Visible luminescence characteristics of Pr3+ ions in TeO2–Sb2O3–WO3 glasses

Optical Materials 101 (2020) 109740

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Visible luminescence characteristics of Pr3þ ions in TeO2–Sb2O3–WO3 glasses V. Himamaheswara Rao a, b, *, P. Syam Prasad b, K. Sowri Babu c a

Division of Physics, Department of Humanities and Sciences, PACE Institute of Technology and Sciences, Valluru, Ongole, Andhra Pradesh state, 523 272, India Department of Physics, National Institute of Technology Warangal, Warangal, 506 004, Telangana State, India c Vignan Foundation for Science Technology and Research (Deemed to be University), Vadlamudi, Guntur District, Andhra Pradesh State, 522 213, India b

A R T I C L E I N F O

A B S T R A C T

Keywords: Tellurite glass Antimony and tungsten oxides Pr3þ ion PL spectra Judd-ofelt theory Decay curve analysis

Pr3þ ion doped TeO2 glasses were synthesized by standard melt quenching method. Optical absorption, photo­ luminescence and decay profiles were recorded to assess the luminescence efficiency of Pr3þ ions in Sb2O3–WO3 mixed tellurite glasses. Judd-Ofelt theory was applied to evaluate radiative properties and also the varying environment of the rare earth ion with varying concentration of Pr3þ ions in the glass matrix. Luminescence spectra recorded in the visible region exhibited several emission transitions and among them the red lumines­ cence transition 3P0 → 3F2 is found to be more intense. Variations of luminescence intensity and life times of excited state energy levels with the concentrations of Pr3þ ions are discussed in the light of variation in the environment of Pr3þ ions in the glass network.

1. Introduction A huge number of glass and crystalline materials are being studied to investigate the influence of host matrix on luminescence characteristics of rare earth ions (RE) [1–4]. Easiness in glass fabrication in any size and shape and recent developments in many potential applications such as optical data storage, medical surgery, military devices, laser wave guides, full color display devices and optical amplifiers etc., attracted researchers to investigate the rare earth ion doped glasses. Recently tellurite glass systems have been the subject of investigation for several researchers; besides the greater moisture resistant, these glasses possess high linear and non-linear refractive indices, low phonon energy, low glass transition and melting temperatures, high mechanical, thermal and chemical stability [5,6]. Praseodymium ion is one of the most promising RE doping ions and it is an attractive optical activator due to its rich energy level spectrum which includes large number of meta-stable multiplets viz., 3P0,1,2,1D2 and 1G4 that makes the radiation to be emitted simultaneously in blue, green and red regions, and also prominent infrared emission at 1.3 μm which is useful in optical amplification [7–10]. Pr3þ doped glasses are attractive for application in short wave length upconversion laser ma­ terials [11–13]. Pr3þ ion doping is also useful in enhancement of

nonlinear optical properties of the materials. Pr3þ ion effect on the non linear optical susceptibility and photo induced second harmonic gen­ eration (SHG) in BiBO glasses is investigated by Majchrow-ski et al. [14–16]. They have reported that the high polarizability of trivalent praseodymium ion strongly influences the output of nonlinear optical SHG parameters which is due to their well organized connections among the fundamental structural clusters of the BiBO glass system [14–16]. 91 fold degenerate orbitals possessed by 4f2 configuration caused for the investigations to be increased on luminescence features of Pr3þ ions in different glass systems in recent years [17]. The more attractive feature of Pr3þ ion is that the energy levels at the lower edge of the 4d5d configuration and the 1S0 level of the 4f2 configuration are separated with very small energy [18]. The position of the 4f5d configuration relative to the 4f levels is also decisive for several applications. The splitting of the 4f5d configuration is strongly influenced by the host environment because of the coupling interaction between the lattice phonons and the 5d electrons. In this manner, a phonon emission cascade can be expected if the 5d states are positioned at higher energy than 1S0 level. On the other hand if the lowest 4f5d energy level is positioned below the 1S0 level, an intense broad emission occurs due to the parity allowed electric dipole 4fN–15d1 → 4fN transition under high energy photon excitation. Visible up-conversion emission in Pr3þ ion

* Corresponding author. Division of Physics, Department of Humanities and Sciences, PACE Institute of Technology and Sciences, Valluru, Ongole, Andhra Pradesh state, 523 272, India. E-mail address: [email protected] (V.H. Rao). https://doi.org/10.1016/j.optmat.2020.109740 Received 23 December 2019; Received in revised form 4 February 2020; Accepted 5 February 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.

