Article pubs.acs.org/JPCA
Effect of Fluorine Ions on 2.7 μm Emission in Er3+/Nd3+-Codoped Fluorotellurite Glass Yanyan Guo,†,‡ Ming Li,†,‡ Lili Hu,† and Junjie Zhang†,* †
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China ‡ Graduate School of Chinese Academy of Science, Beijing 100039, PR China ABSTRACT: The 2.7 μm emission properties of Er3+/Nd3+-codoped fluorotellurite glasses were investigated in the present work. The thermal stability, refractive index, absorption and transmission spectra, and emission spectra were measured and investigated. The 2.7 μm emission in Er3+/Nd3+codoped fluorotellurite glasses was enhanced with the increase of fluorine ions. The Judd−Ofelt analysis based on absorption spectra was performed in order to determine the Judd−Ofelt intensity parameters Ωt (t = 2, 4, 6), spontaneous emission probability, radiative lifetime and branching ratios of Er3+:4I11/2 → 4I13/2 transition. It is found that the Er3+/Nd3+-codoped fluorotellurite glass possesses a lower spontaneous transition probability A (58.95 s−1) but a higher branching ratio β (15.72%) corresponding to the stimulated emission of Er3+:4I11/2 → 4I13/2 transition. Additionally, the transmittance was also tested and reached a maximum when the molar concentration of ZnF2 is 15%. The presence of fluorine ions greatly decreases the population of OH groups, which affects the 2.7 μm emission effectively by means of decreasing the rate of energy transfer to impurities (e.g., OH groups). absorption around 3 μm ascribing to stretching vibration of free OH groups and stronger hydrogen bond −O−H···O− are strong in tellurite glasses which is one of the main impact factors of 2.7 μm emission.11 The OH concentration could be decreased in tellurite glass by introducing properly F ions and then the quenching effect on 2.7 μm emission induced by OH groups may be depressed. Moreover, the fluorine ions introduced into the tellurite glass could modify the spectroscopic properties due to the formation of a local bonding environment of oxygen and fluorine with the cations.11,12 In present study, F ions were introduced into Nd3+/Er3+codoped tellurite glass. The thermal stability, refractive index, absorption, mid-infrared transmittance and emission spectra were investigated. Considering the presence of F ions, the structure features from the Raman spectra were investigated to interpret the physical and optical properties of the glasses.
I. INTRODUCTION The investigation of solid state lasers operating in the midinfrared wavelength region (2−5 μm) has been taken widely in the past few years. Many potential applications, such as medical lasers, sensing, military counter-measures as well as light detection and ranging (LIDAR) were offered by the devices operating in the mid-infrared wavelength region.1−3 Light sources in the 2.7 μm region are of great interest owing to the strong absorption by water in this spectral region. Despite the laser characteristics of the 2.7 μm (Er3+: 4I11/2→4I13/2 selfterminating transition) are not satisfactory, Er3+ is an important candidate to provide 2.7 μm emission. Fortunately, codoping of Nd3+ has been demonstrated to enhance the 2.7 μm emission.4,5 In order to achieve strong emission at the 2.7 μm region, the main effort so far has been concentrated on the Er3+-doped fluoride and chalcogenide glasses because of their low phonon energy which decreases the rate of nonradiative transitions.6,7 However, the applications of fluoride and chalcogenide glasses are limited due to their poor chemical durability and thermal stability. The oxide and oxyfluoride glasses possess better characteristics in mechanical strength, thermal stability, and chemical durability than fluoride and chalcogenide glasses and the successful observation of 2.7 μm emission from oxide and oxyfluoride glasses host stand only in a few glass systems up to now.4,8−10 Because of the low phonon energy and good physicochemical properties of tellurite glasses, it is considered to be a new candidate material for 2.7 μm emission. While, the © 2012 American Chemical Society
