Judd–Ofelt analysis and photoluminescence properties of RE3+ (RE = Er & Nd): Cadmium lithium boro tellurite glasses

Judd–Ofelt analysis and photoluminescence properties of RE3+ (RE = Er & Nd): Cadmium lithium boro tellurite glasses

Solid State Sciences 15 (2013) 102e109 Contents lists available at SciVerse ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/lo...

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Solid State Sciences 15 (2013) 102e109

Contents lists available at SciVerse ScienceDirect

Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie

JuddeOfelt analysis and photoluminescence properties of RE3þ (RE ¼ Er & Nd): Cadmium lithium boro tellurite glasses K. Vemasevana Raju, C. Nageswara Raju, S. Sailaja, B. Sudhakar Reddy* Department of Physics, S.V. Degree College, Kadapa, Y.S.R. District 516003, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 July 2011 Received in revised form 30 July 2012 Accepted 23 August 2012 Available online 13 September 2012

Rare earth (Er3þ and Nd3þ) ions doped cadmium lithium boro tellurite (CLiBT) glasses were prepared by melt quenching method. The viseNIR absorption spectra of these glasses have been analyzed systematically. JuddeOfelt intensity parameters Ul (l ¼ 2, 4, 6) have been evaluated and used to compute the radiative properties of emission transitions of Er3þ and Nd3þ: CLiBT glasses. From the NIR emission spectra of Er3þ: CLiBT glasses a broad emission band centered at 1538 nm (4I13/2 / 4I15/2) is observed and from Nd3þ: CLiBT glasses, three NIR emission bands at 898 nm (4F3/2 / 4I9/2), 1070 nm (4F3/2 / 4I11/2) and 1338 nm (4F3/2 / 4I13/2) are observed with an excitation wavelength lexci ¼ 514.5 nm (Arþ Laser). The FWHM and stimulated emission cross-section values are calculated for Er3þ and Nd3þ: CLiBT glasses. FWHM  sPe values are also calculated for Er3þ: CLiBT glasses.  2012 Elsevier Masson SAS. All rights reserved.

Keywords: Er3þ and Nd3þ: CLiBT glasses viseNIR absorption NIR emission

1. Introduction Over the past several years, tellurite glasses have become a subject of investigation of much interest due to their promising optical properties such as good chemical durability, good thermal and mechanical stability, high refractive index, good transparency in the mid-infrared region (0.35e6 mm), low phonon energy values (700e800 cm1), lower melting temperature, high linear and nonlinear refractive indices, a wide transmission window (0.4e6 mm), excellent transmittance in the visible and near infrared spectral regions and also high solubility for rare earth ions [1]. The application of tellurite glasses in industries such as electrical, optical, electronic and other fields are immense due to their good semiconducting properties. It is well known that a pure TeO2 chemical does not form a glass but it does so, when it is mixed with certain other oxides such as B2O3, CdO and Li2O etc. Further, the glasses upon addition of Li2O as network modifier (NWM) could strengthen (or) enhance certain electrical, thermal and optical properties [2e7]. Tellurite glasses have recently gained wide attention because of their potential as hosts of rare earth ions for the development of fibers and lasers covering all the main telecommunication bands, photonic applications and optical switching devices [8e11]. Among the rare earth ions, Er3þ is the most popular as well as one of the most efficient ion and on the other hand, Er3þ-doped

* Corresponding author. Tel./fax: þ91 8562 259059. E-mail address: [email protected] (B.S. Reddy). 1293-2558/$ e see front matter  2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.08.011

fiber amplifier is one of the important devices for use in 1.55 mm wavelength optical communication window [12e15]. On the other hand Er3þ waveguide laser and up-conversion laser operations have been achieved at room temperature. The optical properties of Er3þ are of great interest because of their use in infrared lasers operating at eye-safe wavelengths and in the fabrication of optical amplifiers [16]. In recent years Er3þ ions doped different host materials have been studied efficiently because of their applications for lasers, temperature sensors, colour display, biomedical diagnostics and optical amplifiers [17,18]. Erbium-doped tellurite fibers are the promising candidates in the fabrication of novel optical amplifiers because they can offer broad amplification bandwidth and high radiative transition efficiencies [19]. Er-doped fiber amplifiers (EDFA) have advantageous properties such as high gain, polarization transparency, low coupling loss, low cost due to these properties Er-doped fiber amplifiers (EDFA) are commonly used in long distance fiber communications [20]. Rare earth ions activated glasses with emission in the NIR region are attracting a great deal of interest because of their use as laser host materials and optical amplifiers for use in telecommunication devices. Among different rare earth ions, Nd3þ is the most efficient ion for obtaining the laser action, frequency up-conversion and optical fiber amplification [21,22]. Neodymium glasses have been proven to be one of the most efficient candidates for photonic devices such as fiber lasers, microchip lasers and planar waveguides [23e25]. In recent years, research has been focused in the near infrared spectral range from 800 to 1500 nm; where Nd3þ ions present emission at 898, 1070 and 1338 nm originating from the

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

electronic transitions between the 4fn levels, 4F3/2 / 4I9/2, 4F3/2 / 4I11/2, 4F3/2 / 4I13/2 respectively. The present work is devoted to the analysis of effect of rare earth ion concentration on luminescence properties of Er3þ and Nd3þ ions doped cadmium lithium boro tellurite glasses. 2. Experimental studies The chemicals used were reagent grade of H3BO3, TeO2, CdO, Li2CO3, Er2O3 and Nd2O3 and (99.9% purity Merck, Himedia, Sigma Aldrich). Rare earth ions doped (Er3þ and Nd3þ) glasses have been prepared by melt quenching method in the following compositions.

