Fluorescence properties of Nd3+-doped tellurite glasses

Spectrochimica Acta Part A 67 (2007) 702–708

Fluorescence properties of Nd3+-doped tellurite glasses K. Upendra Kumar a , V.A. Prathyusha a , P. Babu b , C.K. Jayasankar a,∗ , A.S. Joshi c , A. Speghini d , M. Bettinelli d a Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Government Degree College, Wanaparthy 509 103, India c High Power Laser Optics Laboratory, Laser Plasma Division, Centre for Advanced Technology, Indore 452 013, India d Dipartimento Scientifico e Tecnologico, Universit` a di Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy b

Received 21 June 2006; received in revised form 17 August 2006; accepted 20 August 2006

Abstract The compositional and concentration dependence of luminescence of the 4 F3/2 → 4 IJ (J = 13/2, 11/2 and 9/2) transitions in four Nd3+ -doped tellurite based glasses has been studied. The free-ion energy levels obtained for 60TeO2 + 39ZnO2 + 1.0Nd2 O3 (TZN10) glass have been analysed using the free-ion Hamiltonian model and compared with similar results obtained for Nd3+ :glass systems. The absorption spectrum of TZN10 glass has been analysed using the Judd–Ofelt theory. Relatively longer decay rates have been obtained for Nd3+ -doped phosphotellurite glasses. The emission characteristics of the 4 F3/2 → 4 I11/2 transition, of the Nd3+ :TZN10 glass, are found to be comparable to those obtained for Nd3+ :phosphate laser glasses. The non-exponential shape of the emission decay curves for the 4 F3/2 → 4 I11/2 transition is attributed to the presence of energy transfer processes between the Nd3+ ions. © 2006 Elsevier B.V. All rights reserved. Keywords: Neodymium; Tellurite glasses; Fluorescence; Free-ion Hamiltonian; Laser spectroscopy

1. Introduction Lanthanide (Ln) doped tellurite glasses have been the subject of recent investigations as novel materials for potential applications in photonics. These glasses have a good optical quality and are stable against atmospheric moisture [1]. They have a wide transmission range (0.35–5 ␮m) compared to silicate glasses, good glass stability and corrosion resistance superior to fluoride glasses. In addition, they exhibit the lowest phonon energy cutoff (about 800 cm−1 ) among oxide glass formers and a low process temperature [2,3]. Tellurite glasses possess a high refractive index (∼2) which increases the local field correction at the Ln ion site leading to an enhancement of the radiative decay compared to silicate glasses and lowering thereby the importance of nonradiative relaxation rates of the Ln ion excited states. Moreover, tellurite glasses have high non-linear refractive indices enabling applications for second harmonic generation [3,4]. The Ln ions exhibit high solubility in these materials and therefore glass sam-

∗

Corresponding author. Tel.: +91 877 2248033; fax: +91 877 2225211. E-mail address: [email protected] (C.K. Jayasankar).

1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.08.027

ples at high lanthanide ion concentration can be successfully prepared. Moreover, these glasses are non-hygroscopic as well as highly stable against crystallization [1]. For these reasons, tellurite based glasses show the potential as optical amplifiers for second and third telecommunication windows at 1.3 and 1.5 ␮m, respectively, as frequency up-converters, colour displays, high density optical data reading and storage, biomedical diagnostics, infrared laser viewers and indicators [1–6]. Among the Ln ions, Nd3+ (4f3 ) ion emerges as an excellent lasing ion at 1.06 ␮m (4 F3/2 → 4 I11/2 ). For good laser efficiency, one of the required properties is a long fluorescence lifetime for the 4 F3/2 level of Nd3+ ion which is significantly influenced by non-radiative decay due to multiphonon relaxation and energy transfer processes such as emission self-quenching due to Nd3+ –Nd3+ interactions, etc. [7–9]. All these processes can be tailored by optimization of Nd3+ ion concentration as well as glass composition. In this direction, the present work attempts to report a correlation between tellurite based glass compositions and the spectral properties of Nd3+ ions in the title glasses. The work includes the detailed analysis of the energy levels, intensities, and decay curves of Nd3+ -doped title glasses as carried out in earlier work [10–23].

