Spectroscopic properties of the Pr 3+ ion in TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3 glasses

Transcription

1 Cent. Eur. J. Phys. 12(1) DOI: /s Central European Journal of Physics Spectroscopic properties of the Pr 3+ ion in TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3 glasses Research Article Bożena Burtan 1, Maciej Sitarz 2, Radosław Lisiecki 3, Witold Ryba-Romanowski 3, Piotr Jeleń 2, Jan Cisowski 1, Manuela Reben 2 1 Institute of Physics, Cracow University of Technology, ul. Podchorążych 1, Cracow, Poland 2 AGH - University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, Cracow, Poland 3 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okolna 2, Wrocław, Poland Received 04 May 2013; accepted 10 November 2013 Abstract: The goal of this work was to investigate the spectroscopic properties of Pr 3+ ions, embedded in two different tellurite glass matrices, TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3. The absorption and fluorescence spectra have been recorded and analyzed in terms of the Judd-Ofelt theory along with the luminescence decay of the 3 P 0 and 1 D 2 levels of the Pr 3+ ion. The spectroscopic studies were completed with ellipsometric measurements providing the dispersion relation of the refractive index of the investigated glasses. PACS (2008): Ci, Pg, Cd, Qr, Jg, Kf Keywords: tellurite glasses rare-earth ions absorption photoluminescence Judd-Ofelt theory Versita sp. z o.o. 1. Introduction Tellurite glasses are currently the subject of intense research because of their interesting physical properties, such as extended infrared transmittance [1], high nonlinear optical indices and low melting point [2]. The main component of tellurite glasses - tellurium oxide (TeO 2 ) - manuelar@agh.edu.pl is called a conditional glass former since it cannot form a glass itself. However, a number of heavy metal and rare earth oxides are known to dissolve in the TeO 2 liquid and to create a metastable liquid that, upon quenching, yields a stable glass composition [3]. TeO 2 - based glasses are characterized by maximal phonon frequencies that are lower than those of many common oxide glasses, such as silicates, phosphates and germanates [4]. Due to addition of optically inactive rare earth oxides, i.e. La 2 O 3 or Lu 2 O 3, tellurite glasses exhibit other favorable properties, including good mechanical strength and chemical dura- 57