ð60  xÞB2 O3  10CdO  20TeO2  10Li2 O  xRE3þ ðRE ¼ Er and NdÞ where x ¼ 0.5, 1.0, 2.0 and 3.0 mol %. All the weighed chemicals were finely powdered and then mixed thoroughly before each of batches (10 g) was melt by using alumina crucibles in an electric furnace at 950  C for an hour. These melts were quenched in between two brass plates and thus obtained 2e3 cm diameter optical glasses with a uniform thickness 0.3 cm and these glasses were annealed at 200  C for an hour in order to remove thermal strains if any in them soon after the glasses production. Abbe’s refractometer was employed to measure the glass refractive indices with sodium vapour lamp (589.3 nm). The values of refractive indices of (60  x) B2O3e10CdOe20TeO2e10Li2OexEr3þ (where x ¼ 0.5, 1.0, 2.0 and 3.0 mol %) glasses are 1.754, 1.757, 1.762 and 1.767 respectively and that of (60  x) B2O3e10CdOe20TeO2e10Li2OexNd3þ (where x ¼ 0.5,1.0, 2.0 and 3.0 mol%) glasses are 1.753,1.755,1.759 and 1.762 respectively. The optical absorption spectra (350e2500 nm) for all glasses were measured on a VarianeCary win spectrometer. NIR photoluminescence spectra of the Er3þ and Nd3þ: CLiBT glasses were also measured on a Horiba Triax e 550 grating monochromator (JOBIN YVON HORIBA) equipped with a liquid nitrogen cooled InGaAs photodetector (Electro-optical system Inc) in the wavelength range of 800e2400 nm and a lockin amplifier (SR830 DSP, standard Research systems) with an Arþ laser (514.5 nm) (LEXEL MODEL 85 ION LASER, 5 mWe200 mW) as the excitation source.

103

electronic transitions can be electric dipole or magnetic dipole in character. The qualitative calculations of the intensities of these transitions have been developed independently by Judd [27] and Ofelt [28]. Over the years, the JuddeOfelt theory has been proved to be quite successful for the intensity analysis of the trivalent lanthanide ions. Earlier a lot of pioneering work on the intensities of the fef transitions of the lanthanide series and the systematic intensity parametrization has been done by Carnall et al. [29e31]. A brief outline of the JuddeOfelt theory is given below. 3.3. The oscillatory strengths The spectral intensities are expressed in terms of oscillatorstrengths. Experimentally, the oscillator strength could be calculated from the area of the absorption band under the Gaussian curve [32].

fexp ¼



2303mc2 =NA pe2

Z

˛ðvÞdv

where NA is the Avogadro number and ˛(n) is the molar absorption coefficient. This equation reduces to

fexp ¼ 4:32  109

Z ˛ðvÞdv

The molar absorption coefficient ˛(n) at a given energy is evaluated from the BeereLambert Law [32].

˛ðnÞ ¼

1 I log 0 cl I

where ‘c’ is the concentration of lanthanide ion (mol %), l is the thickness of the glass (cm) and log (I0/I) is the absorptivity (A) or the optical density (OD). According to the JeO theory, an expression for the theoretical evaluation of the electric dipole oscillator strength is given by

f ¼

6 X

l2

   2   Tl n jj U l j1j

3.1. Nephelauxetic effect e bonding parameter

 2   in which the U l  are unit tensor operators of the rank l ¼ 2,4,6 and Tl are the three JeO parameters which can be evaluated from the experimental absorption spectra. A relationship has been established between the Tl, Ul and nd as shown below [33].

The nature of the ReO bond is known by the nephelauxetic ratios (b) and the bonding parameters (d) which are computed by using the following formulae [26]. The nephelauxetic ratio is given by

" # 3h 9nd Ul ¼  2 ð2J þ 1ÞTl 8p2 mc n2d þ 2

3. Theory

b ¼ vg =va where ng is the wavenumber (in cm1) of a particular transition to an ion in the glass and na is the wavenumber (in cm1) of the same transition for the aqua ion. The bonding parameter d is given by



1b

b

For trivalent lanthanides ions, the oscillator strength ‘f’ or allowed magnetic dipole (MD) and forbidden (induced) electric dipole (ED) transition is of the order of 106. According to the JeO theory, the calculated oscillator strength of a transition of average frequency (n) from a level (jJ) to a level (J0J 0 ) is



fcal jJ ; j0j0



where b is the average value of b.

