state (C0 J0 ) depends on three parameters Ol (l ¼2, 4 and 6) as #  " 2 X 8p2 mcn ðn þ 2Þ2 O ðCJ:U l :C0 J 0 Þ2 ð3Þ fcal ¼  l ¼ 2,4,6 l 3hð2J þ1Þ 9n where n is the wavenumber (cm  1) of the transition from ground state (CJ) to excited state (C0 J0 ), n is the refractive index, c is the velocity of light in vacuum, m is the mass of an electron, (n2 þ2)2/ 9n is the Lorentz local field correction accounts for the dipole– dipole transition, J is the total angular momentum of the ground state, Ol is the JO intensity parameter and kUlk2 are the squared doubly reduced matrix elements of the unit tensor operator, which are evaluated from the intermediate coupling approximation for a transition from CJ to C0 J0 . The experimental oscillator strength (fexp) is directly proportional to the area under the absorption curve and is defined as [29,30] Z Z 2:303mc2 fexp ¼ eðnÞdn ¼ 4:318  109 eðnÞdn ð4Þ 2 N pe where N is the Avagadro’s number, e(n) is the molar absorptivity of the band at a wavenumber n(cm  1), e is the charge of an electron. The experimental and calculated oscillator strengths of the various absorption bands along with srms deviation are presented in Table 3. The srms deviation gives the quality of the fit between experimental and calculated oscillator strengths. The srms values are found to be 70.542, 70.551, 70.307, 70.302 and 70.422 corresponding to B0TS, B1TS, B2TS, B3TS and B4TS glasses, respectively. The oscillator strength values of the Sm3 þ :BnTS glasses are similar to the other reported Sm3 þ doped glasses [24,31–33]. The JO intensity parameters are host dependent and are important in exploring the glass structure and transition rate of the rare earth ion energy levels. The trends of the JO parameters are found to be in the order of O4 4 O6 4 O2 for the prepared Sm3 þ :BnTS glasses. The O2, JO intensity parameter of the Sm3 þ doped glasses are found to be associated with the covalency, structural change and symmetry of the ligand field around Sm3 þ site [34]. The JO intensity parameters O4 and O6 refer to the viscosity of the glass matrix and dielectric of the media, which are affected by the vibronic transitions of the RE ions bound to the ligand atoms [35,36]. The spectroscopic quality factor X¼ O4/O6 is used to characterize the quality of the prepared glasses [10,33,37,38]. The JO parameters values, trends and

Table 3 Experimental, calculated oscillator strengths (  10  6) and bonding parameters (b and d) of the Sm3 þ -doped boro-tellurite glasses. Transition

6

6

H15/2 þ F1/2 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2 4 I11/2 þ 4M15/2 þ 4I9/2 4 I13/2 4 G9/2 þ 4M17/2 þ 4F5/2 6 P5/2 4 M19/2 4 L13/2 þ 4F7/2 þ 6P3/2 4 L15/2 6 P7/2 4 D3/2 N 6

srms b d

B0TS

B1TS

B2TS

B3TS

B4TS

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

0.329 0.602 1.666 3.961 2.450 0.447 0.442 0.104 0.581 0.111 0.127 5.828 0.088 1.300 1.155

0.089 0.887 1.893 3.811 2.660 0.439 0.801 0.496 0.181 0.500 0.000 3.974 0.129 1.362 0.839

0.389 0.708 1.746 3.179 2.220 0.241 0.466 0.161 0.396 0.103 0.197 5.955 0.184 1.286 1.106

0.175 0.967 1.925 3.246 2.136 0.346 0.637 0.404 0.148 0.508 0.000 3.977 0.102 1.083 0.845

0.361 0.667 1.851 3.452 2.008 0.296 0.413 0.131 0.376 0.158 0.200 4.854 0.119 1.169 1.099

0.150 0.951 1.913 3.306 2.206 0.358 0.662 0.417 0.153 0.504 0.000 3.959 0.105 1.120 0.842

0.278 0.711 1.714 2.800 1.842 0.274 0.433 0.122 0.409 0.159 0.180 4.515 0.146 1.240 1.193

