Effect of Pr3+ ions concentration on the spectroscopic properties of Zinc telluro-fluoroborate glasses for laser and optical amplifier applications

Journal of Luminescence 187 (2017) 392–402

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Effect of Pr3 þ ions concentration on the spectroscopic properties of Zinc telluro-fluoroborate glasses for laser and optical amplifier applications P. Suthanthirakumar a, Ch. Basavapoornima b, K. Marimuthu a,n a b

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

art ic l e i nf o

a b s t r a c t

Article history: Received 1 November 2016 Received in revised form 16 March 2017 Accepted 22 March 2017 Available online 23 March 2017

A new series of Pr3 þ ions doped Zinc telluro-fluoroborate (PrZTFB) glasses have been prepared by adding up to 2 wt% Pr2O3. Spectroscopic properties were explored through X-ray diffraction, Raman, optical absorption, photoluminescence and decay measurements. The Raman spectra reveal the presence of different vibrational bonds of the borate and tellurite network(s). The bonding parameters have shown the ionic nature of the bonding Pr–X (X¼O,F). The optical band gap energy and Urbach energy have been determined to understand the electronic band structure. The Judd-Ofelt parameters Ωλ (λ ¼2, 4 and 6) have been calculated to explore the bonding environment around the Pr3 þ ions. The luminescence spectra exhibit emission bands in the visible region attributed to the 3P0-3H4, 3F2, 3F3, 3F4 and 3P1-3H5 and 1D2-3H4, 3H5 transitions and a broad near infrared emission band at around 1330 nm corresponding to the 1G4-3H5 transition with a FWHM E 70 nm. The glasses lying in the reddish orange region of CIE 1931 chromaticity diagram have been found suitable for light emitting diode applications. The decay curves of 3P0 and 1D2 levels of Pr3 þ exhibit non exponential behavior for all the glasses and experimental lifetime value is found to decrease while increasing the Pr3 þ ions due to cross-relaxation mechanisms. The radiative parameters corresponding to the prominent 3P0-3H4, 1D2-3H4 and 1G4-3H5 emission transitions have been determined to elucidate the suitability of the studied glasses for the fabrication of photonic devices that includes laser materials and broad band optical amplifiers. & 2017 Elsevier B.V. All rights reserved.

Keywords: Functional groups Band gap Judd-Ofelt parameters Luminescence Gain media Optical amplifiers

1. Introduction Nowadays, rare earth (RE) ions doped crystals as well as glass materials play a vital role in many scientific and technological applications such as solid state lasers, optical amplifiers, bar-code reading, sensors, display devices and telecommunications etc., since they exhibit sharp excitation and emission bands due to the shielding effect of 4f electrons by 5s2 and 5p6 shells. This shielding effect makes the RE ions to retain their emission properties though they are doped into different host matrices due to the less dependency of RE ions on the ligand field environment [1–5]. Glasses are the most favorable one for RE doping because of the fact that they exhibit broad emission and absorption spectral bands compared to the crystalline host materials and further it possess remarkable advantages like flexibility in choosing different chemical composition and ease of fabrication. Among the RE ions, number n

Corresponding author. E-mail address: [email protected] (K. Marimuthu).

http://dx.doi.org/10.1016/j.jlumin.2017.03.052 0022-2313/& 2017 Elsevier B.V. All rights reserved.

of investigations have been carried out on Pr3 þ doped glass matrices towards the development of solid state lasers, up-converters, optical temperature sensors [6–9] and other opto-electronic devices. Furthermore, energy levels of Pr3 þ ions demonstrate several meta-stable states and many of the researchers focus on the 3P0-3H4 (blue) and 1D2-3H4 (orange) laser transitions which offer emission in the visible region [7,10,11]. Among the several glass forming oxides, there is extensive amount of interest in the selection of borate based glass as the host matrix for RE ion doping because of their remarkable physical, mechanical, structural and optical properties [12] like transparency, lower melting temperature, higher dielectric constant and good RE ion solubility despite the fact that they possess larger phonon energy. In addition to that, research community shows enormous interest towards tellurite (TeO2) based glasses due to their advantages which includes high density, high transparency in the mid infrared region, moderate phonon energy, good mechanical and chemical stability, large thermal expansion, good corrosion resistance and importantly high refractive index which

P. Suthanthirakumar et al. / Journal of Luminescence 187 (2017) 392–402

make them potential candidate for the fabrication of optoelectronic devices such as optical amplifiers, planar waveguides, single mode fiber lasers and optical switching etc., [1,13]. Since borate (B2O3) possess larger phonon energy, addition of fluoride compounds such as ZnF2, CaF2 and BaF2 with their lesser phonon energy as network modifiers would result in the phonon energy of the borates (E1300–1500 cm  1) to a relatively lower value (E 600–800 cm  1) thus improves the excited state lifetime and luminescence efficiency of the RE ions by reducing the non-radiative (NR) losses. Furthermore, addition of fluoride compounds into the chosen glass matrix strongly decreases the OH absorption which inturn increases the transparency, mechanical and thermal stability [2,14]. Moreover, presence of Zinc oxide in the chosen glass matrix improves the mechanical strength, chemical stability and lower thermal expansion, hygroscopic nature further enhances the glass forming nature [15]. The suitability of the Zinc telluro-fluoroborate (B2O3–TeO2–ZnO–ZnF2–CaF2–BaF2) host matrix for laser applications have been reported [16–18] by the same authors and the proven results invokes interest to explore the lasing action as well as optical amplification of Pr3 þ ions in the same host matrix. In recent times many researchers pay much attention on Pr3 þ doped glasses due to their versatile photonic and optoelectronic applications since they exhibit an important feature of rich emission that nearly covers the whole visible and NIR spectral region. Kumar et al. [19] reported the fluorescence properties of Pr3 þ doped lead telluroborate (PTBPr) glasses for efficient visible laser applications. Naresh et al. [20] examined the visible and NIR emission characteristics of Pr3 þ doped borosilicate glasses and reported on multiphonon and cross-relaxation (CR) channels for the different emission levels of Pr3 þ ions. Brahmachary et al. [21] studied and reported the concentration effect of Pr3 þ ions on the spectroscopic properties of ZANP glasses. Multichannel emission from Pr3 þ doped borate based heavy-metal oxide glasses have been investigated and reported by Herrera et al. [9]. The aim of the present study is to (i) synthesize Pr3 þ doped Zinc telluro-fluoroborate glasses by varying the Pr3 þ ions concentration (ii) explore the presence of various functional groups in the prepared glasses (iii) investigate the spectroscopic properties employing the JuddOfelt (JO) theory [22,23] and finally (iv) determine the important radiative properties like transition probability (A), stimulated emission cross-section ( σPE ) and branching ratios (βR) for the different emission transitions of Pr3 þ ions and to compare the results with the reported Pr3 þ doped glasses.