8p2 mn ¼ 3hð2J þ lÞe2

"

2 # n2d þ 2 Sed þ n3d Smd 9nd

3.2. The JuddeOfelt theory

where nd is the refractive index of the medium at the sodium D line, Sed is the electric dipole line strength [34].

The optical absorption spectra of rare earth ions doped glasses helps for understanding the radiative properties of the emission transitions. The sharp absorption lines arising from the 4fe4f

Sed ¼ e2

6 X 2

 



Ul jJ U l j0J0

2

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K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

Smd is the line strength for the magnetic dipole transitions that could be obtained by the expression



Smd jJ ; j0J 0





e2 h2

¼

16p2 m2 c2

jJ kL þ 2Skj0J 0

1  ¼ P  A jJ ; j0J 0 j0J0

2

The magnetic dipole line strengths have not been considered, since the sharp lines arising due to fef transitions are essentially electric dipole in nature. The absorption cross-section values have been calculated by using the formula

sabs ðlÞ ¼ 2:303

 

sR jJ

ODðlÞ N0 L

stronger emission probabilities and more transitions from a level lead to faster decay and shorter lifetimes. The stimulated emission cross-section (sPe cm2) for all emission transitions has been computed from the formula.

l4p 8 cn2d Dlp sm

sEp ¼ Q

where lp is the emission peak wavelength and Dlp is the full width at half maximum of the emission transition [35e39].

where ‘N0’ is the concentration of the RE3þ ions in the glass, OD(l) is optical density obtained from the absorption spectrum and ‘L’ is the thickness of the sample.

4. Results and discussion 4.1. Er3þ: CLiBT glasses

3.4. Radiative properties JeO intensity parameters are used to calculate the radiative properties of the emission transitions. The electric and magnetic dipole line strengths of a transition JJ / JJ0 are given by

Sed ¼ e2

6 X

 



Ul jJ U l j0J0

2

2

and

  Smd jJ ; j0J 0 ¼

2 e2 h2  jJ kL þ 2Skj0J 0 2 2 2 16p m c

respectively. For emission (or luminescence) spectra, the spontaneous emission coefficient (also called transition probability for spontaneous emission or the Einstein coefficient for spontaneous emission) A (jJ, J0J 0 ) can be written as

2   ¼ A jJ ; j0J0



2 6nd nd

64p4 n3 6 3hð2J þ 1Þ 4

þ2

9

2

3 7 Sed þ n3d Smd 7 5

The viseNIR absorption spectrum of (2.0 mol %) Er3þ: CLiBT glass is shown in Fig. 1. From this spectrum, sharp absorption bands at 452 nm, 489 nm, 520 nm, 652 nm, 792 nm, 974 nm and 1530 nm are observed and these are assigned to the electronic transitions 4 I15/2 / 4F5/2, 4F7/2, (4G11/2, 2H11/2), 4F9/2, 4I9/2, 4I11/2 and 4I13/2 respectively. Assignments to these bands have been made by our earlier paper [40]. JuddeOfelt theory is a useful method for analyzing the spectral properties of rare earth ions doped glasses. The JuddeOfelt intensity parameters (Ul  1020 cm2) have been derived by using a least square fit analysis [41] and the results are compared with earlier reported literature [12,14,42] which are presented in Table 1. The validity of fitting has been examined by comparing the experimental and calculated oscillatory strengths of Er3þ absorption bands which are also listed in Table 1, along with the estimated root mean square deviations (drms). The low drms value clearly indicates the fairness of fitting. The nature of the ReO bond has been known from the nephelauxetic ratio (b) and bonding parameters (d) which are computed and the values are also presented in Table 1. The nature of the bond will depends on the positive or negative sign of d, for the present glass the value of d is

Like in the case of absorption spectra, for the measured emission spectra also, the electric dipole line strength (Sed) have been calculated for all excited state manifolds of the rare earth glassy materials concerned. Because an excited state jJ is relaxed to several lower-lying states J0J 0 , the radiative branching ratio ßR is defined as



bR jJ ; j0J 0



  A jJ ; j0J 0  ¼ P  A jJ ; j0J 0 j’J0

where the factor in the denominator is the total radiative transition probability. The branching ratios can be used to predict the relative intensities of all emission lines originating from a given excited state. The experimental branching ratio can be found from the relative areas of the emission lines. Once all emission probabilities that depopulate an initial level 2Sþ1LJ have been calculated, they can be used to determine how fast that level is depopulated. This rate is given by the radiative lifetime sR (jJ)

Fig. 1. viseNIR absorption spectrum of (2.0 mol %) Er3þ: CLiBT glass.

