Optical properties and Judd-Ofelt parameters of Sm3+ doped BiO1.5-WO3-TeO2 glasses

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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aPhys. Status Solidi C 8, No. 9, 2597–2600 (2011) / DOI 10.1002/pssc.201084077

Optical properties and Judd-Ofelt parameters of Sm3+ doped BiO1.5-WO3-TeO2 glasses Takeshi Fujiwara1, Tomokatsu Hayakawa*1,3, Masayuki Nogami1, Jean-René Duclère2, and Philippe Thomas2 1 Field of Advanced Energy Conversion, Department of Frontier Materials, Nagoya Institute of Technology, Gokiso, Showa,

Nagoya 466-8555, Japan 2 Science des Procédés Céramiques et de Traitements de Surface (SPCTS), UMR 6638 CNRS, Faculté des Sciences,

Université de Limoges, 123, avenue Albert Thomas, 87060 Limoges Cedex, France

3 Toyota Physical and Chemical Research Institute, Yokomichi 41-1, Nagakute, Aichi 480-1192, Japan

Received 3 October 2010, accepted 4 February 2011 Published online 3 June 2011

Keywords bismuth-tungsten-tellurite glass, optical properties, samarium, Judd-Ofelt analysis, stimulated emission, cross-section * Corresponding author: e-mail [email protected], Phone: +81 52 735 5110, Fax: +81 52 735 5110

In this paper, the optical absorption and emission spectra of Sm3+ ions in 5BiO1.5-20WO3-75TeO2 glasses (0.2, 0.5, 1.0, 2.0 and 3.0 mol% of Sm3+) are presented. These glasses have been developed very recently and are found to exhibit excellent third-order nonlinear properties. A Judd-Ofelt analysis has been performed in order to calcu-late various optical parameters such as Judd-Ofelt pa-rameters, radiative transition probabilities, and stimulated emission cross-section. According to the computed Judd-Ofelt parameters, the symmetry of the sites occupied by the Sm3+ ions increased as the amount of Sm2O3 was in-creased, and the covalency decreased above 1.0 mol%. The results of the stimulated emission cross-section re-vealed that the 4G5/2 → 6H7/2 transition of 1.0 mol% Sm3+ in 5BiO1.5-20WO3-75TeO2 glasses could be promising for lasing action.

500 550 600 650 700 750 800

PL In

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Wavelength / nm

Ω2 = 4.92 pm2

Ω4 = 3.70 pm2

Ω6 = 2.01 pm2

σ(6H9/2) = 15.5 ×10-22cm2

σ(6H7/2) = 8.09 ×10-22cm2

σ(6H5/2) = 1.50 ×10-22cm2

6H11/2

6H9/2

6H7/2

6H5/2

4G5/2 →

Emission spectrum of 1.0 mol% Sm2O3-doped 5BiO1.5-20WO3-75TeO2 glass. Judd-Ofelt parameters (Ωλ) and peak emission stimulated cross-section σ(λp) are shown in the figure. The 4G5/2 → 6H7/2 transition is promising for lasing action.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Enhanced nonlinear optical materials have recently attracted significant interest, especially in novel applications, as alternatives for electronic materials. Tellurite (TeO2) glasses exhibit high optical nonlinearity because of the significant contribution of the virtual transi-tion from the anionic valance p-orbitals to the empty cati-onic 5d-orbitals of the Te atom [1-5]. In addition to having high nonlinearity, TeO2-based glasses are very often rein-forced by the addition of either a second lone pair holder (Bi3+, Pb2+) or cations with empty d-orbitals (W6+, Ti4+, Nb5+) [6, 7]. We have investigated the femto-second third-

order nonlinear properties of binary WO3-TeO2 glasses. Sekiya et al. [8, 9] have shown that WO3-TeO2 glasses comprise two phases; a tellurite molecular phase and a WO3 cluster phase. Very recently, we have reported on the structure and third-order nonlinear properties of BiO1.5-WO3-TeO2 (BWT) glasses and have shown that the size of the WO3 cluster is altered by the BiO1.5 addition [10]. Fur-ther, we have found that the third-order optical nonlinear susceptibility could be increased by adding of a small amount of BiO1.5 (5 mol%) to 20WO3-80TeO2 glass which was well correlated with the WO3 cluster size [10, 11].