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doped TeO2–ZnO glass has been investigated by Rai [19]. Manzani et al. [7] studied the orange emission by fluoro-indate glasses doped with Pr3þ ions. Pawar et al. [20] studied the luminescence properties of the Dy3þ and Pr3þ ions co-doped lithium borate glasses for white LED applica­ tions. Lasing properties of Pr3þ doped tellurofluorophosphate glasses were investigated by Moorthy et al. [21]. Presence of WO3 and Sb2O3 in tellurite glass network enhances the PL output of Pr3þ ion since these oxides modify the environment around the rare earth ion due to their higher polarizability [22,23]. Further, both Sb2O3 and WO3 play an important role in the formation of stable glass network by their active participation as glass formers and as well as modifiers depending on the surrounding structural cluster of the host glass network [2,3,24,25]. The low phonon energy and higher polariz­ abilities of WO3 and Sb2O3 can enhance the luminescence efficiency of tellurite glasses. Besides, Sb2O3 decreases the melting temperature of the tellurite glasses. Presence of WO3 in glasses induces photochromism and electrochromism properties and the glasses become ideal candidates for optoelectronic devices [2]. It is well recognized that the radiative pa­ rameters connected with emission transitions of rare earth ions are very responsive even for little alteration of the chemical network in the glass lattice. Network modifiers are also expected to influence the local environment of rare earth ions to some extent [26,27]. Thus the inclu­ sion of Sb2O3 and WO3 which act as both network formers and modifiers affect the luminescence properties to a greater extent. This manuscript reports the investigation on the radiative properties of the Pr3þ ion by employing the Judd–Ofelt (J-O) theory and also the influence on nature of chemical bonding in the surrounding environ­ ment of Pr3þ ions which varies due to the existence of various modifier oxides in the TeO2–Sb2O3–WO3 glass matrix. However, the applicability of J–O theory to praseodymium ion is found doubtful for several glass systems because of the small energy separation connecting first excited energy state configuration 4f15d1 and the ground configuration 4f2 [28]. This is because of the difficulty experienced in fitting the 3H4 → 3P2 hypersensitive transition due to huge diversity among the calculated and experimental oscillator strengths [29]. In this investigation, the J-O parameters were evaluated using the UV–vis–NIR optical absorption spectra and these parameters are used to determine different radiative parameters such as transition probabilities (AR), radiative lifetimes (τR), branching ratios (βcal), and stimulated peak emission cross-sections (σ(λP)) for different emission transitions of Pr3þ ions in the TSWP glasses. The obtained results were used to assess the suitability of the material for potential applications as lasers and photonic devices.

