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The study of Sm3+-doped low-phonon-energy chalcohalide glasses
Journal of Non-Crystalline Solids 357 (2011) 2463–2467
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
The study of Sm3+-doped low-phonon-energy chalcohalide glasses Gao Tang a,⁎, Huihua Xiong a,b, Wei Chen a, Lan Luo a a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 10 November 2010 Available online 16 December 2010 Keywords: Chalohalide glass; Low-phonon energy; Sm3+ ions; Infrared emission
a b s t r a c t The Sm3+-doped low-phonon-energy (LPE) Ge–Ga–Se–CsI glasses were studied. Upon excitation at 980 nm diode laser, intense 1.25 and 1.49 μm near-infrared fluorescence bands with broad full width at half maximum (FWHM) of 49 and 53 nm were observed, respectively. About 180–300 μs fluorescence lifetimes were obtained for the 1.49 μm emission. The thermal properties and structure of glasses were investigated by differential thermal analysis (DTA) and Raman spectra, respectively. Spectroscopic characteristics of the optical transitions have been calculated by using the Judd–Ofelt theory and evaluated for excited levels. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Optical properties of rare earth ions doped in glass host have been extensively studied for several decades because of their possible application for solid state lasers, fiber amplifiers, up-conversion devices and other opto-electronic components [1–3]. For some rare earth ions, their emissions, especially in near-infrared and midinfrared regions, are very weak or even quenched in usually used oxide and fluoride hosts, mainly owing to serious multi-phonon relaxation (MPR). For Sm3+ ions, energy gaps between 6F11/2, 9/2, 7/2 to their lower levels are ~ 1400, 1000 and 1000 cm−1, respectively [4,5]. Consequently, the infrared luminescence of Sm3+ usually quenched in conventional oxide or fluoride glasses host. Recently research shows that low-phonon-energy (LPE) chalcohalide glasses with much lower vibrational phonon energies may result in much less MPR [3], and therefore are considered promising hosts. In this paper, the thermal properties and near-infrared luminescence of Sm3+ ions in Ge–Ga–Se–CsI glasses are reported. The radiative properties of Sm3+ ions are discussed based on Judd–Oflet analysis. The glass structures are analyzed by Raman spectra to elucidate the effects of the local environment of Sm3+ ions on the luminescence.
2. Experimental procedure The glasses were synthesized by melting mixtures of the constituent elements (Ge, Ga, and Se, all of 99.999% purity, CsI and Sm of 99.9% purity) in evacuated (10−2 Pa) and flame-sealed silica ampoule in a rocking furnace. The mixtures were melted at 900–950 °C for 20 h. After ⁎ Corresponding author. E-mail address: [email protected] (G. Tang). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.060
that, the ampoule was quenched in cold water or air then swiftly moved to a preheated furnace and annealed near corresponding glass transition temperature for 2 h. The bulk sample was obtained by taking it out from the ampoule. Glass rod was finally cut and polished for testing. The samples were about 2 mm thickness. Based on the previous similar glass system research, the lifetime of rare earth in glasses seems sensitive to the I/Ga ratio. The lifetime reaches a relative high value when I/Ga = 1 condition is satisfied [6]. Thus host glasses with I/Ga= 1 have been studied: 70GeSe2–10Ga2Se3–20CsI (G1), 55GeSe2–15Ga2Se3–30CsI (G2), and 40GeSe2–20Ga2Se3–40CsI (G3), and Sm3+ doping concentration is fixed at 0.1 wt.%. The differential thermal analysis (DTA) measurements of bulk glass pieces (40–50 mg) were carried out by a CDR-1P Thermal Analyzer (SBIF, Shanghai, P.R. China) with an accuracy of ±2 °C at a heating rate of 10 °C/min. Pure α-Al2O3 powder was used as the reference material. Glass transition temperatures (Tg) and crystallization onset temperature (Tc) were determined by the slope intercept method from measured DTA curves. The density was obtained using the Archimedes method and the accuracy was ±0.001 g/cm3. The viscosity–temperature measurement was conducted on glass rods (Ф6mm × 6 mm) by a Rheotronic III 1000 parallel plate viscometer (Theta, USA) with a precision of 0.5% at a rate of 2 °C/min. The Vis–NIR absorption spectrum was measured by a VARIAN Cary 500 spectrophotometer (USA) with wavelength resolutions of 1 nm. The IR transmission spectrum was measured a Shinmadzu IRPrestige21 INFRARED spectrophotometer with resolutions of 4 cm−1. Fluorescence spectra and decay curves were measured on the polished plates by a Jobin Yvon Fluorolog-3 Spectrophotometer equipped with a 980 nm diode laser and a Hamamatsu R5509-72 photomultiplier. The fluorescent lifetime is defined as the time when the fluorescent intensity decreases to the e−1-fold of primary intensity. The Raman spectra were measured by a Renishaw Invia Raman Microscope (UK)
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G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467
Table 1 Thermal properties and densities of Ge–Ga–Se–CsI glasses. Glass compositions
Tg (°C)
Tc (°C)
△ T (°C)
ρ (g/cm3)
70GeSe2–10Ga2Se3–20CsI (G1) 55GeSe2–15Ga2Se3–30CsI (G2) 40GeSe2–20Ga2Se3–40CsI (G3)
332 309 286
459 415 414
127 106 128
4.208 4.190 4.176
with a resolution of 0.5 cm−1. The excited wavelength was 785 nm. The 600–1600 nm refractive indices of the glasses were tested by a Jobin Yvon UVISEL/460-VIS-AGAS Ellipsometer (France). All of the spectrums were measured at room temperature. 3. Results and discussion 3.1. Thermal properties The glass transition temperature (Tg) and crystallization onset temperature (Tc), which are determined by DSC curves, are presented in Table 1. The difference between the glass transition temperature and the onset crystallization temperature (△T = Tc−Tg), △T, has been commonly used as a rough criterion of the glass thermal stability against devitrification. The thermal stability criterion △T of glasses is larger than 100 °C, indicating that these glasses have good thermal stability and can easily be obtained in bulk forms. It is shown that Tg and ρ decrease with increasing Ga2Se3 and CsI content when I/Ga = 1 (Table 1). This phenomenon indicates the Ga2Se3 and CsI addition in glasses can open the glass structure and decrease the glass connectivity. The viscosity–temperature relationship of a typical sample, G1 glass, is shown in Fig. 1. In the fiber drawing process, the working temperature window available for G1 glass is between Tg = 332 °C and Tc = 459 °C (Table 1). From the viscosity–temperature curve, the softening temperature of 397.1 °C can be determined. No crystallization appears near the softening temperature, indicating that the glass of this composition is suitable for fiber drawing.