II. EXPERIMENTS A. Preparation of Glass. The glass samples with molar compositions of 60TeO2−15GeO2−20ZnO−5K2O−1Er2O3 (TE) and 60TeO 2 −15GeO 2 −(20−x)ZnO−(5−y)K 2 O− xZnF2−yKF−1Er2O3−0.5Nd2O3 (TF, x = 0, 5, 10, 15, 20; y = 0, 5) were prepared by conventional melting and quenching Received: February 17, 2012 Revised: May 16, 2012 Published: May 23, 2012 5571
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method. Mixed batches of ∼30 g were melted in a alumina crucible at 1150 °C for about 30 min. Then the melts were poured into stainless-steel molds and annealed for 10 h near glass transition temperature. Samples were cut into a rectangular (20 × 20 × 2 mm3) shape and optically polished. B. Measurements. Glass density (ρ) was measured by the Archimede’s method using distilled water as immersion liquid with maximum error of ±2%. Refractive indices of 633 and 1064 nm were measured by a Spectro-Ellipsometer (Woollam W-VASE, error limit ±0.05%). The glass transition temperature (Tg) and crystallization beginning temperature (Tx) were tested by differential scanning calorimetry (DSC) measurement. Raman spectra in the range of 100−1000 cm−1 and 1000− 2000 cm−1 were measured with a FT Raman spectrophotometer (Nicolet MODULE) using the 788 nm excitation line from a spectra physics laser. Midinfrared transmittance spectra were measured with a Perkin-Elmer 1600 series Fourier transform infrared (FTIR) spectrometer in a wavenumber region between 4000 and 2500 cm−1. Absorption spectra were tested in the range of 300−1600 nm with a Perkin-Elmer Lambda 900UV/VIS/NIR spectrophotometer with 1 nm steps. The lifetime of Er: 4I13/2 level was measured by the FLSP920 fluorescence spectrophotometer (Edinburgh Analytical Instruments Ltd.) pumped by 808 nm LD. Emission spectra in the range of 1400−1700 nm and 2550−2820 nm were obtained by a computer-controlled TRIAX 320 type spectrometer pumped by 808 nm laser diode (LD). In order to accurately compare the intensity of 2.7 μm emission, the position and power of 808 nm LD and the width of the slit to collect signal were fixed to the same condition and the samples were set at the same place in the experimental setup. All measurements were carried out at room temperature.
conventional 808 nm laser diode (LD) because of the overlap of Er3+: 4I15/2 → 4I11/2 and Nd3+: 4I9/2 → 4F5/2, 2H9/2 transitions. The radiative transition within the 4fn configuration of a rare earth ion can be analyzed by Judd−Ofelt theory.13 The Judd− Ofelt parameters were obtained from the measured absorption spectra excluding the absorption from the ground state 4I15/2 to the 4I13/2 level. According to previous studies, the value of Ω2 is related with the symmetry of the glass while the value of Ω6 is inversely proportional to the covalency of Er−O bond. Additionally, the values of Ω4 and Ω6 are related to the PH value of host materials. The larger the PH value, the lower the values of Ω4 and Ω6. The value of Ω4/Ω6 is an important parameter to predict the excited emission in a laser active host.8 The larger the Ω4/Ω6 favors, the more desired the laser transition. The Ωt parameters, the spontaneous emission probabilities (A), branching ratios (β), and calculated radiative lifetime for 4I13/2 and 4I11/2 are listed in Table 1. With the presence of F ions, the values of Ωt (t = 2, 4, 6) are increased. It is possible to infer that the covalency of Er−O is decreased and the PH value of host material is decreased by the presence of fluorine ions. With the increase of fluorine ions, the values of Ω4/Ω6 were increased which hints the enhanced 2.7 μm emission. According to the Judd−Ofelt theory, the line strength of the electric dipole components (Sed) of 2.7 μm emission can be calculated as Sed(4I11/2 → 4 I13/2) = ⟨|| U (2) ||⟩ × Ω 2 + ⟨|| U (4) ||⟩Ω4 + ⟨|| U (6) ||⟩ × Ω6 = 0.021Ω 2 + 0.11Ω4 + 1.04Ω6