2. Experimental Pr3 þ doped Zinc telluro-fluoroborate (xPrZTFB) glasses with

393

the chemical composition (30 x)B2O3 þ 30TeO2 þ16ZnO þ 10ZnF2 þ7CaF2 þ7BaF2 þxPr2O3 (xPrZTFB; where x ¼ 0.05, 0.1, 0.25, 0.5, 0.75, 1 and 2 in wt%) have been synthesized by melt quenching technique by taking the high purity (99.99%) analytical grade chemicals such as H3BO3, TeO2, ZnO, ZnF2, CaF2, BaF2 and Pr2O3 as starting materials purchased from Sigma Aldrich following the procedure reported in literature [17]. About 15 g batches were put into a porcelain crucible and melted in an electric furnace at 1050 °C for 45 min. The obtained glass melt was poured on to a preheated brass mold and subsequently annealed at 350 °C for 12 h to remove the thermal strain. The refractive indices of the title glasses were measured using Abbe refractometer at sodium wavelength (5893 Å) having 1-bromonapthaline as a contact liquid. Subsequently, the densities were determined employing Archimedes's principle with xylene as an immersion liquid. In order to ensure the amorphous nature, X-ray diffraction measurements were performed using JEOL 8030 X-ray diffractometer employing CuKα radiation. The Raman spectral analysis was carried out using SJ-301 Mitutoyo surface Profilometer with Imaging Spectrograph STR 500 mm focal length Laser Raman spectrometer. The optical absorption measurements were made using Perkin Elmer Lambda-950 UV–Vis–NIR spectrophotometer in the wavelength range 400–2500 nm. Visible luminescence spectra have been recorded in the wavelength region 470–760 nm using Jobin Yvon Fluorolog-3 Spectrofluorimeter exciting with xenon lamp (450 W) and the NIR luminescence spectra in the wavelength region 1250–1450 nm were recorded using EG&G Princeton Applied Research model 5210 with a spectral resolution of 70.5 nm.

3. Results and discussion 3.1. Physical properties The physical properties which exhibit great influence on the optical properties have been studied for the Pr3 þ doped Zinc telluro-fluoroborate glasses. The densities of the prepared glasses were found to increase due to the replacement of B2O3 by higher molecular weight Pr2O3 content. Refractive index (nD) is one among the most significant properties which decides the suitability of the materials for optical applications and play an important role in calculating the JO intensity parameters and laser parameters. The obtained nD values of the present glasses are given in Table 1 and it is observed that the nD values increases with the increasing concentration of Pr2O3. The direct replacement of B2O3 by Pr2O3 in the present study modifies the boron to oxygen ratio which converts BO3 units into BO4 tetrahedral units thus enhances the formation of number of non-bridging oxygen's

Table 1 Physical properties of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Physical properties 3

Density ρ (g/cm ) Refractive index nD Average molecular weight MT (g) Molar volume VM (cm3) Rare earth ion concentration N (1020 ions/cm3) Polaron radius rp (Å) Inter ionic distance ri (Å) Field strength F (1014 cm–2) Electronic polarizability αe (10–22 cm3) Molar refractivity Rm (cm3/mol) Dielectric constant (ε) Reflection losses R (%) Optical dielectric constant (P

∂t ) ∂P

0.05PrZTFB

0.1PrZTFB

0.25PrZTFB

0.5PrZTFB

0.75PrZTFB

1PrZTFB

2PrZTFB

4.446 1.612 107.66 24.215 0.25 13.79 34.25 0.256 33.345 1.173 2.599 5.49 1.599

4.462 1.614 107.80 24.157 0.50 10.94 27.16 0.407 16.683 1.172 2.605 5.52 1.605

4.482 1.617 108.20 24.138 1.25 8.06 20.01 0.749 6.697 1.171 2.615 5.56 1.615

4.569 1.619 108.87 23.826 2.53 6.37 15.81 1.199 3.315 1.152 2.621 5.59 1.621

4.583 1.622 109.54 23.899 3.78 5.57 13.83 1.568 2.225 1.153 2.631 5.63 1.631

4.688 1.624 110.21 23.507 5.12 5.04 12.49 1.921 1.646 1.130 2.637 5.66 1.637

4.951 1.627 112.89 22.802 10.57 3.96 9.82 3.112 0.801 1.074 2.647 5.70 1.647

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concentration of Pr3 þ ions from 0.25 to 10.57  1020 ions/cm3 which results in the formation of cluster of ions at higher concentration. These physical properties along with the other calculated parameters like field strength, electronic polarizability and reflection losses etc., are presented in Table 1. The variation in density and refractive index of the title glasses with the concentration of Pr3 þ ions is shown in Fig. 1. 3.2. X-ray diffraction analysis The X-ray diffraction spectra of the studied glasses have been recorded in the range 10° r θ r80°. The XRD pattern did not exhibit any notable diffraction crystalline peaks but shows a broad diffused pattern at lower scattering angles thus indicates the long range structural disorder which in-turn confirms the amorphous nature [24]. 3.3. Raman spectra and phonon energy of the glass host

Fig. 1. Variation of density and refractive index with the concentration of Pr3 þ ions.

Fig. 2. Raman spectrum of the Pr3 þ doped 1PrZTFB glass.

