Composition dependent structural and optical properties of Sm3+-doped sodium borate and sodium fluoroborate glasses

Composition dependent structural and optical properties of Sm3+-doped sodium borate and sodium fluoroborate glasses

ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1313–1319 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: ww...

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ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1313–1319

Contents lists available at ScienceDirect

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

Composition dependent structural and optical properties of Sm3 + -doped sodium borate and sodium fluoroborate glasses S. Shanmuga Sundari a, K. Marimuthu a,n, M. Sivraman a, S. Surendra Babu b a b

Department of Physics, Gandhigram Rural University, Gandhigram 624 302, India Laser Instrumentation Design Centre, Instruments Research and Development Establishment, Raipur Road, Dehradun 248 008, India

a r t i c l e in fo

abstract

Article history: Received 27 August 2009 Received in revised form 9 January 2010 Accepted 22 February 2010 Available online 25 February 2010

Sodium borate and fluoroborate glasses doped with trivalent samarium (Sm3 + ) were prepared and their detailed spectroscopic analysis was carried out. The FTIR spectra reveal that, the glasses contain BO3, BO4, non-bridging oxygen and strong OH  bonds. From the optical absorption spectra, Judd–Ofelt intensity parameters (Ol, l = 2, 4 and 6) have been evaluated and are 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 + ions in sodium borate and sodium fluoroborate glasses. The dependence of the spectral characteristics of Sm3 + ions due to compositional changes have been examined and reported. The value is found to decrease with the decrease in the sodium content in the glass. The decay from the 4G5/2 level is found to be non-exponential indicating a cross-relaxation among the Sm3 + ions. & 2010 Elsevier B.V. All rights reserved.

Keywords: Judd–Ofelt parameters Radiative life-time Branching ratio Stimulated emission cross section Optical absorption

1. Introduction Rare earth (RE) ions have been widely investigated in various crystals and glasses and played an important role in the development of many optoelectronic devices such as lasers, light converters, sensors, hole burning high-density memories, optical fibers and amplifiers [1–3]. In these devices, excitations and emissions are due to transitions among 4f electronic states of trivalent RE ions, which are highly sensitive to the symmetry, structure of the local environment and phonon energy of the host matrix. Among the RE ions Sm3 + ions have stimulated extensive interests due to their potential application for high-density optical memory devices [4–6]. The excited 4G5/2 level of Sm3 + ions emits in the visible region which exhibits relatively high quantum efficiency and shows different quenching mechanisms which make Sm3 + ions as an interesting case to analyze the energy transfer process [7–17]. Over the past few years, there has been a considerable interest in the study of borate based glasses due to their structural and optical properties. An interesting characteristic of the borate glasses is the appearance of variations in its structural properties when alkaline or alkaline-earth cations are introduced. The structure of the borate glasses is not a random distribution of BO3 triangles and BO4 tetrahedra, but a gathering of these units to form well defined and stable borate groups such as diborate,

n

Corresponding author. Tel.: + 91 451 2452371; fax: + 91 451 2454466. E-mail address: [email protected] (K. Marimuthu).

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.046

triborate, tetraborate, etc., which constitute the random threedimensional network [18]. These things make the borate glasses as one of the best choices for RE doping. However, there is less interest in borate glasses due to their high phonon energy. As the large phonon energy is not detrimental to Sm3 + ions emission, still borate based glasses are of interest for Sm3 + ion doping. The present work reports the structural and optical properties of Sm3 + doped sodium borate and sodium fluoroborate glasses by measuring XRD, FTIR, optical absorption, luminescence spectra and decay time measurements. Using the Judd–Ofelt parameters, derived from the absorption spectra, various radiative properties such as radiative lifetimes, branching ratios and emission crosssections have been predicted. The theoretically predicted values are compared with the experimental values. A significant improvement in the quantum efficiency of 4G5/2 level is observed when Na2O is completely replaced with NaF, even though there is no systematic change in the experimental or calculated lifetimes. The peak stimulated emission cross section of the 4G5/2-6H9/2, 11/2 transition of Sm3 + ion in the titled glasses is found to be higher than the other reported glasses. All the measured/evaluated values are compared with the similar glasses doped with Sm3 + ions.