2 Spectroscopic properties of the Pr 3+ ion in TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3 glasses bility [5]. These properties make tellurite glasses good candidates for the development of optical devices, especially as amplifiers for optical communication [6, 7]. Tellurite glasses are excellent hosts for active element doping with rare earth (RE) ions, justifying a continuous technological interest [8]. Among trivalent RE ions, the luminescence features of the Pr 3+ ion have became more popular because of its specific 4f 2 configuration, possessing 91 degenerate energy levels [9, 10]. The tellurite glass systems doped with Pr 3+ ions have been widely studied for their structural details and luminescent properties [3, 11 15]. Moreover, due to the low phonon energies mentioned above, the fluorescence intensity of Pr 3+ - doped tellurite glasses has been observed to be several times larger than that of other glass systems [16 18]. The energy level diagram of the Pr 3+ ion contains several metastable states that provide the possibility of simultaneous emissions in the blue, orange, red and infrared regions [19, 20]. In this work, we report the spectroscopic properties of Pr 3+ ions embedded in the TeO 2 -WO 3 -PbO glass host, completed with La 2 O 3 (TWPLa) or Lu 2 O 3 (TWPLu) containing optically inactive La 3+ or Lu 3+ ions and added to modify the local ligand field between the Pr 3+ ions and the host, as well as to improve the thermal properties of the glass. The absorption and luminescence spectra of Pr 3+ ions in both tellurite matrices have been analyzed using the standard Judd-Ofelt theory yielding the intensity parameters, transition probalilities and branching ratios (see e.g. [5, 21, 22]). Additionally, dispersion of the refractive index obtained from ellipsometric measurements has been taken into acount during the analysis. 2. Experimental The molar host glass compositions were selected as 60% TeO 2-27% WO 3-10% Pb-3% La 2 O 3 (TWPLa) and 60% TeO 2-27% WO 3-10% PbO-3% Lu 2 O 3 (TWPLu). Both host glasses show relevant high RE stability and resistance against crystallization during reheating. The glass densities were determined from Archimedes law as being equal to 6.43 and 6.55 g/cm 3 for TWPLa and TWPLu, respectively. In order to introduce the Pr 3+ ions into the TW- PLa and TWPLu glass matrix, the respective amounts of Pr 6 O 11 were added to the batches. All the chemicals were mixed properly to ensure the homogeneity. Tellurite glasses were obtained by melting in a gold crucible in an electric furnace at 850 C in an air atmosphere. The crucible was covered with a platinum plate to avoid vaporization losses. The melt was poured onto a steel plate, preheated to 400 C, forming a layer a few mm thick layer and subsequently annealed in the temperature range C. The amorphous nature of the Pr 3+ - doped TW- PLa and TWPLu glasses was verified by X-ray diffraction analysis. The resulting concentrations of Pr 3+ ions were equal to cm 3 and cm 3 for TWPLa:Pr and TWPLu:Pr, respectively. For spectroscopic measurements, the annealed glass samples were sliced and polished to dimensions of about mm 3. The absorption spectra were recorded with a Varian 5E UV-Vis-NIR spectrophotometer and the emission spectra were measured using an Optron Dong Woo fluorometer system containing an ozone-free Xe lamp, a primary monochromator with 150 mm focal length, an emission monochromator with 750 mm focal length and a Hamamatsu R-955 photomultiplier. The luminescence decay curves were measured following a short pulse excitation provided by an optical parametric oscillator (OPO) pumped by a third harmonic of a I Nd:YAG laser. The resulting luminescence signal was filtered using a monochromator GDM-1000, detected by a Hamamatsu R928 photomultiplier and recorded with a Tektronix TDS3052 oscilloscope. The emission spectra, measured at 300 K, have been corrected for the equipment response and the relative error in measurements of the fluorescence lifetime was estimated to be ± 2%. The ellipsometric data were collected with a M-2000 Woollam ellipsometer in the spectral range nm. All the measurements were performed at room temperature. 3. Results and discussion The ellipsometric data, gathered as a function of wavelength λ, have been fitted in the range of low absorption, i.e nm, to the Sellmeier refractive index (n) normal dispersion of the form [23, 24]: n 2 = A + Bλ 2 /(λ 2 - C 2 ) - Dλ 2, presented in Fig. 1 along with the fitting parameters A, B, C and D. The refractive index dispersions of investigated tellurite glasses are quite similar to each other and exhibit a very high value, greater than 2.1, considerably higher than that obtained for standard optical glasses. Differences between particular glasses (< 5%) were not considered significant since ellipsometry is not a direct method to determine the refractive index. A practical measure of the dispersion of light is the Abbe number [25], the values of which for the investigated glasses are shown in the last column of the Table inset within Fig. 1. The obtained Abbe numbers cover a narrow range ( ), confirming a general, inverse relationship between the Abbe number and refractive index in the optical glasses, including other tellurite glasses [25]. The optical absorption spectra of TWPLa:Pr 3+ and TWPLu:Pr 3+ glasses are presented in 58

3 Bożena Burta et al. Figure 1. Dispersion of the refraction coefficient of undoped- and Pr 3+ - doped tellurite glasses obtained from elipsometric measurements. Fig. 2. The spectra are quite similar and reveal the presence of 8 absorption bands in the investigated wavenumber range that are related to transitions from the ground state 3 H 4 to various higher energy states of the Pr 3+ ion, as indicated in Fig. 2. absorption edge of the matrices and, for this reason, cannot be taken into account in further analysis. Applying the standard J-O procedure to the observed absorption bands [5, 21, 22], the experimental and calculated oscillator strengths of Pr 3+ ions in TWPLa and TWPLu hosts have been determined and compiled in Table 1, along with derived J-O parameters and root-mean square (rms) deviations. During calculation, the refractive index dispersion relation has been taken into account (see Fig. 1) and the values of the reduced matrix elements for particular electric-dipole transitions from [21] have been used. For the sake of completeness, generally weak magnetic-dipole transitions have been taken into account, with the values of the oscillator strengths and radiative probabilities from [26]. Due to similarity of the absorption spectra of TWPLa:Pr and TWPLu:Pr glasses (see Fig. 2), it is not surprising that the values of the J-O intensity parameters for both systems are also similar and comparable with those obtained for other tellurite glasses [27, 28]. Determined J-O parameters have been used to estimate the radiative transition rates W r, luminescence branching ratios β and the total radiative lifetimes τ rad of Pr 3+ excited states, and especially those of the 1 D 2 and 3 P 0 states that can be compared with the experimental data obtained from photoluminescence (PL) measurements. All of these data are presented in Table 2. The emission spectra of TWPLa:Pr 3+ and TWPLu:Pr 3+ glasses are shown in Fig. 3. Figure 2. Absorption coefficient spectra of Pr 3+ - doped tellurite glasses. In the near infrared (NIR), the absorption spectra are characterized by two intense bands corresponding to transitions from the ground state 3 H 4 to excited levels 3 F 2, 3, 4 and relatively weak band with a maximum at 9900 cm 1 that is identified as a spin-forbidden electron transition 3 H 4 1 G 4. In the absorption coefficient spectra there are also two absorption bands appeared between cm 1 of unknown origin. In the visible spectral range, the absorption spectra are characterized by a single band at cm 1 corresponding to the transition 3 H 4 1 D 2 and two absorption bands at higher energies, assigned to transitions to 3 P 0 and ( 3 P I 6 ) levels; the hyper-sensitive transition 3 H 4 3 P 0 [21, 22] at higher energy is not observed, since it is masked by the Figure 3. Emission spectra of Pr 3+ ions in the TWPLa and TWPLu glasses corresponding to transitions from the 3 P 0 and 1 D 2 levels to lower - lying states. The spectra have been taken at somewhat different excitation wavelengths (467 and 478 nm for TWPLa:Pr 3+ and TWPLu:Pr 3+ glasses, respectively), but it has been checked that any excitation wavelength from the range nm yields practically the same PL spectrum for a given sample. Observed emission bands, slightly different for both glasses, correspond to the electron tran- 59