(NBO's). It is a well known fact that polarizability is directly proportional to the refractive index and the NBO ions possess higher polarizability when compared to the bridging oxygen. Hence the nD values are found to increase as the number of NBO increases in the present investigation. The inter ionic distance was found to decrease from 34.25 to 9.82 Å with the increase in the

The Raman spectra of the title glasses have been recorded in the wave number range 50–1600 cm  1. Since all the Raman spectra of the title glasses lookalike, Raman spectrum of the Pr3 þ doped 1PrZTFB glass is shown in Fig. 2 as a representative case and the band positions corresponding to their band assignments are given in Table 2. The Raman band observed at 60 cm  1 is called as boson peak and is associated with the BO3–TeO4 bridged local collective vibrations [25]. The band at 340 cm  1 is due to the formation of TeO3 trigonal pyramidal (tp) structural units with NBO's. The band at 460 cm  1 is assigned to the bending vibrations of Te–O–Te or O–Te–O linkages of TeO4 units [26]. The presence of band around 696 cm  1 is attributed to the stretching vibrations of TeO4 trigonal bipyramidal (tbp) units. The strong band observed at 776 cm  1 is due to the combined vibrations of Te–O from TeO3 tp units and symmetric breathing vibrations of six membered rings with a BO3 triangle replaced by the BO4 tetrahedra [1,27]. The band at 1005 cm  1 is mainly due to the presence of pentaborate groups in the borate network [12]. The band observed at around 1426 cm  1 is due to the BO2O  triangles linked with the other borate triangular (BO3) units [28]. The highest vibrational energy observed from the Raman spectrum can be represented as the phonon energy of the glass host and in the present study maximum phonon energy observed is found to be 776 cm  1, which is comparatively lower than the reported phosphate (1120 cm  1) [29] and silicate (889 cm  1) [30] based glasses. 3.4. Absorption spectra and Bonding parameters The room temperature absorption spectrum of the Pr3 þ doped 2PrZTFB glass recorded in the UV–vis-NIR region is shown in Fig. 3. The spectrum exhibit eight absorption peaks which corresponds to the transitions from the 3H4 ground state to the various excited states of Pr3 þ ions such as 3F2, 3F3, 3F4, 1G4, 1D2, 3P0, 3P1, and 3P2 positioned at around 1938, 1535, 1447, 1014, 590, 482, 468 and

Table 2 Raman spectral peak positions and the band assignments for the Pr3 þ doped 1PrZTFB glass. Sl.No. Peak positions (cm  1) Peak assignments 1 2 3 4 5

60 340 460 696 776

6 7

1005 1426

Boson peak associated with the BO3–TeO4 bridged local collective vibrations TeO3 trigonal pyramidal (tp) structural units with non-bridging oxygen (NBO's) Bending vibrations of Te–O–Te or O–Te–O linkages of TeO4 units Stretching vibrations of TeO4 trigonal bipyramidal (tbp) units Stretching vibrations of Te–O from TeO3 tp units or TeO3 þ 1 polyhedra and symmetric breathing vibrations of six membered rings with a BO3 triangle replaced by a BO4 tetrahedra Presence of pentaborate groups BO2O  triangles linked with other borate triangular (BO3) units

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395

the polarization of oxygen upon Pr3 þ ions. Hence, decrease in the formation of B  O bonds lead to have a gradual increase in the ionicity of the corresponding Pr  O bonds. 3.5. Oscillator strengths and JO intensity parameters The spectral intensity of the absorption bands are generally expressed in terms of their oscillator strength values. The experimental oscillator strengths (fexp) corresponding to the f–f induced electric-dipole transition can be obtained by integrating the areas under each absorption band of the absorption spectrum using the following expression [10]

fexp = 4.318 × 10−9

Fig. 3. Absorption spectrum of the Pr3 þ doped Zinc telluro-fluoroborate glass in the UV–Vis–NIR region.

443 nm respectively [9,20]. Among the absorption bands, 3 H4-3P2 and 3H4-3F3 transitions are found to be more intense due to the change in the local environment around the Pr3 þ ions and are called as hypersensitive transitions (HSTs). Further these transitions obey the selection rules, |ΔS| ¼ 0, |ΔL| r2 and |ΔJ| r2. Once the glass host matrix is doped with RE ions, a significant wavelength shift is observed in the absorption spectrum compared to that of aqua-ion and this effect is called as nephelauxetic effect. This may happen due to the deformation of electronic orbitals within the 4f configuration. Based on this phenomenon, the nephelauxetic ratios (β) and bonding parameter (δ) values have been calculated from the positions of the absorption spectra in order to identify the nature of metal-ligand bond in the studied glasses using the mathematical expressions reported in literature [16]. The nature of the Pr3 þ ligand bonding can be covalent or ionic depending upon the positive or negative sign of δ value [18]. The calculated bonding parameter (δ) values of the title glasses are presented in Table 3. The results confirms the fact that the Pr3 þ ions with the surrounding ligands O2  /F  in the prepared glasses possess ionic nature and is found to increase gradually with the increase in Pr3 þ ion concentration. According to the electronegativity theory among the two network formers, interaction of boron B þ III with the surrounding ligands is found to be weaker compared to the tellurium Te þ IV ions in the studied glasses and further replacement of B2O3 content by Pr2O3 results reduction in

∫ ε(ν)dν

(1)

where, ε(ν) is the molar absorptivity of the band at a wave number ν (cm–1). The theoretical oscillator strengths correspond to the f–f transition of the Pr3 þ ions can be evaluated using Judd-Ofelt (JO) theory [22,23]. According to this theory, calculated oscillator strength (fcal) of the electric-dipole transitions from the ground state (ΨJ) to the excited state (Ψ’J’) can be expressed using the below given expression

fcal

⎡ ⎡ 8π 2mcν ⎤⎢ n2 + 2 ⎢ ⎥ = ⎢⎣ 3h( 2J + 1) ⎥⎦⎢ 9n ⎢⎣

(

2⎤

) ⎥×∑ ⎥ ⎥⎦

λ = 2,4,6

(

Ωλ ΨJ‖U λ‖Ψ ′J′

2

)

(2)

where, ν is the energy in wave number (cm ) of the transition, n is the refractive index, c is the velocity of light in vacuum, m is the mass of an electron, J is the total angular momentum of the ground λ state, Ωλ (λ ¼2, 4 and 6) are the JO intensity parameters and ║U ║2 are the doubly reduced matrix elements of the unit tensor operator evaluated from the intermediate coupling approximation for the transition from ΨJ to Ψ’J’ and the values for the present study have been taken from the reported literature by Carnall et al. [31]. The experimental (ƒexp) and calculated (ƒcal) oscillator strength values along with the rms deviation (srms) for the title glasses are presented in Table 4. It is observed from the table that, the hypersensitive transitions (HSTs) possess higher oscillator strength values than the other observed transitions thus indicate the fact that Pr3 þ ions are located in a higher asymmetrical environment in the prepared glasses [32]. The Judd-Ofelt intensity parameters, Ωλ (λ ¼ 2, 4, 6) have been evaluated following the least square fitting approximation procedure using the refractive index, experimental (ƒexp) and calculated (ƒcal) oscillator strength values. While taking all the ƒexp values for the fitting procedure, some irregular results like larger rms deviation (srms) and negative ƒcal values have been observed. These inconsistent results are expected and are mainly due to the smaller energy difference between the 4f2 ground state configuration and the 4f15d1 first excited state of –1