spectra of the TSWP glasses were measured on a Flourolog-3 spectro­ fluorometer procured from Horiba Jobin Yvon, Japan which uses xenon arc lamp of 450 W as a radiation source. Decay profiles were also recorded using a xenon lamp of 60 W in the same spectrofluorometer. The densities of the TSWP glasses were evaluated by means of Archi­ medes’ principle using water as the immersion liquid. The refractive indices of the TSWP glass samples were recorded with a prism coupler, SPA-4000. All the experimental measurements were carried out at room temperature. The physical and optical parameters calculated using densities and refractive indices are presented in Table 1. 3. Results and discussion 3.1. X-ray diffraction The X-ray diffraction patterns shown in Fig. 1 exhibited the broad diffused peak at lower angles which is the characteristic amorphous nature of the glasses; this observation proves the non-crystalline char­ acter of the prepared samples. 3.2. Absorption spectra and Judd-Ofelt analysis Optical absorption spectrum of TSWP10 glass is shown in Fig. 2. The absorption spectra for all TSWP glasses are observed to be similar; however, but exhibited considerable variations in the intensity of different absorption bands. The absorption spectra exhibited nine ab­ sorption bands at 446, 473, 486, 595, 1015, 1448, 1539, 1950 and 2355 identified as being due to excitation of Pr3þ ion from its ground state 3H4 to 3P2, 3P1, 3P0, 1D2, 1G4, 3F4, 3F3, 3F2 and 3H6 upper levels, respectively. These bands are attributed following the report by Carnall et al. [30] and are in well agreement with the earlier reports [31,32]. Among these bands, the band due to 3H4 → 3P2 transition is observed to be hyper­ sensitive following the selection rules |ΔS| ¼ 0, |ΔL|�2, and |ΔJ|�2 and it is highly responsive to the neighbouring environment of the rare earth ion, whereas the ligand environment surrounding the RE ion influences the intensity of all the absorption bands. The absorption band peak positions (in cm 1) of Pr3þ ion doped TSWP glasses are presented in Table 2. The peak positions of the corresponding bands of the aqua ion as per the earlier reports [30,33,34] are also given in the table alongside. The nephelauxetic ratio (β) and bonding parameters (δ) which are calculated by using the peak positions of absorption bands are useful in evaluating the rare earth ion bonding behaviour with the surrounding ligand. The calculated values of β‾ (average of β) and δ are presented in Table 2. The positive values of δ observed for the glasses specify the covalent character of the bonding between surrounding ligand and Pr3þ ion [35]. The higher values of bonding parameter around 0.6 suggested the higher covalence nature of the bond between the ligand and Pr3þ ion in these glasses. The intensity of absorption bands of TSWP glasses were evaluated in terms of experimental oscillator strengths, fexp which were evaluated using the integrated area under the corresponding absorption peaks, whereas fcal were estimated using the Judd-Ofelt (J-O) theory [36,37] and are presented in Table 3. The Judd-Ofelt intensity parameters (Ωλ, λ ¼ 2, 4, 6) were determined from the fexp and fcal using the least squares fitting method and are given in Table 4. The RMS deviation (δrms) be­ tween the fexp and fcal values is assessed (with the inclusion and exclusion of hypersensitive transition 3H4 → 3P2 in the fit) and presented in Table 3. As given in the Table, δrms values show the better agreement in the least square fitting when the hypersensitive transition is excluded: however, there is no significant change in J-O parameters. The Ω2 and Ω6 values decrease with the exclusion of the hypersensitive transition while the Ω4 values are increasing with the exclusion of the transition. However, the trend of the Ωλ values (Ω4> Ω2> Ω6) is same for all the glasses with the inclusion and the exclusion of hypersensitive transition. The obtained small δrms values signify the good quality of the fitting between the fexp and fcal values and the accuracy in the J-O parameters.

2. Experimental Tellurite glasses mixed with Sb2O3 and WO3 and doped with varying concentrations of Pr3þ (TSWP glasses) were prepared by melt quenching method. The compositions of the glasses are (75-x) TeO2–15 Sb2O3–10 WO3–x Pr6O11 (where x ¼ 0.2, 0.5, 0.8, 1.0 and 1.5 mol %) which are labelled TSWP2, TSWP5, TSWP8, TSWP10 and TSWP15, respectively. For the preparation of the glasses high purity chemicals of TeO2 (99.9%), WO3 (99.9%), Sb2O3 (99.99%) and Pr6O11 (99%) were thor­ oughly mixed to obtain homogeneous batch composition. The batches of 10 g each were melted in platinum crucibles in an electric muffle furnace at 800 � C for 20 min. The molten liquid was poured on to a preheated (at 200 � C) brass mould of disc shape (diameter of 1 cm) and pressed with another flat brass plate. The samples were transferred to another furnace and annealed at 200 � C for 8 h to remove internal strains and later the samples were polished to the optical quality. The thickness of the pol­ ished samples ranges from 2.2 to 3.3 mm. The X-ray diffraction patterns of the samples were recorded in the diffraction angle (2θ) from 10� to 80� using a PANalytical XPERT-PRO Xray diffractometer through radiation of CuKα (λ ¼ 1.5406) using a scanning rate of 4� /min and a step size of 0.02. Absorption spectra of the TSWP glasses were recorded in the Visible-NIR region of 400–2300 nm using a Jasco V-670 spectrophotometer. The excitation and emission 2

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Table 1 Physical and optical properties of TSWP glasses. S.No.