0.147 0.881 1.773 2.822 1.832 0.294 0.542 0.348 0.127 0.467 0.000 3.641 0.087 0.917 0.778

0.259 0.468 1.617 2.304 1.350 0.221 0.343 0.178 0.439 0.110 0.220 4.785 0.122 1.312 1.123

0.035 0.767 1.685 2.314 1.394 0.220 0.403 0.273 0.098 0.450 0.000 3.479 0.064 0.691 0.748

15 7 0.542 1.0117

15 7 0.551 1.0092

15 7 0.307 1.0089

15 70.302 1.0077

15 7 0.422 1.0072

 1.1578

 0.9110

 0.8834

 0.7667

 0.7137

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the spectroscopic quality factor of the prepared Sm3 þ :BnTS glasses are quite comparable to the other reported Sm3 þ -doped glass systems and are presented in Table 4. It is observed from the table that, among the prepared Sm3 þ :BnTS glasses, B4TS appeared to be a better optical glass. From the obtained JO intensity parameter values, the other radiative parameters such as transition probability (A), stimulated emission cross-section (sEP ) and branching ratios (bR) for the excited levels of Sm3 þ ions corresponding to the 4G5/26 H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2-6H11/2 transitions have also been calculated referring earlier reports [18,24,39,40] and the results are presented in Table 5. The radiative properties can be calculated through the Ol values. The radiative transition probability (A) is expressed as [24] ! 64p4 nðn2 þ 2Þ2 0 0 3  Sed þ n Smd AðCJ, C J Þ ¼ Aed þ Amd ¼ 3 9 3hl ð2J þ 1Þ

electric dipole transitions, and n3 is the local field correction for magnetic dipole transitions. Sed and Smd correspond to electric and magnetic-dipole line strengths, which are expressed as X Sed ¼ e2 O ðCJ:U l :C0 J 0 Þ2 ð6Þ l ¼ 2,4,6 l and Smd ¼

Table 4 Judd–Ofelt (  10  20 cm2) parameters, trends of Ol parameters and spectroscopic quality factor (O4/O6) of the Sm3 þ -doped boro-tellurite glasses and reported Sm3 þ glasses. Glass composition

B0TS B1TS B2TS B3TS B4TS P2O5–Al2O3–Na2O Glass-C Glass-F Glass-G L5FBS PbO–PbF2–B2O3 PbO–PbF2 N3BS PKFBASm10

JO parameters

O2

O4

O6

0.200 0.466 0.374 0.372 0.066 1.370 1.180 1.080 1.460 2.340 1.280 1.160 2.560 1.500

3.478 3.435 3.273 2.980 2.819 3.070 4.070 4.700 4.540 7.540 2.780 2.600 3.450 3.750

3.023 2.327 2.312 1.867 1.362 1.720 3.250 3.850 4.120 5.400 1.970 1.400 2.720 1.890

Trends of Ol

O4/O6

Reference

O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2 O4 4 O6 4 O2

1.151 1.476 1.415 1.596 1.785 2.070 1.252 1.220 1.020 1.396 1.411 1.860 1.268 1.984

Present Present Present Present Present [36] [32] [32] [32] [24] [37] [38] [18] [2]

ð7Þ

The total radiative transition probability AT is given as the sum of A (CJ,C0 J0 ) terms calculated over all terminal levels and is given as X AT ðCJÞ ¼ AðCJ, C0 J 0 Þ ð8Þ The radiative lifetime (tR) of the CJ level is given by

ð5Þ where Aed is electric-dipole and Amd is magnetic-dipole contributions, respectively. n(n2 þ2)2/9 is the local field correction for the

e2 h2 O ðCJ:L þ 2S:C0 J 0 Þ2 16 p2 m2 c2 l

tR ðCJÞ ¼ ½AT ðCJÞ1

ð9Þ

The branching ratio (bR) can be obtained from the equation given below

bR ðCJ, C0 JÞ ¼

AðCJ, C0 J0 Þ AT CJ

ð10Þ

The relative values of the branching ratios can be obtained from the areas under the emission curves. The peak stimulated emission cross-section (sEP ) can be calculated using the expression

sEP ¼

l4p A 8pcn2 Dleff

ð11Þ

where n is the refractive index, lp is the emission transition peak wavelength and Dleff is the effective line width of the transition and is given by

Dleff ¼

1

Z

Imax

IðlÞdl

ð12Þ

where I is the fluorescence intensity and Imax is the intensity at band maximum.