2. Experimental The chemicals used in the present work are Na2CO3, NaF, H3BO3 and Sm2O3 (99.99% purity grade). The glass samples were prepared by conventional melt quenching technique. The batch

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Table 1 Physical properties of Sm3 + doped sodium borate and sodium fluoroborate glasses. Average mol.wt (M)g

Density (d) g cm  3

Refractive index nd (589.3 nm)

Sm3 + ions (N)  1022 ions/cm  3

N5BS N4BS N3BS NBNfS NfBS

86.54 82.12 77.71 70.70 54.86

2.9772 3.7720 3.2765 2.9242 2.9815

1.648 1.646 1.645 1.649 1.648

2.0720 2.7662 2.5394 2.4908 3.2728

composition (in mol%) of the Sm3 + doped sodium borate and sodium fluoroborate glasses are as follows:

N5BS

% Transmission

Glasses

N4BS N3BS NBNfS

N5BS: 49.5 Na2O+49.5 B2O3 +1.0 Sm2O3 N4BS: 39.5 Na2O+59.5 B2O3 +1.0 Sm2O3 N3BS: 29.5 Na2O+69.5 B2O3 +1.0 Sm2O3 NBNfS: 24.75Na2O+49.5 B2O3 +24.75 NaF+ 1.0 Sm2O3 NfBS: 49.5 NaF + 49.5 B2O3 +1.0Sm2O3 About 7 g batches of the above compositions were taken in an agate mortar and the same were ground thoroughly to attain homogeneity and melted in an electric furnace in the temperature range 950–1000 1C for about 45 min. Two percent of the NaF content is taken in excess to compensate the F  ion losses. The melts were air quenched by pouring it on a pre heated brass plate. The glasses were annealed at 350 1C for 6 h in order to remove strains during the quenching process. The glasses were slowly cooled to room temperature (RT) and then were polished to obtain a planer faces for optical measurements. The glass density was measured employing the Archimedes principle, using xylene as an immersion liquid. The refractive indices of the prepared glasses have been measured at sodium wavelength (589.3 nm) using an Abbe refractometer with monobromonapthalene as the contact liquid. Table 1 presents the physical properties of the titled glasses. The X-ray powder diffraction (XRD) analysis was carried out using JEOL 8030 X-ray diffractometer employing CuKa radiation. Fourier transform infrared (FTIR) spectra of the glasses were taken using a Perkin-Elmer Peragon 500 FTIR system with a spectral resolution of 4 cm  1. Optical absorption spectra of the Sm3 + doped glasses were recorded using Perkin Elmer Lambda 35 UV–vis spectrometer in the range 350–1700 nm with a spectral resolution of 1 nm. Photoluminescence (PL) was measured with a monochromator (Dongwoo, DM 701) equipped with PMT (Hamamatsu R928) as a detector. 488 nm from an Ar + ion laser (Spectra-Physics, BeamLok 2060) was used as the excitation source for PL and decay time measurements. Decay curves were measured by using a mechanical chopper in connection with a LeCroy 9350A 500 MHz digital oscilloscope and the data was collected with a computercontrolled data acquisition system. Each fluorescence decay curve was averaged over 500 pulses.

3. Results and discussion 3.1. Structural investigations The X-ray diffraction (XRD) pattern of the titled glasses (51r y r801) have been recorded. The XRD pattern has shown a broad diffuse scattering at lower angles, which is the characteristic of long range structural disorder and hence the XRD spectra are not shown. This confirms the amorphous nature of the prepared glasses. To identify the local structure, the FTIR spectra of the sodium borate and sodium fluoroborate glasses have been recorded and are shown in Fig.1. The IR spectral vibrations of the

NfBS

750

1500

2250

3000

3750

Wavenumber (cm -1) Fig. 1. FTIR spectra of Sm3 + doped sodium borate and sodium fluoroborate glasses.

