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Enhanced 2.7 m emission from Er3+ doped oxyfluoride tellurite glasses for a diodepump mid-infrared laser
F. F. Zhang, W. J. Zhang, J. Yuan, D. D. Chen, Q. Qian, and Q. Y. Zhang
Citation: AIP Advances 4, 047101 (2014); doi: 10.1063/1.4870581 View online: http://dx.doi.org/10.1063/1.4870581
View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/4?ver=pdfcov Published by the AIP Publishing
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AIP ADVANCES 4, 047101 (2014)
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Enhanced 2.7 //m emission from Er3+ doped oxyfluoride tellurite glasses for a diode-pump mid-infrared laser
F. F. Zhang,1 W. J. Zhang,2 J. Yuan,1 D. D. Chen,1 Q. Qian,1 and Q. Y. Zhang1a
1 State Key Laboratory of Luminescence Materials and Devices, and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, People's Republic of China
2 Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, People's Republic of China
(Received 29 January 2014; accepted 25 March 2014; published online 2 April 2014)
The influence of fluoride and shielding gas (O2 or Ar) on the physical and spectroscopic properties of Er3+ doped TeO2-ZnO-ZnF2 glass system is investigated. The larger electronegativity of F than O accounts for the gradual decrease of refractive index, density, and J-O parameters with increasing ZnF2. An analysis on Fourier transform infrared transmission spectra reveals that the absorption coefficient of OH-around 3 ¡m as low as 0.247 cm-1 can be achieved when 30 mol% ZnF2 containing sample is treated with Ar gas during glass melting process. The reduction of OH-groups combined with the low multiphonon relaxation rate (207 s-1) contributes to the enhanced emissions at 1.5 and 2.7 ¡m, along with prolonged lifetimes of 4I11/2 and4I13/2 levels. A high branching ratio (17.95%) corresponding to theEr3+: 4I11/2 ^ 4I13/2 transition, the large absorption and emission cross section (0.44 x 10-20 cm2 and 0.45 x 10-20 cm2), and good gain cross section demonstrate that oxyfluoride tellurite glass could be a promising material for a diode-pump 2.7 ¡m fiber laser. © 2014 Au-thor(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/L4870581]
I. INTRODUCTION
Recently, considerable interest has been aroused concerning the development of mid-infrared lasers operating in the 3-¡m region, where water exhibits intense absorption, thus making them suitable for enormous potential applications including laser surgery, military optoelectrionic coun-termeasure, and remote sensing, etc.1,2 Lasing at ~2.7 ¡m has been achieved in various Er3+ doped crystals, ceramics and ZBLAN fibers taking advantage of Er3+: 4I11/2 ^ 4I13/2 radiative transition, as depicted in Fig. 1.3 However, the quite narrow energy gap between 4I11/2 and 4I13/2 states (about 3600 cm-1) makes such radiative transition susceptible to nonradiative decays with larger probability for hosts with higher phonon energy. Thus, intense 2.7 ¡xm emission can only be obtained in host with low phonon energy such as fluoride, chalcogenide, fluorophosphate and germinate glasses.4-7 Among them, tellurite glass has attracted a great deal of interest not only for its relatively lower maximum phonon energy (about 700 cm-1), but also for the improvement in chemical and mechanical stability as well as the high refractive index, excellent infrared transmissivity.8 On the other hand, the residual hydroxyl groups (OH-) in glass could be the quenching center for 2.7 ¡m emission as the vibrational energy of OH- is comparable to the energy difference between 4I11/2 and 4I13/2 level. For this reason, several approaches have been proposed to remove OH- groups from the glass, which usually involve melting glass in vacuum or dry-air-bubbling through molten
1 Author to whom correspondence should be addressed; electronic mail: qyzhang@scut.edu.cn
2158-3226/2014/4(4)/047101/11
4, 047101-1
) Author(s) 2014 i
FIG. 1. Energy level schemes of Er3+ and OH .
glass.9 Additionally, it has been demonstrated that the content of OH- groups can be reduced with the addition of fluorine ions.10
In the present study, a dry technique combining the addition of fluoride with shielding gas (O2 or Ar) has been exploited to remove OH- and enhance the mid-infrared emission in TeO2-ZnO-ZnF2 glass system. The dehydrating results were evaluated based on the Fourier transform infrared (FTIR) transmission spectra. Furthermore, the influence of ZnF2 content and/or shielding gas on the physical and spectroscopic properties of Er3+-doped samples has been investigated in detail.