III. RESULTS AND DISCUSSIONS A. Absorption Spectra and Judd−Ofelt Analysis. The absorption spectra of Er3+/Nd3+-codoped fluorotellurite glasses are shown in Figure 1 and the spectra of all samples are similar; no shifts of the wavelength was found in the absorption peaks. The absorptions bands belong to the transition of Er3+ and Nd3+ ions from the ground state to the labeled levels are shown in Figure 1. The strong absorption at around 808 nm indicates that these glasses can be excited quite efficiently by a
(1)
which is a function of glass structure and composition.14 It can be found that large Ωt will produce a large Sed value. And the large Sed indicates increased electric-dipole transition which brings to flat emission spectra. In addition, with the increase of fluorine ions, the calculated values of τ(4I11/2) and τ(4I13/2) increased. However, according to the energy transfer processes prescribed in previous researches, the values of τ(4I13/2) were quenched greatly by the presence of Nd3+ ions.4,5,8 The experimental values of τ(4I13/2) of Er3+/Nd3+-codoped fluorotellurite glasses were about 400 μs (shown in Figure 2). The energy transfer efficiency ηt of Er3+:4I13/2 + Nd3+:4I9/2 → Er3+:4I15/2 + Nd3+:4I15/2 transition can be evaluated from the lifetime values by the following equation:9 ηt = 1 −
τEr/Nd τEr
(2)
Here τEr/Nd and τEr are the lifetimes of the Er : I13/2 level with and without Nd3+ ions, respectively. The value of ηt is up to 81.5%. The slight decrease of ηt could be attributed to the impact of the variation of refractive index and the density caused by the increased fluorine ions. The high ηt demonstrates that Nd3+ ions can efficiently quench the lower level of Er3+ ions for 2.7 μm emissions. B. Physical Properties. The thermal stability was characterized by the value of ΔT (Tx− Tg), where Tg is the glass transition temperature and Tx is the onset crystallization temperature. A high Tg gives glass good thermal stability to resist thermal damage at high pumping intensity. ΔT presents 3+ 4
Figure 1. Absorption spectra of Er3+/Nd3+-codoped fluorotellurite glasses. 5572
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Table 1. Judd−Ofelt Intensity Parameters of Er3+ Ions, the Branching Ratio, the Spontaneous Emission Probabilities, Calculated Radiative Lifetime for 4I13/2 and 4I11/2 Levels, and Measured Lifetime of 4I13/2 Level in Er3+-Doped and Er3+/Nd3+Codoped Fluorotellurite Glasses parameters −20
Ω2 (×10 cm ) Ω4 (×10−20 cm2) Ω6 (×10−20 cm2) Ω4/Ω6 δ (×10−6) Sed(4I11/2 → 4I13/2) A(4I11/2 → 4I13/2) (s−1) β(4I11/2 → 4I13/2) (%) A*β(4I11/2 → 4I13/2) τ(4I11/2) (calculated) (ms) τ(4I13/2) (calculated) (ms) τ(4I13/2) (experimental) (μs) ηt (%) 2
TE
TF1
TF2
TF3
TF4
TF5
TF6
4.38 3.05 1.04 2.93 0.46 1.51 50.02 18.36 918 3.67 3.95 1.76 (ms)
8.32 4.19 2.43 1.72 0.16 3.16 88.04 14.21 1251 1.61 2.04 325 81.5
7.65 3.48 1.44 2.41 0.81 2.04 64.31 15.66 1007 2.44 2.96 365 79.3
7.66 3.44 1.55 2.22 0.73 2.15 63.56 15.44 981 2.43 2.97 411 76.6
7.26 3.74 1.38 2.71 0.59 2.00 64.31 15.66 1007 2.44 2.96 419 76.2
7.74 3.58 1.48 2.42 1.03 2.10 58.95 15.72 927 2.67 3.23 443 74.8
7.79 3.94 1.72 2.29 1.1 2.38 64.81 15.35 995 2.37 2.89 450 74.4