borate based glasses are divided into three main regions. The first region lies between 1200 and 1600 cm  1 which is due to the asymmetric stretching relaxation of the B–O bond stretching of trigonal BO3 units and the second region lies between 800 and 1200 cm  1 is assigned to the B–O stretching of the BO4 units. The third band around 700 cm  1 is due to the B–O–B linkages in the borate network. The IR spectral vibrations of the borate glasses are categorized and the peak assignments are given accordingly. The observed bands at the following positions around 3434, 2925, 2855, 1540, 1425, 1036 and 725 cm  1 could be seen prominently in the spectral records of these glasses and the same is presented in Table 2 along with band assignments. The peaks around 3434 cm  1 is due to the stretching of OH group. The band around 2925 and 2855 cm  1 is indicative of hydrogen bonding. The peak around 1540 cm  1 is assigned for B–O  bonds from isolated pyroborate groups. The broad band around 1425 cm  1 is the characteristic of B–O stretching vibrations of the trigonal BO3  units in metaborate, pyroborate, and orthoborate groups [18]. The peaks around 1036 cm  1 is attributed to the stretching of BO4 group and broad bands around 725 cm  1 is due to the bending of B–O–B linkages in the borate network [19]. 3.2. Optical absorption spectra The room temperature absorption spectra of titled glasses consists of several inhomogeneously broadened bands assigned to f-f transitions from the ground 6H5/2 state to various excited states of Sm3 + ions in the host glasses. As an example, Fig. 2 shows the optical absorption spectra of one selected Sm3 + -doped sodium borate glass measured in the 350–1700 nm spectral region. The spectra of all the titled glasses are similar to each other and are comparable to other Sm3 + -doped glasses [8–17]. However, a slight variation in peak positions and intensities are noticed which is due to variations in the glass compositions. For all the Sm3 + -doped titled glasses, 20 excited levels which includes 6F1/2, 3/2, 5/2, 7/2, 9/2, 6 4 4 4 4 6 11/2, H15/2, G5/2, 7/2, F3/2, I9/2, 11/2, 13/2, M15/2, 17/2, P5/2, 3/2, 7/2, 4 4 L15/2 and D3/2 that span up to 350 nm have been observed. The band assignments are shown in Fig. 2 and their positions for N5BS and N4BS glasses are given in Table 3. It is clear from Fig. 2 that the most intense transitions of Sm3 + ions are found in the near infrared

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Table 2 Peak table of FTIR spectra of Sm3 + doped sodium borate and sodium fluoroborate glasses. Sl. No. 1 2 3 4 5 6 7

N5BS (cm  1)

N4BS (cm  1)

N3BS (cm  1)

NBNfS (cm  1)

NfBS (cm  1)

Assignments

3434 2925 2855 1540 1425 1036 725

3442 2922 – 1546 1434 1038 722

3446 2926 2851 – 1431 1034 727

3435 2925 2852 1540 1436 1034 –

3437 2924 2856 1545 1431 1030 721

Stretching of OH group Hydrogen bonding Hydrogen bonding B–O  bond in isolated pyroborate groups B–O stretching in (BO3)3  Stretching of BO4 groups Bending of B–O–B linkages

4

D3/2

6

6

P3/2

F7/2

Optical density (a.u)

6F 9/2 6P

6F 5/2 6F 3/2

7/2

6

F1/2

F5/2 , 4G9/2

4

4

4M , 15/2

, I11/2

4I 9/2

6

F11/2

6

H15/2

4 4

M17/2

350

400

I13/2 4G 7/2

450

4F 3/2

500

550

4G 5/2

600 800 1000 Wavelength (nm)

1200

1400

1600

Fig. 2. Absorption spectra of the Sm3 + :sodium borate glass in the UV–vis-NIR region (N5BS-Glass).