Table 3 Observed band positions (in cm–1) and bonding parameters ( β and δ) of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Transition3H4-

0.05PrZTFB

0.1PrZTFB

0.25PrZTFB

0.5PrZTFB

0.75PrZTFB

1PrZTFB

2PrZTFB

Aqua- ion [31]

3

5155 6529 6916 9859 16964 20768 21363 22614 1.00025

5152 6530 6923 9879 16972 20781 21377 22609 1.00078

5160 6518 6922 9884 16984 20799 21358 22624 1.00098

5159 6533 6941 9891 16938 20786 21386 22635 1.00145

5159 6527 6924 9874 17024 20803 21395 22619 1.00152

5160 6535 6935 9871 17004 20777 21390 22630 1.00160

5157 6527 6937 9892 17001 20794 21404 22650 1.00195

5200 6500 6950 9900 16840 20750 21300 22520 –

 0.025

 0.078

 0.098

 0.144

 0.152

 0.159

 0.194

–

F2 F3 F4 1 G4 1 D2 3 P0 3 P1 3 P2 β δ 3 3

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Table 4 Experimental and Calculated oscillator strengths (  10–6) of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Transition3H4-

3

F2 F3 F4 1 G4 1 D2 3 P0 3 P1 N s 3 3

0.05PrZTFB

0.1PrZTFB

0.25PrZTFB

0.5PrZTFB

0.75PrZTFB

1PrZTFB

2PrZTFB

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fcal

fcal

4.399 8.059 4.221 0.385 2.191 1.823 1.245 7 0.558

4.418 7.761 4.704 0.414 1.390 2.241 2.263

4.752 8.649 4.425 0.558 2.478 0.993 1.042 7 0.720

4.778 8.229 5.105 0.445 1.486 2.007 2.026

5.210 9.102 4.545 0.510 2.999 1.477 1.573 7 0.827

5.236 8.685 5.239 0.462 1.551 2.522 2.546

5.487 9.826 4.933 0.408 2.986 0.931 2.086 7 0.894

5.516 9.358 5.693 0.500 1.677 2.602 2.626

5.457 9.934 5.001 0.446 2.211 0.828 1.560 7 0.786

5.488 9.411 5.807 0.507 1.696 2.383 2.406

5.457 9.512 5.144 0.320 2.384 0.437 0.939 7 0.680

5.482 9.088 5.807 0.498 1.657 1.665 1.681

5.334 9.380 5.961 0.390 2.722 0.519 1.357 7 0.465

5.339 9.291 6.135 0.519 1.718 1.163 1.175

Table 5 Judd-Ofelt parameters (  10–20 cm2), Spectroscopic quality factor (Ω4/Ω6) of the Pr3 þ doped Zinc telluro-fluoroborate glasses along with the reported Pr3 þ doped glasses. Glass composition

0.05PrZTFB 0.1PrZTFB 0.25PrZTFB 0.5PrZTFB 0.75PrZTFB 1PrZTFB 2PrZTFB TeO2–WO3–PbO–La2O3 B2O3–Li2O–CaO B2O3–PbO–ZnO–Li2O Li2B4O7–BaF2–NaF–PbO TeO2–WO3–PbO–La2O3 B2O3–P2O5–Na2O–Al2O–ZnF2 Li2B4O7–Nb2O5 B2O3–PbO–GeO2–Bi2O3

3þ

JO parameters Ω2

Ω4

Ω6

4.987 5.917 6.215 6.536 6.682 7.531 7.687 4.77 2.087 8.06 5.11 3.09 2.74 0.104 0.67

3.525 3.168 3.973 4.091 3.739 2.608 1.818 1.21 1.591 6.13 4.87 1.32 1.77 0.909 1.99

7.343 8.051 8.161 8.884 9.084 9.152 9.752 5.16 3.780 9.79 21.72 4.13 8.31 1.28 3.75

Trends of Ωλ

References

Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω2 4Ω4 Ω6 4 Ω4 4Ω2 Ω6 4 Ω4 4Ω2

Present Present Present Present Present Present Present [35] [36] [37] [38] [39] [40] [41] [9]

the Pr ions [33]. In order to overcome this uncertain behavior, 3 H4-3P2 transition has been neglected in the JO analysis to reduce the srms values. While excluding this transition in the analysis, the quality of the fit between the experimental and calculated oscillator strength values is much improved and similar results have been observed and reported in many of the Pr3 þ ions doped glasses [21,34]. The JO intensity parameters (Ωλ) play an important role in elucidating the local structure and bonding nature of the rareearth ions with their surrounding ligands [19]. The JO parameters obtained for the title glasses are given in Table 5 along with the reported Pr3 þ doped glasses. It is observed from Table 5 that, the intensity parameters follow the order as Ω6 4 Ω2 4 Ω4 uniformly for all the studied glasses similar to the reported Pr3 þ doped glasses [35–41]. However, Ω2 values contribute both for the asymmetry of ligand field environment around the RE3 þ ions site and covalency of the metal-ligand bond, it is highly sensitive to the asymmetry of the ligand field and exhibit less dependency on the covalency. The Ω2 parameter values obtained in the present study are found to be higher than the reported Pr3 þ doped B2O3–PbO– GeO2–Bi2O3 [9], TeO2–WO3–PbO–La2O3 [35], B2O3–Li2O–CaO [36], Li2B4O7–BaF2–NaF–PbO [38], TeO2–WO3–PbO–La2O3 [39], B2O3– P2O5–Na2O–Al2O–ZnF2 [40], Li2B4O7–Nb2O5 [41] glasses thus indicates the higher asymmetry between Pr3 þ ions and the ligand field environment in the present glasses. Furthermore, higher magnitude of the Ω6 intensity parameter among three JO intensity parameters Ωλ (λ ¼2, 4, 6) implies the higher rigidity of the chosen host.

Fig. 4. Tauc's plot for the indirect allowed transitions of the Pr3 þ doped Zinc telluro-fluoroborate glasses [Inset shows the variation of band gap and Urbach energy values as a function of Pr3 þ ions concentration].