Fig. 2. Vis–NIR optical absorption cross section of glasses (thickness = 2 mm).
where d is thickness of the samples, and N is the number of Sm3+ ions in 1 cm−3 samples. From Figs. 2 and 3, several absorption bands, which associated with the transitions from 6H5/2 ground state to 6F11/2, 6F9/2, 6 F7/2, 6F5/2 6H15/2, and 6F3/2 6F1/2 excited states, can be observed. The absorption in the 910–1000 nm range is important for 980 nm diode laser pumping. The short-wavelength absorption edge of studied glasses lies in the 600–750 nm region and has a blue-shift when CsI and Ga2Se3 addition increases. The long-wavelength absorption edge of glasses was found near 16 μm and can be assigned to the Ge–Se multi-phonon vibration (Fig. 4). The transmission range of Sm3+-doped Ge–Ga–Se–CsI glass is similar to that of undoped glasses [7]. The Judd–Ofelt theory was frequently used to calculate the spectroscopic parameters such as intensity parameters Ωλ (λ = 2, 4, and 6) [8,9]. According to the Judd–Ofelt theory, the experimental oscillator strengths of dipole transitions of Sm3+ ions can be calculated from absorption spectra using Eq. (2):
3.2. Absorption spectra and Judd–Ofelt analysis The room-temperature Vis–NIR optical absorption cross sections of glasses are shown in Fig. 2. The optical absorption cross section is obtained using Eq. (1):
σðνÞ =
ln I0ðνÞ = IðνÞ dd N
;
fexp =
ð2Þ
where m is electron mass, c is velocity of light in vacuum and e is charge of the electron. The oscillator strength fcal of a transition between two multiplets is given by the formula (3):
ð1Þ 2
fcal =
Fig. 1. The viscosity–temperature curve of G1 glass.
mc2 ∫σðνÞdν; πe2
2
2
8π mcν0 ðn + 2Þ d d 3hð2J + 1Þ 9n
N λ N 2 ∑ Ωλ 〈f ΨJ‖U ‖ f Ψ′ J ′ 〉
λ = 2;4;6
Fig. 3. Schematic diagram of Sm3+ ions in Ge–Ga–Se–CsI glasses.
ð3Þ
G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467
Fig. 5. Emission spectra of 0.1%wt. Sm3+-doped Ge–Ga–Se–CsI glasses.
Fig. 4. IR optical absorption cross section of glasses (thickness = 2 mm).
where h is Plank's constant, J is the ground state angular momentum and n is the refractive index of glass. The Judd–Ofelt intensity parameters Ωλ can be evaluated by the least-square fitting between experimental and calculated oscillator strengths. The reduced matrix elements 〈f N ΨJ‖U λ ‖ f N Ψ′ J ′ 〉 required for the analysis have been taken from ref. [10]. The intensity parameters Ωλ are important for investigating the local structure and the bonding in the vicinity of rare earth ions. The Ω2 is indicative of asymmetry of the local structure and the amount of covalent bonding, while the Ω6 is related to the ionicity of the host [11,12]. Table 2 shows the composition dependence of Judd–Ofelt intensity parameters Ωλ for Sm3+ ions in Ge–Ga–Se–CsI glass and other glasses. From the variation of Ω2 and Ω6 parameter values in Ge-Ga-Se-CsI glass (G1 N G2 N G3), it can be concluded that the covalence of the bonds between Sm3+ ions and surrounding atoms decreases with the Ga2Se3 and CsI addition. Furthermore, the chemical bonds in Ge–Ga–Se–CsI glass appear less covalent than other glasses.