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2. Experimental details The tellurite based glasses were prepared from a mixture of powders of TeO2 , ZnO, LiF, (NH4 )H2 PO4 and Nd2 O3 with the following molar percentages: TZN01: 60TeO2 + 39.9ZnO + 0.1Nd2 O3 . TZN10: 60TeO2 + 39ZnO + 1.0Nd2 O3 . TZPN01: 59.9TeO2 + 30ZnO + 10(NH4 )H2 PO4 + 0.1Nd2 O3 . TLFPN01: 59.9TeO2 + 20LiF + 20(NH4 )H2 PO4 + 0.1Nd2 O3 . The appropriate amounts of chemicals were put in a platinum crucible and heated at ∼850 ◦ C for 1 h. The melts were poured on a preheated brass plate and annealed at 300 ◦ C for 8 h to remove thermal strains. Finally, the glass samples were polished in order to carry out the optical measurements. The densities of the samples were determined by Archimedes’ method with water as an immersion medium. The refractive index was measured on the Abb´e refractometer using sodium yellow wavelength (589.3 nm) with 1-bromonaphthalene as a contact liquid. The optical absorption spectrum (300–1000 nm) of the TZN10 glass was recorded using a Hitachi U-3400 spectrophotometer. The NIR emission spectra (800–1600 nm) were recorded using a Jarell-Ash 3/4m Czerny–Turner single monochromator. The signal was detected by a liquid nitrogen-cooled Northcoast EO-817P germanium detector connected to a computer-controlled Stanford Research SR510 lock-in-amplifier. The emission decay curves were measured by exciting the samples with a pulsed Nd:YAG laser (λexc = 355 nm). The signal was then analysed using a 0.5 m monochromator equipped with a 150 lines/mm grating and detected with a GaAs PMT and a digital oscilloscope. 3. Theory 3.1. Energy level analysis Optical absorption spectra of triply ionized lanthanide ions originate from transitions between levels of 4fn configuration. The free-ion Hamiltonian model (HFI ) which describes the position of these levels for Nd3+ ions is written as [11,14–16,24]  ˆ SO + αL( ˆ 2) ˆ FI = EAVG + ˆ L ˆ + 1) + βG(G H F k fˆ k + ξ A k

ˆ 7) + + γ G(R

 i

T i ˆti +

 k

P k pˆ k +



Mjm ˆj

(1)

j

where EAVG involves the kinetic energy of the electrons and their interaction with the nucleus. It shifts only the barycentre of the whole 4f configuration. Fk (k = 2, 4, 6) are free electron repulsion parameters and ξ is the spin–orbit coupling constant. fˆ k and ˆ SO represent the angular part of the electrostatic and spin–orbit A interaction, respectively. α, β and γ are the Trees interaction ˆ (G2 ) and G ˆ ˆ is the total angular momentum. G parameters. L (R7 ) are the Casimir’s operators for the groups G2 and R7 . Ti (i = 2, 3, 4, 6, 7 and 8) are the Judd parameters, which describe the corresponding three-body interactions. ˆti represents the operator