Parameter

TSWP2

TSWP5

TSWP8

TSWP10

TSWP15

1 2 3 4 5 6 7 8 9 11 10 12 13 14

Molar Mass, M (g/mol) Molar volume, Vm (cm3/mol) Path length, l (mm) Density, ρ (g/cm3) Refractive index, n Pr3þ ion concentration, C (1020 ions/cm3) Polaron radius, rp (Å) Inter ionic distance, ri (Å) Field strength, F (1016/cm2) Molar refractivity, Rm (cm3) Electronic polarizability, αe (Å3) Reflection losses, R (%) Dielectric constant, ε Metallization factor, Mt

188.336 32.299 3.310 5.831 2.138 0.373 12.063 29.932 0.454 17.553 0.696 13.152 4.571 0.457

190.921 32.703 2.300 5.838 2.153 0.921 8.925 22.146 0.829 17.917 0.711 13.372 4.635 0.452

193.507 33.118 2.200 5.843 2.161 1.455 7.663 19.014 1.124 18.222 0.723 13.490 4.670 0.450

195.230 33.367 2.520 5.851 2.175 1.805 7.131 17.695 1.298 18.495 0.734 13.696 4.731 0.446

199.539 34.005 2.670 5.868 2.194 2.656 6.269 15.556 1.679 19.033 0.755 13.975 4.814 0.440

dependent on the bulk properties such as rigidity, viscosity and dielec­ tric of the medium and are also influenced by the vibronic transition of the bond between ligand and rare earth ions. The Ω2 parameters of the titled TSWP glasses are higher than that of the several glasses reported earlier [28,34,39–44] and are comparable with that of the Pr3þ doped LTT glasses reported by Venkateswarlu et al. [32]. The Judd-Ofelt pa­ rameters are observed to not vary much with Pr3þ ion concentration, but for the doping of 1.5 mol% of Pr3þ ion, the Ω2 and Ω4 values are found to decrease sharply indicating the decrease in asymmetric and covalent nature of the surrounding environment of the rare earth ion. The higher values of all Judd-Ofelt intensity parameters are because of the higher polarizabilities of the components TeO2, Sb2O3 and WO3 chosen in our host composition. The higher values of the Ω2 parameters observed for all TSWP glasses indicate higher degree of covelancy of Pr3þ ion bonding with the oxygen ligand as confirmed from the bonding parameters and it also be a sign of the higher asymmetric nature of the ion sites sur­ rounding the Pr3þ ion [45]. Conversely, the higher values of Ω4 and Ω6 indicate the higher rigidity of the host medium around the rare earth ion. In addition, Ω4 and Ω6 values can be used to determine the spec­ troscopic quality factor (Ω4/Ω6), an essential laser characteristic parameter employed in guessing the stimulated emission for any laser active medium. The calculated values of spectroscopic quality factor for TSWP glasses are observed to be higher as compared to the reported Pr3þ doped glasses presented in Table 4; this observation suggests higher stimulated emission of Pr3þ ion in the titled host glass.

Fig. 1. X-ray diffraction patterns of TSWP glasses.

3.3. Emission spectra In order to study the luminescence characteristics of the Pr3þ ion doped tellurite glasses, excitation spectrum of TSWP10 glass is recorded by keeping the emission wavelength (λem) at 645 nm and is exhibited in Fig. 3; the spectrum exhibited three excitation bands at 447, 472 and 485 nm corresponding to the transitions 3H4 → 3P0, 3P1, 3P2, respec­ tively. Among these bands, the band at 447 nm is observed to possess more intensity and the wavelength is used for excitation for recording the emission spectra of all the TSWP glasses which are shown in Fig. 4. The emission spectra measured in the visible region of electromag­ netic radiation showed numerous inhomogeneous broad emission bands. The spectra consist of bands peaked at 475, 487, 530, 542, 598, 614, 646, 685, 707 and 731 corresponding to the emission transitions 3 P1 →3H4, 3P0 →3H4, 3P1 →3H5, 3P0 →3H5, 1D2 →3H4, 3P0 →3H6, 3P0 →3F2, 3P0 →3F3, 3P1 →3F4 and 3P0 →3F4, respectively. The band at 646 nm exhibited the highest luminescence intensity among all the transi­ tions and the intensity of this band exhibited increasing trend with in­ crease of Pr3þ ion concentration up to 1.0 mol% and beyond 1.0 mol% concentration, it is found to decrease due to luminescence quenching. This quenching arises due to non-radiative transitions such as multi phonon relaxation, cross-relaxation channels and resonance energy transfer processes. However, the variation of emission intensity of

Fig. 2. Optical absorption spectrum of TSWP10 glass.