3.6. Bandgap and Urbach’s energy analysis To understand the basic mechanism behind the optically induced transitions in crystalline as well as amorphous materials and their band structure, the fundamental absorption edge is one of the significant parameter which can be obtained from the absorption spectral data. Mott and Davis [42] proposed the relation between absorption coefficient (α( ν ν)) and photon energy (h νν ν) to evaluate the direct, indirect transitions occur in the band gap and is expressed as (αhν) ¼B(hν–Eg)n where ‘B’ is the band tailing parameter, ‘Eg’ is optical energy gap and ‘n’ is the index value which characterizes the type of transition (n¼ ½ or 2 represents direct or indirect allowed transitions). The band gap values of the prepared glasses have been determined by extrapolating the Tauc’s curve [43] between (αhν αhν )n and h νν ν and the same is shown in Fig. 4. The obtained direct, indirect band gap and band tailing parameter values of the title glasses are presented in Table 6. It is observed from the table that, both the band gap values decreases with the increase in Pr3 þ ions concentration and is mainly due to the formation of increasing amount of NBO’s which alter the structure of the glasses. The NBO’s are mainly related to the valence band maximum (VBM) hence increasing the NBO’s with the addition of Pr3 þ concentration shifts the VBM towards the conduction band (CB) which in turn lead to have a fall in the band gap values. The information pertaining to the disorderliness of the

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397

Table 6 Direct and indirect band gap energy (Eg), band tailing parameter (B) and Urbach's energy (ΔE) of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Sample code Direct allowed transitions (n¼ ½) B(cm–2 eV)

Eg(eV)

Indirect allowed transitions (n¼ 2) Eg(eV)

ΔE(eV)

B(cm eV)–1/ 2

0.05PrZTFB 0.1PrZTFB 0.25PrZTFB 0.5PrZTFB 0.75PrZTFB 1PrZTFB 2PrZTFB

2.891 2.870 2.859 2.848 2.814 2.807 2.803

816.222 797.732 840.815 819.513 822.687 761.339 979.467

2.754 2.739 2.729 2.723 2.714 2.705 2.701

15.398 14.763 14.888 15.594 15.631 15.497 17.314

0.385 0.387 0.395 0.396 0.399 0.408 0.436

amorphous materials can be provided as a function of Urbach energy (ΔE) values. According to the Urbach’s rule [44] optical absorption coefficient (α) near the absorption edge exhibits exponential dependency on the photon energy (h νν ν) and is ex-

( ) where ΔE is the Urbach’s energy cal-

pressed as α(ν )=α0exp

hν ∆E

culated by taking reciprocal of the slopes of linear portion of lnα (ν) plotted against h νν ν curves in the lower photon energy region. The calculated ΔE values are presented in Table 6 and the values are found to increase with the addition of Pr2O3 content into the glass matrix. It is known that materials with larger Urbach energy values have higher tendency to convert the weak bonds into defects. Therefore, the increasing trend of Urbach energy values with the Pr3 þ ions concentration validates the number of defects as well as the disorderliness present in the title glasses. The band gap and Urbach energy values obtained for the present glasses are found to vary inversely with each other and is clearly shown in the inset of Fig. 4. 3.7. Visible luminescence spectra The excitation spectra of the title glasses have been recorded in the wavelength range 420–520 nm by monitoring the emission wavelength at 610 nm and as a representative case excitation spectrum of the 1PrZTFB glass is shown in Fig. 5. The spectrum exhibit three well resolved bands centered at around 448, 471 and

Fig. 5. Excitation spectrum of the Pr3 þ doped 1PrZTFB glass.

Fig. 6. Luminescence spectra of the Pr3 þ doped Zinc telluro-fluoroborate glasses in the visible region.

485 nm correspond to the transitions such as 3H4-3P2, 3H4-3P1 and 3H4-3P0 respectively. Among them, the 3H4-3P2 transition observed at 448 nm possesses higher intensity than the other transitions and the same is used as a pumping wavelength to record the luminescence spectra. Fig. 6 shows the luminescence spectra of the studied glasses recorded in the visible wavelength region 470–760 nm by monitoring an excitation wavelength at 448 nm. The emission spectra exhibit seven emission bands at around 486, 529, 610, 645, 685, 708 and 730 nm corresponding to the transitions 3P0-3H4, 3 P1-3H5, 1D2-3H4, 3P0-3F2, 1D2-3H5, 3P0-3F3 and 3P0-3F4 respectively [20]. It is evident that the emission bands almost cover the entire visible spectrum and among the emission bands, 3 P0-3H4 (blue) and 1D2-3H4 (orange) transitions exhibit higher intensity than the other transitions. Furthermore, emission intensity of the 3P0-3H4 transition observed at 486 nm is found to increase when the Pr3 þ ion concentration increases upto 0.75 wt% and beyond that luminescence quenching occurs due to the nonradiative energy transfer (ET) process takes place between the nearby Pr3 þ ions in the studied glasses. Subsequently, emission intensity of the 1D2-3H4 transition is found to decrease with the increase in Pr3 þ ion concentration beyond 0.1 wt% which signifies the fact that, at lower concentrations the population of the 1D2 state is dominated by the fast multiphonon non-radiative relaxation from the higher 3P0,1 states [35] and for higher Pr3 þ ion concentrations the non-radiative ET through CR channel becomes more thus in-turn leads to have luminescence quenching which is apparently evident from the intensity of the emission spectra. When NR decay process takes place, the unstable Pr3 þ ions in the 3 P0 state quickly get relaxed and populate the 1D2 state. The unusual trend pertaining to the emission intensities corresponds to the 1D2-3H4 transition may occur due to the CR and ET process which takes place through ion-ion interactions or interaction of Pr3 þ ions with hydroxyl groups in the studied glasses. Thus inturn leads to have a change in the intensity with respect to the red emissions in the present study. Fig. 7 shows the partial energy level diagram of the different emission transitions takes place in the prepared Pr3 þ doped title glasses along with the prominent CR channel from 1D2 level.