3.3. Infrared emission and glass structure The near-infrared emission spectra of Sm3+ in the Ge–Ga–Se–CsI glasses under 980 nm pumping are shown in Fig. 5. The three emission bands at 1.09, 1.25 and 1.49 μm were observed, which are attributed to the optical transitions 6F11/2 → 6H7/2, 6F11/2 → 6H9/2 and 6 F11/2 → 6H11/2 (Fig. 3), respectively. The full width at half maximum (FWHM) of 1.25 μm and 1.49 μm emission in the glasses is about 49 and 53 nm, respectively. The distorted emission spectra may be related to the distribution of 3H4 stark sublevels which can be influenced by the crystal fields around Sm3+ ions. The relative intensity of 1.25 μm emission and 1.49 μm emission both decreases with increased amount of Ga2Se3 and CsI.
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From the Judd–Ofelt analysis, the radiative transition probability for a transition ΨJ–Ψ′J′ is given by: AðΨJ; Ψ′ J ′ Þ =
" # 64π4 ν3 nðn2 + 2Þ2 3 × Sed + n × Smd 3hð2J + 1Þ 9
ð4Þ
where the Sed and Smd are the line strength for electric dipole (ed) and magnetic dipole (md) transitions respectively between J manifolds. 2
Sed = e ×
N λ N 2 ∑ Ωλ 〈f ΨJ‖U ‖ f Ψ′ J ′ 〉
ð5Þ
λ = 2;4;6 2 2
Smd =
e h → → N N 2 〈f ΨJ‖ L + 2 S ‖ f Ψ′ J ′ 〉 16π2 m2 c2
ð6Þ
where L is the angular momentum operator and S is the spin operator. The total spontaneous emission transition probability (Atotal) for the excited ΨJ state is the sum of electric and magnetic dipole transition to all terminal states Ψ′J′. The calculated radiative lifetime τcal of the excited state and the fluorescence branching ratio β can be obtained by the following equations: 1 Atotal = ∑ AðΨJ; Ψ′ J ′ Þ = : τ cal Ψ′ J ′
ð7Þ
These important spectroscopic parameters of typical G1 glass were calculated from Eqs. (4)–(7) and listed in Table 3. The quantum efficiencies can be obtained from the ratio of the measured lifetime τmea to the calculated radiative lifetime τcal, η = τmea/τcal. Fig. 6 indicates the decay curves of 1.49 μm fluorescence of the Ge–Ga–Se– CsI glasses doped 0.1%wt. Sm3+. Lifetimes of 1.49 μm emission Table 3 Radiative transition probability, fluorescence branching ratio, calculated radiative lifetime and quantum efficiency for emission from the 6F11/2 level of Sm3+ in G1 glass. Glass
Transition
Energy (cm−1)
Aed + Amd (s−1)
τcal (μs)
η
G1
6
1400 2400 3400 3800 4100 4400 5500 7000 8300 9500 10,600
0.33 0.65 0.86 102.70 0.70 0.43 124.96 114.40 76.80 38.22 10.54
2125
8.9%
F11/2 → F9/2 F11/2 → 6F7/2 F11/2 → 6F5/2 6 F11/2 → 6H15/2 6 F11/2 → 6F3/2 6 F11/2 → 6F1/2 6 F11/2 → 6H13/2 6 F11/2 → 6H11/2 6 F11/2 → 6H9/2 6 F11/2 → 6F7/2 6 F11/2 → 6F5/2 6
Table 2 Judd–Oflet intensity parameters (10−20 cm2) of Sm3+ in different glasses.
6
Host glasses
Ω2
Ω4
Ω6
Reference
Ge30Ga5Se60 Ge15Ga5Se75 PbO–PbF2 glass G1 glass G2 glass G3 glass
8.61 4.82 1.16 1.92 0.89 0.88
14.10 10.51 2.60 1.16 0.56 1.07
6.82 5.64 1.40 0.25 0.08 0.03
Ref. [13] Ref. [13] Ref. [14] This work This work This work
6
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G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467 Table 4 Raman shift and assignment of the bands in Ge–Ga–Se–CsI glasses. Raman shift (cm−1)
Assignments
137 153 164–168 ~ 174
[Ga2I7]− units [Ge(Ga)I4] mixed-anion units [Ge(Ga)I3Se] mixed-anion units [Ge(Ga)I2Se2] mixed-anion units [Se3Ge(Ga)–(Ga)GeSe3] ethane-like units [Ge(Ga)ISe3] mixed-anion units [Ge(Ga)Se4] ν1(A1) symmetric stretching modes edge-sharing [Ge(Ga)Se4] νc1(Ac1) modes
190 202–205 214
Fig. 6. Decay curves of 1.49 μm emission of the Ge–Ga–Se–CsI: 0.1%wt. Sm3+.
