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Enhanced luminescence and energy transfer study in Tb:Sm codoped lead fluorotellurite glass
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 177–181
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Enhanced luminescence and energy transfer study in Tb: Sm codoped lead fluorotellurite glass A. Bahadur, Y. Dwivedi, S.B. Rai ⇑ Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, UP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ to Sm3+ ions has been observed. 4 6 The transitions from I9/2 to Hj are 3+ observed in Sm ion emission spectra. 5 3+ The lifetime of D4 level of Tb ion decreases in the presence of Sm3+ ion. Cross-relaxation process dominantly populates the 5D4 level of Tb3+ ion.
Energy transfer from Tb
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 11 August 2013 Accepted 15 August 2013 Available online 31 August 2013 Keywords: Fluorotellurite glass Samarium Terbium Luminescence Energy transfer
a b s t r a c t Spectroscopic properties of Sm3+, Tb3+ doped and Tb3+:Sm3+ co-doped lead fluorotellurite glasses have been studied in detail. On the excitation with 355 and 532 nm laser wavelengths, the luminescence properties of the singly and doubly doped glasses have been analyzed. Intensity of characteristic emission bands due to Sm3+ ions is appreciably enhanced in the presence of Tb3+ ions. This is due to the energy transfer from Tb3+ to Sm3+ ions. Different energy transfer parameters have also been calculated, which affirm an efficient energy transfer from Tb3+ to Sm3+ ions. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Tellurite glasses have been extensively investigated in past due to their intriguing properties such as good transparency in the mid-infrared region, low phonon vibration (700–800 cm1), high refractive index (2), and high dielectric constants [1–3]. The physical properties e.g. refractive index, melting point, dielectric constant, etc. of tellurite glasses can be fairly tuned in controlled ⇑ Corresponding author. Tel.: +91 542 2307308; fax: +91 542 2368390. E-mail address: [email protected] (S.B. Rai). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.078
fashion by adding suitable heavy metal modifiers viz. Ba, Bi, Pb, etc. For example, addition of PbF2 not only supply fluoride which lowers the phonon energy considerably (140 cm1) less than the phonon energy of the tellurite (Te–O 800 cm1), but also lead to improved fluorine ion conductivity [4]. In particular, lead tellurite glasses are interesting as they are bi-functional: network modifier and former (at higher concentration). The covalent character of PbO is due to strong interaction of easily polarizable valance shells of Pb2+ ions and highly polarizable O2 ions; and also due to its efficient third order nonlinearity [5]. Doping of rare earth ions in tellurite glasses makes them optically active, which develop
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multifunctional characters in them and improves their commercial value [2,3]. Tb and Sm ions are highly desirable in the optical field because they show characteristic emissions in two different important spectral regions. Tb doped tellurite glasses have large potential for the development of amplifiers and lasers covering the main telecom windows [6,7]. It emits a bright green colour along with weak blue and red colours which are often used in fluorescent lamps, X-ray intensified screens, and television tubes. Several combination of Tb with other rare earth ions e.g. Tb–Tm, Tb–Eu, Tb–Ce, Tb–Yb, etc. [8–10] have been studied in glasses and it has been reported that the luminescent properties of Tb3+ are affected significantly. On the other hand, Sm3+ ion emits mainly in orange red region and so regarded as one of the significant rare earth ions and its demand is increasing day by day in various fields including fluorescent devices, high-density optical storage, under sea communication, colour displays and in visible solid-state lasers [11]. The 4G5/2 level of Sm3+ ion exhibits relatively high quantum efficiency and thus various populating and quenching emission channels results interesting fluorescence properties [12]. Sm3+ ion has also been studied in pairs of rare earth ions e.g. Sm–Tb, Sm–Ce, Sm–Eu, Sm–Dy and Er–Sm, etc. Results suggest that there is a good hope for efficient emission in suitable host matrix [13–15]. A more detail study on a pair of Sm:Tb is important as it may provide wide colour tunability extending from blue to NIR region and also white light emission. Though, energy transfer characteristics in Tb and Sm ion pair have been studied earlier [16–18], a detailed analysis of energy transfer is still lacking in commercially important modified tellurite host. In the present research work, luminescence and energy transfer characteristics of Sm, Tb and Sm:Tb codoped lead fluorotellurite glasses have been studied by using 532 nm and 355 nm laser as excitations sources. Our studies reveal an efficient energy transfer from Tb3+ to Sm3+ ion, which is verified from time resolved spectroscopy. Experimental Glasses were prepared with following chemical compositions:
½80 ðx þ yÞTeO2 þ 20PbF2 þ x Sm2 O3 þ yTb4 O7 where x = 0, 0.3, 0.4, 1 mol% and y = 0, 0.3, 1, and 1.5 mol%. Purity of the compounds used was better than 99.9%. Typical, melt and quench method [19] was adopted to prepare these glasses (heating temperature 1123 K). Good optical quality glasses of dimensions 1.0 1.0 0.3 cm were prepared. They were cleaned, polished and then used for further optical measurements. The absorption spectra of the samples were measured using JASCO V-670 double beam spectrophotometer in the range of 200–1700 nm. Excitation spectra were recorded on Fluoromax–4 spectrofluorometer (Horiba JobinYvon). The fluorescence measurements were carried out using the second and third harmonics (532, 355 nm) of a pulsed Nd:YAG (Spitlight 600, Innolas, Germany) laser as the excitation source. A iHR320 computer controlled trix monochromator equipped with grating (blazed at 500 nm) and PMT (model-1424 M) was used as a detector to detect luminescence. Photoluminescence decay curves were recorded using 532 and 355 nm pulsed radiations (repetition rate 10 Hz, pulse width 7 ns) of Nd:YAG laser. The collected signal was fed to 150 MHz digital oscilloscope (model No. HM 1507, Hameg Instruments) and the decay curves were obtained for further analysis. Life time of the radiative levels was estimated by exponential fitting.
Results and discussion Absorption Absorption spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped lead fluorotellurite glasses have been recorded in the region 200– 1700 nm (see Fig. 1). The absorption band edge of this glass is found to extend up to 490 nm due to which most of the absorption peaks of Tb3+ ion and some of the peaks of Sm3+ ions are not seen in glass absorption region. Trivalent Sm ion contains five electrons in the 4f shell and its ground state is 6H5/2. The absorption bands of Sm3+ ion appear in the visible and the NIR region are due to 4f–4f electronic transitions. The bands observed in the NIR region at 942, 1082, 1234, 1384, 1487, 1549 and 1588 nm are corresponds to 6F11/2, 6F9/2, 6 6 F7/2, 6F5/2, 6F3/2, 6H15/2 and 6F1/2 H5/2 transitions of Sm3+ ion, respectively, which are basically of electric dipole–dipole character. Few absorption bands of Sm3+ ion also lie in the visible region overlapped with the absorption band edge of lead fluorotellurite glass and they could not be seen.