The J-O intensity parameters (Ωλ) play a vital role in elucidating the bonding character of the RE ions with the neighbouring ligand envi­ ronment and the local structural arrangement of the atoms. Jorgensen and Reisfeld [38] have reported that, the Ω2 values are dependent on the covalence of ligand-RE3þ bond in addition to the asymmetry of local ion sites in the region of the RE3þ ion, while the values of Ω4 andΩ6 are 3

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Table 2 Peak positions of the absorption transitions (in cm

1

) and bonding parameters (β‾ and δ) of TSWP glasses.

Transition 3 H4 →

TSWP2

TSWP5

TSWP8

TSWP10

TSWP15

Aqua ion

3

22523 21209 20576 16835 9857 6906 6496 5126 4243 0.99531 0.471

22421.52 21097.05 20576.13 16778.52 9847.366 6901.311 6497.726 5128.205 4240.882 0.99372 0.632

22421.525 21186.441 20618.557 16806.723 9852.2167 6901.3112 6493.5065 5122.9508 4251.7007 0.99476 0.527

22422 21142 20576 16807 9852 6906 6498 5128 4246 0.99441 0.562

22371 21142 20576 16849 9852 6901 6496 5128 4248 0.99438 0.565

22520 21300 20750 16840 9900 6950 6500 5200 4215 – –

P2 P1 P0 1 D2 1 G4 3 F4 3 F3 3 F2 3 H6 β‾ δ 3 3

Table 3 Experimental oscillator strengths (fexp, 10 6), calculated oscillator strengths (fcal, 10 6) and rms deviation (δ, 10 the Table fcal* is the oscillator strength evaluated with the omission of hyper-sensitive transition.

6

) of fexp and fcal of Pr3þions doped TSWP glasses. In

Transition 3 H4 →

TSWP2 fexp

fcal

fcal*

fexp

fcal

fcal*

fexp

fcal

fcal*

fexp

fcal

fcal*

fexp

fcal

fcal*

3

15.27 10.70 13.15 15.16 2.02 3.27 27.57 31.90 0.30

10.65 21.30 20.92 3.56 0.97 9.04 23.88 32.04 1.96 �0.39

– 21.38 21.00 3.53 0.96 8.89 23.72 32.06 1.94 �0.40

25.64 12.84 11.85 15.02 1.99 2.26 25.47 30.59 0.36

9.76 20.87 20.60 3.29 0.90 8.12 22.43 30.70 1.70 �0.46

– 21.14 20.87 3.18 0.86 7.62 21.89 30.74 1.63 �0.40

38.09 14.56 11.49 13.66 2.14 2.43 27.93 32.50 0.27

10.97 22.51 22.18 3.68 1.00 9.24 24.76 32.60 2.03 �0.53

– 22.98 22.64 3.48 0.94 8.39 23.82 32.67 1.89 �0.39

41.82 15.04 12.54 16.02 2.35 3.42 28.97 33.41 0.23

11.78 22.77 22.43 3.92 1.08 10.12 26.13 33.49 2.04 �0.54

– 23.29 22.94 3.70 1.01 9.17 25.09 33.58 1.89 �0.38

39.82 13.59 10.23 14.63 2.16 3.42 27.79 30.41 0.24

11.41 20.86 20.55 3.78 1.04 9.96 24.86 30.49 2.13 �0.55

– 21.35 21.03 3.57 0.98 9.06 23.88 30.57 1.98 �0.40

P2 3 P1 3 P0 1 D2 1 G4 3 F4 3 F3 3 F2 3 H6 δrms

TSWP5

TSWP8

TSWP10

TSWP15

Table 4 The values of JO intensity parameters Ω2, Ω4, Ω6 (10 20 cm2), their trend and spectroscopic quality factor (Ω4/Ω6) of TSWP glasses along with that of the earlier reported Pr3þ doped glasses. (*indicates values evaluated with the omission of hypersensitive transition 3H4 → 3P2in the fit.). Glass name

Ω2

Ω4

Ω6

Trend

Ω4/Ω6

Ref.