Table 3 Energy level positions and experimental and calculated oscillator strengths of Sm3 + doped sodium borate glasses. Transition N5BS

6

H15/2 F1/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 F11/2 4 G5/2 4 F3/2 4 G7/2 4 I11/2 4 F5/2 6 P3/2 N 6

s

N4BS

Wavelength (nm)

fexp

1593 1533 1473 1373 1227 1073 950 560 529 497 462 447 410

0.301 0.049 1.940 1.940 2.710 4.380 6.480 6.810 9.709 9.655 6.523 6.055 0.886 0.965 0.329 0.041 0.369 0.006 0.416 0.215 3.096 1.260 0.713 0.191 11.15 12.01 13 70.775

fcal

Wavelength (nm)

fexp

fcal

1594 1535 1472 1367 1224 1077 947 559 526 499 462 446 407

0.228 0.009 0.866 0.866 1.144 1.579 2.093 2.207 3.189 2.245 2.028 1.164 0.174 0.174 0.168 0.014 0.189 0.003 0.335 0.057 1.516 0.224 0.407 0.044 3.905 3.858 13 70.489

(NIR) region and the assignment of free-ion levels in the UV–vis regions is not easy because of the overlap of different 2S + 1LJ levels. The transition from the ground 6H5/2 state to 6H and 6F terms are spin-allowed (DS=0) and hence, these transitions lying in the NIR region are intense. On the other hand the spin allowed 6H5/2-6P3/2

transition in the UV/vis region is also found to be more intense than the other transitions. 3.2.1. Nephelauxetic effect—bonding parameter To have an idea about the nature of Sm3 + -ligand bond, nephelauxetic ratios and bonding parameters have also been evaluated. The nephelauxetic ratio (b) is given by [20]



nc na

ð1Þ

where nc is the wavenumber (in cm  1) of a particular transition for an ion in the host under investigation and na the wavenumber (in cm  1) of the same transition for the aquo ion. From the average values of b, (taken as b) the bonding parameter, d, is given by



1b

b

ð2Þ

The bonding will be covalent or ionic depending upon the positive or negative sign of d. For the present glasses the d value is found to be N5BS (  0.339)oN4BS (0.418)oN3BS (  0.497)oNBNfS (  0.547)oNfBS ( 0.907) and is presented diagrammatically in Fig. 3. The negative sign of d for the titled glasses indicate the ionic nature of the Sm–O bond in the titled glasses. This kind of ionic nature is also found in Sm3 + doped borate and fluoroborate [16], phosphate [17], lead niobium phosphate [15] glasses. Among the titled glasses the ionic bonding is getting stronger with the decrease in the sodium

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where N is the concentration on Sm3 + ions, e the charge of an electron, n the wavenumber and a(n) the absorbance. The oscillator strengths of various observed transitions are evaluated through Eq. (4) and are used in Eq. (3). A least-square fitting approach is adopted for Eq. (3) to determine the JO parameters, which gives the best-fit between the experimental and calculated oscillator strengths. The experimental and theoretical oscillator strengths obtained from the JO analysis for Sm3 + doped N5BS and N4BS glasses were collected and presented in Table 3. Table 4 compares the JO parameters obtained in the present work with those of the similar other Sm3 + -doped glass systems. As seen from Table 4, it is interesting to note that the magnitude of trends of Ol parameters for Sm3 + -doped sodium borate and sodium fluoroborate glasses are found to be O4 4 O2 4 O6, where as a mixed trend is observed in the other Sm3 + doped glasses [15–17]. These trends may be attributed to the presence of similar sites around the Sm3 + ions, consequently it can be evidence for the good quality of the glass host for optical application [24]. The O2 parameter exhibits the dependence of the covalency between Sm3 + ions and ligand anions, because O2 is connected to the asymmetry of the local environment around the Sm3 + sites. The lesser the value of O2, the more centrosymmetrical the ion site and the more ionic its chemical bond with the ligands.

content and it is strongest with the addition of the fluoride content, which is a characteristic of the fluoride glasses [16].