Photoluminescence spectra The emission spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped glasses has been recorded using 532 and 355 nm laser excitations and are depicted in Figs. 2 and 3a and b. On excitation with 532 nm (18797 cm1) laser radiation, the Sm3+ ions promoted to 4 G5/2 level. Here, the energy mismatch was compensated by phonon vibrations of telluride lattice. Excited Sm3+ ions radiatively relax from 4G5/2 level to different lower lying energy levels. In emission spectra of Sm3+ doped glass, the peaks observed at 566, 601, 644, 706 and 794 nm attributed to 4G5/2 ? 6Hj (j = 5/2, 7/2, 9/2, 11/2, and 13/2) transitions of Sm3+ ion, respectively. Among these the most intense peak is at 601 nm which is due to 4 G5/2 ? 6H7/2 transition. The concentration of Sm3+ ions is optimized with the emission intensity. It is found that the concentration quenching took place beyond 0.3 mol% concentration [20]. The concentration quenching is presumed to be due to energy migration to the unexcited Sm3+ ions and impurities [21,22]. No energy level of Tb3+ ion matches with the photon energy of 532 nm laser radiation. Hence, luminescence is not observed due to Tb3+ ion with 532 nm laser excitation. When Sm3+ doped glass is excited with 355 nm radiation then the observed emission spectra contains all the emission bands which were observed on 532 nm excitation along with these few new weak bands viz. at 482, 510, 542, 571, 751, 844 are also
Fig. 1. Absorption spectra of 1 Tb3+, 0.3 Sm3+ doped and 0.3 Sm3+:1 Tb3+ codoped lead tellurite glass.
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Fig. 2. (Left) Photoluminescence spectra of Sm3+ doped and Sm3+:Tb3+ codoped glasses on excitation with 532 nm laser radiations. (Right) Energy level diagrams of trivalent Sm, Tb ion and excitations; emissions; cross-relaxation (CR); nonradiative relaxations; and energy transfer pathways.
Fig. 3a. Normalized Luminescence spectra of 0.3 Sm3+ doped glass on excitation with 355 nm and 532 nm lasers. Lower graph shows the enlarged portion of upper graph in the range of 450–550 nm (Left) and 735–870 nm (Right).
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observed (see Fig. 3a). These peaks are due to 4I9/2,11/2,13/2 + 4 M15/2 ? 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2, 15/2) transitions, respectively. On excitation with 355 nm laser, Sm3+ ions are excited to 6 D1/2, 4H7/2 levels. There are many closely spaced levels between 4 H7/2 and 4I9/2 level due to which the excited ions non-radiatively relax from 4H7/2 to 4I9/2 level [23] and a sufficient number of Sm3+ ions are collected at 4I9/2. Some of these Sm3+ ions radiatively relax from 4I9/2 to 6Hj levels while most of these relax non-radiatively to long live 4G5/2 level (ms). The energy gap between 4 G5/2 and the next lower lying energy level 6F11/2 (10,500 cm1) is 7100 cm1 which is very large, therefore, the excited Sm3+ ions in 4G5/2 level preferably relax radiatively to the lower levels [24]. Usually, the transitions from 4I9/2 to the lower levels are not visible in Sm3+ ions in most of the hosts, though these appear in our case perhaps due to low lattice vibration in the presence of heavy metal (Pb) in host. In the case of Tb3+ doped glass, the bright green emission is seen on excitation with 355 nm radiation. The seven emission bands observed at 488, 543, 583, 620, 641, 666.5 and 678 nm correspond to 5 D4 ? 7Fj (j = 6, 5, 4, 3, 2, 1, 0) transitions of Tb3+ ion (see Fig. 3b). The 5D4 ? 7F5 transition peaked at 543 nm is the brightest. The mechanism involved in the observed transitions can be explained as follows: the Tb3+ ions absorb the incident photons from 355 nm (28170 cm1) wavelength radiation and promoted to 5L10 level and then they nonradiatively relax to the 5D3 level. The energy gap between 5D3 and 5D4 level is 4500 cm1, which seems difficult to bridge by lattice vibration (Te–O 800 cm1) as it needs at least 6–7 phonons. Hence, the non radiative relaxation of Tb3+ ions from 5D3 to 5D4 is less probable at room temperature. The important channels of depopulation of 5D3 level are cross relaxation and radiative decay. Since cross relaxation process is fast hence, it will dominate to populate the 5D4 level following ultraviolet excitation [25,26]. The energy of 5D3 ? 7F0 and 5 D4 ? 7F6; 5D3 ? 5D4 and 7F0 ? 7F6 transitions are nearly equal which provides a pathway for cross relaxation process and populate the 5D4 level [25,27]. The Tb3+ ions present in 5D4 level relax radiatively to different low lying levels (7Fj (j = 6, 5, 4, 3, 2, 1, 0)) and emit blue, green and red colour light. The concentration of Tb3+ ions was optimized with luminescence intensity, which is optimum for 61 mol% concentration [28]. The concentration quenching takes place for higher concentration of Tb3+ ions which results the considerable reduction in photoluminescence intensity. When the trivalent Tb and Sm both the ions are present in the lead fluorotellurite glass, the photoluminescence spectrum extends from blue to NIR region. On comparison, it is found that the emission intensity of Sm3+ ion peaks is enhanced in the presence of Tb3+ ion, while the emission intensity of Tb3+ ion peaks is reduced significantly (see Fig. 3b). This shows an efficient energy transfer from Tb3+ to Sm3+ ions. From a close look of the energy level diagram of trivalent Tb and Sm ion, it is evident that several energy levels of both the ions are partially or fully overlapped to each other. For example 5D4 (20600 cm1) level of Tb3+ ion is close to 4M15/2 (20580 cm1) level of Sm3+ ion and 5D3 (26520 cm1) level of Tb3+ ion is close to 6P3/2 (26809 cm1) level of Sm3+ ion. It is possible that some of the excited Tb3+ ions transfer their excitation energy nonradiatively to the Sm3+ ions and promote them to 4 M15/2/6P3/2 levels and consequently improve the population of Sm3+ ions in 4G5/2 level, resulting improved luminescence. Excitation spectra
3+
3+
3+
3+
Fig. 3b. Luminescence spectra of 1.0 Tb , 0.3 Sm doped 0.3 Sm :1.0 Tb and 0.3 Sm3+:1.5Tb3+ codoped glasses on excitation with 355 nm laser source. Inset figure shows the luminescence spectra of 1.0 Tb3+ on enlarged scale in the range of 630– 680 nm.