2598 T. Fujiwara et al.: Optical properties and Judd-Ofelt parameters of Sm3+ doped BiO1.5-WO3-TeO2

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TeO2-based glasses have proven to be an interesting host for rare-earth ions owing to their low phonon energy and wide optical window [1, 2]. The optical properties of rare-earth ions depend on their host materials; therefore, the Judd-Ofelt (JO) theory [12, 13] is very often used to analyze the environment that surrounds rare-earth elements when doped into a glass matrix.

In this paper, we report on the optical properties and Judd-Ofelt parameters of xSm2O3-doped 5BiO1.5-20WO3-75TeO2 glasses (x = 0.2, 0.5, 1.0, 1.5, 2.0, 3.0). Optical ab-sorption and emission spectra were measured and analyzed, and the phenomenological JO parameters (Ω2, Ω4, Ω6), transition probabilities and stimulated emission cross-section of the 4G5/2 state were calculated.

2 Theoretical background The measured oscillator strength was determined experimentally from the absorp-tion spectrum using the following formula [12-16]:

∫−×= ννε df )(10318.4 9exp ,

(1)

where ε(ν)is the molar extinction coefficient at wavenum-ber (cm-1). This should be equal to the sum of the electric dipole and magnetic dipole oscillator strengths for the tran-sition band of interest, fed and fmd. Therefore, it can be re-written as fexp = fed + fmd, with

eded Sn

nJhmcf

9)2(

)12(38 222 +

+=

νπ, (2)

mdmd nSJhmcf

)12(38 2

+=

νπ, (3)

where ν is the wavenumber of transmission J→J’, n is the refractive index, h is Planck’s constant, and Sed and Smd are the line strengths for the electric dipole and magnetic di-pole transitions, respectively, as given in [12, 13,15, 16]:

2

6,4,2|'|∑

=

><Ω=λ

λλ bJUaJSed ,

(4)

22222 |'2|)16/( >+<= bJSLaJcmhSmd π, (5)

The Judd-Ofelt parameters, Ωλ, are closely related to the active ion environment, as given in [17]:

∑ −++=ps

sps sA,

1,

22, )12()12( λλ ΞλΩ

, (6)

where As,p are the crystal-field parameters of rank s and are related to the structure surrounding the rare-earth ions. Ξ2

s,λ is related to the matrix elements between the two ra-dial wave functions of 4f and the admixing levels, e.g., 5d, 5g, and the energy difference between these two levels. The term |<aJ||Uλ||bJ’>|2 is the square of the matrix ele-ments of the tensorial operator Uλ, which connects states |aJ> to |bJ’> and is considered to be independent of the

host matrix. The value of |<aJ||Uλ||bJ’>|2 has previously been reported by Carnall et al. [18]

The spontaneous emission probabilities (A) of the dif-ferent electronic transitions are given by

⎥⎦

⎤⎢⎣

⎡+

++

= mded SnSnnJh

A 32234

9)2(

)12(364 νπ

. (7)

The peak-stimulated emission cross-section (σp) [19] is given as

Acn

pp

λπ

λσ

Δ= 2

4

8 , (8)

where λp is the wavelength of the emission peak, and Δλ is the effective half-width of the emission bands.

3 Experimental 5BiO1.5-20WO3-75TeO2 (mol%) glasses [10] doped with xSm2O3(x = 0.2, 0.5, 1.0, 1.5, 2.0, 3.0) were prepared using optical grade Bi2O3 (Kishida Chem. Co.), WO3 (Kishida Chem. Co.), and TeO2 crystal-line powders obtained by the thermal decomposition of Te(OH)6 (Aldrich) [5]. A mixed batch was melted in a Pt crucible at 900 oC for 20 min. Once completely melted, the glass thus obtained was poured into a warm stainless mold on a brass base and annealed at 300 oC for 10 h. Then, the glass was well polished to meet the requirements of high quality optical measurements.