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

105

having negative sign that indicates the ionic nature of the ReO bond. For Er3þ: CLiBT glass, the JeO intensity parameters follow the trend of U2 > U4 > U6. In addition to that the spectroscopic quality factor (U4/U6) has been calculated to characterize the glasses concerned. The absorption cross-section (sabs) for the transition (4I15/2 / 4I13/2) at 1530 nm of (2.0 mol%) Er3þ: CLiBT glass was calculated and the value is 0.709  1020 cm2, which is comparable with the value reported in the literature [43]. Fig. 2, shows the NIR emission spectra of Er3þ: CLiBT glasses. From the measured NIR emission spectra, we have observed a broad emission centered at 1538 nm (4I13/2 / 4I15/2) with 514.5 nm (Arþ laser) as the excitation source. Because of the overlap of the 4I13/2 / 4I15/2 emission band and the 4I15/2 / 4I13/2 absorption band at 1.53 mm, the broadening of the emission band was observed and should mainly be attributed to the radiative trapping and the emission

intensity at this wavelength is very high than other transitions. Because of self-absorption process, the emission performance of Er3þ ions is significantly altered with the increase of the erbium concentration. Thus, we can observe bandwidth broadening with the increase of the erbium content [44]. The NIR emission spectra shows that emission intensity gradually increases upto the critical concentration (2.0 mol %) and beyond that it decreases indicating the concentration quenching phenomena. This is because, at higher concentrations i.e. beyond 2.0 mol % the interactions between Er3þ ions are very high because the distance between them is decreased and this causes the non-radiative processes, formation of ion pairs and clusters. Further, the stepwise increase of the concentration of Er3þ ions leads to the weak emission intensity. Hence 2.0 mol % is the optimized concentration. Fig. 3 shows the emission intensity of Er3þ as a function of its doping concentration (mol %). The inset of Fig. 3 shows the graph plotted between intensity/concentration and Er3þ ion concentration. This graph gives the evidence for the concentration quenching and the quenching of emission intensity may be assigned to the non-radiative energy transfer [36]. The excitation-dependent photoluminescence spectra for (2.0 mol %) of Er3þ: CLiBT glass are shown in Fig. 4. It indicates that these emission peaks do not show any considerable shift with the change in excitation power but the NIR emission intensity has linear dependence on excitation source power as shown in Fig. 5. By using the JuddeOfelt intensity (Ul) parameters, radiative properties of emission band of Er3þ: CLiBT glass have been computed and the results relating to spontaneous emission transition probability (A s1), radiative transition rate (AT s1), radiative life time (sR ms) and branching ratio (b) of the emission transition 4I13/2 / 4I15/2 (1538 nm) are presented in Table 2. The full width at half maximum (FWHM, Dlp) and the stimulated emission cross-section values of the NIR emission band at 1538 nm (4I13/2 / 4I15/2) for all the Er3þ: CLiBT glasses have been computed and the values are presented in Table 3. From the Table it is observed that FWHM value is maximum for 2.0 mol % of Er3þ: CLiBT glass and the value is 84 nm. The stimulated emission cross-section (sPe ) values of all Er3þ: CLiBT glasses are greater than other glass systems which are listed out in Table 3. Further, the product of the stimulated emission cross-

Fig. 2. NIR emission spectra of Er3þ: CLiBT glasses.

Fig. 3. Dependence of 1.53 mm emission line intensity on the Er2O3 concentration and inset shows the intensity/concentration as a function of erbium concentration.

Table 1 Experimental and calculated oscillator strengths (f  106) of absorption transitions, JuddeOfelt intensity parameters (Ul  1020 cm2), spectroscopic quality factor (U4/ U6), drms (RMS deviation), b (Nephelauxetic ratio), d (Bonding parameter) values of 2.0 mol % of Er3þ ions doped CLiBT glasses. Wavelength

Oscillator strengths (f  106) CLiBT: Er3þ glass fexp

fcal

I13/2 4 I11/2 4 I9/2 4 F9/2 (4G11/2, 2H11/2) 4 F7/2 4 F5/2 drms (RMS deviation) b (Nephelauxetic ratio) d (Bonding parameter)

1530 974 792 652 520 489 452 0.9201 1.0029 0.0029

e 1.21 0.41 3.42 16.12 2.11 e

0.93 2.38 0.89 3.53 16.04 1.83 0.89

JeO intensity parameters

CLiBT: Er3þ glass 5.213 1.932 1.054 1.833

References [35] [36] [37]

Absorption transition from ground state 4

I15/2/ 4

U2 U4 U6 U4/U6

106

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109 Table 2 Emission transition, wavelength, estimated values of spontaneous emission transition probabilities (A s1), radiative rate (AT s1), radiative lifetime (sR ms) and branching ratios (b%) of emission transitions of 2.0 mol % of Er3þions doped CLiBT glasses. Emission transition

4



Fig. 4. Excitation-dependent photoluminescence spectra of (2.0 mol %) Er glass.

: CLiBT

section (sPe ) and the FWHM i.e. sPe  FWHM, is an important factor to characterize laser materials, because the gain bandwidth of an amplifier can be evaluated by sPe  FWHM value. If the product of these two factors is larger, then the properties of the amplifier are also better. From Table 3, it is observed that FWHM  sPe product values of all Er3þ: CLiBT glasses are also greater than that of other glass systems such as germanates [45], silicates [46], and phosphates [47]. Thus, tellurite glass is the best host material for Er3þdoped broadband amplifiers than other glass host materials. Particularly 2.0 mol % of Er3þ-doped tellurite glass is ideal for broadband amplification [14,42].