We have also monitored the excitation spectra of 0.3 Sm3+, 1 Tb3+ doped and 0.3 Sm3+:1 Tb3+ codoped glasses for 601, 543 and 601 nm emissions, respectively (see Fig. 4). The excitation spectra of 1 mol% Tb3+ doped glass for 543 nm emission shows several high 7 7 lying energy levels at 272, 284 nm (5Ij F6); 304 nm (5Hj F6);
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7 7 318, 326 nm (5D1,0 F6); 341, 352, 358 nm: (5D2 F6) 5 7 5 7 5 7 ( G6 F6); 369 nm ( L10 F6); 377 nm ( D3 F6); 402 nm 7 7 (5G6 F5) and 484 nm (5D4 F6). On the other hand, 0.3 mol% 3+ Sm doped glass shows several excitation peaks corresponding 6 to 601 nm emission. These peaks are at 344 nm (4D7/2 H5/2); 6 6 361 nm (4D3/2 H5/2); 374 nm (6P7/2 H5/2); 390, 401 nm 6 6 6 (6P3/2 H5/2); 416 nm (6P5/2 H5/2); 437 nm (4M17/2 H5/2); 6 6 471 nm (4I11/2 H5/2); 489 nm (4I9/2 H5/2); 499 nm (4G7/2 6 H5/2). Fig. 4 clearly shows that the energy levels of Tb3+ and Sm3+ ions are considerably overlapped with each other in the region 330–390 nm and 475–500 nm. On comparing the intensity of excitation spectra in Sm3+ doped and Sm3+:Tb3+ codoped glasses for 601 nm emission, it is observed that (see Fig. 4) the emission intensity in Tb3+:Sm3+ codoped glass increases in overlapping regions and remains almost constant in non-overlapping regions compared to Sm3+ doped glass. This shows that Tb3+ ion is also contributing to populate the 4G5/2 level of Sm3+ ion and enhance the emission intensity of Sm3+ ion bands. This clearly shows an efficient energy transfer from Tb3+ to Sm3+ ions.
Decay curve analysis To understand the luminescence dynamics, the decay curves correspond to 4G5/2 ? 6H5/2 (566 nm) transition of Sm3+ ion and 5 D4 ? 7F6 (488 nm) transition of Tb3+ in singly and doubly doped samples have been recorded using pulsed 532 and 355 nm lasers
(pulsed width 7 ns). The observed decay curves for 355 nm excitation are shown in Fig. 5. When only Sm3+ ion (0.3 mol%) is present in the host, the decay curve observed for 4G5/2 level of Sm3+ ion on excitation with 532 nm radiation fits well with a single exponential function. However, the decay curve becomes non-exponential for 0.4 mol% concentration of Sm3+ ions. Moreover, the lifetime of 4G5/2 level is also reduced from 593 ls (0.3 mol%) to 550 ls (0.4 mol%). This is due to diffusion of energy amongst the Sm3+ ions at the higher concentrations. The possibility of cross relaxation process cannot be ignored under close proximity of the Sm3+–Sm3+ ion pair formation at higher concentration. Cross-relaxation process would also be possible since the fluorescence from 4G5/2 (17730 cm1) to 6F9/2 (9320 cm1), 6F7/2 (8180 cm1) and 6F5/2 (7319 cm1) matches well 6 with the absorption from 6Fi H5/2 (i = 5/2, 7/2 and 9/2) transitions, respectively [29]. In the case of 1 mol% concentration of Tb3+ ions, the decay curve of 5D4 level of Tb3+ ion corresponds to 5D4 ? 7F6 transition with 355 nm laser excitation needed double exponential function to fit the curve i.e.
NðtÞ ¼ A½expðt=sd Þ þ expðt=sr Þ where A is a constant and sd and sc are the decay and rise time which are found to be 350 ls and 21.9 ls, respectively (see Fig. 5). On increasing the concentration of the Tb3+ ions, the decay curve becomes non-exponential and lifetime of 5D4 level reduces (see Table 1). The appearance of rise time is due to slow population feeding (slow internal conversion rate) from the 5D3 to 5D4 level. At 1.5 mol% concentration of Tb3+ ions, the decay curve of 5D4 level of Tb3+ ion show small deviation from exponential nature and decay time reduced (290 ls) compared to its value for 1 mol% concentration (see Table 1). This shows that concentration quenching effect takes place at >1 mol% concentration of Tb3+ ions. Thus in 0.3 Sm3+:1.5 Tb3+, the emission intensity of the bands of both the ions, Sm3+ as well Tb3+, reduced considerably (see Fig. 3b). The cross relaxation rate (WCR) has been estimated in singly doped glasses using the relation:
W CR ¼
Fig. 4. Excitation spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped glasses. Inset shows the enlarged portion of same spectra in the range of 260–330 nm.
1
1
sðxÞ sðyÞ
where x is the mol% concentration of Sm3+/Tb3+, y = 0.3 mol% for Sm3+ ions and 1.0 mol% for Tb3+ ions and s is the life time of observed level of rare earth ion. The cross relaxation rate in Sm3+ doped glass (0.3 ? 0.4 mol%) is 1.32 s1 which increased up to 3.55 s1 for 1.0 mol% of Sm3+ ions. Similarly in case of Tb3+ doped
Fig. 5. (Left) The photo luminescence decay curve of 4G5/2 of Sm3+ ion corresponding to 4G5/2 ? 6H5/2 (566 nm) transition (Right) and 5D4 level of Tb3+ ion corresponding to 5 D4 ? 7F6 (488 nm) transition.