703

for electrostatic correction. Pk (k = 2, 4 and 6) represents the electrostatically corrected spin-orbit interactions and Mj (j = 0, 2 and 4) are the spin-other-orbit interaction parameters. pˆ k and m ˆ j represent the operators for the magnetic correction. Among various interactions, the inter-electronic repulsion and the spin–orbit interaction, the second and the third terms, respectively, are the main ones which give rise to the 2S+1 LJ levels. The rest only make corrections to the energy of these levels without removing their degeneracy. For Nd3+ (4f3 ) configuration the free-ion energy level Hamiltonian involves a square matrix of size 41 × 41 (comprising of 41 |SLJ basisset). Thematrix elements of the free-ion Hamiltonian within the 4f 3 SLJ basis can be calculated more accurately as detailed in Ref. [16]. However, the various radial integrals cannot be determined accurately. A semi-empirical approach is adopted in which these radial integrals are treated as variable parameters in fitting the eigen values of the complete basis set free-ion Hamiltonian matrix to the observed band positions. Thus, in performing calculated to experimental energy level fits, a least squares fitting procedure was used to minimize the root mean square (r.m.s.) deviation, σ, defined as   (Eobs − Ecal )2 i i σ= (2) N where Eiobs and Eical are the observed and calculated energies, respectively, for level i and N denotes the total number of levels included in the energy-level fit. 3.2. Judd–Ofelt analysis and radiative properties The intensities of the absorption bands have been calculated using the formula  f = 4.32 × 10−9 ε(ν) dν (3) where ε is the molar absorptivity at ν is the wavenumber (cm−1 ). According to Judd–Ofelt (JO) theory [19,20] the oscillator strength of an electric-dipole transition is given by f =

8π2 mcν (n2 + 2) 3h(2J + 1) 9n

2



  2 Ωλ (ΨJ U λ  Ψ  J  )

(4)

λ=2,4,6

where n is the refractive index, ν the wavenumber of the transition, Ωλ the JO intensity parameters and ||Uλ ||2 are the doubly reduced square matrix elements of the unit tensor operator calculated from the intermediate coupling approximation [25]. The electric (Sed ) and magnetic (Smd ) dipole line strengths were calculated from the formulae [25],    2 Sed = e2 Ωλ (ΨJ U λ  Ψ  J  ) (5) λ=2,4,6

and Smd =

e 2 h2 2 (ΨJ L + 2S Ψ  J  ) 16π2 m2 c2

(6)

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The radiative transition probability for a Ψ J → Ψ  J transition is given by  2 64π4 ν3 n(n2 + 2)   3 (7) Sed + n Smd A(ΨJ, Ψ J ) = 3h(2J + 1) 9 Since fluorescent level relaxation generally involves transitions to all low-lying levels and therefore define the total radiative transition probability as  AT (ΨJ) = A(ΨJ, Ψ  J  ) (8) Ψ J 

The fluorescent branching ratio (βR ), and radiative lifetime (τ R ) are given by [26,27] βR (ΨJ, Ψ  J  ) =

A(ΨJ, Ψ  J  ) AT (ΨJ)

τR (ΨJ) = [AT (ΨJ)]−1

Fig. 1. Room temperature absorption spectrum of the Nd3+ -doped TZN10 glass.

(9) (10)

Thus Eqs. (4)–(10) have been used to calculate various spectroscopic parameters [25–27] which asses the quality of the ion-host combination for specific purpose. 3.3. Emission and fluorescence decay For direct excitation, the luminescence quantum efficiency (η) is defined as η=

τexp τR

(11)

where τ exp is the observed lifetime determined from the luminescence decay curve and τ R refers to the calculated lifetime using JO analysis [Eq. (10)]. For a Ψ J → Ψ  J transition with a spontaneous emission probability A(Ψ J, Ψ  J ), the stimulated emission cross-section at the peak wavelength is estimated with the formula [21] σ(ΨJ, Ψ  J  ) =

λ4P A(ΨJ, Ψ  J  ) 8πcn2 λeff

4. Results and discussion 4.1. Energy level analysis and free-ion parameters The room temperature absorption spectrum of the Nd3+ doped TZN10 glass is shown in Fig. 1 together with the assignment of the absorption bands. The positions of the absorption bands are similar to those observed for other Nd3+ :glasses [9,10,29,30]. The energy band positions are shown in Table 1 for Nd3+ :TZN10 glass. Emission spectra have been recorded for all the four glasses under investigation (λexc = 355 nm). Fig. 2 shows the emission spectra (4 F3/2 → 4 IJ , J = 9/2, 11/2, 13/2) measured for TZN01, TZPN01 and TLFPN01 glasses. As can be seen from Fig. 2, the emission bands are affected by sizable inhomogeneous broadening due to the large crystal-field distribution around the Ln ions which is characteristic of glasses. The fitting process of the energy levels as well as the conventional notation and meaning of the free-ion parameters are reported in our earlier papers [11,31]. The energy level analysis