CLiBT: Er3þ glass

Wavelength

4

I13/2 / I15/2 AT (s1) sR (ms) 4 I11/2 / 4I13/2 4 I15/2 AT (s1) sR (ms) 4 F9/2 / 4I9/2 4 I11/2 4 I13/2 4 I15/2 AT (s1) sR (ms) 4 S3/2 / 4I9/2 4 I11/2 4 I13/2 4 I15/2 AT (s1) sR (ms) 2 H9/2 / 4F9/2 4 I9/2 4 I11/2 4 I13/2 4 I15/2 AT (s1) sR (ms)

1538

2746 987

3466 1956 1143 660

1676 1226 843 547

1075 825 693 549 404

A (s1)

b%

412.5 412.5 2.424 78.2 480.1 558.3 1.791 22.01 240.8 203.4 4479.2 4945.41 0.2022 198.6 134 1593.4 4379 6305 0.1586 78.3 370 301.6 359.8 850 1959.4 0.510

100

14 85.99

0.445 4.869 4.112 90.572

3.149 2.125 25.272 69.452

3.996 18.883 15.377 18.362 43.380

4.2. Nd3þ: CLiBT glasses The absorption spectrum of (2.0 mol %) Nd3þ-doped glass is shown in Fig. 6. From this spectrum, absorption bands at 472 nm, 511 nm, 524 nm, 583 nm, 624 nm, 686 nm, 745 nm, 802 nm and 874 nm are identified and these are assigned to the electronic transitions 4I9/2 / 2G9/2, 4G9/2, 4G7/2, (4G5/2, 2G7/2), 2H11/2, 4F9/2, (4F7/ 4 4 4 4 2, S3/2), ( F5/2, H9/2) and F3/2 respectively. Assignments to these bands have been made by our earlier paper [48]. In the case of Nd3þ ion, the absorption takes place from the ground state 4I9/2 to various excited states which are predominately 4fe4f induced electric dipole in nature. The JuddeOfelt intensity parameters (Ul  1020 cm2) and root mean square deviation (drms) have been calculated and the results are compared with earlier reported literature [38,49,50] which are presented in Table 4. From Table 4, it is clear that the oscillator strength of the transition 4I9/2 / (4G5/2, 2 G7/2) at 583 nm is high compared with the other absorption transitions. Thus, the transition 4I9/2 / (4G5/2, 2G7/2) is known as the hypersensitive transition (HST) and follows the selection rules

Table 3 FWHM (DlP nm), stimulated emission cross-section sPe  1020 cm2 and FWHM  sPe (1020) values for the transition at 1.53 mm of all Er3þ: CLiBT glasses. Glasses

Fig. 5. The emission (at 1538 nm) intensity dependence on excitation source power change for (2.0 mol %) Er3þ: CLiBT glass.

Present study 0.5 mol % Er3þ: CLiBT 1.0 mol % Er3þ: CLiBT 2.0 mol % Er3þ: CLiBT 3.0 mol % Er3þ: CLiBT Other glass systems Silicate glasses Germanate glasses Phosphate glasses

glass glass glass glass

FWHM (DlP nm)

sPe  1020 cm2

FWHM  sPe (1020) (cm2 nm)

63 69 84 82

1.580 1.438 1.174 1.196

99.54 99.22 98.65 98.09

45 53 37

0.55 0.57 0.64

22.00 30.21 23.68

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

107

Fig. 6. viseNIR absorption spectrum of (2.0 mol %) Nd3þ: CLiBT glass.

DS ¼ 0, DL ¼ 2 and DJ  2. Because of its hypersensitivity this transition shows the maximum intensity in the absorption spectra compared with the other transitions. For Nd3þ: CLiBT glass, the JeO intensity parameters follow the trend of U2 < U4 < U6. The nephelauxetic ratio (b) and bonding parameters (d) have been computed to know the nature of the ReO bond and the values are presented in Table 4. The JeO intensity parameter U2 describes the asymmetry of the local environments around Nd3þ sites. Thus it depends on the covalency between Nd3þ ions and ligand anions. The smaller value of intensity parameter (U2) indicates the lower asymmetric nature of the ion sites and the ionic nature of the chemical bond between Nd3þ ions and the ligands [51]. In the present investigation also U2 is having the smaller value indicating the ionic nature and lower asymmetry of NdeO bond. For this Nd3þ: CLiBT glasses, the value of d (bonding parameter) is also having negative sign that confirms the ionic nature of the ReO bond. The spectroscopic quality factor

Table 4 Experimental and calculated oscillator strengths (f  106) of absorption transitions, JuddeOfelt intensity parameters (Ul  1020 cm2), spectroscopic quality factor (U4/ U6), drms (RMS deviation), b (Nephelauxetic ratio), d (Bonding parameter) of 2.0 mol % of Nd3þ ions doped CLiBT glasses. Absorption transition from ground state 4

Wavelength

I9/2/

Oscillator strengths (f  106) CLiBT: Nd3þ glass fexp

fcal

F3/2 4 F5/2, 4H9/2 4 F7/2, 4S3/2 4 F9/2 2 H11/2 4 G5/2, 2G7/2 4 G7/2 4 G9/2 2 G9/2 drms (RMS deviation) b (Nephelauxetic ratio) d (Bonding parameter)

874 802 745 686 624 583 524 511 472 2.7351 1.0013 0.0012

e 7.95 e e e 17.53 12.40 10.96 e

2.42 8.43 8.72 0.62 0.21 17.96 11.76 8.38 1.56

JeO intensity parameters

CLiBT: Nd3þ glass 2.43 3.41 3.96 0.861

References

4

U2 U4 U6 U4/U6

Fig. 7. NIR emission spectra of Nd3þ: CLiBT glasses.