A. Bahadur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 177–181 Table 1 Lifetime values of Sm3+ and Tb3+ ion in singly and codoped glasses by varying the individual concentrations. x Sm3+/y Tb3+ (mol%)
4
G5/2 level (Sm3+) lifetime (ls)
5
D4 level (Tb3+) lifetime (ls)
0.3/0 0.4/0 1.0/0 0/0.3 0/1.0 0/1.5 0.3/0.3 0.3/1.0 1.0/1.0
593 550 490 – – – 620 630 –
– – – 345 345 290 220 172 144
glass, the CR rate changes 11.84 s1, when its concentration increased from 1 ? 1.5 mol%. In the case of codoped system, the 355 nm laser radiation excites Tb3+ as well as Sm3+ ions both hence; fluorescence is seen for both the ions. In Sm3+ doped and Tb3+:Sm3+ codoped glasses, the decay curves were monitored for 4G5/2 ? 6H5/2 transition of Sm3+ ion with 355 nm excitation (see Fig. 5). In 0.3 mol% Sm3+ doped glass, the life time of 4G5/2 level was found to be 593 ls. However in Tb3+:Sm3+ codoped glass by fixing the Sm3+ ions concentration to 0.3 mol% and varying the concentration of Tb3+ ions, the life time of the 4G5/2 level of Sm3+ ion increases with increase in the concentration of Tb3+ ion. It seems that 4G5/2 level of Sm3+ ion is continuously populating by energy transfer from Tb3+ ions (Table 1). We have also recorded the decay curves of 5D4 level of Tb3+ ion for 488 nm transition in Tb3+ and Tb3+:Sm3+ codoped glasses for 355 nm excitation (see Fig. 5). In 1 mol% Tb3+ doped glass, there appears a rise time associated with decay time, the life time of 5D4 level was found to be 345 ls. But in Tb3+:Sm3+ codoped glass by fixing the Tb3+ ions concentration to 1.0 mol% and varying the concentration of Sm3+ ions, it is observed that correspond to 5D4 level of Tb3+ ions, the rise time disappear and the life time decreases with increase in the concentration of Sm3+ ion (Table 1). This clearly shows an efficient energy transfer from Tb3+ to Sm3+ ion. Energy transfer probability has been calculated using lifetime value of donor, in presence and absence of acceptor using following relation:
W¼
1
1
sðSm : TbÞ sðTbÞ
We have also calculated the energy transfer efficiency using the relation [30]:
sTbSm ET ¼ 1
sTb
Energy transfer rate estimated using the above relation is 16.47 s1 for a fixed concentration of Sm3+ ion i.e. 0.3 Sm:0.3 Tb which is increased to 29.6 s1 when Tb3+ concentration is increased to 1 mol% (i.e. for 0.3 Sm3+:1.0 Tb3+) and the energy transfer efficiency from Tb3+ to Sm3+ ion is found to increase from 36.24 ? 50.1% for the same change of concentration.
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Conclusions In summary, luminescence and energy transfer properties of trivalent Sm3+ and Tb3+ ions have been studied in lead fluorotellurite glasses on excitation with 532 and 355 nm laser radiations. The bright red, green and greenish red perception has been observed in Sm3+, Tb3+ and Sm3+:Tb3+ codoped samples, respectively. A significant enhancement in emission intensity of Sm3+ ion peaks has been observed in the presence of Tb3+ ion while the emission intensity of Tb3+ peaks is reduced appreciably. To understand the energy transfer kinetics, time resolved spectroscopy has been used. An analysis of emission spectra, excitation spectra and decay curve reveals the occurrence of significant energy transfer from Tb3+ ? Sm3+ ion. References [1] R.A.H. El-Mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC Boca Raton, FL, 2001. [2] J.S. Wang, E.M. Vogel, E. Snitzer, J.L. Jackel, V.L. da Silva, Y. Silberberg, J. NonCryst. Solids 178 (1994) 109–113. [3] Y. Dwivedi, A. Rai, S.B. Rai, J. Appl. Phys. 104 (2009) 043509–043512. [4] S.H. Morgan, D.O. Henderson, R.H. Magruder III, J. Non-Crys. Solids 128 (1991) 146–153. [5] E.M. Vogel, J. Am. Ceram. Soc. 72 (1989) 719–724. [6] A.J. Kenyon, Prog. Quant. Elec. 26 (2002) 225–284. [7] A.F.H. Librantz, L. Gomes, G. Pairier, S.J.L. Ribeiro, Y. Messaddeq, J. Lum. 128 (2008) 51–59. [8] W. Stambouli, H. Elhouichet, B. Gelloz, M. Ferid, N. Koshida 132 (2012) 205– 209. [9] S. Xinyuan, G. Mu, Z. Min, H. Shiming, J. Rare Earth 28 (2010) 340–344. [10] Z. Zhou, A. Lin, H. Guo, X. Liu, C. hou, M. Lu, W. Wei, B. Peng, W. Zhao, J. Toulouse, J. Non-Cryst. Solids 356 (2010) 2896–2899. [11] L. Huang, A. Jha, S. Shen, Opt. Commun. 281 (2008) 4370–4373. [12] C.G. Walrand, K. Binnemans, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 25, Elsevier Science, New York, 1998, pp. 101–264. [13] G. Blasse, A. Bril, J. Chem. Phys. 47 (1967) 1920–1926. [14] P.R. Biju, G. Lose, V. Thomas, V.P.N. Nampoori, N.V. Unnikrishnan, Opt. Mater. 24 (2004) 671–677. [15] P. Nachimuthu, R. Jagannathan, V. Kumar, D. Narayana, J. Non Cryst. Solids 217 (1997) 215–223. [16] W.W. Holloway, M. Kestigian, J. Opt. Soc. Am. 56 (1966). 1171-1171. [17] A.R. Zanatta, A. Khan, M.E. Kordesch, J.Phys.: Condens. Matter 19 (2007) 436230–436237. [18] G. Lakshminarayana, R. Yang, J.R. Qiu, M.G. Brik, G.A. Kumar, I.V. Kityk, J. Phys. D: Appl. Phys. 42 (2009) 015414–015425. [19] Y. Dwivedi, S.B. Rai, Opt. Mater. 31 (2008) 87–93. [20] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A 59 (2009) 917–925. [21] G. Baldacchini, in: B. diBartolo (Ed.), Advances in Energy Transfer Process, World scientific, Singapore, 2001, p. 359. [22] Y. Dwivedi, A. Rai, S.B. Rai, J. Lum. 129 (2009) 629–633. [23] R. Praveena, V. Venkatramu, P. Babu, C.K. Jayasankar, Physica B 403 (2008) 3527–3534. [24] J.M.F. van Dijk, M.F.H. Schuurmans, J. Chem. Phys. 78 (1983) 5317–5323. [25] G.K. Liu, J. Lum. 60 (1994) 860–863. [26] A. Hoakesey, J. Woods, K.N.R. Taylor, J. Lum. 17 (1978) 385–400. [27] G.C. Kim, T.W. Kim, S.I. Mho, S.G. Kim, H.L. Park, J. Kor, Phys. Soc. 34 (1999) 97– 99. [28] V.K. Rai, S.B. Rai, D.K. Rai, J. Mater. Sci. 39 (2004) 4971–4975. [29] A.K. Agarwal, N.C. Lohani, T.C. Pant, K.C. Pant, J. Solid State Chem. 54 (1984) 219–225. [30] B.C. Joshi, J. Non-Cryst. Solids 180 (1995) 217–220.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Enhanced luminescence and energy transfer study in Tb: Sm codoped lead fluorotellurite glass A. Bahadur, Y. Dwivedi, S.B. Rai ⇑ Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, UP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ to Sm3+ ions has been observed. 4 6 The transitions from I9/2 to Hj are 3+ observed in Sm ion emission spectra. 5 3+ The lifetime of D4 level of Tb ion decreases in the presence of Sm3+ ion. Cross-relaxation process dominantly populates the 5D4 level of Tb3+ ion.