The linear refractive indices n were measured using an automatic ellipsometer (FiveLab, MARY-102) at 632.8 nm with a He-Ne laser. The density was measured using Ar-chimedes’ principle, and water was used for the immersion. The transmission spectra were recorded in the range 250-900 nm using a UV-Vis spectrophotometer (JASCO, V-570), and emission spectra were obtained using a spectro-photofluorometer (HITACHI, F-7000).

Figure 1 Optical absorption spectra of Sm3+ in 5BiO1.5-20WO3-75TeO2 (BWT) glasses. The transitions are from the 6H5/2 ground level to the levels indicated.

4000 5000 6000 7000 8000 9000 1000011000

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ical

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(a.u

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Wavenumber / cm-1

x=0.2

x=0.5

x=1.0

x=1.5

x=2.0

x=3.0 6H13/2

6H15/2

6F1/2

6F3/2

6F5/26F7/2

6F9/2

6F11/2

6H5/2 →

Phys. Status Solidi C 8, No. 9 (2011) 2599

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4 Experimental results The absorption spectra of xSm2O3-doped 5BiO1.5-20WO3-75TeO2 glasses (x = 0.2, 0.5, 1.0, 1.5, 2.0, 3.0) are shown in Fig. 1. All the features observed correspond to the f-f transitions of Sm3+ from the ground 6H5/2 state to the different excited levels of the 4f 6 configuration. Using the absorption spectra, a Judd-Ofelt analysis was performed to determine the JO parameters Ω2, Ω4, and Ω6 [12, 13]. The assignment of these absorption bands and the values of the experimental and calculated oscillator strengths, as obtained from the Judd-Ofelt analy-sis, are presented in Table 1.

The emission spectra of Sm3+ in 5BiO1.5-20WO3-75TeO2 glasses have been recorded in the spectral range 500-750 nm, when excited with 478 nm radiation from a Xe lamp, as shown in Fig. 2. Four emission peaks were as-signed to the transitions 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, 11/2). It was also found that the intensity of the photolumi-nescence was maximized at 1 mol% doping of Sm2O3.

5 Discussion The JO parameters Ωλ are important for investigating local structures and bonding in the vicin-ity of rare-earth ions. Ω2 is associated with the asymmetry of the ligand field at the rare-earth site, whereas Ω6 is an indicator of the covalency of the Sm-O bond. Ω2 decreased as the Sm2O3 content was increased up to 2.0 mol%. As the amount of Sm2O3 content was increased, Ω6 remained roughly constant, although a slight increase in Ω6 was ob-served with 1.0 mol% doping. These results suggest an in-crease in the symmetry of the site occupied by the Sm3+ ions and Sm-O covalency, in accordance with an increase in the Sm2O3 content up to 1.0 mol%. This led to closer packing of the oxygen anions around the Sm3+ ion. How-ever, Sm2O3 content in excess of 1.0 mol% seemed to pro-duce an ionic glass structure surrounding the Sm3+ ion.

The transition probability and stimulated cross-section σp were calculated in order to predict the optical proper ties of the probable lasing transitions. Their values are

Figure 2 Emission spectra of Sm3+ in 5BiO1.5-20WO3-75TeO2 glasses. The transitions are from the 4G5/2 level to the levels indi-cated. listed in Table 2. The 4G5/2 → 6H9/2 transition showed the highest radiative transition probability and stimulated cross-section. The radiative transition probability corre-sponded to the emission intensity, and from Table 2, the 4G5/2 → 6H9/2 transition seemed to have the strongest inten-sity. However, it is evident from Fig. 2 that the intensity of the 4G5/2 → 6H9/2 transition was actually weaker than the 4G5/2 → 6H7/2 transition. Except for very large value of the Ω2 parameter of Sm3+ in an organic chelate with a very asymmetric structure [20], the above-mentioned behavior has generally been reported, although this is still under de-bate [21]. From our results, we conclude that the 4G5/2 → 6H7/2 transition (at around 600 nm) provides a favorable lasing action for Sm3+-doped BWT glass and the optimal Sm2O3 doping level is 1.0 mol%. The stimulated cross-section of the Sm2O3-doped 5BiO1.5-20WO3-75TeO2 glasses was similar to that of other anticipative Sm3+ doped

500 550 600 650 700 750

PL In

tens

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.u.)