(12)

where λP is the peak wavelength of the emission band and λeff is the effective bandwidth determined by the relation [21]  1 I(λ) dλ (13) λeff = Ip where Ip is the peak intensity of the emission band. Following pulsed excitation the fluorescence decay curves of all glasses under investigation exhibit a non-exponential shape and can be well described by the equation [28]



 −t −t I(t) = A1 exp + A2 exp (14) τ1 τ2 and the average lifetime τ can be estimated as τ=

A1 τ12 + A2 τ22 A 1 τ1 + A 2 τ2

(15)

Fig. 2. Room temperature emission spectra of Nd3+ -doped: (a) TZN01, (b) TZPN01 and (c) TLFPN01 glasses.

K.U. Kumar et al. / Spectrochimica Acta Part A 67 (2007) 702–708

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Table 1 Experimental (Eexp ) and calculated (Ecal ) energy levels (cm−1 ) and best fit free ion parameters (cm−1 ), net electrostatic effect ( Fk ) and hydrogenic ratios (F4 /F2 and F6 /F2 ) for TZN10 glass and Nd3+ :systems Transition 4 I9/2 →

4I 9/2 4I 11/2 4I 13/2 4I 15/2 4F 3/2 4F 5/2 2H 9/2 4F 7/2 4S 3/2 4F 9/2 2H 11/2 4G 5/2 2G 7/2 4G 7/2 4G 9/2 2K 13/2 2G 9/2 2D 3/2 4G 11/2 2K 15/2 2P 1/2 2D 5/2 2P 3/2 4D 3/2 4D 5/2 4D 1/2 2I 11/2

σ (N)a EAVG F2 F4 F6 ξ Fk 4 F /F2 F6 /F2 a

TZN10

ZNP [10]

LiBO [12]

Free-ion [13]

Eexp

Ecal

Eexp

Ecal

Eexp

Ecal

Eexp

Ecal

0 1753 3753 – 11,373 12,406 – – 13,435 14,619 15,948 17,035 – 18,955 19,406 – – 21,159 – – 23,132 – – – – – –

−22 1812 3774 5822 11,336 12,358 12,619 13,326 13,408 14,601 15,914 17,088 17,262 19,014 19,376 19,607 21,035 21,137 21,339 21,569 23,173 23,799 26,038 28,032 28,174 28,532 29,353

0 – – – 11,584 12,559 – 13,565 – 14,767 – – 17,356 19,115 19,526 – – – – – – – – – – – –

25 1851 3802 5838 11,570 12,580 12,569 13,522 13,625 14,735 15,815 17,423 17,310 19,163 19,571 19,609 21,027 21,370 21,613 21,576 23,384 23,847 26,266 28,499 28,689 29,016 29,666

0 – – – 11,581 12,444 12,560 13,382 13,492 14,663 15,590 17,141 17,313 19,029 19,516 – 21,142 – – 21,701 23,250 – – – – – –

14 1881 3873 5950 11,411 12,451 12,624 13,426 13,510 14,708 15,965 17,176 17,298 19,090 19,485 19,643 21,121 21,242 21,487 21,637 23,281 23,869 26,220 28,142 28,299 28,671 29,464

0 1398 2893 4454 9371 10,138 10,033 10,859 10,950 11,762 12,495 14,187 13,888 15,443 15,705 16,089 16,764 17,096 17,410 17,642 18,694 19,046 20,857 23,092 23,246 23,465 24,358