Energy transfer from Tb
a r t i c l e
i n f o
Article history: Received 31 May 2013 Received in revised form 11 August 2013 Accepted 15 August 2013 Available online 31 August 2013 Keywords: Fluorotellurite glass Samarium Terbium Luminescence Energy transfer
a b s t r a c t Spectroscopic properties of Sm3+, Tb3+ doped and Tb3+:Sm3+ co-doped lead fluorotellurite glasses have been studied in detail. On the excitation with 355 and 532 nm laser wavelengths, the luminescence properties of the singly and doubly doped glasses have been analyzed. Intensity of characteristic emission bands due to Sm3+ ions is appreciably enhanced in the presence of Tb3+ ions. This is due to the energy transfer from Tb3+ to Sm3+ ions. Different energy transfer parameters have also been calculated, which affirm an efficient energy transfer from Tb3+ to Sm3+ ions. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Tellurite glasses have been extensively investigated in past due to their intriguing properties such as good transparency in the mid-infrared region, low phonon vibration (700–800 cm1), high refractive index (2), and high dielectric constants [1–3]. The physical properties e.g. refractive index, melting point, dielectric constant, etc. of tellurite glasses can be fairly tuned in controlled ⇑ Corresponding author. Tel.: +91 542 2307308; fax: +91 542 2368390. E-mail address: [email protected] (S.B. Rai). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.08.078
fashion by adding suitable heavy metal modifiers viz. Ba, Bi, Pb, etc. For example, addition of PbF2 not only supply fluoride which lowers the phonon energy considerably (140 cm1) less than the phonon energy of the tellurite (Te–O 800 cm1), but also lead to improved fluorine ion conductivity [4]. In particular, lead tellurite glasses are interesting as they are bi-functional: network modifier and former (at higher concentration). The covalent character of PbO is due to strong interaction of easily polarizable valance shells of Pb2+ ions and highly polarizable O2 ions; and also due to its efficient third order nonlinearity [5]. Doping of rare earth ions in tellurite glasses makes them optically active, which develop
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multifunctional characters in them and improves their commercial value [2,3]. Tb and Sm ions are highly desirable in the optical field because they show characteristic emissions in two different important spectral regions. Tb doped tellurite glasses have large potential for the development of amplifiers and lasers covering the main telecom windows [6,7]. It emits a bright green colour along with weak blue and red colours which are often used in fluorescent lamps, X-ray intensified screens, and television tubes. Several combination of Tb with other rare earth ions e.g. Tb–Tm, Tb–Eu, Tb–Ce, Tb–Yb, etc. [8–10] have been studied in glasses and it has been reported that the luminescent properties of Tb3+ are affected significantly. On the other hand, Sm3+ ion emits mainly in orange red region and so regarded as one of the significant rare earth ions and its demand is increasing day by day in various fields including fluorescent devices, high-density optical storage, under sea communication, colour displays and in visible solid-state lasers [11]. The 4G5/2 level of Sm3+ ion exhibits relatively high quantum efficiency and thus various populating and quenching emission channels results interesting fluorescence properties [12]. Sm3+ ion has also been studied in pairs of rare earth ions e.g. Sm–Tb, Sm–Ce, Sm–Eu, Sm–Dy and Er–Sm, etc. Results suggest that there is a good hope for efficient emission in suitable host matrix [13–15]. A more detail study on a pair of Sm:Tb is important as it may provide wide colour tunability extending from blue to NIR region and also white light emission. Though, energy transfer characteristics in Tb and Sm ion pair have been studied earlier [16–18], a detailed analysis of energy transfer is still lacking in commercially important modified tellurite host. In the present research work, luminescence and energy transfer characteristics of Sm, Tb and Sm:Tb codoped lead fluorotellurite glasses have been studied by using 532 nm and 355 nm laser as excitations sources. Our studies reveal an efficient energy transfer from Tb3+ to Sm3+ ion, which is verified from time resolved spectroscopy. Experimental Glasses were prepared with following chemical compositions:
½80 ðx þ yÞTeO2 þ 20PbF2 þ x Sm2 O3 þ yTb4 O7 where x = 0, 0.3, 0.4, 1 mol% and y = 0, 0.3, 1, and 1.5 mol%. Purity of the compounds used was better than 99.9%. Typical, melt and quench method [19] was adopted to prepare these glasses (heating temperature 1123 K). Good optical quality glasses of dimensions 1.0 1.0 0.3 cm were prepared. They were cleaned, polished and then used for further optical measurements. The absorption spectra of the samples were measured using JASCO V-670 double beam spectrophotometer in the range of 200–1700 nm. Excitation spectra were recorded on Fluoromax–4 spectrofluorometer (Horiba JobinYvon). The fluorescence measurements were carried out using the second and third harmonics (532, 355 nm) of a pulsed Nd:YAG (Spitlight 600, Innolas, Germany) laser as the excitation source. A iHR320 computer controlled trix monochromator equipped with grating (blazed at 500 nm) and PMT (model-1424 M) was used as a detector to detect luminescence. Photoluminescence decay curves were recorded using 532 and 355 nm pulsed radiations (repetition rate 10 Hz, pulse width 7 ns) of Nd:YAG laser. The collected signal was fed to 150 MHz digital oscilloscope (model No. HM 1507, Hameg Instruments) and the decay curves were obtained for further analysis. Life time of the radiative levels was estimated by exponential fitting.
Results and discussion Absorption Absorption spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped lead fluorotellurite glasses have been recorded in the region 200– 1700 nm (see Fig. 1). The absorption band edge of this glass is found to extend up to 490 nm due to which most of the absorption peaks of Tb3+ ion and some of the peaks of Sm3+ ions are not seen in glass absorption region. Trivalent Sm ion contains five electrons in the 4f shell and its ground state is 6H5/2. The absorption bands of Sm3+ ion appear in the visible and the NIR region are due to 4f–4f electronic transitions. The bands observed in the NIR region at 942, 1082, 1234, 1384, 1487, 1549 and 1588 nm are corresponds to 6F11/2, 6F9/2, 6 6 F7/2, 6F5/2, 6F3/2, 6H15/2 and 6F1/2 H5/2 transitions of Sm3+ ion, respectively, which are basically of electric dipole–dipole character. Few absorption bands of Sm3+ ion also lie in the visible region overlapped with the absorption band edge of lead fluorotellurite glass and they could not be seen.