3.2.2. Oscillator strengths and JO parameters The absorption spectra associate with the transitions within the 4f5 configuration of Sm3 + have been analysed within the frame work of Judd–Ofelt (JO) theory [21,22]. According to the JO dipole–dipole theory, the oscillator strength, fcal, of an electric   ðS; LÞJ , to the final absorption transition from the initial state   state ðS0; L0ÞJ0 , depends on three Ol parameters (l = 2, 4 and 6) as fcal ½ðS; LÞJ; ðS0 ; L0 ÞJ0  ¼

8p2 mc ðn2 þ 2Þ2 9n 3hlð2J þ 1Þ D E2 X   Ol  ðS; LÞJ99U ðlÞ 99ðS0 ; L0 ÞJ 0  

ð3Þ

l ¼ 2;4;6

where l is the mean wavelength of the transition, m the mass of the electron, n the refractive index, Ol the JO parameters and  ðlÞ  :U : the doubly reduced matrix elements of unit tensor operator which are considered to be independent of the host matrix. These reduced matrix elements were taken from the work of Jayasankar and Rukmini [23] for the JO analysis. The experimental oscillator strength (fexp) of any absorption transition can be obtained by integrating the absorbance of each band given by Z mc2 fexp ¼ aðnÞdu; ð4Þ pe2 N

3.5

Under the excitation of 488 nm the titled glasses has shown a strong visible emission in the orange-red region. The emission spectra of the titled glasses are shown in Fig. 4, which correspond

γ Ω2x10-20 cm

1.75 Normalised Intensity (arb. units)

3.0

Parameters

3.3. Emission spectra and radiative properties

2.5 2.0 0.0

-0.5

6H

1.50

4G 5/2 6H

1.25

7/2 6

H9/2

5/2 6H

1.00

11/2

0.75

N4BS

0.50

N3BS NBNfS

0.25

NfBS

0.00

-1.0 N5BS

N3BS N4BS NBNfS Glass composition

550

NfBS

N5BS

600

650 Wavelength (nm)

700

Fig. 4. Luminescence spectra of Sm3 + doped sodium borate and sodium fluoroborate glasses.

Fig. 3. Variation of nephelauxetic ratio and Judd–Ofelt parameter, O2 with the studied glass composition.

Table 4 Judd–Ofelt parameters (Ol  10  20 cm2) and radiative lifetimes (ms) (4G5/2) of Sm3 + :glasses and other reported Sm3 + :glasses. Glass system

O2

O4

O6

Trend of Ol

N5BS N4BS N3BS NBNfS NfBS 50GeO2–43PbO–5PbF2–2SmF3 LBTAF 75NaPO3–24CaF2–1SmF3 75NaPO4–24BaF2–1SmF2 Phosphate L5FBS PKBAS

2.86 2.52 2.56 2.15 3.23 8.56 0.27 2.18 2.23 4.31 2.34 4.49

4.13 3.62 3.45 3.95 5.33 3.02 2.52 3.80 3.82 4.28 7.54 7.36

1.92 1.14 2.72 1.89 2.26 2.37 2.47 2.15 2.18 5.78 5.40 3.87

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

trad of 4G5/2 (ms)

Reference

2852 3375 2988 3090 2369 2080 4880 5900 – 2800 2154 2090

Present Present Present Present Present Ref [8] Ref [9] Ref [13] Ref [14] Ref [15] Ref [16] Ref [17]

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to the 4G5/2-6HJ (J= 5/2, 7/2, 9/2 and 11/2) emission transitions at 562, 597, 645 and 705 nm, respectively. H. Lin et al. [10] observed this kind of visible emission under 488 nm excitation from Sm3 + doped CdO–Al2O3–SiO2 glasses. Some of the important radiative properties such as total spontaneous transition probability (A), lifetime (trad) and branching ratio (bR) can be calculated using the Ol parameters. The total spontaneous transition probability is given by A½ðS; LÞJ; S0 ; L0 ÞJ0  ¼ Aed þ Amd ¼ 

nðn2 þ2Þ2 Sed þ n3 Smd 9

!