11 1389 2895 4475 9370 10,137 10,021 10,859 10,948 11,759 12,519 14,193 13,888 15,445 15,695 16,096 16,749 17,083 17,407 17,627 18,690 19,044 20,841 23,104 23,252 23,469 24,381

±39 (13) 24,133 72,095 53,858 35,496 868 161,449 0.75 0.49

±36 (8) 24,395 73,836 52,033 35,692 863 161,562 0.70 0.48

±57 (15) 24,258 72,542 53,098 35,720 881 161,360 0.73 0.49

±10 (27) 19,717 59,960 39,937 26,429 664 126,326 0.66 0.44

For details, see Eq. (2).

of Nd3+ :LaCl3 [31] was extensively studied and the free-ion parameters were well optimized. Therefore, in our present analysis, the starting free-ion parameters for energy level fits are those obtained for Nd3+ :LaCl3 [31]. The energy levels obtained from absorption as well as emission measurements are collected in Table 1. For TZN10 glass, 13 levels have been used for parameterization and a good fit was obtained with σ value of ±39 cm−1 . The fitted free-ion parameters along with experimental and calculated energies of Nd3+ in TZN10 glass are collected in Table 1. Table 1 also shows the r.m.s deviation (σ) and the number (N) of energy levels used in the fit as σ(N) along with k F k and the hydrogenic ratios, F4 /F2 and F6 /F2 . The energy levels reported in the literature for Nd3+ -doped ZNP (zinc sodium phosphate) [10], LiBO (Li2 CO3 + H3 BO3 ) [12] glasses and free-ion [13] have also been analysed systematically for better comparison. In order to maintain consistent and meaningful values for the free-ion parameter values, only Fk and ξ parameters are varied during the energy level fits and the other parameters are fixed to

those values obtained for Nd3+ :LaCl3 [31]. The fixed parameter values are (in cm−1 ): T2 = 372, T3 = 40, T4 = 61, T6 = −291, T7 = 347, T8 = 355, M0 = 1.84, M2 = 0.56M0 , M4 = 0.38M0 , P2 = 281, P4 = 0.75P2 and P6 = 0.5P2 . The fitted free-ion parameters obtained for Nd3+ :TZN10 glass are comparable to other Nd3+ -doped glasses  and crystals [11,31]. Also the

[10,12] k , hydrogenic ratios (F4 /F2 ) and net electrostatic effect F k (F6 /F2 ) have been found to be 161,449 cm−1 , 0.75 and 0.49, respectively, for the TZN10 glass; these values are comparable to those obtained for other phosphate [10] and oxide glasses [12]. 4.2. Intensities and radiative parameters The oscillator strengths of the absorption bands have been determined using Eq. (3), and the JO parameters have been obtained via least square fitting [Eq. (4)]. The experimental and calculated oscillator strengths and JO parameters obtained

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Table 2 Experimental (fexp ) and calculated (fcal ) oscillator strengths (×10−6 ), JO parameters (Ωλ ± 5%, ×10−20 cm2 ), spectroscopic quality factor (χ), calculated (τ R ) and experimental (τ exp ) lifetimes (␮s) for the 4 F3/2 level of Nd3+ ions in the TZN10 glass and in reported Nd3+ :glasses Transition 4 I9/2 →

TZN10 fexp

4F 3/2 4F , 2H 5/2 9/2 4F , 4S 7/2 3/2 4F 9/2 2H 11/2 4G , 2G 5/2 7/2 4G 7/2 4G 9/2 2 G ,4 G 2 9/2 11/2, K15/2 2P 1/2 4D , 4D 3/2 1/2