increase gradually from 190 μs, 288 μs to 300 μs with increased Ga2Se3 and CsI addition. Based on the τmea and τcal data, the quantum efficiency of 1.49 μm emission in typical G1 glass is only 8.9%. From the schematic diagram of Sm3+ energy levels (Fig. 3), the 6F11/2 state is separated from the next lower lying state by not more than about 1400 cm−1. Thus, it can be deduced that the appearance of infrared emission of Sm3+ is mainly due to the low-energy-phonon nature of Ge–Ga–Se–CsI glasses. However, the multi-phonon relaxation transition of Sm3+ is serious even if the Sm3+ is surrounded by a matrix that possesses low lattice vibrational energies. Fig. 7 shows the Raman spectra of Ge–Ga–Se–CsI glasses and Table 4 gives the assignments of the Raman bands. To evaluate the variation of Raman bands, the intensities of the bands are normalized to that of the ~200 cm−1 band. There are three important characteristic of Raman spectral evolution following the addition of Ga2Se3 and CsI. Firstly, the 214 cm−1 band is decreased; secondly, the amplitude of the 174 cm−1 band is reduced; and thirdly, new low-phonon bands at 137 cm−1, 153 cm−1, 168 cm−1 and 190 cm−1 appear and get larger. With CsI addition increase, the Se atoms of glass-former [GeSe4] and [GaSe4] units may be substituted for I atoms. Furthermore, the Ge(Ga)–(Ga)Ge bonds of [Se3Ge(Ga)–(Ga)GeSe3] units may be broken by I atoms to form [Se 3 Ge(Ga)-I]. Therefore, the 214 cm −1 and 174 cm −1 bands which associate with the glass-former [Ge(Ga)Se4] units and [Se3Ge (Ga)–(Ga)GeSe3] ethane-like units decrease. From ref. [15] and ref. [16], the 137 cm−1 and 153 cm−1 new band can be attributed to vibration of [Ga2I7]− unit and [Ge(Ga)I4] units, respectively. It is known that the
qffiffiffiffiffiffiffi ffi vibration frequency of units can be simply proportional to k μ , where k refers to force constant related to bond strength and μ is the reduced mass [16]. Because the vibration frequencies of [Ge(Ga)Se4] and [Ge(Ga)I4] are 202 cm−1 and 153 cm−1, respectively [16,17]. From the theoretical analysis [18], the increase in reduced mass and decreasing coupling strength of [Ge(Ga)IxSe4-x] units, attributed to I atoms, result in their vibration frequencies should shift to a lower wave number than that of [Ge(Ga)Se4] units. As a result, the 168 cm−1 and 190 cm−1 bands which attribute to [Ge(Ga)I3Se] units and [Ge(Ga)ISe3] increase abruptly with increasing Ga2Se3 and CsI addition. Compared with the [Ge(Ga)Se4] vibration units, the appearance of new low-energy-phonon bands mainly result in the τmea increase of 1.49 μm emission. Theoretically, the ionicity of new units containing I atoms could be more than that of [Ge(Ga)Se4]. When Ga2Se3 and CsI increase, the enhancement of new ionicity bands coincides with the variant of Ω2 and Ω6 parameters discussed above. 4. Conclusion The thermal stability, spectroscopic properties and structure of Sm3+doped Ge–Ga–Se–CsI glasses were investigated. The glasses present good thermal stability and are suitable for fiber drawing. The three Judd–Oflet parameters for Sm3+ indicate that the local site symmetry of the glass is high and the covalence of the glass is low. The emission bands located at 1.25 μm and 1.49 μm can be observed and the 1.49 μm fluorescence lifetime increased with increasing Ga2Se3 and CsI addition and these phenomena may be attributed to the lower local vibrational phonon energy of Sm3+ in the glass matrix. The Raman spectra supported this point. The results show that Sm3+-doped Ge-Ga-Se-CsI glasses may be good candidates for infrared emission spectral region. Acknowledgement This work was financially supported by the Shanghai Institute of Ceramics's Science and Technology Innovation project (No. Y05ZC9190G). References
Fig. 7. Raman spectra of Ge–Ga–Se–CsI glasses.
[1] J.-L. Adam, Lanthanides in non-oxide glasses, Chem. Rev. 102 (2002) 2461–2476. [2] B. Cole, L.B. Shaw, P.C. Pureza, R. Mossadegh, J.S. Sanghera, I.D. Aggarwal, Rareearth doped selenide glasses and fibers for active applications in the near and mid-IR, J. Non-Cryst. Solids 256–257 (1999) 253–259. [3] G. Tang, Z. Yang, L. Luo, W. Chen, Dy3+-doped chalcohalide glass for 1.3 μm optical fiber amplifiers, J. Mater. Res. 23 (4) (2008) 954–961. [4] B. Frumarová, M. Bílková, M. Frumar, M. Repka, J. Jedelský, Thin films of Sb2S3 doped by Sm3+ ions, J. Non-Cryst. Solids 326–327 (2003) 348–352. [5] Z. Yang, G. Tang, L. Luo, W. Chen, Modified local environment and enhanced nearinfrared luminescence of Sm3+ in chalcohalide glasses, Appl. Phys. Lett. 89 (2006) 131117. [6] G. Tang, C. Liu, Z. Yang, L. Luo, W. Chen, Near-Infrared emission properties and energy transfer of Tm3+-doped and Tm3+/Dy3+-codoped chalcohalide glasses, J. Appl. Phys. 104 (2008) 113116. [7] Z. Yang, L. Luo, W. Chen, Red color GeSe2-based chalcohalide glasses for infrared optics, J. Am. Ceram. Soc. 89 (2006) 2327–2329. [8] B.R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750–761. [9] G.S. Ofelt, Intensities of crystal spectra of rare-earth Ions, J. Chem. Phys. 37 (1962) 511–520.