64p4 3

3hl ð2J þ 1Þ ð5Þ

where Aed and Amd are the electric- and magnetic-dipole contributions, respectively, and Smd and Sed are electric and magnetic-dipole line strengths. The radiative lifetime of an emitting state is related to the total spontaneous emission probability for all transitions from this state by 8
The fluorescence branching ratio of transitions from initial manifold (S,L)J to (S0 ,L0 )J0 is given by

bR ¼ P

A½ðS; LÞJ; ðS0 ; L0 ÞJ 0 Þ 0 0 0 s0 ;l0 j0 A½ðS; LÞJ; ðS ; L ÞJ Þ

ð7Þ

The experimental bR for titled glasses are found to be in the order of 4G5/2-6H7/2 4 6H9/2 4 6H5/2 4 6H11/2. The stimulated emission cross-section (sEP ) is an important parameter and its value signifies the rate of energy extraction from the lasing material. The sEP is given by

sEP ¼

l4p A 8pcn2 Dleff

ð8Þ

1317

where n is the refractive index, lp the emission peak wavelength and Dleff the emission effective linewidth of the transition given by Z 1 Dleff ¼ ð9Þ IðlÞdl Ip The values of sEP for the 4G5/2 emission transitions are in the order of 4G5/2-6H9/2, 6H7/2, 6H11/2 and 6H5/2 for title glasses. Table 5 reports the emission band position (lp), effective band width (Dleff), radiative transition probability (A), experimental and calculated branching ratios (bR) along with the peak stimulated emission cross section (sEP ) for some of the transition originating from 4G5/2 level of Sm3 + ion in the titled glasses along with the other reported glasses. The stimulated emission cross section values of the prepared Sm3 + glasses lies on the higher side than the reported borate glasses [16,25]. The stimulated emission cross section of the Sm3 + :N5BS glass corresponding to 4G5/2-6H7/2.9/2 transition is found to be on the higher side among the prepared glasses. The large stimulated emission cross section value is the attractive feature for low threshold, high gain applications which are used to obtain laser action, and in the present work Sm3 + :N5BS glass could be suggested for laser medium, since it exhibits higher stimulated cross section values may be due to its higher sodium content and the value decreases with the decrease in the value of sodium content 3.4. Decay times and quantum efficiencies Decay curves of the 4G5/2 level of Sm3 + ion have been measured and same has been shown Fig. 5 for NBNfS glass. As seen from the figure, it is clear that the decay curves exhibit a non-exponential behavior, which is the characteristic of the Sm3 + doped glasses with high dopent concentration. The effective decay times are determined by using the following expression: R tIðtÞdt texp ¼ teff ¼ R ð10Þ IðtÞdt

Table 5 Emission band position (lp, nm), effective band width (Dleff, nm), radiative transition probability (A, s  1), peak stimulated emission cross section (sEP  1022 cm2 ), experimental and calculated branching ratios (bR) for 4G5/2 transition level of Sm3 + :NXBS glasses Transition parameters

Sm3 + :N5BS

Sm3 + :N4BS

Sm3 + :N3BS

Sm3 + :NBNfS

Sm3 + :NfBS

Sm3 + :L5FBS [16]

Sm3 + :ZFBP [25]

Sm3 + :L5FBS [26]

Sm3 + :LSG [27]

4

558 15 25.65 0.8099

560 15 24.32 0.7812

561 15 22.05 0.7139

560 12.5 22.10 0.8489

562 12.5 17.985 0.7013

562 20 25.18 0.703

557 14 – –

562 10 25.17 1.401

563 12 102 4.25

0.170 0.054

– –

0.094 0.054

598 27 216.05 5.744

598 12.5 57.35 3.448

598 14 215.98 11.091

G5/2-6H5/2

lp Dleff A

sEP bR(Exp) bR(Cal) 4

G5/2-6H7/2

lp Dleff A

sEP bR(Exp) bR(Cal) 4

G5/2-6H9/2

lp Dleff A

sEP bR(Exp) bR(Cal) 4

G5/2-6H11/2

lp Dleff A

sEP bR(Exp) bR(Cal)