Table 5 Radiative transition probability (A, s  1), peak stimulated emission cross-section (sEP  10–22 cm2), experimental and calculated branching ratios (bR), O/R ratio, experimental (texp) and radiative (trad) lifetimes (ms) and quantum efficiency (Z, %) of the Sm3 þ :BnTS glasses and other reported Sm3 þ glasses. Transition

Parameters

4

A

4

4

4

G5/2- 6H5/2

G5/2- 6H7/2(Orange)

G5/2- 6H9/2 (Red)

G5/2-6H11/2

(4G5/2-6H7/2)/(4G5/2-6H9/2)

BOTS

B1TS

B2TS

B3TS

B4TS

N5BS [18]

L5FBS [24]

LGT10 [39]

PTBSm10 [40]

sEP

20.94 1.19

22.37 1.55

24.48 1.06

24.82 1.56

30.36 1.66

25.65 0.809

25.18 0.703

48.77 1.573

13.85 5.10

bR (Exp) bR (Cal)

0.101 0.082

0.079 0.087

0.084 0.089

0.074 0.096

0.087 0.107

0.108 0.045

0.170 0.054

0.155 0.076

0.21 0.12

A

sEP

112.58 7.43

122.83 9.24

133.52 9.19

122.28 8.59

128.48 9.61

246.47 8.741

216.05 5.744

296.15 9.209

54.70 20.00

bR (Exp) bR (Cal)

0.440 0.476

0.548 0.506

0.449 0.483

0.457 0.473

0.599 0.578

0.529 0.436

0.546 0.466

0.542 0.461

0.48 0.49

A

sEP

51.05 5.88

56.69 5.59

59.28 6.11

46.93 4.80

66.80 6.89

174.92 9.739

125.76 3.814

159.64 6.727

19.18 9.00

bR (Exp) bR (Cal)

0.316 0.201

0.376 0.221

0.416 0.215

0.437 0.216

0.418 0.198

0.335 0.309

0.244 0.271

0.286 0.248

0.27 0.17

A

sEP

28.06 2.70

29.90 5.63

32.16 4.30

29.65 3.35

31.09 6.56

61.429 7.627

55.43 1.399

79.64 4.145

10.58 5.80

bR (Exp) bR (Cal)

0.036 0.119

0.027 0.116

0.051 0.116

0.033 0.115

0.055 0.119

0.025 0.109

0.038 0.119

0.018 0.124

0.03 0.09

O/R

1.732 1142 4228 27

1.381 994 3865 26

1.078 1069 3618 30

1.045 1348 3893 35

1.054 1888 3936 48

– 2175 2852 76

– – 2154 –

– 795 1550 51

– 2350 8940 –

texp (ms) trad (ms) Z(%)