G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467 [10] A.A. Kaminskii (Ed.), Crystalline Lasers: Physical Processes and Operation Schemes, CRC press, Boca Raton, New York, London and Tokyo, 1996, pp. 227–306. [11] R.D. Peacock, The intensities of lanthanide f–f transitions, Struct. Bonding 22 (1975) 83–122. [12] M.B. Saisudha, J. Ramakrishna, Effect of host glass on the optical absorption properties of Nd3+, Sm3+, and Dy3+ in lead borate glasses, Phys. Rev. B 53 (1996) 6186–6196. [13] P. Nĕmec, J. Jedelský, M. Frumar, On the effect of composition on the Judd–Ofelt parameters of Sm3+-doped chalcogenide glasses, J. Non-Cryst. Solids 326&327 (2003) 325–329. [14] P. Nachimuthu, R. Jagannathan, V.N. Kumar, D.N. Rao, Absorption and emission spectral studies of Sm3+ and Dy3+ ions in PbO–PbF2 glasses, J. Non-Crystal Solids 217 (1997) 215–223.
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[15] M.J. Taylor, L.A. Kloo, Raman investigations of indium iodide complexes: evidence for the In2I− 7 ion, J. Raman Spectrosc. 31 (2000) 465–468. [16] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, third ed.John Wiley & Sons, New York, 1978. [17] P. Němec, B. Frumarová, M. Frumar, Structure and properties of the pure and Pr3+doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses, J. Non-Cryst. Solids 270 (2000) 137–146. [18] G. Tang, C. Liu, Z. Yang, L. Luo, W. Chen, Micro-structural studies of GeSe2–Ga2Se3–MX (MX = CsI and PbI2) glasses using Raman spectra, J. Non-Cryst. Solids 355 (2009) 1585–1589.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
The study of Sm3+-doped low-phonon-energy chalcohalide glasses Gao Tang a,⁎, Huihua Xiong a,b, Wei Chen a, Lan Luo a a b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 10 November 2010 Available online 16 December 2010 Keywords: Chalohalide glass; Low-phonon energy; Sm3+ ions; Infrared emission
a b s t r a c t The Sm3+-doped low-phonon-energy (LPE) Ge–Ga–Se–CsI glasses were studied. Upon excitation at 980 nm diode laser, intense 1.25 and 1.49 μm near-infrared fluorescence bands with broad full width at half maximum (FWHM) of 49 and 53 nm were observed, respectively. About 180–300 μs fluorescence lifetimes were obtained for the 1.49 μm emission. The thermal properties and structure of glasses were investigated by differential thermal analysis (DTA) and Raman spectra, respectively. Spectroscopic characteristics of the optical transitions have been calculated by using the Judd–Ofelt theory and evaluated for excited levels. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Optical properties of rare earth ions doped in glass host have been extensively studied for several decades because of their possible application for solid state lasers, fiber amplifiers, up-conversion devices and other opto-electronic components [1–3]. For some rare earth ions, their emissions, especially in near-infrared and midinfrared regions, are very weak or even quenched in usually used oxide and fluoride hosts, mainly owing to serious multi-phonon relaxation (MPR). For Sm3+ ions, energy gaps between 6F11/2, 9/2, 7/2 to their lower levels are ~ 1400, 1000 and 1000 cm−1, respectively [4,5]. Consequently, the infrared luminescence of Sm3+ usually quenched in conventional oxide or fluoride glasses host. Recently research shows that low-phonon-energy (LPE) chalcohalide glasses with much lower vibrational phonon energies may result in much less MPR [3], and therefore are considered promising hosts. In this paper, the thermal properties and near-infrared luminescence of Sm3+ ions in Ge–Ga–Se–CsI glasses are reported. The radiative properties of Sm3+ ions are discussed based on Judd–Oflet analysis. The glass structures are analyzed by Raman spectra to elucidate the effects of the local environment of Sm3+ ions on the luminescence.