0.108 0.045

0.109 0.050

597 17.5 246.47 8.7411

600 15 207.57 8.7806

0.599 0.436 643 15 174.921 9.7395 0.335 0.309 710 10 61.429 7.6269 0.025 0.109

0.544 0.430 647 17.5 148.507 7.2833 0.318 0.308 708 7.5 54.478 8.9392 0.027 0.113

0.106 0.059 598 22.5 145.86 4.0948 0.567 0.391 645 20 124.68 5.2912 0.295 0.334 708 12.5 39.607 3.9041 0.030 0.106

0.101 0.052 598 20 184.55 5.7585 0.552 0.438 648 17.5 126.478 6.2186 0.315 0.300 710 15 45.607 3.7704 0.030 0.108

0.096 0.054 598 17.5 154.321 5.5098 0.549 0.466 645 17.5 90.542 4.3752

0.546 0.466 645 32 126.76 3.814

0.318 0.271

0.244 0.271

708 15 39.542 3.885

708 56 55.43 1.399

0.035 0.119

0.038 0.119

0.3668 – 645 14.5 71.02 5.013 0.4543 – 705 15.5 23.02 2.165 0.1474 –

0.458 0.466 645 16 125.64 7.657 0.313 0.271 705 28 55.40 2.797 0.134 0.119

– 0.239 600 12 178 8.20 – 0.418 649 9 83 5.79 – 0.196 714 8 44 3.88 – 0.103

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Fig. 5. Decay curve the of the 4G5/2 level of Sm3 + ions in NBNfS glass. Inset shows the logarithmic intensity plot with time,

where I(t) is the emission intensity at time t. The texp for the G5/2 level is found to be 2175, 2493, 2258, 2424 and 2172 ms for N5BS, N4BS, N3BS, NBNfS and NfBS glasses, respectively. These values are very well within the range of the glasses commonly found in the literature [7–10,15–17]. In the case of RE doped materials, the measured lifetime (texp) can be expressed as [25]

4

1

texp

¼

1

trad

þ WMPR þWET þWOH þ   

ð11Þ

where trad is the radiative lifetime calculated from the JO theory. WMPR the multiphonon relaxation (MPR) rate, WET the rate of energy transfer (ET) and WOH the energy transfer rate between RE ion and OH groups. The energy difference between the 4G5/2 level and the next lower level of about 7000 cm  1 which is large enough to suppress the multiphonon relaxation in the titled glasses. Thus, we can expect that in the experimental luminescence lifetime of 4G5/2 level should be close to radiative lifetime evaluated from the JO analysis. The non-radiative decay rate WNR is given by, WNR ¼

1

texp



1

trad

ð12Þ

The WNR (s  1) for the 4G5/2 level is found to be 109, 105, 108, 89 and 38 for N5BS, N4BS, N3BS, NBNfS and NfBS glasses, respectively. From these values it is interesting to note that the non-radiative decay rate or energy transfer rate is almost constant with the decrease in the sodium content but considerably decreases with the addition or complete replacement of sodium oxide with the sodium fluoride content in the glass. The pure fluoride glass is having a low energy transfer rate. For direct excitation, the luminous quantum efficiency (Z) is defined as the ratio of the emitted light intensity to the absorbed pump intensity. For RE ion systems, it is equal to the ratio of the measured lifetime (texp) to the calculated lifetime (trad)for respective levels given by [25]

texp  100 ð13Þ trad The Z value is found to be 76%, 74%, 75%, 78% and 92% for N5BS,



N4BS, N3BS, NBNfS and NfBS glasses, respectively. The variation of Z is also following the same trend as that of WNR. Hence the replacement of fluoride content to the glass increases the quantum efficiency of the fluorescent 4G5/2 level by decreasing the non-radiative decay rate. The considerable value of the experimental and radiaitve lifetimes and the non-exponential nature of the decay profile in