4 Spectroscopic properties of the Pr 3+ ion in TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3 glasses Table 1. Measured and calculated oscillator strengths as well as the J-O intensity parameters for Pr 3+ ions in tellurite glasses. Oscillator strength P ( 10 6 ) Transition TWPLa:Pr 3+ TWPLu:Pr 3+ from 3 H 4 to: Energy (cm 1 ) P exp P cal Energy (cm 1 )P exp P cal 3 H F F F G D P P I Ω 2 =3.09-,Ω 4 =1.32-, Ω 6 = cm 20 ; Ω 2 =4.74-,Ω 4 =1.22-, Ω 6 = cm 20 ; rms deviation= rms deviation= sition from the excited 3 P 0 and 1 D 2 states to lower - lying states of the Pr 3+ ion, as indicated in Fig. 3. The time - dependent decay of the Pr 3+ - ion luminescence I(t) is slightly non-exponential (see Figs. 4 and 5); therefore, we have determined the effective lifetime, defined as τ eff = t I(t) * dt/ I(t) * dt [29], of the 3 P 0 and 1 D 2 levels that are inserted in Figs. 4 and 5 and presented in Table 2 as τ exp. Figure 5. Luminescence lifetime of the 3 P 0 (a) and 1 D 2 (b) states of Pr 3+ ions in the TeO 2 -WO 3 -PbO-Lu 2 O 3 glass. Figure 4. Luminescence lifetime of the 3 P 0 (a) and 1 D 2 (b) states of Pr 3+ ions in the TeO 2 -WO 3 -PbO-La 2 O 3 glass. The quantum efficiency of a luminescent level is expressed as η =τ exp /τ rad. Having determined the radiative and experimental lifetimes of the 3 P 0 and 1 D 2 levels (see Table 2), we estimated the quantum efficiency of both levels in TWPLa and TWPLu glasses. For the TWPLa:Pr 3+ glass, η( 3 P 0 ) = 0.12 and η( 1 D 2 ) = 0.47, while for the TWPLu:Pr 3+ glass, η( 3 P 0 ) = 0.17 and η( 1 D 2 ) = It appears that the quantum efficiency of the 3 P 0 level in 60

5 Bożena Burta et al. Table 2. Values of the radiative transition rates (W r ), luminescence branching ratios (β), calculated (τ rad ) and measured (τ exp ) lifetimes of excited states of Pr 3+ ions in tellurite glasses. Transition TWPLa:Pr 3+ TWPLu:Pr 3+ from 3 P 0 to: Energy (cm 1 ) W r (s 1 ) β Energy (cm 1 ) W r (s 1 ) β 1 D G F F F H H H Transition from 3 D 2 to: τ rad = 16.1 µs; τ exp = 2.0 µs τ rad = 12.3 µs; τ exp = 2.1 µs 1 G F F F H H H τ rad = µ s; τ exp = 69.0 µs τ rad = µs; τ exp = 18.4 µs both glasses is comparable and relatively low. Conversely, the quantum efficiency of the 1 D 2 level differs considerably between the two hosts, indicating that the TWPLa:Pr 3+ glass is more promising than the TWPLu:Pr 3+ glass from the point of view of possible applications. 4. Conclusions The results of spetroscopic studies of Pr 3+ - doped TWPLa and TWPLu glasses can be summarized as follows: Both glasses are characterized by high a refractive index (n > 2.1). The absorption spectra of both glasses are similar, but the Pr 3+ - doped TWPLa glass appears to be more transparent than the Pr 3+ - doped TWPLu glass. The intensity parameters obtained from the Judd- Ofelt analysis of the absorption spectra of both glasses are similar. However, a higher quantum efficiency (47%) was predicted for the 1 D 2 level of Pr 3+ ion embedded in the TWPLa host, in comparison to the Pr 3+ - doped TWPLu host (18%), indicating that the Pr 3+ - doped TWPLa glass is quite promising for applications. Acknowledgments This work was supported by the Statutory Activities No (2013) of Faculty of Materials Science and Ceramics, AGH - University of Science and Technology, al. Mickiewicza 30, Cracow, Poland. References [1] M. J. Weber, J. D. Myers, D. H. Blackburn, J. Appl. Phys. 52, 2944 (1981) [2] K. Shioya, T. Komatsu, H. G. Kim, R. Sato, K.Matusita, J. Non-Cryst. Solids, 189, 16 (1995) [3] A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, P. Joshi, X. Jiang, J. Lousteau, Progress Mater. Sci. 57, 1426 (2012) [4] J. Wasylak, M. Reben, Phys. Chem. Glasses- European J. Glass Sci. Techn. B 48, 246 (2007) [5] B. Burtan et al., Opt. Mater. 34, 2050 (2012) [6] H. Ebendorff-Heidepriem, D. Ehrt, M. Bettinelli, A. Speghini, In: S. Jiang, S. Hokkanen (Eds.), SPIE Proc. Vol. 3622, Rare-Earth-Doped Materials and Devices III (SPIE, Bellingham, Washington, USA, 199) 19 [7] J. S. Wang, E. M. Vogel, E. Snitzer, J. L. Jackel, V. L. 61