2. Experimental procedure The glasses were synthesized by melting mixtures of the constituent elements (Ge, Ga, and Se, all of 99.999% purity, CsI and Sm of 99.9% purity) in evacuated (10−2 Pa) and flame-sealed silica ampoule in a rocking furnace. The mixtures were melted at 900–950 °C for 20 h. After ⁎ Corresponding author. E-mail address: [email protected] (G. Tang). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.060
that, the ampoule was quenched in cold water or air then swiftly moved to a preheated furnace and annealed near corresponding glass transition temperature for 2 h. The bulk sample was obtained by taking it out from the ampoule. Glass rod was finally cut and polished for testing. The samples were about 2 mm thickness. Based on the previous similar glass system research, the lifetime of rare earth in glasses seems sensitive to the I/Ga ratio. The lifetime reaches a relative high value when I/Ga = 1 condition is satisfied [6]. Thus host glasses with I/Ga= 1 have been studied: 70GeSe2–10Ga2Se3–20CsI (G1), 55GeSe2–15Ga2Se3–30CsI (G2), and 40GeSe2–20Ga2Se3–40CsI (G3), and Sm3+ doping concentration is fixed at 0.1 wt.%. The differential thermal analysis (DTA) measurements of bulk glass pieces (40–50 mg) were carried out by a CDR-1P Thermal Analyzer (SBIF, Shanghai, P.R. China) with an accuracy of ±2 °C at a heating rate of 10 °C/min. Pure α-Al2O3 powder was used as the reference material. Glass transition temperatures (Tg) and crystallization onset temperature (Tc) were determined by the slope intercept method from measured DTA curves. The density was obtained using the Archimedes method and the accuracy was ±0.001 g/cm3. The viscosity–temperature measurement was conducted on glass rods (Ф6mm × 6 mm) by a Rheotronic III 1000 parallel plate viscometer (Theta, USA) with a precision of 0.5% at a rate of 2 °C/min. The Vis–NIR absorption spectrum was measured by a VARIAN Cary 500 spectrophotometer (USA) with wavelength resolutions of 1 nm. The IR transmission spectrum was measured a Shinmadzu IRPrestige21 INFRARED spectrophotometer with resolutions of 4 cm−1. Fluorescence spectra and decay curves were measured on the polished plates by a Jobin Yvon Fluorolog-3 Spectrophotometer equipped with a 980 nm diode laser and a Hamamatsu R5509-72 photomultiplier. The fluorescent lifetime is defined as the time when the fluorescent intensity decreases to the e−1-fold of primary intensity. The Raman spectra were measured by a Renishaw Invia Raman Microscope (UK)
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G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467
Table 1 Thermal properties and densities of Ge–Ga–Se–CsI glasses. Glass compositions
Tg (°C)
Tc (°C)
△ T (°C)
ρ (g/cm3)
70GeSe2–10Ga2Se3–20CsI (G1) 55GeSe2–15Ga2Se3–30CsI (G2) 40GeSe2–20Ga2Se3–40CsI (G3)
332 309 286
459 415 414
127 106 128
4.208 4.190 4.176
with a resolution of 0.5 cm−1. The excited wavelength was 785 nm. The 600–1600 nm refractive indices of the glasses were tested by a Jobin Yvon UVISEL/460-VIS-AGAS Ellipsometer (France). All of the spectrums were measured at room temperature. 3. Results and discussion 3.1. Thermal properties The glass transition temperature (Tg) and crystallization onset temperature (Tc), which are determined by DSC curves, are presented in Table 1. The difference between the glass transition temperature and the onset crystallization temperature (△T = Tc−Tg), △T, has been commonly used as a rough criterion of the glass thermal stability against devitrification. The thermal stability criterion △T of glasses is larger than 100 °C, indicating that these glasses have good thermal stability and can easily be obtained in bulk forms. It is shown that Tg and ρ decrease with increasing Ga2Se3 and CsI content when I/Ga = 1 (Table 1). This phenomenon indicates the Ga2Se3 and CsI addition in glasses can open the glass structure and decrease the glass connectivity. The viscosity–temperature relationship of a typical sample, G1 glass, is shown in Fig. 1. In the fiber drawing process, the working temperature window available for G1 glass is between Tg = 332 °C and Tc = 459 °C (Table 1). From the viscosity–temperature curve, the softening temperature of 397.1 °C can be determined. No crystallization appears near the softening temperature, indicating that the glass of this composition is suitable for fiber drawing.
Fig. 2. Vis–NIR optical absorption cross section of glasses (thickness = 2 mm).
where d is thickness of the samples, and N is the number of Sm3+ ions in 1 cm−3 samples. From Figs. 2 and 3, several absorption bands, which associated with the transitions from 6H5/2 ground state to 6F11/2, 6F9/2, 6 F7/2, 6F5/2 6H15/2, and 6F3/2 6F1/2 excited states, can be observed. The absorption in the 910–1000 nm range is important for 980 nm diode laser pumping. The short-wavelength absorption edge of studied glasses lies in the 600–750 nm region and has a blue-shift when CsI and Ga2Se3 addition increases. The long-wavelength absorption edge of glasses was found near 16 μm and can be assigned to the Ge–Se multi-phonon vibration (Fig. 4). The transmission range of Sm3+-doped Ge–Ga–Se–CsI glass is similar to that of undoped glasses [7]. The Judd–Ofelt theory was frequently used to calculate the spectroscopic parameters such as intensity parameters Ωλ (λ = 2, 4, and 6) [8,9]. According to the Judd–Ofelt theory, the experimental oscillator strengths of dipole transitions of Sm3+ ions can be calculated from absorption spectra using Eq. (2):
3.2. Absorption spectra and Judd–Ofelt analysis The room-temperature Vis–NIR optical absorption cross sections of glasses are shown in Fig. 2. The optical absorption cross section is obtained using Eq. (1):
σðνÞ =
ln I0ðνÞ = IðνÞ dd N
;
fexp =
ð2Þ
where m is electron mass, c is velocity of light in vacuum and e is charge of the electron. The oscillator strength fcal of a transition between two multiplets is given by the formula (3):
ð1Þ 2
fcal =
Fig. 1. The viscosity–temperature curve of G1 glass.
mc2 ∫σðνÞdν; πe2
2
2
8π mcν0 ðn + 2Þ d d 3hð2J + 1Þ 9n
N λ N 2 ∑ Ωλ 〈f ΨJ‖U ‖ f Ψ′ J ′ 〉
λ = 2;4;6
Fig. 3. Schematic diagram of Sm3+ ions in Ge–Ga–Se–CsI glasses.