K. Maheshvaran et al. / Journal of Luminescence 131 (2011) 2746–2753

3.4. Luminescence spectra The luminescence spectra of the Sm3 þ :BnTS glasses recorded in the wavelength region 500–750 nm with 488 nm excitation at room temperature is shown in Fig. 4, which is similar to other reported Sm3 þ -doped glasses [10,18,39]. The luminescence spectra exhibits four emission peaks at 17750, 16663, 15467 and 14074 cm  1 corresponding to 4G5/2-6H5/2, 6H7/2, 6H9/2 and 6H11/2 transitions, respectively. Among the four observed bands, the 4 G5/2-6H7/2 transition is more intense and 4G5/2-6H11/2 transition is found to be weak in intensity. The 4G5/2-6H7/2 and 4 G5/2-6H9/2 transitions correspond to the orange and red luminescence, respectively, and in the present work all the glasses exhibit strong orange and red luminescence. The orange to red (O/R) luminescence intensity ratio has been calculated for the studied glasses and is shown in Table 5. This ratio gives more information about covalency and symmetry of the studied glasses. The 4G5/2-6H7/2 transition is magnetic dipole allowed but electric dipole dominated with the selection rule DJ¼ 71, and the 4 G5/2-6H9/2 band is purely electric dipole transition. The intensity ratio between the electric dipole and magnetic dipole transition is used to measure the symmetry of the local environment of the trivalent RE ions. Higher intensity of the electric dipole transition exhibit more asymmetric nature [41]. In the present work, spectral intensity of the 4G5/2-6H9/2 (ED) transition of the Sm3 þ ion is more than the 4G5/2-6H5/2 (MD) transition, which indicates that the asymmetric nature is predominant in the prepared glasses, which is further confirmed through the lower O2 JO parameter values of the titled glasses compared to the reported Sm3 þ -doped glasses [2,18,24,32,36–38]. The radiative properties of the prepared Sm3 þ :BnTS glasses are compared with similar Sm3 þ -doped glasses [18,24,39,40] and presented in Table 5. The experimental branching ratio (bR) values for the different transitions have been obtained from the relative areas under the emission peaks of the luminescence spectra and the same is compared with the calculated branching ratio values derived from the JO theory. The experimental and calculated bR values for the Sm3 þ :BnTS glasses are found to follow the order 4G5/2 -6H7/2 4 6H9/2 4 6H5/2 4 6H11/2. The stimulated emission cross-section value is found to be higher for the 4 G5/2-6H7/2 transition compared to other excited state transitions for all the prepared glasses. The stimulated emission crosssection values are found to be 7.43, 9.24, 9.19, 8.59 and 9.61 for 4 G5/2-6H7/2 transition corresponding to B0TS, B1TS, B2TS, B3TS and B4TS glasses, respectively. Higher stimulated emission cross-section value is the attractive feature for low threshold,

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high gain laser applications, which are used to obtain continuous wave (CW) laser action [42]. Among the prepared Sm3 þ :BnTS glasses, Sm3 þ :B4TS glass is found to be suitable for developing the visible laser and fiber optic amplifier, since it exhibits larger stimulated emission cross-section and branching ratios.

3.5. Decay rate analysis Decay rates of the 4G5/2 level of the prepared Sm3 þ :BnTS glasses have been measured at room temperature and are shown in Fig. 5. It is clearly observed from the figure that the decay rate exhibit a non-exponential behavior, which is typical of Sm3 þ -doped glasses when the RE ion concentration is high. The effective decay time can be determined by following the expression [29,43]: R tIðtÞdt texp ¼ teff ¼ R ð13Þ IðtÞdt where I(t) is the emission intensity at time ‘t’. The measured lifetime of the excited 4G5/2 level has been obtained by taking first e-folding times of decay curves. These values are compared with the reported Sm3 þ -doped glasses commonly found in the literature [2,25,26,44]. The shortening of the lifetime and the deviation from the exponential law are the characteristic of the existence of a concentration quenching mechanism in the lifetime of 4G5/2 level at higher concentration. In the case of RE-doped glasses the experimental lifetime (texp) can be expressed using the following expression [45]: 1

texp

¼

1

trad

þ WMPR þ WET þ WOH þ . . .::

ð14Þ

where trad is the radiative lifetime calculated from JO theory. WMPR is the multiphonon relaxation (MPR) rate, WET is the energy transfer (ET) rate and WOH is the energy transfer rate between RE ion and OH groups. The energy transfer rate strongly depends on the RE ion concentration in the host matrix [46]. In the present work, since the concentration of the Sm3 þ ions is incorporated equally in all the prepared glasses, the energy transfer rate between active ions (Sm3 þ ) should be similar in all the glasses and therefore, it can be ignored. The multiphonon relaxation rate (WMPR) is given by WMPR ¼

1

texp



1

ð15Þ

trad

1 7/2

5/2

6H

6

H9/2

5/2 6H

11/2

B4TS B3TS B2TS B1TS B0TS

550

600

650 Wavelength (nm)

700

750

Fig. 4. Luminescence spectra of the Sm3 þ -doped boro-tellurite glasses.

Normalised intensity (arb. units)

Intensity (a.u)

4G

6H

0.1

B0TS B1TS B4TS B2TS B3TS

0.01

1E-3 0

1000

2000

3000

Time (μs) Fig. 5. Luminescence decay rate for the 4G5/2 level of Sm3 þ :BnTS glasses.