Ω2 Ω4 Ω6 σ (N)a χ = Ω4 /Ω6 τ R (4 F3/2 ) τ exp (4 F3/2 ) a

2.80 10.06 8.75 0.55 0.27 25.33 4.76 3.52 1.74 0.38 –

Phosphate [9] fcal 3.37 9.46 9.24 0.73 0.20 25.33 5.00 2.06 1.79 0.96 –

3.80 4.94 4.54 ±0.59 (10) 1.08 153 104

fexp 1.82 6.76 6.63 0.75 – 15.68 4.30 – 4.57 2.00 10.02

Zinc borate [29]

TLF [32]

fcal

fexp

fcal

fexp

1.94 8.85 4.79 1.30 – 15.59 4.05 – 1.73 3.09 2.48

2.00 6.90 7.62 0.60 0.15 21.22 7.20 – 1.73 0.43 10.04

2.05 7.13 7.62 0.59 0.16 21.30 5.56 – 1.21 0.50 10.60

3.28 3.54 4.67 ±2.20 (9) 0.76 430 190

fcal

2.82 9.64 11.99 0.76 0.20 31.73 6.00 2.34 – 3.43 –

5.2 3.6 5.0 ±0.69 (10) 0.72 311 –

3.20 10.74 11.28 0.88 0.245 31.71 6.25 2.17 – 0.79 –

5.61 4.17 5.44 ±1.69 (11) 0.76 209 –

For details see Eq. (2)

for the present glass systems are reported in Table 2 and these are compared with the other reported Nd3+ –glass systems that include phosphate (57P2 O5 + 28.5BaO + 14.5K2 O) [9], zinc borate (4ZnO + 3B2 O3 ) [29] and fluorotellurite (TLF:70TeO2 + 30LiF) [32] glasses. As can be seen from Table 2, the calculated oscillator strengths from the JO theory agree quite well with the experimental oscillator strengths. The quality of the fit is estimated by the σ value [Eq. (2)] which is equal to ±0.59 for the TZN10 glass. The oscillator strengths and σ values of reported Nd3+ :glass systems [9,29,32] are also given in Table 2 for comparison. The experimental oscillator strengths obtained for the Nd3+ :TZN10 glass differs considerably with those obtained for phosphate, zinc borate and fluorotellurite glasses. According to Jacobs and Weber [26], the emission intensity of the 4 F3/2 → 4 I11/2 transition of Nd3+ could be characterized uniquely by the ratio of Ω4 and Ω6 parameters, the so-called spectroscopic quality factor (χ). The smaller this parameter is more intense is the 4 F3/2 → 4 I11/2 laser transition. The χ value for the present TZN10 glass system is 1.08. In this range both 4F 4 4 4 3/2 → I9/2 and F3/2 → I11/2 laser channels possess more or less equal probability for laser action [10,26].

From the JO parameters (Ωλ ), various spectroscopic parameters such as Sed , Smd , A, βR and τ R for fluorescent levels of Nd3+ in the TZN10 glass have been calculated using Eqs. (5)–(10). 4.3. Emission properties The emission spectra of the Nd3+ -doped tellurite glasses consist of three broad and asymmetric bands centered nearly at 890, 1061 and 1338 nm (Fig. 2) which are assigned to the 4 F3/2 → 4 I9/2 , 4 I11/2 and 4 I13/2 transitions, respectively. Table 3 shows the experimental peak positions (λP ), effective bandwidths (λeff ) and branching ratios (βR ) for 4 F3/2 → 4 IJ (J = 9/2, 11/2 and 13/2) transitions and experimental lifetimes (τ exp ) of the 4 F3/2 level for Nd3+ -doped tellurite based glasses. As can be seen from Table 3, among the four glasses under investigation, the Nd3+ :TLFPN01 glass possesses a relatively longer lifetime (200 ␮s) for the 4 F3/2 level and a higher βR and a lower effective bandwidth for the 4 F3/2 → 4 I11/2 transition. The experimental and calculated βR for 4 F3/2 → 4 IJ (J = 13/2, 11/2 and 9/2) transitions, experimental peak positions (λP ), effective bandwidths (λeff ) and peak stimulated emission cross-sections [σ(λp )] for the 4 F3/2 → 4 I11/2 transition and experimental life-