Photoluminescence spectra The emission spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped glasses has been recorded using 532 and 355 nm laser excitations and are depicted in Figs. 2 and 3a and b. On excitation with 532 nm (18797 cm1) laser radiation, the Sm3+ ions promoted to 4 G5/2 level. Here, the energy mismatch was compensated by phonon vibrations of telluride lattice. Excited Sm3+ ions radiatively relax from 4G5/2 level to different lower lying energy levels. In emission spectra of Sm3+ doped glass, the peaks observed at 566, 601, 644, 706 and 794 nm attributed to 4G5/2 ? 6Hj (j = 5/2, 7/2, 9/2, 11/2, and 13/2) transitions of Sm3+ ion, respectively. Among these the most intense peak is at 601 nm which is due to 4 G5/2 ? 6H7/2 transition. The concentration of Sm3+ ions is optimized with the emission intensity. It is found that the concentration quenching took place beyond 0.3 mol% concentration [20]. The concentration quenching is presumed to be due to energy migration to the unexcited Sm3+ ions and impurities [21,22]. No energy level of Tb3+ ion matches with the photon energy of 532 nm laser radiation. Hence, luminescence is not observed due to Tb3+ ion with 532 nm laser excitation. When Sm3+ doped glass is excited with 355 nm radiation then the observed emission spectra contains all the emission bands which were observed on 532 nm excitation along with these few new weak bands viz. at 482, 510, 542, 571, 751, 844 are also
Fig. 1. Absorption spectra of 1 Tb3+, 0.3 Sm3+ doped and 0.3 Sm3+:1 Tb3+ codoped lead tellurite glass.
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Fig. 2. (Left) Photoluminescence spectra of Sm3+ doped and Sm3+:Tb3+ codoped glasses on excitation with 532 nm laser radiations. (Right) Energy level diagrams of trivalent Sm, Tb ion and excitations; emissions; cross-relaxation (CR); nonradiative relaxations; and energy transfer pathways.
Fig. 3a. Normalized Luminescence spectra of 0.3 Sm3+ doped glass on excitation with 355 nm and 532 nm lasers. Lower graph shows the enlarged portion of upper graph in the range of 450–550 nm (Left) and 735–870 nm (Right).
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observed (see Fig. 3a). These peaks are due to 4I9/2,11/2,13/2 + 4 M15/2 ? 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2, 15/2) transitions, respectively. On excitation with 355 nm laser, Sm3+ ions are excited to 6 D1/2, 4H7/2 levels. There are many closely spaced levels between 4 H7/2 and 4I9/2 level due to which the excited ions non-radiatively relax from 4H7/2 to 4I9/2 level [23] and a sufficient number of Sm3+ ions are collected at 4I9/2. Some of these Sm3+ ions radiatively relax from 4I9/2 to 6Hj levels while most of these relax non-radiatively to long live 4G5/2 level (ms). The energy gap between 4 G5/2 and the next lower lying energy level 6F11/2 (10,500 cm1) is 7100 cm1 which is very large, therefore, the excited Sm3+ ions in 4G5/2 level preferably relax radiatively to the lower levels [24]. Usually, the transitions from 4I9/2 to the lower levels are not visible in Sm3+ ions in most of the hosts, though these appear in our case perhaps due to low lattice vibration in the presence of heavy metal (Pb) in host. In the case of Tb3+ doped glass, the bright green emission is seen on excitation with 355 nm radiation. The seven emission bands observed at 488, 543, 583, 620, 641, 666.5 and 678 nm correspond to 5 D4 ? 7Fj (j = 6, 5, 4, 3, 2, 1, 0) transitions of Tb3+ ion (see Fig. 3b). The 5D4 ? 7F5 transition peaked at 543 nm is the brightest. The mechanism involved in the observed transitions can be explained as follows: the Tb3+ ions absorb the incident photons from 355 nm (28170 cm1) wavelength radiation and promoted to 5L10 level and then they nonradiatively relax to the 5D3 level. The energy gap between 5D3 and 5D4 level is 4500 cm1, which seems difficult to bridge by lattice vibration (Te–O 800 cm1) as it needs at least 6–7 phonons. Hence, the non radiative relaxation of Tb3+ ions from 5D3 to 5D4 is less probable at room temperature. The important channels of depopulation of 5D3 level are cross relaxation and radiative decay. Since cross relaxation process is fast hence, it will dominate to populate the 5D4 level following ultraviolet excitation [25,26]. The energy of 5D3 ? 7F0 and 5 D4 ? 7F6; 5D3 ? 5D4 and 7F0 ? 7F6 transitions are nearly equal which provides a pathway for cross relaxation process and populate the 5D4 level [25,27]. The Tb3+ ions present in 5D4 level relax radiatively to different low lying levels (7Fj (j = 6, 5, 4, 3, 2, 1, 0)) and emit blue, green and red colour light. The concentration of Tb3+ ions was optimized with luminescence intensity, which is optimum for 61 mol% concentration [28]. The concentration quenching takes place for higher concentration of Tb3+ ions which results the considerable reduction in photoluminescence intensity. When the trivalent Tb and Sm both the ions are present in the lead fluorotellurite glass, the photoluminescence spectrum extends from blue to NIR region. On comparison, it is found that the emission intensity of Sm3+ ion peaks is enhanced in the presence of Tb3+ ion, while the emission intensity of Tb3+ ion peaks is reduced significantly (see Fig. 3b). This shows an efficient energy transfer from Tb3+ to Sm3+ ions. From a close look of the energy level diagram of trivalent Tb and Sm ion, it is evident that several energy levels of both the ions are partially or fully overlapped to each other. For example 5D4 (20600 cm1) level of Tb3+ ion is close to 4M15/2 (20580 cm1) level of Sm3+ ion and 5D3 (26520 cm1) level of Tb3+ ion is close to 6P3/2 (26809 cm1) level of Sm3+ ion. It is possible that some of the excited Tb3+ ions transfer their excitation energy nonradiatively to the Sm3+ ions and promote them to 4 M15/2/6P3/2 levels and consequently improve the population of Sm3+ ions in 4G5/2 level, resulting improved luminescence. Excitation spectra
3+
3+
3+
3+
Fig. 3b. Luminescence spectra of 1.0 Tb , 0.3 Sm doped 0.3 Sm :1.0 Tb and 0.3 Sm3+:1.5Tb3+ codoped glasses on excitation with 355 nm laser source. Inset figure shows the luminescence spectra of 1.0 Tb3+ on enlarged scale in the range of 630– 680 nm.