ð3Þ
G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467
Fig. 5. Emission spectra of 0.1%wt. Sm3+-doped Ge–Ga–Se–CsI glasses.
Fig. 4. IR optical absorption cross section of glasses (thickness = 2 mm).
where h is Plank's constant, J is the ground state angular momentum and n is the refractive index of glass. The Judd–Ofelt intensity parameters Ωλ can be evaluated by the least-square fitting between experimental and calculated oscillator strengths. The reduced matrix elements 〈f N ΨJ‖U λ ‖ f N Ψ′ J ′ 〉 required for the analysis have been taken from ref. [10]. The intensity parameters Ωλ are important for investigating the local structure and the bonding in the vicinity of rare earth ions. The Ω2 is indicative of asymmetry of the local structure and the amount of covalent bonding, while the Ω6 is related to the ionicity of the host [11,12]. Table 2 shows the composition dependence of Judd–Ofelt intensity parameters Ωλ for Sm3+ ions in Ge–Ga–Se–CsI glass and other glasses. From the variation of Ω2 and Ω6 parameter values in Ge-Ga-Se-CsI glass (G1 N G2 N G3), it can be concluded that the covalence of the bonds between Sm3+ ions and surrounding atoms decreases with the Ga2Se3 and CsI addition. Furthermore, the chemical bonds in Ge–Ga–Se–CsI glass appear less covalent than other glasses.
3.3. Infrared emission and glass structure The near-infrared emission spectra of Sm3+ in the Ge–Ga–Se–CsI glasses under 980 nm pumping are shown in Fig. 5. The three emission bands at 1.09, 1.25 and 1.49 μm were observed, which are attributed to the optical transitions 6F11/2 → 6H7/2, 6F11/2 → 6H9/2 and 6 F11/2 → 6H11/2 (Fig. 3), respectively. The full width at half maximum (FWHM) of 1.25 μm and 1.49 μm emission in the glasses is about 49 and 53 nm, respectively. The distorted emission spectra may be related to the distribution of 3H4 stark sublevels which can be influenced by the crystal fields around Sm3+ ions. The relative intensity of 1.25 μm emission and 1.49 μm emission both decreases with increased amount of Ga2Se3 and CsI.
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From the Judd–Ofelt analysis, the radiative transition probability for a transition ΨJ–Ψ′J′ is given by: AðΨJ; Ψ′ J ′ Þ =
" # 64π4 ν3 nðn2 + 2Þ2 3 × Sed + n × Smd 3hð2J + 1Þ 9
ð4Þ
where the Sed and Smd are the line strength for electric dipole (ed) and magnetic dipole (md) transitions respectively between J manifolds. 2
Sed = e ×
N λ N 2 ∑ Ωλ 〈f ΨJ‖U ‖ f Ψ′ J ′ 〉
ð5Þ
λ = 2;4;6 2 2
Smd =
e h → → N N 2 〈f ΨJ‖ L + 2 S ‖ f Ψ′ J ′ 〉 16π2 m2 c2
ð6Þ
where L is the angular momentum operator and S is the spin operator. The total spontaneous emission transition probability (Atotal) for the excited ΨJ state is the sum of electric and magnetic dipole transition to all terminal states Ψ′J′. The calculated radiative lifetime τcal of the excited state and the fluorescence branching ratio β can be obtained by the following equations: 1 Atotal = ∑ AðΨJ; Ψ′ J ′ Þ = : τ cal Ψ′ J ′
ð7Þ
These important spectroscopic parameters of typical G1 glass were calculated from Eqs. (4)–(7) and listed in Table 3. The quantum efficiencies can be obtained from the ratio of the measured lifetime τmea to the calculated radiative lifetime τcal, η = τmea/τcal. Fig. 6 indicates the decay curves of 1.49 μm fluorescence of the Ge–Ga–Se– CsI glasses doped 0.1%wt. Sm3+. Lifetimes of 1.49 μm emission Table 3 Radiative transition probability, fluorescence branching ratio, calculated radiative lifetime and quantum efficiency for emission from the 6F11/2 level of Sm3+ in G1 glass. Glass
Transition
Energy (cm−1)
Aed + Amd (s−1)
τcal (μs)
η
G1
6
1400 2400 3400 3800 4100 4400 5500 7000 8300 9500 10,600
0.33 0.65 0.86 102.70 0.70 0.43 124.96 114.40 76.80 38.22 10.54
2125
8.9%
F11/2 → F9/2 F11/2 → 6F7/2 F11/2 → 6F5/2 6 F11/2 → 6H15/2 6 F11/2 → 6F3/2 6 F11/2 → 6F1/2 6 F11/2 → 6H13/2 6 F11/2 → 6H11/2 6 F11/2 → 6H9/2 6 F11/2 → 6F7/2 6 F11/2 → 6F5/2 6
Table 2 Judd–Oflet intensity parameters (10−20 cm2) of Sm3+ in different glasses.