Wavelength / nm

x=0.2

x=0.5

x=1.0

x=1.5

x=2.0

x=3.06H5/2

6H7/26H9/2 6H11/2

4G5/2 →

Table 1 Oscillator strengths (fexp, fcal / × 10-6) for various transitions from indicated levels to ground level 6H5/2, Judd-Ofelt parameters Ω2, Ω4, and Ω6, and root-mean-square (δ RMS/ × 10-6) value [17], which indicates fitting quality and theoreti-cal and experimental results of xSm3+ in 5BiO1.5-20WO3-75TeO2 glasses.

Transition from 6H5/2 x=0.2 x=0.5 x=1.0 x=1.5 x=2.0 x=3.0

fexp fcal fexp fcal fexp fcal fexp fcal fexp fcal fexp fcal 6F3/2+6H15/2+6F1/2 5.54 5.54 5.20 5.21 5.19 5.19 4.69 4.69 4.54 4.55 4.63 4.64 6F5/2 3.42 3.42 3.24 3.24 3.24 3.24 2.87 2.88 2.82 2.90 2.82 2.83 6F7/2 3.94 3.94 4.08 4.08 4.19 4.20 3.86 3.86 3.73 3.73 3.50 3.50 6F9/2 2.78 2.30 2.78 2.49 2.74 2.60 2.51 2.43 2.30 2.30 2.29 2.12 6F11/2 0.41 0.36 0.41 0.39 0.42 0.41 0.33 0.39 0.34 0.36 0.30 0.34 Ω2 / pm2 5.51 5.08 4.92 4.63 4.36 4.59 Ω4 / pm2 4.07 3.80 3.70 3.38 3.39 3.32 Ω6 / pm2 1.82 1.98 2.01 1.96 1.81 1.69 δ RMS 0.34 0.20 0.10 0.071 0.060 0.12

2600 T. Fujiwara et al.: Optical properties and Judd-Ofelt parameters of Sm3+ doped BiO1.5-WO3-TeO2

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materials [20-22]. Hence, the BiO1.5-WO3-TeO2 glass is a promising host material for application in photonics, owing to its high optical nonlinearity and stimulated emission cross-section.

6 Conclusion The Judd-Ofelt parameters Ω2, Ω4, and Ω6 were obtained from the absorption spectra of Sm2O3-doped 5BiO1.5-20WO3-75TeO2 glasses with high optical nonlinearities. The oscillator strength, radiative transition probability, and stimulated cross-section were calculated. As the Sm2O3 content was increased, the sym-metry of the site occupied by the Sm3+ ions also increased, and the covalency decreased above 1.0 mol%. The 4G5/2 → 6H7/2 transitions in 1.0 mol% Sm2O3-doped 5BiO1.5-20WO3-75TeO2 glass are promising as lasing action be-cause of their high stimulated emission cross-section, i.e., of the order of 10-22 cm2. Finally, we conclude that Sm2O3-doped 5BiO1.5-20WO3-75TeO2 glasses exhibit both high stimulated emission cross-sections and high optical nonlin-earities.

Acknowledgements This work was supported by the JSPS International Training Program (ITP) “Young Scientist-Training Program for World Ceramics Network.”

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Table 2 Emission band position (λp / nm), effective line width (Δλ / nm), radiative transition probability (A / s-1), and peak stimu-lated emission cross-section (σ (λp) / ×10-22 cm2) of Sm3+ in 5BiO1.5-20WO3-75TeO2 glasses.

x=0.2 x=0.5

λp Δλ A σ (λp) λp Δλ A σ (λp) 4G5/2 → 6H9/2 645 11.27 316.31 16.6 646 11.97 315.81 15.4 6H7/2 599 13.53 246.03 7.98 599 13.53 250.50 7.97 6H5/2 563 9.82 50.97 1.78 563 10.4 51.06 1.65

x=1.0 x=1.5


x=2.0 x=3.0


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Optical properties and Judd-Ofelt parameters of Sm3+ doped BiO1.5-WO3-TeO2 glasses