Because the polarization of fluorine ions (F) is much lower than that of oxygen ions (O2), the n decreases with the increase of fluorine ions concentration. In Table 2, except the TF2 sample, the fluorotellurite glasses maintain the good thermal stability. For the 60TeO2−15GeO2−(20−x)ZnO− xZnF2−(5−y)K2O−yKF glasses, the ΔT and KH are considerable with 60TeO2−15GeO2−20ZnO−5K2O glass. The value of ΔT approaches to 176 in TF3 sample indicating that the thermal stability against crystallization is good. Additionally, large ΔT indicates a wide working-temperature range for high quality fiber drawing. Figure 3 shows the Raman spectra of fluorotellurite glasses. All the spectra were normalized by the intensities of 820 cm−1 and a least-squares fit was made for the Raman spectra of TF1 sample, assuming a Gaussian shape for each Raman band. As can be seen from Figure 3a, six individual modes are distinguished for all the glasses: five medium peaks around 129, 172, 293, 443, and 677 cm−1 and one strong peak around 767 cm−1. And it is clear in Figure 3b that the peaks do not shift with the increase of fluorine ions. It is well-known that Raman peaks around 725−780 and 305 cm−1 are attribute to the [TeO3] groups and these around 650−670 and 455 cm−1 are attribute to the [TeO4] groups in tellurite glasses. The fluorine ions replace bridging-oxygen (BO) ions in the [TeO3] and [TeO4] groups and these groups convert to [Te(O, F)3] and [Te(O, F)4] groups with bridging-fluorine (BF).15 The network structures are composed of the mixed [Te(O, F)3] and [Te(O, F)4] groups in fluorotellurite glasses. It can be concluded that the peaks around 767 and 293 cm−1 are assigned to the [Te(O, F)3] groups and the peaks around 677 and 443 cm−1 are assigned to the [Te(O, F)4] groups in Figure 1a. Because of the stronger heteropolar bond strength of fluorine ions than oxygen ions, the BF bond will stabilize the chain structure while two
Figure 2. Luminescence decay curve of 1535 nm emission in Er3+/ Nd3+-codoped fluorotellurite glasses.
the temperature interval during the nucleation and higher value of ΔT indicates the potential for fiber fabrication without crystallization. Hruby’s parameter KH = (Tc − Tg)/(Tm − Tc) evaluates glass forming ability, where Tc is the crystallization temperature and Tm is the melting temperature.13 According to this Hruby’s equation, the larger KH is, the stronger inhibition to nucleation and crystallization processes is, and consequently, the better glass forming ability is. The refractive index (n), density (ρ), characteristic temperatures and thermal parameters of fluorotellurite glasses were listed in the Table 2. It is clear from Table 2 that the values of n decrease while the values of ρ increase with the increase of fluorine ions. The n of glasses is related with the polarization in glasses. The lower the polarization, the lower the value of n.
Table 2. Refractive Index, Density, Characteristic Temperatures, and Thermal Parameters of Fluorotellurite Glasses samples
n (λ = 633 nm)
ρ (g/cm3)
N (1020 cm−3)
Tg (°C)
Tx (°C)
Tc (°C)
ΔT (°C)
KH
TE TF1 TF2 TF3 TF4 TF5 TF6
1.8875 1.9198 1.9206 1.8939 1.8707 1.8718 1.8639
4.730 4.7302 4.8664 4.8972 4.8273 4.7862 4.8047
4.229 4.198 4.377 4.368 4.271 4.270 4.184
421 372 338 386 388 384 381
561 540 443 562 559 532 531
590 577 462 582 587 570 558
140 168 105 176 171 148 150
0.302 0.358 0.180 0.345 0.353 0.320 0.299
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Figure 3. Peak deconvolution of the Raman spectra of TF1 sample (a); Raman spectra of Er3+/Nd3+-codoped fluorotellurite glasses (b).
Table 3. Absorption Coefficient αH of Er3+-Doped and Er3+/Nd3+-Codoped Fluorotellurite Glasses sample
TE
TF1
TF2
TF3
TF4
TF5
TF6
αOH (cm−1)
0.990
0.466
0.544
0.291
0.204
0.181
0.196
Figure 4. Mid-infrared transmittance: the trends of (a) the highest and (b) the lowest transmittance of Er3+/Nd3+-codoped fluorotellurite glasses.