6
Host glasses
Ω2
Ω4
Ω6
Reference
Ge30Ga5Se60 Ge15Ga5Se75 PbO–PbF2 glass G1 glass G2 glass G3 glass
8.61 4.82 1.16 1.92 0.89 0.88
14.10 10.51 2.60 1.16 0.56 1.07
6.82 5.64 1.40 0.25 0.08 0.03
Ref. [13] Ref. [13] Ref. [14] This work This work This work
6
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G. Tang et al. / Journal of Non-Crystalline Solids 357 (2011) 2463–2467 Table 4 Raman shift and assignment of the bands in Ge–Ga–Se–CsI glasses. Raman shift (cm−1)
Assignments
137 153 164–168 ~ 174
[Ga2I7]− units [Ge(Ga)I4] mixed-anion units [Ge(Ga)I3Se] mixed-anion units [Ge(Ga)I2Se2] mixed-anion units [Se3Ge(Ga)–(Ga)GeSe3] ethane-like units [Ge(Ga)ISe3] mixed-anion units [Ge(Ga)Se4] ν1(A1) symmetric stretching modes edge-sharing [Ge(Ga)Se4] νc1(Ac1) modes
190 202–205 214
Fig. 6. Decay curves of 1.49 μm emission of the Ge–Ga–Se–CsI: 0.1%wt. Sm3+.
increase gradually from 190 μs, 288 μs to 300 μs with increased Ga2Se3 and CsI addition. Based on the τmea and τcal data, the quantum efficiency of 1.49 μm emission in typical G1 glass is only 8.9%. From the schematic diagram of Sm3+ energy levels (Fig. 3), the 6F11/2 state is separated from the next lower lying state by not more than about 1400 cm−1. Thus, it can be deduced that the appearance of infrared emission of Sm3+ is mainly due to the low-energy-phonon nature of Ge–Ga–Se–CsI glasses. However, the multi-phonon relaxation transition of Sm3+ is serious even if the Sm3+ is surrounded by a matrix that possesses low lattice vibrational energies. Fig. 7 shows the Raman spectra of Ge–Ga–Se–CsI glasses and Table 4 gives the assignments of the Raman bands. To evaluate the variation of Raman bands, the intensities of the bands are normalized to that of the ~200 cm−1 band. There are three important characteristic of Raman spectral evolution following the addition of Ga2Se3 and CsI. Firstly, the 214 cm−1 band is decreased; secondly, the amplitude of the 174 cm−1 band is reduced; and thirdly, new low-phonon bands at 137 cm−1, 153 cm−1, 168 cm−1 and 190 cm−1 appear and get larger. With CsI addition increase, the Se atoms of glass-former [GeSe4] and [GaSe4] units may be substituted for I atoms. Furthermore, the Ge(Ga)–(Ga)Ge bonds of [Se3Ge(Ga)–(Ga)GeSe3] units may be broken by I atoms to form [Se 3 Ge(Ga)-I]. Therefore, the 214 cm −1 and 174 cm −1 bands which associate with the glass-former [Ge(Ga)Se4] units and [Se3Ge (Ga)–(Ga)GeSe3] ethane-like units decrease. From ref. [15] and ref. [16], the 137 cm−1 and 153 cm−1 new band can be attributed to vibration of [Ga2I7]− unit and [Ge(Ga)I4] units, respectively. It is known that the
qffiffiffiffiffiffiffi ffi vibration frequency of units can be simply proportional to k μ , where k refers to force constant related to bond strength and μ is the reduced mass [16]. Because the vibration frequencies of [Ge(Ga)Se4] and [Ge(Ga)I4] are 202 cm−1 and 153 cm−1, respectively [16,17]. From the theoretical analysis [18], the increase in reduced mass and decreasing coupling strength of [Ge(Ga)IxSe4-x] units, attributed to I atoms, result in their vibration frequencies should shift to a lower wave number than that of [Ge(Ga)Se4] units. As a result, the 168 cm−1 and 190 cm−1 bands which attribute to [Ge(Ga)I3Se] units and [Ge(Ga)ISe3] increase abruptly with increasing Ga2Se3 and CsI addition. Compared with the [Ge(Ga)Se4] vibration units, the appearance of new low-energy-phonon bands mainly result in the τmea increase of 1.49 μm emission. Theoretically, the ionicity of new units containing I atoms could be more than that of [Ge(Ga)Se4]. When Ga2Se3 and CsI increase, the enhancement of new ionicity bands coincides with the variant of Ω2 and Ω6 parameters discussed above. 4. Conclusion The thermal stability, spectroscopic properties and structure of Sm3+doped Ge–Ga–Se–CsI glasses were investigated. The glasses present good thermal stability and are suitable for fiber drawing. The three Judd–Oflet parameters for Sm3+ indicate that the local site symmetry of the glass is high and the covalence of the glass is low. The emission bands located at 1.25 μm and 1.49 μm can be observed and the 1.49 μm fluorescence lifetime increased with increasing Ga2Se3 and CsI addition and these phenomena may be attributed to the lower local vibrational phonon energy of Sm3+ in the glass matrix. The Raman spectra supported this point. The results show that Sm3+-doped Ge-Ga-Se-CsI glasses may be good candidates for infrared emission spectral region. Acknowledgement This work was financially supported by the Shanghai Institute of Ceramics's Science and Technology Innovation project (No. Y05ZC9190G). References
Fig. 7. Raman spectra of Ge–Ga–Se–CsI glasses.
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