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The gain bandwidth ( σPE  λeff) and optical gain ( σPE  τcal) are the important parameters used for the exploration of amplification of the medium in which the RE ions are located. In the present work, σPE  λeƒƒ and σPE  τcal values corresponding to the 3P0-3H4 transition of the prepared xPrZTFB glasses with x ¼0.05, 0.1, 0.25, 0.5, 0.75, 1 and 2 are found to be 30.651, 39.500, 42.369, 52.895, 58.007, 51.606, 20.211 (  10  27 cm3) and 10.217, 12.445, 11.802, 11.964, 13.412, 12.041, 5.357 (  10  25 cm2 s) respectively. The important lasing parameters such as βR, σPE , ( σPE  λeff) and σPE  τcal have been estimated for the prepared glasses. From the above studies it is concluded that the 3P0-3H4 (486 nm) emission transition corresponds to the 0.75PrZTFB glass possess higher lasing parameter values and the same is suggested for the fabrication of solid state lasers and optical amplifiers operating in the visible region. 3.9. CIE color chromaticity diagram Fig. 7. Partial energy level diagram of the Pr3 þ doped Zinc telluro-fluoroborate glasses along with the non-radiative (NR) and cross-relaxation (CR) channels.

3.8. Radiative properties The important lasing parameters such as radiative transition probability (AT), effective line width (Δλeff), stimulated emission cross-section ( σPE ) and branching ratio (βR) values of the prepared Pr3 þ ions doped Zinc telluro-fluoroborate glasses have been calculated with the help of JO parameters and emission spectra along with the refractive index values using the theoretical expressions reported in the literature [10,19,45]. The obtained radiative parameters for the prominent emission transitions 3P0-3H4 and 1 D2-3H4 of the studied glasses are presented in Table 7 along with the other reported Pr3 þ doped glasses. The rate of energy extraction of laser materials can be evaluated from the stimulated emission cross-section ( σPE ) of the emission bands using the JO intensity parameters. The values for the 3P0-3H4 transition were calculated and σPE are found to be 4.257, 5.411, 5.901, 6.297, 6.706, 5.734 and 2.435 (  10  22 cm2) corresponding to the studied xPrZTFB glasses with x ¼ 0.05, 0.1, 0.25, 0.5, 0.75, 1 and 2, respectively. It is clearly observed from the studies that, among the prominent emission transitions 3P0-3H4 transition possesses higher branching ratio (βR) and stimulated emission cross-section ( σPE ) values for all the studied glasses. Furthermore among the prepared glasses, Pr3 þ doped 0.75PrZTFB glass exhibits higher βR and σPE values which are comparable to the other reported Pr3 þ doped glasses [9,19,46,47].

The emission spectra of the Pr3 þ doped Zinc telluro-fluoroborate glasses have been characterized within the framework of CIE 1931 color chromaticity diagram to explore the dominant characteristic emission color and the same is shown in Fig. 8. The color chromaticity coordinate values (x, y) [18] are found to be (0.599, 0.368), (0.594, 0.368), (0.539, 0.378), (0.464, 0.395), (0.489, 0.389), (0.458, 0.396) and (0.437, 0.402) corresponding to the xPrZTFB glasses with x ¼0.05, 0.1, 0.25, 0.5, 0.75, 1 and 2, respectively. It is observed from the CIE 1931 chromaticity diagram shown in Fig. 8 that the x, y coordinates of all the studied glasses are found to be located in the reddish-orange region and found to have slight deviation with the change in Pr3 þ ion concentration. Hence, it is suggested that the prepared Pr3 þ doped Zinc tellurofluoroborate glasses are suitable for the visible light emitting diodes and display device applications. The correlated color temperatures (CCT) have also been calculated using the x, y chromaticity coordinate values following the McCamy's approximation formula [18] in order to check the quality of the emitted light. In general, CCT is one of the significant characteristic feature which describes the temperature corresponding to the closest Planckian black-body radiator to the operating point in the CIE diagram [48] and the obtained CCT values are found to be in the range 1665–2982 K pertaining to the reddish-orange light for the title glasses and can be considered as a warm (o4000 K) source. The color purity and the correlated color temperature (CCT) values are found to be 90, 89, 76, 59, 64, 57, 53% and 1683, 1665, 1699, 2506, 2131, 2601, 2982 K corresponding to the present xPrZTFB glasses with x ¼0.05, 0.1, 0.25, 0.5, 0.75, 1 and

Table 7 Emission band position (λP, nm), effective bandwidth (Δλeff, nm), transition probability (A, s  1), calculated and experimental branching ratios (βR), stimulated emission crosssection ( σPE  10  20 cm2), radiative lifetime (τcal, ms), for the 3P0-3H4 and 1D2-3H4 transitions of the 0.75PrZTFB glass and other reported Pr3 þ doped glasses. Transition

Parameters

0.75PrZTFB

ZBP3 [8]

PTBPr10 [19]

PrPbLiFB [38]

Pr-LP [46]

15PrLi-K [47]

3

λp Δλeff A

486 8.431 16215 6.706

493 – 11189 5.071

492 22.56 10354 4.151

488 6.76 19466 3.34

485 14.581 30380 5.530

490 – – 1.174

0.511 0.328 20.00

– 0.324 28.96

0.412 0.884 39.00

0.25 0.32 16.00

0.328 0.699 23.00

0.410 0.442 6.3

613 12.906 1555 0.858

608 – 7003 1.731

613 26.82 1015 0.842

604 7.34 3593 0.87

600 14.506 1129 0.484

603 – – 0.818

0.264 0.286 183

– 0.202 –

0.391 1.000 388

0.70 0.48 133

0.966 0.975 864

0.589 0.388 236

3

P0- H4

σPE βR(Exp) βR(Cal) τcal 1

D2-3H4

λp Δλeff A

σPE βR(Exp) βR(Cal) τcal

P. Suthanthirakumar et al. / Journal of Luminescence 187 (2017) 392–402

399

Fig. 8. CIE 1931diagram of the Pr3 þ doped Zinc telluro-fluoroborate glasses. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

2, respectively. The color purity pertaining to the dominant emission color exhibited by the title glasses are found to be in the range 90–53% which signifies the fact that with the increase in Pr3 þ ions concentration the change in emission color from bright reddish-orange to yellowish-orange occurs due to the overlapping of blue emission. 3.10. Luminescence decay spectral analysis The decay curves from the 3P0 and 1D2 excited levels have been recorded by monitoring an excitation at 448 nm and emissions at 486 and 610 nm and are shown in Fig. 9(a) and (b) respectively. It is clearly observed from the figure that the decay curves exhibit non-exponential behavior for all the glasses and the same may be due to the ET process takes place between the nearby Pr3 þ ions. The deconvoluted decay profiles for the (a) 3P0 excited level of the 0.1PrZFTB glass and (b) 1D2 excited level of the 0.75PrZFTB glass is presented in Fig. 10 as a representative case. The effective decay time can be determined by subjecting the decay curves of the title glasses to the non-exponential curve fitting method using the below given expression