We have also monitored the excitation spectra of 0.3 Sm3+, 1 Tb3+ doped and 0.3 Sm3+:1 Tb3+ codoped glasses for 601, 543 and 601 nm emissions, respectively (see Fig. 4). The excitation spectra of 1 mol% Tb3+ doped glass for 543 nm emission shows several high 7 7 lying energy levels at 272, 284 nm (5Ij F6); 304 nm (5Hj F6);
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7 7 318, 326 nm (5D1,0 F6); 341, 352, 358 nm: (5D2 F6) 5 7 5 7 5 7 ( G6 F6); 369 nm ( L10 F6); 377 nm ( D3 F6); 402 nm 7 7 (5G6 F5) and 484 nm (5D4 F6). On the other hand, 0.3 mol% 3+ Sm doped glass shows several excitation peaks corresponding 6 to 601 nm emission. These peaks are at 344 nm (4D7/2 H5/2); 6 6 361 nm (4D3/2 H5/2); 374 nm (6P7/2 H5/2); 390, 401 nm 6 6 6 (6P3/2 H5/2); 416 nm (6P5/2 H5/2); 437 nm (4M17/2 H5/2); 6 6 471 nm (4I11/2 H5/2); 489 nm (4I9/2 H5/2); 499 nm (4G7/2 6 H5/2). Fig. 4 clearly shows that the energy levels of Tb3+ and Sm3+ ions are considerably overlapped with each other in the region 330–390 nm and 475–500 nm. On comparing the intensity of excitation spectra in Sm3+ doped and Sm3+:Tb3+ codoped glasses for 601 nm emission, it is observed that (see Fig. 4) the emission intensity in Tb3+:Sm3+ codoped glass increases in overlapping regions and remains almost constant in non-overlapping regions compared to Sm3+ doped glass. This shows that Tb3+ ion is also contributing to populate the 4G5/2 level of Sm3+ ion and enhance the emission intensity of Sm3+ ion bands. This clearly shows an efficient energy transfer from Tb3+ to Sm3+ ions.
Decay curve analysis To understand the luminescence dynamics, the decay curves correspond to 4G5/2 ? 6H5/2 (566 nm) transition of Sm3+ ion and 5 D4 ? 7F6 (488 nm) transition of Tb3+ in singly and doubly doped samples have been recorded using pulsed 532 and 355 nm lasers
(pulsed width 7 ns). The observed decay curves for 355 nm excitation are shown in Fig. 5. When only Sm3+ ion (0.3 mol%) is present in the host, the decay curve observed for 4G5/2 level of Sm3+ ion on excitation with 532 nm radiation fits well with a single exponential function. However, the decay curve becomes non-exponential for 0.4 mol% concentration of Sm3+ ions. Moreover, the lifetime of 4G5/2 level is also reduced from 593 ls (0.3 mol%) to 550 ls (0.4 mol%). This is due to diffusion of energy amongst the Sm3+ ions at the higher concentrations. The possibility of cross relaxation process cannot be ignored under close proximity of the Sm3+–Sm3+ ion pair formation at higher concentration. Cross-relaxation process would also be possible since the fluorescence from 4G5/2 (17730 cm1) to 6F9/2 (9320 cm1), 6F7/2 (8180 cm1) and 6F5/2 (7319 cm1) matches well 6 with the absorption from 6Fi H5/2 (i = 5/2, 7/2 and 9/2) transitions, respectively [29]. In the case of 1 mol% concentration of Tb3+ ions, the decay curve of 5D4 level of Tb3+ ion corresponds to 5D4 ? 7F6 transition with 355 nm laser excitation needed double exponential function to fit the curve i.e.
NðtÞ ¼ A½expðt=sd Þ þ expðt=sr Þ where A is a constant and sd and sc are the decay and rise time which are found to be 350 ls and 21.9 ls, respectively (see Fig. 5). On increasing the concentration of the Tb3+ ions, the decay curve becomes non-exponential and lifetime of 5D4 level reduces (see Table 1). The appearance of rise time is due to slow population feeding (slow internal conversion rate) from the 5D3 to 5D4 level. At 1.5 mol% concentration of Tb3+ ions, the decay curve of 5D4 level of Tb3+ ion show small deviation from exponential nature and decay time reduced (290 ls) compared to its value for 1 mol% concentration (see Table 1). This shows that concentration quenching effect takes place at >1 mol% concentration of Tb3+ ions. Thus in 0.3 Sm3+:1.5 Tb3+, the emission intensity of the bands of both the ions, Sm3+ as well Tb3+, reduced considerably (see Fig. 3b). The cross relaxation rate (WCR) has been estimated in singly doped glasses using the relation:
W CR ¼
Fig. 4. Excitation spectra of Sm3+, Tb3+ doped and Sm3+:Tb3+ codoped glasses. Inset shows the enlarged portion of same spectra in the range of 260–330 nm.
1
1
sðxÞ sðyÞ
where x is the mol% concentration of Sm3+/Tb3+, y = 0.3 mol% for Sm3+ ions and 1.0 mol% for Tb3+ ions and s is the life time of observed level of rare earth ion. The cross relaxation rate in Sm3+ doped glass (0.3 ? 0.4 mol%) is 1.32 s1 which increased up to 3.55 s1 for 1.0 mol% of Sm3+ ions. Similarly in case of Tb3+ doped
Fig. 5. (Left) The photo luminescence decay curve of 4G5/2 of Sm3+ ion corresponding to 4G5/2 ? 6H5/2 (566 nm) transition (Right) and 5D4 level of Tb3+ ion corresponding to 5 D4 ? 7F6 (488 nm) transition.