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K. Maheshvaran et al. / Journal of Luminescence 131 (2011) 2746–2753

The WMPR (s  1) for the 4G5/2 level is found to be 639, 747, 659, 485 and 276 for B0TS, B1TS, B2TS, B3TS and B4TS glasses, respectively. It is observed from these values that the multiphonon relaxation rate decreases linearly with the increase in TeO2 content. It is suggested that the decay rate and multiphonon relaxation rate strongly depends on the host matrix [47,48]. Therefore, it can be concluded that, the compositional dependence of measured lifetime is mainly determined by the joint effect of the radiative decay rate and the multiphonon decay rate. Since B2O3 tends to bring a large amount of H2O with it and the phonon energy of the tellurite glass is lower than that of borate glass, it is due to the decline of nonradiative decay process that the substitution of TeO2 for B2O3 increases the experimental lifetime [48]. The quantum efficiency of the 4G5/2 excited level is obtained using the following expression [26]:

t Z ¼ exp 100% trad

ð16Þ

The quantum efficiency values of the Sm3 þ :BnTS glasses are presented in Table 5. The absence of fluoride content in the glass matrix probably decreases the quantum efficiency of the fluorescent 4G5/2 level by increasing the non-radiative decay rate. The radiative lifetime (trad) of the prepared Sm3 þ :BnTS glasses is found to follow the order B2TS o B1TSo B3TS o B4TSo B0TS glasses and it is clear that radiative lifetime (trad) is found to be longer than the experimental lifetime (texp) and the difference may be due to non-radiative relaxation process. There are two important mechanisms to explain the energy transfer processes resulting in luminescence quenching. The first one is due to the cross-relaxation between the pairs of Sm3 þ ions and the second process is connected with the migration of the excitation energy, which can accelerate the decay by energy transfer to the structural defects acting as energy sinks. The energy transfer among the excited Sm3 þ ions is due to the cross relaxation [4G5/2 ,6H5/2]-[6F5/2, 6F11/2] (  10,400 cm  1). This cross-relaxation is due to the energy transfer from the excited 4G5/2 emission level to the nearby Sm3 þ ions in the ground level 6H5/2. This transfer of cross-relaxation occurs through 4G5/2-6F5/2 transition on one ion and 6H5/2-6F11/2 transition on the other. After that, both ions quickly decay non-radiatively to the ground state [40]. Lavin et al. [49] reported a detailed analysis of such energy migration processes. Along with the above two mentioned mechanisms, the non-radiative decay rate of Sm3 þ ions due to OH  groups should also be considered. The presence of strong OH  groups explored in the FTIR spectra of the prepared glasses also play a considerable role in the quenching of excited state lifetime of Sm3 þ ions.

4. Conclusions The structural and optical behavior of the Sm3 þ -doped borotellurite glasses were studied through XRD, FTIR, UV absorption, luminescence and decay rate measurements. The XRD pattern confirms the amorphous nature of the prepared glasses. The FTIR spectra reveal the attachment of the borate network with the B– O  vibrations. The Te–O–Te linkage and Te–O bending vibrations associated with the TeO3 and TeO6 units were identified and reported. The ionic character of the Sm3 þ ions with its surrounding ligands have been confirmed through the optical spectra and it is concluded that the ionic character gradually decreases with the increase in TeO2 content. JO parameters O2, O4 and O6, for the Sm3 þ -doped BnTS glasses were derived from the absorption spectra and the lower values of the O2 parameter exhibits asymmetric nature of the ion site, which is further confirmed through its ionic character and the intensity ratio between the

electric-dipole and magnetic-dipole transitions. Intense reddish– orange emission corresponding to 4G5/2-6H7/2 transition has been observed in the Sm3 þ :BnTS glasses under 488 nm excitation. Based on the studied optical behavior, it is concluded that Sm3 þ :B4TS glass may be used as a laser active medium for emission at 600 nm corresponding to 4G5/2-6H7/2 transition. The quantum efficiency of the 4G5/2 fluorescent level is found to be less due to the cross relaxation between the pair of Sm3 þ ions, energy transfer to the structural defects and also due to the presence of strong OH  groups in the studied glasses.

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