I =A1exp( − t /τ1)+A2 exp( − t /τ2)

(3)

where, A1 and A2 are the decay constants, τ1 and τ2 are the lifetimes of the two channels involved in the decay processes. The experimental lifetime (τexp) values of the non-exponential decay curves can be estimated from the following equation,

τexp =

(A1τ12 + A1τ22) (A1τ1 + A2 τ2)

(4)

The measured τexp values of the 3P0 and 1D2 excited levels are gathered in Table 8. In the present study, lifetime values are found to larger for the 1D2 level than the 3P0 level and the corresponding values are comparable to the reported Pr3 þ doped fluorotellurite [5], lead telluroborate [19], tellurite-tungstate [35], fluoro-phosphate [49], sodium fluoro-borate [50], and alkaline potassium-titanium-phosphate [51] glasses. It is observed from both the cases that, the τexp values are found to decrease monotonically with the increase in Pr3 þ ions concentration and the same may be due to the non-radiative ET process takes place either through CR or multi-phonon relaxation mechanism. It is known that the energy difference between the 3P0 emission level and the next lower lying

Fig. 9. (a): Decay profile of the 3P0 excited level of the Pr3 þ doped Zinc tellurofluoroborate glasses [Inset shows the IH fit for the 0.1PrZTFB glass]. (b): Decay profile of the 1D2 excited level of the Pr3 þ doped Zinc telluro-fluoroborate glasses [Inset shows the IH fit for the 0.1PrZTFB glass].

1 D2 level is found to be  4000 cm  1. Also the energy gap between the 1D2 level and the next lower lying 1G4 level is found to be  6600 cm  1 which are much higher than the phonon energy of the Zinc telluro-fluoroborate glasses (  780 cm  1). Hence, there is no possibility for the occurrence of multi-phonon decay in the present glasses and reduction in the lifetime (τexp) value is mainly due to the ET process takes place through CR between excited (donor) and non-excited (acceptor) Pr3 þ ions. During the CR process, excited ions offer part of its energy to the unexcited ions and then both of them relax to the metastable state. Finally, the donor and acceptor ions in the metastable state reach the ground state through NR decay process. The possible CR channels involved in the 3P0 level are found to be A: (3P0-3H6: 3H4-1D2), B: (3P0-1G4: 3H4-1G4), C: (3P0-1D2: 3H4-3H6) and simultaneously the CR channels takes place in the 1D2 level are D: (1D2-1G4: 3 H4-3F4), E: (1D2-3F4: 3H4-1G4) respectively and the same is shown in the energy level diagram of Pr3 þ ions in Fig. 7.

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Fig. 10. Deconvolution of decay profile(s) of (a) 3P0 excited level for 0.1PrZFTB glass (b) 1D2 excited level for 0.75PrZFTB glass [Red line indicates the fitting of the decay curve, Dashed line indicates the fast decay component and Dashed Dot line indicate the slow decay component]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 8 The experimental lifetime (τexp, ms), Energy transfer parameter (Q), critical transfer distance (Ro, nm) and the donor-acceptor interaction parameter CDA (  10  38 cm6/s) of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Transition

Parameters

0.05PrZTFB

0.1Pr ZTFB

0.25PrZTFB

0.5PrZTFB

0.75PrZTFB

1Pr ZTFB

2Pr ZTFB

3

P0-3H4

τexp Q Ro CDA

45 0.226 1.06 2.62

33 0.454 1.07 2.64

6.8 1.222 1.09 3.06

6.6 3.008 1.17 4.51

4.8 4.545 1.18 4.60

4.6 6.489 1.19 5.11

4 13.522 1.20 5.21

1

D2-3H4

τexp Q Ro CDA

76 0.279 1.14 2.43

56 0.568 1.15 2.49

37 1.429 1.15 2.52

13 2.918 1.16 2.57

12 4.385 1.16 2.59

11 6.168 1.17 2.79

10 12.869 1.18 2.86

The quantum efficiency (η) is an important parameter which decides the performance of any medium for the suitable laser and planar waveguide applications. The η values of the studied glasses have been obtained as the ratio of the experimental lifetime to the radiative lifetimes of the excited state following the below given expression,

η=

τexp τcal

× 100

(5)

The quantum efficiency value of the 0.75PrZTFB glass for the prominent 3P0 laser level is found to be 25% and is comparably higher than the reported Pr3 þ doped TWPLaPr-3 (13%) [35], PTBPr20 (23%) [19], LBTAF (19%) [49], PbO–B2O3–TiO2–AlF3 (19%) [34] and TeO2–BaF2–NaF (21.1%) [5] glasses. The ET process takes place between the Pr3 þ ions have been carried out by fitting the non-exponential decay curves into Inokuti-Hirayama model (IH) [52] using the following expression,

⎧ ⎛ t ⎞3/ S ⎫ ⎪ t ⎪ I (t ) = I0exp⎨ − − Q ⎜ ⎟ ⎬ ⎪ ⎝ τ0 ⎠ ⎪ ⎩ τ0 ⎭

(6)

where, Q is the ET parameter, τ0 is the intrinsic decay time of the donors (without acceptor ions) which is the lifetime of the glass containing minimum concentration (0.01 wt%) of Pr3 þ ions and it was obtained by fitting the decay curves of 3P0 and 1D2 excited levels into single exponential curve fitting method. The τ0 values are found to be 57ms for 3P0-3H4 and 94ms for 1D2-3H4 levels respectively. t is the time after excitation and S can have values as 6, 8 and 10 depending upon the dipole-dipole (D–D), dipolequadrupole (D–Q) and quadrupole-quadrupole (Q–Q) interactions respectively. The Q values can be calculated using the following expression,

Q=

4π ⎛ 3⎞ Γ ⎜ 1 − ⎟N0R 03 3 ⎝ S⎠

(7)

P. Suthanthirakumar et al. / Journal of Luminescence 187 (2017) 392–402

Fig. 11. NIR luminescence spectra of the Pr3 þ doped Zinc telluro-fluoroborate glasses.

where, N0 is the acceptor ion concentration which is approximately equal to the RE ion concentration, the Γ(x) function is equal to 1.77 for D–D, 1.43 for D–Q and 1.3 for Q–Q interactions respectively and R0 is the critical transfer distance equal to the decay rate (τo  1) of the donor ions. The donor-acceptor interaction parameter (CDA) can be expressed as,