(X) is the ratio of the intensity parameters U4, U6 and this factor indicates the stronger emission intensity from the 4F3/2 level of Nd3þ ions. If X value is smaller than unity, then the intensity of 4F3/ 4 2 / I11/2 transition is having stronger emission intensity than that of the 4F3/2/4I9/2. In the present work the value of spectroscopic quality factor (X) is 0.861 indicating that the transition 4F3/2 / 4I11/2 at 1070 nm is having the greater intensity [38]. The absorption cross-section (sabs) value was calculated for the transition (4G5/2, 2 G7/2) at 583 nm of (2.0 mol %) Nd3þ: CLiBT glass and the value is 0.1417  1020 cm2. The NIR emission spectra of Nd3þ: CLiBT glass in the region 800e1500 nm obtained by 514.5 nm (Arþ laser) as the excitation source is shown in Fig. 7. From this spectra, three NIR emission bands at 898 nm, 1070 nm and 1338 nm are observed which are corresponding to the transitions (4F3/2 / 4I9/2), (4F3/ 4 4 4 2 / I11/2) and ( F3/2 / I13/2) respectively [37]. The radiative properties of all the emission bands of (2.0 mol %) Nd3þ: CLiBT glass have been computed and the results relating to spontaneous emission transition probability (A s1), radiative transition rate (AT s1), radiative life time (sR ms) and branching ratio (b) are presented in Table 5. From the measured NIR emission spectra, it is found that the emission intensity of Nd3þ (0.5, 1.0, 2.0 and 3.0 mol %)-doped glasses enhances with the increase of Nd3þ-doping ratio

Table 5 Emission transition, wavelength peak position (lP nm), band width (DlP nm) and estimated values of spontaneous emission transition probabilities (A s1), radiative rate (AT s1), radiative lifetime (sR ms) and branching ratios (b%) of emission transitions of (2.0 mol %) Nd3þions doped CLiBT glasses. Emission transition

[40] [41] [14]

4

4

F3/2 / I13/2 4 I11/2 4 I9/2 AT (s1) sR (ms)

lP (nm) 1338 1070 898 5305.7 0.1884

DlP (nm) 46 40 49

CLiBT: Nd3þ glass A (s1)

b%

748 2804.4 1696.8

14.098 52.856 31.980

108

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

Fig. 8. Dependence of emission (at 1070 nm) intensity on the Nd2O3 concentration and inset shows the intensity/concentration as a function of neodymium concentration.

and the maximum emission intensity appears at x ¼ 2.0 mol %. When the Nd3þ-doping ratio is higher above 2.0 mol % the luminescent intensity was reduced. The decrease of luminescence intensity is due to the fact that, with the increase in the concentration of dopant ions, the spacing among the ions decreases and it may be small which allows the rare earth ions to interact and also to transfer energy [52]. Thus concentration quenching occurs and 2.0 mol % is the optimized concentration. Fig. 8 shows the emission intensity of Nd3þ as a function of doping concentration (mol %). Another graph plotted between intensity/concentration and Nd3þ

Fig. 10. The emission (at 1070 nm) intensity dependence on excitation source power change for (2.0 mol %) Nd3þ: CLiBT glass.

ion concentration is shown in the inset of Fig. 8, this graph is useful for the confirmation of the concentration quenching and the quenching of emission intensity may be due to the non-radiative energy transfer. For (2.0 mol %) of Nd3þ: CLiBT glass, the excitation-dependent photoluminescence spectra are shown in Fig. 9. It indicates that, the emission intensity varies with the excitation power linearly as shown in Fig. 10. The FWHM and stimulated emission cross-section values of the emission band at 1070 nm for all Nd3þ: CLiBT glasses are calculated and compared with the other glass systems which are listed out in Table 6. From the table it is observed that FWHM is maximum for (2.0 mol %) of Nd3þ: CLiBT glass i.e. 40 nm. Moreover, the value of stimulated emission cross-section (sPe ) explains the laser performance of a material and also its value estimates the rate of energy extracted from the lasing material. From Table 6 it is clear that the stimulated emission cross-section values of all Nd3þ: CLiBT glasses are comparable with the commercial laser glasses such as LHG-80 [38,51], LG-770 [38,51], Q-88 [38,51], LHG-8 [38,51], LG-750 [39,53]. Thus, it can be concluded that tellurite glasses are suitable as laser hosts. Especially, 2.0 mol % of Nd3þ: CLiBT glass is a good candidate for laser materials.