Fig. 6. Partial energy level diagram of Sm3 + ions in the NBNfS glass along with the cross-relaxation channels.

the titled glasses may be mainly due to energy transfer process among the neighbouring Sm3 + ions. There are two important mechanisms to explain the energy transfer processes resulting in luminescence quenching. The first one is attributed to the crossrelaxation between pairs of Sm3 + ions. This mechanisms, which is indicated in Fig. 6 (energy level diagram) is the cross-relaxation with the following cross-relaxation channels     4 G 5=2 ; 6 H 5=2 - 6 F 5=2 ; 6 F 11=2 This cross-relaxation is due to the energy transfer from the Sm3 + ions in an excited 4G5/2 state, to a nearby Sm3 + ion in the ground 6H5/2 state. This transfer leaves the first ion in the intermediate level of 6F5/2 at around 1375 nm and the second one in 6F11/2 at around 942 nm, which occur in resonance with the 4 G5/2-6F5/2 transition at around 947.6 nm in the case of NBNfS glass. Later both ions quickly decay non-radiatively to the ground state. Fig.6 demonstrates the other possible mechanism of crossrelaxation process involving the channel     4 G 5=2 ; 6 H 5=2 - 6 F 9=2 ; 6 F 7=2 Similar arguments can be drawn in the case of other glasses except some changes in the energy values [7–9,15–17]. The second process is connected with the migration of the excitation energy, which can accelerate the decay by an energy transfer to the structural defects acting as energy sinks. In the case of high pressure dependent luminescence studies, Lavin et al. [7] had given a detailed analysis of such energy migration processes, where the existence of pressure induced structural defect centres is a common phenomenon. Lavin et al. [7] also concluded that the contribution due to energy migration process may be neglected. The available data do not allow us to discriminate between the possible mechanisms. This fact opens a wide scope for the future work on Sm3 + ions in glasses. Along with the above two mechanisms, the non-radiative decay rate of Sm3 + ions to OH-groups should also be considered. The presence of strong OH  groups in the titled glasses also plays a considerable role in the quenching of excited state lifetime of Sm3 + ions.

4. Conclusion The present paper report results on structural and photoluminescence properties of Sm3 + :NXBS glasses containing different compositions. XRD, FTIR, optical absorption and luminescence

ARTICLE IN PRESS S. Shanmuga Sundari et al. / Journal of Luminescence 130 (2010) 1313–1319

and lifetime measurements have been recorded at room temperature. The ionic character of Sm3 + ions with surrounding ligands have been confirmed through optical absorption spectra and it is concluded that ionic character is getting stronger with the decrease in the sodium content and it becomes strongest with the addition of fluoride content. Judd–Ofelt parameters O2, O4 and O6 for the Sm3 + doped NXBS glasses were derived from the absorption spectra, and the lesser values of O2 parameter exhibits the more centrosymmetrical nature of the ion site which further confirms the more ionic character of the Sm3 + ions with the ligands. Intense reddish-orange emission corresponding to 4 G5/2-6H7/2 transition has been observed in the Sm3 + :NXBS glasses under 488 nm excitation. It is concluded from the studies that the quantum efficiency of the 4G5/2 fluorescent level increases due to the replacement of fluoride content by decreasing the non-radiative decay rate, even though there is no systematic variation of the radiative and fluorescent lifetimes. The peak stimulated emission cross section of the 4G5/2-6H7/2.9/2 transitions of Sm3 + ion in the titled glasses is found to be higher than the other reported glasses. Based on the studied optical properties it is concluded that Sm3 + :N5BS glass may be used as a laser active medium for emission at 600 nm corresponding to the 4 G5/2-6H7/2 transition. References [1] A. Agnesi, P. Dallocchio, F. Pirzio, G. Reali, Opt. Commun. 282 (2009) 2070. [2] Hamit Kalaycioglu, Huseyin Cankaya, Gonul Ozen, Lutfu Ovecoglu, Alphan Sennaroglu, Opt. Commun. 281 (2008) 6056.

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