CDA =

R (0S) (8)

τo

The decay curves of the prepared glasses were found to be well fitted to the IH model for S¼ 6 compared to S ¼8 and S¼10. This fitting confirms that the dipole-dipole (D  D) interactions of Pr3 þ ions give major contribution to the occurrence of ET process takes place in the prepared glasses. The Q, R0 and CDA values obtained from the IH fitted curves for the 3P0 and 1D2 excited states are presented in Table 8. It is observed that the Q, R0 and CDA values increases with the increase in Pr3 þ ion concentration thus illustrates the fact that the ET taking place between the Pr3 þ ions is through dipole-dipole (D–D) interactions. 3.11. NIR luminescence spectra The broadband infrared luminescence behavior of the Pr3 þ ions doped Zinc telluro-fluoroborate glasses have been studied by recording the NIR luminescence spectra in the spectral region 1250– 1450 nm exciting at 448 nm and is depicted in Fig. 11. The spectra

401

exhibits an in-homogeneously broadened emission band at around 1336 nm which corresponds to the 1G4-3H5 transition which covers the entire O, E and S bands which are useful in the low loss optical fiber communication window. Further, the full width at half maximum (FWHM) of the emission band is one of the essential parameters useful to understand the gain bandwidth properties of the broadband optical amplifiers and the large magnitude of the same are useful in enhancing the performance of wavelength division multiplexing (WDM) networks without having any gain damages [53]. In the present study, FWHM values obtained for the 1G4-3H5 transition are presented in Table 9 and the values are found to vary from 50 to 74 nm with varying Pr3 þ ions concentration and the inhomogeneous broadening observed in the NIR emission bands are due to the variation in the local ligand field and the coordination number from the Pr3 þ ion site [17]. It is observed from the spectra that the emission intensities are found to decrease beyond 0.75 wt% Pr3 þ ions concentration and further the emission bands slightly broadened towards the longer wavelength side. The luminescence quenching is found to occur beyond 0.75 wt% Pr3 þ ions concentrations and the same may be ascribed to the reduction in the inter ionic distances resulting the ET taking place between the nearby Pr3 þ ions in the prepared glasses. The stimulated emission cross-section ( σPE ), FWHM, gain bandwidth ( σPE  FWHM) and the figure of merit (FOM) for optical gain ( σPE  τcal) are the key factors in understanding the near-infrared broadband amplification of the medium and the calculated values are presented in Table 9 along with the radiative parameters. The values of σPE calculated for the 1G4-3H5 transition ranging from 1.47 to 2.85 (  10  20 cm2) are found to be higher than the reported Pr3 þ doped ZBLAN (0.35  10  20 cm2), sulphide (1.0  10  20 cm2), bismuth gallate (0.70  10  20 cm2) [54] and GeGa-S (1.33  10  20 cm2) [55] glasses. The larger values of critical amplification parameters such as gain bandwidth ( σPE  FWHM) and FOM ( σPE  τcal) decides the utility of the prepared glasses for broadband optical amplifier applications and the values are presented in Table 9. It is observed from the table that among all the prepared glasses, the gain bandwidth (210.96  10  28 cm3) and FOM (27.310  10  24 cm2 s) values are found to be higher for the 0.75PrZTFB glass. The present investigation concludes that the radiative as well as optical amplification parameters such as ( σPE ), FWHM, (σPE  FWHM), and (σPE  τcal) are found to be higher for the 0.75 wt% Pr3 þ doped Zinc telluro-fluoroborate glass corresponding to the 1G4-3H5 (1330 nm) NIR emission transition and the same is suggested for the fabrication of tunable lasers, broadband optical amplifiers and low loss telecommunication applications.

4. Conclusion In the present study, structural and spectroscopic properties of

Table 9 Emission peak positions (λp,nm), radiative transition probability (A, s  1), FWHM (nm), stimulated emission cross-section ( σPE  10  20 cm2), radiative lifetime (τcal, ms), calculated branching ratios (βR), gain bandwidth (  10  27 cm3) and FOM (  10  24 cm2 s) for the 1G4-3H5 emission transition of the Pr3 þ doped Zinc telluro-fluoroborate glasses. Transition

Parameters

0.05PrZTFB

0.1PrZTFB

0.25PrZTFB

0.5PrZTFB

0.75PrZTFB

1PrZTFB

2PrZTFB

1

λp A FWHM

1334 520.54 52 1.47

1335 565.28 50 1.65

1335 588.8 53 1.65

1336 638.82 71 2.19

1336 650.6 74 2.85

1334 645.47 54 2.15

1334 632.76 55 1.85

0.623 1197 7.62 17.55

0.624 1104 8.25 18.21

0.618 1049 8.77 17.34

0.621 972 15.55 21.29

0.623 958 21.10 27.31

0.624 966 11.59 20.72

0.632 934 10.19 17.30

G4-3H5

σPE βR(Cal) τcal Gain bandwidth FOM

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Pr3 þ doped Zinc telluro-fluoroborate glasses were explored to optimize the Pr3 þ ions concentration for the fabrication of photonic devices. The various vibrational modes of the borate and tellurite network(s) like B–O stretching vibrations of BO3 units and BO4 units involved in the title glasses were identified and the presence of Te–O–Te linkage stretching vibrations were also confirmed from the Raman spectral analysis. The phonon energy of the glass host was found to be 776 cm  1. The optical band gap (Eopt) and Urbach energy (ΔE) values were calculated from the absorption spectra and the Eopt values are found to decrease with the increase in Pr3 þ ion concentration and the same may be due to the structural rearrangement takes place between the metal ions and ligands in the glass network. The JO intensity parameters suggest the higher covalence between Pr–O bonds and/or higher asymmetry around the Pr3 þ ion site. The x, y coordinates of the studied glasses found to lie in the reddish-orange region of the CIE 1931 chromaticity diagram. Among the studied glasses, 0.75PrZTFB glass exhibit higher laser as well as amplification parameter values such as σPE , βR and σPE  λeff ,σPE  τcal corresponding to the 3P0-3H4 and 1G4-3H5 transitions and the same may be suggested for the fabrication of solid state lasers and broadband optical amplifiers.

Acknowledgement Dr. Ch. Basavapoornima is thankful to University Grants Commission, New Delhi, for the award of Post Doctoral Fellowship for Women for the year of 2011-12 (F.15-1/2011-12/PDFWM-2011-12OB-AND-9964 (SA-II), dt. 1-11-2013).

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