TSWP2 TSWP2* TSWP5 TSWP5* TSWP8 TSWP8* TSWP10 TSWP10* TSWP15 TSWP15* TeO2–WO3–PbO–La2O3 1PrZTFB Glass D: LTT TeO2–WO3–ZnO–TiO2–Na2O ZnAlBiB Glass A ZANP25 ZBP5 Pb:LiF–B2O3 40GaS3/2 .40GeS2.20Cs:Br Pr3þ: GeAsGaSe

16.69 16.68 15.58 15.56 16.24 16.21 16.52 16.49 14.62 14.59 4.77 7.531 16.69 42.81 7.36 4.747 26.53 3.94 5.11 12.3 9.05

21.74 21.82 21.14 21.42 22.56 23.03 22.60 23.11 20.38 20.86 1.21 2.608 5.559 36.69 4.46 2.228 12.30 1.34 4.87 6.33 7.26

7.09 6.92 6.08 5.50 7.06 6.09 7.86 6.79 7.83 6.83 5.16 9.152 5.305 23.73 4.17 5.445 5.19 1.23 21.72 5.67 7.28

Ω4> Ω4> Ω4> Ω4> Ω4> Ω4> Ω4> Ω4> Ω4> Ω4> Ω6> Ω6> Ω2> Ω2> Ω2> Ω2> Ω2> Ω2> Ω6> Ω2> Ω2>

3.07 3.15 3.48 3.89 3.20 3.78 2.88 3.41 2.60 3.05 0.24 0.20 1.05 1.546 1.06 0.398 2.36 1.09 0.22 1.12 0.997

Present Present Present Present Present Present Present Present Present Present [39] [34] [32] [46] [40] [41] [47] [42] [28] [43] [44]

different bands in the spectrums altered with RE3þ ion concentration which may be due to the spreading of energy levels within small energy separation that cause non-radiative transitions assisted by phonon en­ ergy within these energy levels. The partial energy level diagram shown in Fig. 5 depicts the process of emission of Pr3þ ion in TSWP glasses at excitation of 447 nm and also shows the various possible cross relaxation channels among the different energy levels. The acceptor ion in ground level absorbs the emission energy released by the donor ion from higher energy level and excites to the intermediate energy level and this ion relaxes to ground level non-

Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω6 Ω2> Ω4 Ω2> Ω4 Ω4> Ω6 Ω4> Ω6 Ω4> Ω6 Ω6> Ω4 Ω4> Ω6 Ω4> Ω6 Ω2> Ω4 Ω4> Ω6 Ω6> Ω4

work work work work work work work work work work

radiatively and as a consequence the emission intensity will be quenched. This relaxation happens due to the close match of the energy separation between those energy levels and phonon subsystems could be the responsible in assisting the non-radiative transitions among these cross relaxation channels. The cross relaxation channels and the corre­ sponding energy separation are given in Table 5. 3.4. Radiative properties Luminescence efficiency of TSWP glasses is assessed using the 4

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Table 5 Cross-relaxation channels and matching of the energy separation between donor and acceptor ion transitions. Cross relaxation channel

Donor emission channel

Acceptor absorption channel

Transition

Energy (in cm 1)

Transition

Energy (in cm 1)