Table 3 Experimental laser parameters (λP , λeff , βR and τ exp ) for Nd3+ -doped tellurite glasses under investigationa Glass

TZN01 TZN10 TZPN01 TLFPN01 a

λP (nm) (4 F3/2 → 4 IJ )

λeff (nm) (4 F3/2 → 4 IJ )

βR (4 F3/2 → 4 IJ )

τ exp (␮s)

4I 9/2

4I 11/2

4I 13/2

4I 9/2

4I 11/2

4I 13/2

4I 9/2

4I 11/2

4I 13/2

4F 3/2

890.6 891.5 888.9 892.3

1061.9 1061.2 1060.7 1061.4

1338.7 1338.6 1338.5 1338.3

35.9 45.1 30.1 36.9

31.3 30.9 32.4 28.1

45.9 55.5 50.9 50.1

0.295 0.273 0.265 0.247

0.595 0.6 0.617 0.629

0.109 0.126 0.117 0.123

168 104 175 200

τ exp represents the average lifetime τ (see Eq. (15)).

η

0.68 0.44 – – 0.67 0.74

τ exp (␮s)

104 190 – – 144 194 30.9 29.3 37.9 – 25.5 28.8

λeff (nm)

0.44 – 0.38 0.25 0.39 0.40 0.27 0.35 – – 0.28 0.21 a

τ exp represents the average lifetime τ (see Eq. (15)).

0.47 – 0.51 0.73 0.50 0.49 0.60 0.53 – – 0.59 0.68 0.09 – 0.10 0.01 0.10 0.01 TZN10 Phosphate [9] Zinc borate [29] TLF [32] PKBAN10 [33] PKMAN10 [34]

0.13 0.11 – – 0.13 0.10

Experimental Calculated Experimental

Calculated

Experimental

Calculated

1061.2 1059 1060 – 1054.4 1053.5

λp (nm) 4I 9/2 4I 11/2 4I 13/2

707

Fig. 3. Room temperature emission decay curves for Nd3+ -doped: (a) TZN01, (b) TZPN01 and (c) TLFPN01 glasses.

4.27 2.78 2.64 – 6.48 4.41

σ (λp ) (4 F3/2 → 4 I11/2 ) (10−20 cm2 ) (4 F3/2 → 4 I11/2 ) βR (4 F3/2 → 4 IJ ) Glass

Table 4 Experimental and calculated βR for 4 F3/2 → 4 IJ (J = 13/2, 11/2 and 9/2) transitions, experimental λP , λeff , σ(λp ) and τ exp for the 4 F3/2 → 4 I11/2 transition and quantum efficiency (η) of the 4 F3/2 level of Nd3+ in the TZN10 glass and in reported Nd3+ :glass systemsa