[Te(O, F)4] groups are connected by the BF bond. It can be found that all the peaks decrease in intensity as the fluorine ions increase, especially the peak around 767 cm−1. The mixed fluorine and oxygen anions are presented in this fluorotellurite glasses system, and the vibration strength of symmetry ligand field decreases as increasing fluorine ions. With the increased concentration of fluorine ions, the ratio of [Te(O, F)4] groups to the [Te(O, F)3] and [Te(O, F)4] groups increases and the stability in these glasses becomes better, as evidenced by the changes in characteristic temperatures. With conventional melting processes, the glasses containing little OH groups cannot be prepared without an elaborate setup. A simple way of removing the OH groups from the glass during melting process is to add a fluoride to liberate hydrogen species from the melt.15 The added fluorine ions will react with OH groups according to eq 3:16
2[Te−OH] + 2F− → Te−O−Te+O2 − + 2HF(g)↑
(3)
The vibration of OH groups (2700−3700 cm−1) matches with the energy gap between the Er:4I11/2 → 4I13/2 transition (about 3650 cm−1). The introducing fluorine ions deplete the OH groups which participates in the energy transfer (ET) of rare-earth ions. The upper level (Er3+:4I11/2) could transfer to the lower level (Er3+:4I13/2) by nonradiative transition easily with the assistance of OH groups.9,17 The content of OH groups in the glass can be evaluated by the absorption coefficient αOH of the OH vibration band at 3 μm (listed in Table 3), which can be given by αOH = ln(T0/T )/l 5574
(4)
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Table 4. Calculated Rates of Radiative Transition (WR), Energy Transfer (WET), and Multiphonon Relaxation (WMPR) and the Calculated Total Rates (1/τmea) in Er3+-Doped Tellurite Glass and Er3+/Nd3+-Codoped Fluorotellurite Glasses sample
TE
TF1
TF2
TF3
TF4
TF5
TF6
WR (s−1) WET (s−1) WMPR (s−1) 1/τmea (s−1)
50.02 586.1 3.477 640.2
88.04 273.9 3.477 365.4
64.31 333.3 3.477 401.1
63.56 178.0 3.477 245.0
64.31 121.9 3.477 189.7
58.95 108.3 3.477 170.7
64.81 114.8 3.477 183.1
Figure 5. 1.5 μm (a) and 2.7 μm emission (b) spectra of Er3+-doped tellurite glass and Er3+/Nd3+-codoped fluorotellurite glasses.
energy transfer to impurities (i.e., OH) and multiphonon relaxation, respectively. Since the rare earth concentrations in present glasses do not change clearly with the increase of fluoride ions, the WCR of each sample can be thought of roughly equal. The WET is proportional to the concentration of Er3+ ions (NEr) and the measured content of OH groups (αOH) which can be defined as18,19
where l is the thickness of the sample equal to 2 mm, and the T0 and T are the incident and transmitted transmittance intensities, respectively. The mid-infrared transmittance spectra, the highest transmittance (around 2730 nm) and the lowest transmittance (around 3100 nm) of fluorotellurite glasses are shown in Figure 4 and the value of αOH is shown in Table 3. It can be found that TF5 sample has the highest maximum transmittance (approach to 80%). As shown in Figure 4, the transmittance of glass decreases with a small amount introduction of fluorine ions and it increases clearly with the increase of fluorine ions. It can be demonstrated by the effect of introduced fluorine ion on the structure of glass: a small amount introduction of fluorine ions will destroy the stablestructure of tellurite glass and the equilibrium between fluorine and oxygen ions were achieve with the increase of fluorine ions. Then, the glasses’ structure tends to be much more stable and homogeneous, which is registered as better thermal stability and transmission and which matches with the variation of [Te(O, F)4] groups. It can be deduced from Table 3 that the introduced fluorine ions can eliminate the OH content and The TF5 sample possesses the lowest αOH in all samples. The excellent properties hints that the TF5 sample might possesses better emission properties especially 2.7 μm emission. C. Radiative Properties. Generally, considering the nonradiative processes, the total rate (1/τmea) of the 4I11/2 → 4 I13/2 transition can be evaluated by the reciprocal of the measured fluorescence lifetime (τmea) which is given by 1 τmea
= WR + WCR + WET + WMPR