A. Bahadur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 177–181 Table 1 Lifetime values of Sm3+ and Tb3+ ion in singly and codoped glasses by varying the individual concentrations. x Sm3+/y Tb3+ (mol%)
4
G5/2 level (Sm3+) lifetime (ls)
5
D4 level (Tb3+) lifetime (ls)
0.3/0 0.4/0 1.0/0 0/0.3 0/1.0 0/1.5 0.3/0.3 0.3/1.0 1.0/1.0
593 550 490 – – – 620 630 –
– – – 345 345 290 220 172 144
glass, the CR rate changes 11.84 s1, when its concentration increased from 1 ? 1.5 mol%. In the case of codoped system, the 355 nm laser radiation excites Tb3+ as well as Sm3+ ions both hence; fluorescence is seen for both the ions. In Sm3+ doped and Tb3+:Sm3+ codoped glasses, the decay curves were monitored for 4G5/2 ? 6H5/2 transition of Sm3+ ion with 355 nm excitation (see Fig. 5). In 0.3 mol% Sm3+ doped glass, the life time of 4G5/2 level was found to be 593 ls. However in Tb3+:Sm3+ codoped glass by fixing the Sm3+ ions concentration to 0.3 mol% and varying the concentration of Tb3+ ions, the life time of the 4G5/2 level of Sm3+ ion increases with increase in the concentration of Tb3+ ion. It seems that 4G5/2 level of Sm3+ ion is continuously populating by energy transfer from Tb3+ ions (Table 1). We have also recorded the decay curves of 5D4 level of Tb3+ ion for 488 nm transition in Tb3+ and Tb3+:Sm3+ codoped glasses for 355 nm excitation (see Fig. 5). In 1 mol% Tb3+ doped glass, there appears a rise time associated with decay time, the life time of 5D4 level was found to be 345 ls. But in Tb3+:Sm3+ codoped glass by fixing the Tb3+ ions concentration to 1.0 mol% and varying the concentration of Sm3+ ions, it is observed that correspond to 5D4 level of Tb3+ ions, the rise time disappear and the life time decreases with increase in the concentration of Sm3+ ion (Table 1). This clearly shows an efficient energy transfer from Tb3+ to Sm3+ ion. Energy transfer probability has been calculated using lifetime value of donor, in presence and absence of acceptor using following relation:
W¼
1
1
sðSm : TbÞ sðTbÞ
We have also calculated the energy transfer efficiency using the relation [30]:
sTbSm ET ¼ 1
sTb
Energy transfer rate estimated using the above relation is 16.47 s1 for a fixed concentration of Sm3+ ion i.e. 0.3 Sm:0.3 Tb which is increased to 29.6 s1 when Tb3+ concentration is increased to 1 mol% (i.e. for 0.3 Sm3+:1.0 Tb3+) and the energy transfer efficiency from Tb3+ to Sm3+ ion is found to increase from 36.24 ? 50.1% for the same change of concentration.
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Conclusions In summary, luminescence and energy transfer properties of trivalent Sm3+ and Tb3+ ions have been studied in lead fluorotellurite glasses on excitation with 532 and 355 nm laser radiations. The bright red, green and greenish red perception has been observed in Sm3+, Tb3+ and Sm3+:Tb3+ codoped samples, respectively. A significant enhancement in emission intensity of Sm3+ ion peaks has been observed in the presence of Tb3+ ion while the emission intensity of Tb3+ peaks is reduced appreciably. To understand the energy transfer kinetics, time resolved spectroscopy has been used. An analysis of emission spectra, excitation spectra and decay curve reveals the occurrence of significant energy transfer from Tb3+ ? Sm3+ ion. References [1] R.A.H. El-Mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC Boca Raton, FL, 2001. [2] J.S. Wang, E.M. Vogel, E. Snitzer, J.L. Jackel, V.L. da Silva, Y. Silberberg, J. NonCryst. Solids 178 (1994) 109–113. [3] Y. Dwivedi, A. Rai, S.B. Rai, J. Appl. Phys. 104 (2009) 043509–043512. [4] S.H. Morgan, D.O. Henderson, R.H. Magruder III, J. Non-Crys. Solids 128 (1991) 146–153. [5] E.M. Vogel, J. Am. Ceram. Soc. 72 (1989) 719–724. [6] A.J. Kenyon, Prog. Quant. Elec. 26 (2002) 225–284. [7] A.F.H. Librantz, L. Gomes, G. Pairier, S.J.L. Ribeiro, Y. Messaddeq, J. Lum. 128 (2008) 51–59. [8] W. Stambouli, H. Elhouichet, B. Gelloz, M. Ferid, N. Koshida 132 (2012) 205– 209. [9] S. Xinyuan, G. Mu, Z. Min, H. Shiming, J. Rare Earth 28 (2010) 340–344. [10] Z. Zhou, A. Lin, H. Guo, X. Liu, C. hou, M. Lu, W. Wei, B. Peng, W. Zhao, J. Toulouse, J. Non-Cryst. Solids 356 (2010) 2896–2899. [11] L. Huang, A. Jha, S. Shen, Opt. Commun. 281 (2008) 4370–4373. [12] C.G. Walrand, K. Binnemans, in: K.A. Gschneidner, L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 25, Elsevier Science, New York, 1998, pp. 101–264. [13] G. Blasse, A. Bril, J. Chem. Phys. 47 (1967) 1920–1926. [14] P.R. Biju, G. Lose, V. Thomas, V.P.N. Nampoori, N.V. Unnikrishnan, Opt. Mater. 24 (2004) 671–677. [15] P. Nachimuthu, R. Jagannathan, V. Kumar, D. Narayana, J. Non Cryst. Solids 217 (1997) 215–223. [16] W.W. Holloway, M. Kestigian, J. Opt. Soc. Am. 56 (1966). 1171-1171. [17] A.R. Zanatta, A. Khan, M.E. Kordesch, J.Phys.: Condens. Matter 19 (2007) 436230–436237. [18] G. Lakshminarayana, R. Yang, J.R. Qiu, M.G. Brik, G.A. Kumar, I.V. Kityk, J. Phys. D: Appl. Phys. 42 (2009) 015414–015425. [19] Y. Dwivedi, S.B. Rai, Opt. Mater. 31 (2008) 87–93. [20] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A 59 (2009) 917–925. [21] G. Baldacchini, in: B. diBartolo (Ed.), Advances in Energy Transfer Process, World scientific, Singapore, 2001, p. 359. [22] Y. Dwivedi, A. Rai, S.B. Rai, J. Lum. 129 (2009) 629–633. [23] R. Praveena, V. Venkatramu, P. Babu, C.K. Jayasankar, Physica B 403 (2008) 3527–3534. [24] J.M.F. van Dijk, M.F.H. Schuurmans, J. Chem. Phys. 78 (1983) 5317–5323. [25] G.K. Liu, J. Lum. 60 (1994) 860–863. [26] A. Hoakesey, J. Woods, K.N.R. Taylor, J. Lum. 17 (1978) 385–400. [27] G.C. Kim, T.W. Kim, S.I. Mho, S.G. Kim, H.L. Park, J. Kor, Phys. Soc. 34 (1999) 97– 99. [28] V.K. Rai, S.B. Rai, D.K. Rai, J. Mater. Sci. 39 (2004) 4971–4975. [29] A.K. Agarwal, N.C. Lohani, T.C. Pant, K.C. Pant, J. Solid State Chem. 54 (1984) 219–225. [30] B.C. Joshi, J. Non-Cryst. Solids 180 (1995) 217–220.