Table 6 FWHM (DlP nm), stimulated emission cross-section sPe  1020 cm2 values for the transition at 1.07 mm of all Nd3þ: CLiBT glasses. Glasses

Fig. 9. Excitation-dependent photoluminescence spectra of (2.0 mol %) Nd3þ: CLiBT glass.

Present study 0.5 mol % Nd3þ: CLiBT 1.0 mol % Nd3þ: CLiBT 2.0 mol % Nd3þ: CLiBT 3.0 mol % Nd3þ: CLiBT Other glass systems LHG-80 LG-770 Q-88 LHG-8 LG-750

glass glass glass glass

FWHM (DlP nm)

sPe  10-20 cm2

26 33 40 31

3.912 2.502 2.110 3.216

24 25 22 e e

4.2 3.9 4.0 3.7 3.6

K.V. Raju et al. / Solid State Sciences 15 (2013) 102e109

5. Conclusions Er3þ and Nd3þ: CLiBT glasses were prepared by melt quenching method. By applying the JuddeOfelt theory, the intensity parameters U2, U4 and U6 and radiative properties of the emission levels of Er3þ and Nd3þ: CLiBT glasses were evaluated. Er3þ: CLiBT glasses have shown broad infrared emission at 1.53 mm which is having stronger intensity. FWHM values of Er3þ: CLiBT glasses are changing from 63 nm to 84 nm and stimulated emission crosssection values are in the range from 1.174  1020 cm2 to 1.580  1020 cm2. The Er3þ: CLiBT glasses exhibit higher FWHM  sPe (99.54  1020 cm2 nm) value than other glass hosts. These experimental results indicate that (2.0 mol %) Er3þ: CLiBT glass is a promising host material for broadband optical amplifiers. In the case of Nd3þ: CLiBT glasses, the lower value of JeO intensity parameter (U2) indicates the lower asymmetry nature around Nd3þ ions and ionic nature of the chemical bond between Nd3þ ions and the ligands. The smaller value of spectroscopic quality factor (less than unity) indicates that, 4F3/2 / 4I11/2 transition is having stronger emission intensity. The stimulated emission cross-section values of all Nd3þ: CLiBT glasses are comparable with the commercial laser glasses. Thus, it can be concluded that tellurite glasses are suitable as laser hosts and 2.0 mol % of Nd3þ: CLiBT glass is a good candidate for laser materials. Acknowledgement This work was supported by the University Grants Commission, New Delhi in the form of Major Research Project (F.No.35-1/2008 SR) sanctioned to the author (BSR) and also provided the financial assistance to one of the co-author (KVR) as a Project Fellow, who would like to thank, the Joint Secretary, UGC, New Delhi, India. References [1] I. Jiassi, H. Elhouichet, M. Ferid, R. Chtourou, M. Oueslati, Opt. Mater. 32 (2010) 743e747. [2] A. Pan, A. Ghosh, Phys. Rev. B 62 (2003) 3190. [3] A. Pan, A. Ghosh, J. Mater. Res. 17 (2002) 1941. [4] A. Pan, A. Ghosh, Phys. Rev. B 60 (1999) 3224. [5] D. Dutta, A. Ghosh, Phys. Rev. B 72 (2005) 024201. [6] A. Ghosh, A. Pan, Phys. Rev. Lett. 84 (2000) 2188. [7] S. Hazra, S. Mandal, A. Ghosh, Phys. Rev. B 56 (1997) 8021. [8] G. Nunziconti, S. Bemeschi, M. Beltinelli, M. Breei, B. Chen, S. Pelli, A. Speghini, G.C. Righini, J. Non-Cryst. Solids 345 (2004) 343. [9] M.A. Sidkey, M.S. Goafar, Phys. B 46 (2004) 348. [10] Joris Lousteau, Nadia Boetti, Alessandro Chiasera, Maurizio Ferrari, Silvio Abrate, Giuseppe Scarciglia, Alberto Venturello, Daniel Milanese, IEEE Photonics J. 4 (2012) 194. [11] Nadia G. Boetti, Joris Lousteau, Alessandro Chiasera, Maurizio Ferrari, Emanuele Mura, Gerardo C. Scarpignato, Silvio Abrate, Daniel Milanese, J. Lumin. 132 (2012) 1265.