WET = WOH = KOH − ErNErαOH
(6)
where KOH‑Er is a constant which is determined by interactions between Er3+ ions and OH groups and is independent of the concentrations of Er3+ ions and OH groups. The KOH‑Er is 14 × 10−19 cm4·s−1 in tellurite glasses. The multiphonon relaxation rate WMPR is a function of temperature, which can be expressed as WMPR(T ) = WMPR(0)[1 − e−ℏwmax / kT ]−p = Ce−αΔE[1 − e−ℏwmax / kT ]−p
(7)
where C and α are positive-definite constants related to the host materials, and ΔE is the energy gap between the 4I11/2 and 4 I13/2 levels. The values of C and α in tellurite glasses have been determined previously to be C = 4.2 × 107 s−1 and α = 4.5 × 10−3 cm.20 The ℏwmax is the highest phonon energy obtained from Raman spectra and p=ΔE/ℏwmax. The calculated WR, WET, WMPR, and 1/τmea are listed in Table 4. It is found that the value of WET decreases greatly with the increase of fluorine ion concentration and the TF5 sample possesses the lowest value of WET. As the radiatively low value of WMPR and WR, the value of 1/τmea is greatly influenced by the changes of WET. Therefore, it can be concluded that the trends of 2.7 μm emission is similar to that of the transmission around 3.0 μm. D. Optical Spectra. The 1.5 and 2.7 μm emission spectra of Er3+-doped tellurite glass and Er3+/Nd3+-codoped fluorotellur-
(5)
where WR is the radiative transition rate obtained from Judd− Ofelt theory (the spontaneous emission probability A); WCR is the rate of cross-relaxation, and WET and WMPR are the rate of 5575
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ite glasses are shown in Figure 5. It is found that the 1.5 μm emission is considerably quenched and the 2.7 μm emission is considerably enhanced by the presence of Nd3+ ions. It is also found that the TF5 sample presented the highest 2.7 μm emission and the variation tendency of 1.5 and 2.7 μm emission are similar. According to the energy transfer mechanism elaborated in previous papers,4,8 there is a closed correlation between the 1.5 and 2.7 μm emission. The 4I13/2 level, which is the lower level of and the upper level of 1.5 μm emission, is one of main factors influencing the property of 2.7 μm emission. Under the same energy transfer mechanism, high 2.7 μm emission will lead to high 1.5 μm emission. Therefore, the variation tendency of 1.5 μm emission is similar to that of 2.7 μm emission with the increase of fluorine ions concentration. On the basis of the analysis of radiative properties, the 2.7 μm emission is closely associated with the transmission around 3.0 μm, which is related to the content of OH groups. It can be concluded that the added fluorine ions affect the 2.7 μm emission greatly by reducing the content of OH groups.
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IV. CONCLUSION Generally, the physical and optical properties of Er3+/Nd3+codoped fluorotellurite glasses were investigated, especially the 2.7 μm emission. On the basis of the absorption spectra, the Judd−Ofelt and radiative parameters were calculated. According to the analysis of Raman spectra, the phonon energy of fluorotellurite glasses did not change and the glass-network structures of fluorotellurite glasses were composed of [Te(O, F)3] and [Te(O, F)4] groups which gives the evidence of changes of thermal properties. With the increase of fluorine ions, the content of OH groups decreased as well as the values of WET decreased. The value of 1/τmea, which is greatly affected by the value of WET, is an important factor for 2.7 μm emission. As a result, the 2.7 μm emission is greatly enhanced by the presence of fluorine ions and the TF5 sample possesses the strongest 2.7 μm emission while the molar concentration of ZnF2 is 15%. The improved thermal, raditive and 2.7 μm emission properties in fluorotellurite glasses indicates that the presence of fluorine ions is good for 2.7 μm emission and the Er3+/Nd3+-codoped fluorotellurite glasses is promising for midinfrared laser devices.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86 21 5991 4297. Fax: +86 21 5991 4516. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 51172252).
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REFERENCES
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