6 Spectroscopic properties of the Pr 3+ ion in TeO 2 -WO 3 -PbO-La 2 O 3 and TeO 2 - WO 3 -PbO-Lu 2 O 3 glasses da Silva, Y. Silberberg, J. Non-Cryst. Solids 178, 109 (1994) [8] R. A. F. El-Mallawany, Tellurite Glasses Handbook- Physical Properties and Data (CRC Press, Boca Raton, Calif, USA, 2001) [9] J. S. Kumar et al., Adv. Mater. Res , 1235 (2010) [10] A. K. Singh, Opto-Electron. Rev. 20, 226 (2012) [11] V. K. Rai, S. B. Rai, D. K. Rai, Spectrochim. Acta A 62, 302 (2005) [12] Y. K. Sharma, R. P. Dubedi, V. Joshi, K. B. Karnataka, S. S. L. Surana, Indian J. Eng. Mater. Sci. 12, 65 (2005) [13] M. PrajnaShree, W. Akshatha, Y. Raviprakash, B. Sangeetha, D. K. Sudha, European Sci. J. 9, 83 (2013) [14] K. Annapurna, R. Chakrabarti, S. Buddhudu, J. Mater. Sci. 42, 6755 (2007) [15] A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, C. J. Hill, J. Lousteau, P. Joshi, Int. Mater. Rev. 57, 357 (2012) [16] H. Takebe et al., Appl. Opt. 36, 5839 (1997) [17] C. B. de Araujo, D. S. da Silva, T. A. A. de Assumpcao, L. R. P. Kassab, D. M. da Silva, Sci. World J. 2013, (2013) [18] L. R. P. Kassab, C. B. de Araujo, Germanate and tellurite glasses for photonic applications, in Photonics Research Developments (Nova Science, New York, 2008) 385 [19] L. P. Naranjo, C. B. de Araujo, O. L. Malta, P. A. S. Cruz, L. R. P. Kassab, Appl. Phys. Lett. 87, (2005) [20] V. K. Rai, K. Kumar, S. B. Rai, Opt. Mater. 29, 873 (2007) [21] P. Babu, C. K. Jayasankar, Physica B 301, 326 (2001) [22] M. P. Hehlen, M. G.Brik, K. W. Kramer, J. Lumin. 136, 221 (2013) [23] T. Mito, S. Fujino, H. Takebe, K. Moringa, S. Todoroki, S. Sakaguchi, J. Non-Cryst. Solids 210, 155 (1997) [24] J. Liu et al., Opt. Laser Techn. 39, 558 (2007) [25] R. El-Mallawany, M. D. Abdalla, I. A. Ahmed, Mater. Chem. Phys, 109, 291 (2008) [26] C. M. Dodson, R. Zia, Phys. Rev. B 86, (2012) [27] G. Lakshminarayana, H. Yang, J. Qiu, J. Alloys Compounds 475, 569 (2009) [28] M. V. V. Kumar, K. R. Gopal, R. R. Reddy, G. V. L. Reddy, N. S. Hussain, B. C. Jamalaiah, J. Non-Cryst. Solids 364, 20 (2013) [29] C. Duverger Arfuso, B. Boulard, Y. Jestin, M. Ferrari, A. Chiasera, Opt. Mater. 28, 441 (2006) 62