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time (τ exp ) of the 4 F3/2 level for the Nd3+ : TZN10 glass are shown in Table 4 and these results are compared with those reported for other Nd3+ -doped glasses [9,29,32–34]. The 4F 4 3/2 → I11/2 transition is the most intense and has an effective bandwidth within the range (25.1–37.9 nm) found for the glass hosts reported in Table 4. The stimulated emission cross-section of the 4 F3/2 → 4 I11/2 transition, calculated using Eq. (12), is found to be 4.27 × 10−20 cm2 for the Nd3+ :TZN10 glass. The emission decay curves of the 4 F3/2 → 4 I9/2 transition for the TZN01, TZPN01 and TLFPN01 glasses are shown in Fig. 3. The experimental average lifetime τ exp (obtained from Eq. (15) of the 4 F3/2 level is shorter (104 ␮s) for the TZN10 glass than for the TLFPN01 (200 ␮s) glass. The quantum efficiency (η) is found to be 0.68 for the Nd3+ :TZN10 glass which is comparable with those of PKBAN10 and PKMAN10 glasses [33,34] (Table 4). The emission decay curves of the 4 F3/2 → 4 I11/2 transition are slightly non-exponential for all the glasses under investigation. It is worth noting that for the same glass host, when the concentration of the Nd3+ ions is reduced from 1.0 mol% (TZN10) to 0.1 mol% (TZN01), the experimental lifetime increases from 104 to 168 ␮s. This behavior is consistent with the presence of crossrelaxation processes between the Nd3+ ions in the host, even in the most diluted lanthanide doped glass sample. Possible mechanisms responsible for the cross-relaxation processes could be of the type 4 F3/2 + 4 I9/2 → 4 I15/2 + 4 I15/2 (resonant mechanism) or 4 F3/2 + 4 I9/2 → 4 I15/2 + 4 I13/2 (phonon assisted mechanism, involving the emission of phonons). These mechanisms were also suggested for other Nd3+ doped glass hosts [35,36]. 5. Conclusions The observed energy levels of Nd3+ in the TZN10 glass are similar to those observed for other Nd3+ :glasses. Energy level analysis yields calculated energies which agree quite well with the experimental ones and the calculated oscillator strengths from Judd–Ofelt theory are closer to experimental oscillator strengths. The decay curves of emission from the 4 F3/2 state are found to be slightly non-exponential for all the investigated

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tellurite glass compositions. The Nd3+ :phosphotellurite glasses under investigation (TZPN, TLFPN) show a longer lifetime for the 4 F3/2 level compared to the Nd3+ :zinc tellurite (TZN) glass samples. The shortening of decay time for the 4 F3/2 level of Nd3+ ions for the higher concentration Nd3+ -doped zinc tellurite glass (TZN10) is most probably due to the presence of energy transfer processes, such as cross relaxations, between the Nd3+ ions. Acknowledgements One of the authors (CKJ) is grateful to DAE-BRNS, Govt. of India for the sanction of major research project (Sanction No. 2003/34/4-BRNS/600) and MIUR of Italy for the financial support. References [1] H.T. Amarim, M.V.D. Vermelho, A.S. Gouveia-Neto, F.C. Cassanjes, S.J.L. Ribeiro, Y. Messaddeq, J. Alloys Compd. 346 (2002) 282. [2] E.R. Taylor, L. Nang, N.P. Sessions, H. Buerger, J. Appl. Phys. 92 (2002) 112. [3] R. Rolli, M. Montagna, S. Chaussedent, A. Monteil, V.K. Tikhomirov, M. Ferrari, Opt. Mater. 21 (2003) 743. [4] S. Balnchandin, P. Thomas, P. Marchet, J.C. Champarnand-Mesjard, B. Frit, J. Alloys Compd. 347 (2002) 206. [5] P.V. dos Santos, M.V.D. Vermelho, E.A. Gouveia, M.T. de Aroujo, A.S. Gouveia-Neto, F.C. Cassanjes, S.J.L. Ribeiro, Y. Messaddeq, J. Alloys Compd. 344 (2002) 304. [6] P. Charton, P. Armand, J. Non-Cryst. Solids 316 (2003) 189. [7] R. Rolli, K. Gatterer, M. Wachtler, M. Bettinelli, A. Speghini, D. Ajo, Spectrochim. Acta A 57 (2001) 2009. [8] D.L. Sidebottom, M.A. Hruschka, B.G. Potter, R.K. Brow, J. Non-Cryst. Solids 222 (1997) 282. [9] M. Ajroud, M. Haouari, H. Ben Ouada, H. Maaref, A. Brenier, C. Garapon, J. Phys.: Condens. Matter 12 (2000) 3181. [10] C.K. Jayasankar, V.V. Ravi Kanth Kumar, Physica B 226 (1996) 313. [11] E. Rukmini, C.K. Jayasnakar, Physica B 212 (1995) 167. [12] A. Renuka Devi, C.K. Jayasankar, Mat. Chem. Phys. 42 (1995) 106.

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