A B C D E

3

16330 10724 3769 6955 9901

3

16807 9852 4246 6906 9852

P0 →3H6 P0 →1G4 3 P1 →1D2 1 D2 →1G4 1 D2 →3F4 3

H4 →1D2 H4 →1G4 3 H4 →3H6 3 H4 →3F4 3 H4 →1G4 3

6) (with the equations reported in our earlier papers [48,49]) and are given in Table 6 along with the effective bandwidth of the emission bands. The experimental branching ratios (βexp) were determined using the integrated area beneath the emission peaks and are given in Table 6 to compare with radiative branching ratios. The branching ratios are useful in predicting the relative intensities of all emission bands origi­ nating from a particular excited level [50]. It is a significant parameter in designing a laser which is used to describe the possibility of achieving the stimulated emission for any particular transition. An emission transition with branching ratio ~50% is generally considered as effi­ cient laser radiation [51]. Among all the transitions, the emission 3 P0→3H4 shows higher magnitude of βexp and the calculated branching ratios (βcal) of 3P0→3H4 and 3P0→3F2 transitions exhibited higher values (around 40% and 45%, respectively) for all the TSWP glasses. The values of AR and σ (λP) of these emission transitions are also found to be maximum and hence these two transitions can be considered as useful in laser devices and light emitting applications. Choosing the low phonon energy materials viz. TeO2, Sb2O3 and WO3 for our host glass made it possible to achieve these higher values of AR and σ(λP). Among various glasses studied, The TSWP10 glass exhibited higher values of AR and σ (λP) and hence it is an attractive glass for low threshold and high gain laser applications [47]. The laser performance of the TSWP glasses has been examined through important parameter viz., optical gain band­ width (σ(λp)xΔλeff) which is found to be the highest for 3P0→3F2 tran­ sition (Table 6). This observation confirms the laser potentiality of the emission in red region. The radiative life time of the excited energy level is calculated using the values of total radiative transition probability and are found to be in the range of 15–17 μs for these glasses.

Fig. 3. Excitation spectrum of TSWP5 glass under the emission of 645 nm.

Fig. 4. Emission spectra of TSWP glasses under the excitation of 447 nm.

3.5. Quantum efficiency and CIE chromaticity coordinates The quantum efficiency of the glasses calculated as per the relation η ¼ (τexp/τcal)100 are given in Table 7 and the values are found to be higher than that of the Pr3þ doped TeO2–WO3–PbO–La2O3 glasses re­ ported by Bozena Burtan et al. [39]. The luminescence emission color of the TSWP glasses under the excitation of 447 nm is assessed with the CIE chromaticity coordinates calculated using simulation of emission spectra. The evaluated CIE chromaticity coordinates of TSWP glasses are given in Table 7; the values are observed to be considerably invariant with the variation of Pr3þion concentration. The chromaticity co­ ordinates of TSWP10 glass are found to be in the region of orange colour (Fig. 6). 3.6. Decay curve analysis The fluorescence lifetime of 3P0 (λexc ¼ 447 nm, λemi ¼ 646 nm) excited state of Pr3þ ion in TSWP glasses is evaluated by recording the decay profiles (Fig. 7) of all the glasses and are given in Table 6; the lifetime values are seen to reduce with increasing of Pr3þ ion content. The energy transfer through the multiphonon and cross relaxation channels [56,57] which are shown in Fig. 5 are predicted to be responsible for such decrease. The lifetime values of 3P0 level of Pr3þ ion in the titled glasses are found to be in good agreement with that of

Fig. 5. Energy level diagram of Pr3þ ion in TSWP10 glass. Excitation, emission and cross relaxation channels are shown in the diagram.

radiative parameters such as radiative transition probability (AR), stimulated emission cross-section (σ (λP)), radiative branching ratio (βR) and radiative lifetimes. These parameters were calculated for the prominent emission transitions by using the J-O parameters Ωλ (λ ¼ 2, 4, 5

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Table 6 Peak wavelength of emission (λp, nm), effective bandwidth (Δλeff, nm), spontaneous radiative transition probability (AR, s 1), stimulated emission cross-section (σ(λp), 10 22 cm2), experimental (βexp) and calculated (βR) branching ratios and optical gain bandwidth (σ(λp)xΔλeff, 10 28 cm3) for the prominent emission transitions of Pr3þ ion in TSWP glasses. Total transition probability (AT, s 1) and calculated and experimental life times (μs) of 3P0 excited energy level are also given in this Table. Transition 3

3

P0→ H4

3

P0→3H6

3

P0→3F2

3

P0→

Parameter

TSWP2

TSWP5

TSWP8

TSWP10

TSWP15

λ Δλeff AR σ(λP) βexp βcal σ(λP)xΔλeff λ Δλeff AR σ(λP) βexp βcal σ(λP)xΔλeff λ Δλeff AR σ(λP) βexp βcal σ(λP)xΔλeff AT

487 13.22 26931.28 33.26 0.405 0.447 439.84 614 16.36 1238.41 3.12 0.207 0.021 51.11 646.00 8.12 27131.60 168.94 0.30 0.45 1371.92 60270.85 16.6 11.73