109

[12] Isabella-Ioana Oprea, Hartmut Hesse, Klaus Betzler, Opt. Mater. 28 (2006) 1136e1142. [13] Yanmin Yang, Zhiping Yang, Panlai Li, Xu Li, Qinglin Guo, Baojiu Chen, Opt. Mater. 32 (2009) 133e138. [14] Hong-Wei Li, Shi-Qing Man, Opt. Commun. 282 (2009) 1579e1583. [15] Chunlei Yu, Dongbing He, Guonian Wang, Junjie Zhang, Lili Hu, J. Non-Cryst. Solids 355 (2009) 2250e2253. [16] Y.K. Sharma, S.S.L. Surana, R.K. Singh, R.P. Dubedi, Opt. Mater. 29 (2007) 598e604. [17] Boyuan Lai, Li Feng, Jing Wang, Qiang Su, Opt. Mater. 32 (2010) 1154e1160. [18] Chengguo Ming, Feng Song, Yin Yu, Gong Zhang, Qingru Wang, Hua Yu, Tongqing Sun, Jianguo Tian, Opt. Commun. 284 (2011) 1868e1871. [19] Junjie Zhang, Shixun Dai, Guonian Wang, Hongtao Sun, Liyan Zhang, Lili Hu, J. Lumin. 115 (2005) 45e52. [20] Kang ill Cho, Ki Hyun Cho, Seung Hyun Cho, Dong Wook Shin, Opt. Mater. 28 (2006) 888e892. [21] B. Jacquier, A. Remillieux, M.F. Joubert, P. Christensen, H. Poingnent, J. NonCryst. Solids 161 (1993) 241. [22] J. Fernandez, R. Balda, A. Mendioroz, M. Sanz, J.L. Adam, J. Non-Cryst. Solids 287 (2001) 437. [23] Ki-Soo Lim, Chul-Woo Lee, Sung-Taek Kim, J. Lumin. 87 (2000) 1008. [24] L.C. Courrol, E.P. Maldonado, L. Gomes, N.D. Vieira, Opt. Mater. 14 (2000) 81. [25] L.B. Shaw, R.S.F. Chang, N. Djeu, Phys. Rev. B 50 (1994) 6609. [26] S.P. Sinha, Complexes of the Rare Earths, Pergamon Press, Oxford, 1966. [27] B.R. Judd, Phys. Rev. 127 (1962) 750. [28] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [29] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797. [30] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [31] W.T. Carnall, The absorption and fluorescence spectra of rare earth ions in solutions, In: Handbook on the Physics and Chemistry of Rare Earths North Holland, Amsterdam 1974. [32] J.C.G. Bunzil, G.R. Choppin, Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, Amsterdam, 1989. [33] S. Hufner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978. [34] W.M. Yen, in: I.Z. Zschokke (Ed.), Oprical Spectroscopy of Glasses, Reidel D. Publishing Company, Doredrecht, 1986. [35] K. Upendra Kumar, P. Babu, Kyoung Hyuk Jang, Hyo Jin Seo, C.K. Jayasankar, A.S. Joshi, J. Alloy. Compd. 458 (2008) 509e516. [36] N. Jaba, H. Ben Mansour, K. Knaoun, A. Brenier, B. Champagnon, J. Lumin. 129 (2009) 270e276. [37] P. Babu, Hyo Jin Seo, C.R. Kesavulu, Kyoung Hyuk Jang, C.K. Jayasankar, J. Lumin. 129 (2009) 444e448. [38] S. Surendra Babu, R. Rajeswari, Kiwan Jang, Cho Eun Jin, Kyoung Hyuk Jang, Hyo Jin Seo, C.K. Jayasankar, J. Lumin 130 (2010) 1021e1025. [39] L. Jyothi, V. Venkatramu, P. Babu, C.K. Jayasankar, M. Bettinelli, G. Mariotto, A. Speghini, Opt. Mater. 33 (2011) 928e936. [40] B. Sudhakar Reddy, S. Buddhudu, Spectrosc. Lett. 41 (2008) 1e8. [41] W. Krupke, IEEE J. Quantum Electron. 10 (1974) 450. [42] Tiefeng Xu, Xudong Zhang, Guangpo Li, Shixun Dai, Qiuhua Nie, Xiang Shen, Xianghua Zhang, Spectrochim. Acta Part A 67 (2007) 559e563. [43] G. Bilir, G. Ozen, D. Tatar, M.L. Ovecoglu, Opt. Commun. 284 (2011) 863e866. [44] N. Jaba, M. Ajroud, G. Panczer, M. Férid, H. Maaref, Opt. Mater. 32 (2010) 479e483. [45] H. Lin, E. Pun, S.Q. Man, J. Opt. Soc. Am. B 18 (2001) 602. [46] X.L. Zou, T. Izumitani, J. Non-Cryst. Solids 162 (1993) 68. [47] G.C. Righini, S. Pelli, M. Fossi, Proc. SPIE 4282 (2001) 210. [48] B. Sudhakar Reddy, S. Buddhudu, Ind. J. Phys. 82 (2008) 1e13. [49] K. Upendra Kumar, V.A. Prathyusha, P. Babu, C.K. Jayasankar, A.S. Joshi, A. Speghini, M. Bettinelli, Spectrochim. Acta Part A 67 (3e4) (2007) 702e708. [50] Hong Li, S. Kamakshi Sundaram, P.A. Blanc-Pattison, Liyu Li, J. Am. Ceram. Soc. 85 (2002) 1377e1382. [51] C.K. Jorgensen, R. Reisfeld, J. Less Common Met. 93 (1983) 107. [52] G.C. Righini, M. Ferrari, Rivista Del Nuovo Cimento 28 (2006) 1e53. [53] J.H. Campbell, T.I. Suratwala, J. Non-Cryst. Solids 263 (2000) 318.