487 12.75 27148.14 34.30 0.405 0.459 437.23 614 15.47 1011.40 2.66 0.213 0.017 41.16 646.00 8.03 25984.54 161.32 0.28 0.44 1295.68 59153.66 16.9 9.18

487 14.32 29598.13 33.04 0.357 0.465 473.16 614 17.46 1134.72 2.62 0.202 0.018 45.83 646.00 8.30 27456.47 163.80 0.33 0.43 1358.96 63651.00 15.7 8.79

487 13.73 30450.55 34.99 0.398 0.461 480.54 614 16.44 1296.62 3.14 0.213 0.020 51.70 646.00 8.25 28620.08 169.59 0.30 0.43 1398.37 65986.22 15.2 8.18

487 13.42 28409.11 32.83 0.399 0.464 440.60 614 16.32 1350.08 3.24 0.213 0.022 52.91 646.00 8.20 26189.36 153.28 0.30 0.43 1257.54 61190.81 16.3 7.94

τcal τexp

Table 7 CIE Chromaticity coordinates and quantum efficiency η (%). Glass sample

CIE chromaticity coordinates

η

TSWP2 TSWP5 TSWP8 TSWP10 TSWP15

(0.43, (0.45, (0.42, (0.43, (0.43,

70.66 54.32 55.99 53.82 48.71

0.36) 0.36) 0.36) 0.36) 0.36)

several Pr3þ doped glasses reported earlier [21,42,52–55]. 4. Conclusions TSWP glasses doped with different concentrations of praseodymium ions were prepared and their luminescence features were investigated Optical absorption spectra of the TSWP glasses exhibited several ab­ sorption bands from the ground state 3H4 of Pr3þ ion. The higher values of bonding parameter around 0.6 suggested the higher covalence nature of the bond between the ligand and Pr3þ ion in these glasses. Judd-Ofelt intensity parameters were determined by employing the Judd-Ofelt theory to understand the surrounding environment of the rare earth ion and also to evaluate various radiative parameters of the Pr3þ ion. The obtained higher Ω2 values for all the TSWP glasses is an indication of higher symmetric environment around the rare earth ion and also the higher covelency of the ligand bond with the Pr3þ ion. Spectroscopic quality factor values more than 3 for the titled glasses facilitate to exhibit higher stimulated emission. The emission spectra under the excitation of 447 nm showed inhomogeneous broad emission bands in the entire visible region of the electromagnetic radiation. Among the titled glasses, TSWP10 exhibited higher values of AR and σ (λP) and thus it is attractive for low threshold and high gain laser applications. Higher values of AR, σ(λP) and σ(λP)xΔλeff (28620 s 1, 17 � 10 21 cm2 and 14 � 10 26 cm3, respectively) associated with 3P0→3F2 transition of TSWP10 glass confirm the lasing potentiality of the transition in orange-red re­ gion of this glass. Low phonon energy and higher polarizabilities of the chosen composition made the glasses to attain higher emission crosssection and luminescence efficiency. The radiative life time of the excited energy level 3P0 calculated using the values of total radiative

Fig. 6. CIE color chromaticity diagram of TSWP10 glass. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

transition probability are found to be in the range of 15–17 μs for the studied glasses. Experimental lifetime values evaluated from decay profiles are found to decrease from 11.73 to 7.94 μs with increasing Pr3þ ion concentration; this decrease is attributed to the different multi­ phonon and cross relaxation channels among different energy levels.

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Optical Materials 101 (2020) 109740

[11] [12] [13] [14]

[15] [16] [17] [18] [19]

Fig. 7. Decay profiles of TSWP glasses under the excitation of 447 nm moni­ toring the emission at 646 nm. The solid lines show the fitting of the profiles.

[20]

Declaration of competing interest

[21]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[22] [23]

CRediT authorship contribution statement [24]

V. Himamaheswara Rao: Conceptualization, Methodology, Soft­ ware, Investigation, Writing - review & editing. P. Syam Prasad: Writing - original draft, Supervision. K. Sowri Babu: Software, Valida­ tion, Visualization, Investigation.

[25] [26]

Acknowledgment

[27]

One of the authors V. Himamaheswara Rao would like to thank MHRD, Government of India for providing financial assistance in the form of Fellowship.

[28]

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