II. EXPERIMENTS
Oxyfluoride tellurite glasses with the nominal compositions of (80-x)TeO2-20ZnO-xZnF2-Er2O3 (x = 0, 5,10,15,20,25, 30) in mol% were prepared in air atmosphere by traditional melt-quenching technique. It is worthwhile noting that the glass tends to devitrify when x is beyond 30. The obtained glass samples were correspondingly denoted as TZF0, TZF5, TZF10, TZF15, TZF20, TZF25 and TZF30. For the sake of comparison, shielding gas (O2 or Ar) was then applied to TZF30 sample for the final 20 minutes of melting process. The samples were hereafter labeled as TZF30-O2 and TZF30-Ar. High purity reagents of TeO2 (5N), ZnO (AR), ZnF2 (AR), and E^O3 (4N) were used as raw materials. Each 20-g batch was well mixed in an agate mortar and melted in a
_l_i_I_._I_I_I_._I_i_I_I_I_
400 600 800 1000 1200 1400 1600 Wavelength (nm)
FIG. 2. Absorption spectra of Er3+ doped glasses.
platinum crucible at 850 °C for 40 minutes. Then, the melts were poured onto a preheated stainless steel plate followed by annealing at a temperature close to the glass transition temperature for 2 hours. The annealed specimens were cut and well polished into about 1.5 mm thick for subsequent measurements.
The optical absorption spectra were measured on a Perkin-Elmer Lambda 900/UV/VIS/NIR spectrophotometer. The refractive index was obtained using the prism coupling method (Metricon Model 2010M) and the density was determined by the Archimedes' liquid immersion method. The FTIR transmission spectra were acquired on a Vector-33 FTIR spectrometer (Bruker, Switzerland). The Raman spectrum was tested by Renishaw inVia Raman spectrometer. For fluorescence spectra in the range of 2400-3000 nm, an Edinburgh FLS920 fluorescence spectrometer was utilized equipped with the liquid nitrogen cooled InSb detector and a 980 nm laser diode (LD) excitation source. The 1400-1700 nm fluorescence spectra pumped by 980 nm LD and lifetime measurement were performed on a Triax 320 type spectrometer (Jobin-Yvon Corp.) with the InGaAs detector. Additionally, the luminescence decay curves were captured by a Tektronix TDS 3012c Digital Phosphor Oscilloscope with pulsed 980 and 808 nm LDs. All the measurements were carried out at room temperature.
III. RESULTS AND DISCUSSIONS
A. Absorption spectra and J-O analysis
Judd-Ofelt (J-O) theory is widely used to analyze the radiative transition within 4f n configuration of rare-earth (RE) ions. The intensity parameters and reflecting the local structure and
bonding in the vicinity of RE ions, can be calculated from absorption data and refractive index.11,12 The values of reduced matrix elements U(A) used in the J-O calculation are quoted from Ref. 13.
TABLE I. Refractive index (n) at 633 nm, density (p), ionic concentration (NEr3+), and J-O intensity parameters (^2, ^6) for Er3+ doped glasses.
Parameter TZF0 TZF5 TZF10 TZF15 TZF20 TZF25 TZF30
n (I = 633 nm) 2.0246 1.9827 1.9568 1.9345 1.9068 1.8772 1.8502
p (g/cm3) 5.349 5.292 5.276 5.268 5.252 5.238 5.185
NEr3+ (x 1020 cm-3) 4.3576 4.3949 4.4682 4.5516 4.6308 4.7157 4.7681
(x10-20 cm2) 5.72 5.83 5.78 5.57 5.23 4.31 3.78
(x10-20 cm2) 1.62 1.67 1.65 1.61 1.54 1.34 1.28
(x10-20 cm2) 1.04 1.05 1.04 1.01 1.0 0.92 0.91
^4/^6 1.558 1.590 1.587 1.594 1.54 1.457 1.407
S (x10-6) 0.10 0.12 0.12 0.11 0.12 0.10 0.11
Fig. 2 illustrates the visible and near-infrared absorption spectra of Er3+-doped glass samples. Six absorption bands corresponding to the intrinsic transitions from the ground state 4I15/2 level to the excited levels 4I11/2, 4I9/2, 4F9/2,4S3/2,2H11/2, and 4F7/2 were utilized for the calculation of oscillator strengths, as marked in Fig. 2. The detailed calculation process can be seen elsewhere.14 The relevant parameters and results are listed in Table I.
As can be seen from Table I, both density and refractive index decrease with the addition of ZnF2. It is accepted that fluorine is more electronegative than oxygen and usually considered as a powerful network disrupter. The disruption of glass network occurs as one bridging oxygen is substituted by two fluorine ions, contributing to the formation of non-bridging oxygens. Thus, the decline of density is ascribed to the more loosely packed glass network with increasing ZnF2 content. According to the Gladstone-Dale formula, the refractive index (n) is given by n = R/V-1, where R is the molecular refraction and V is the molecular volume.15 It means that the increment in molecular volume brought by fluorine ions and the smaller molecule refraction of fluorine ion relative to oxygen ion would lead to the decrease of refractive index with increasing fluorine ion concentration, which is well affirmed by the results in Table I.
At the same time, the calculated J-O parameters (^2, and ^6) also exhibit a downward trend as ZnF2 content increases from 5 to 30 mol%. The abrupt fluctuations between TZF0 and TZF5 might be caused by the compositional change from binary to ternary glass system. In principle, parameter is very sensitive to the covalence between RE ion and its ligand anions. The higher electronegativity of F (4.0) compared to that of O (3.44) makes the Er-F bond more ionic than Er-O bond, which is responsible for the decrease of versus the addition of ZnF2. Parameter is known to be proportional with the rigidity of host glass.16 The slight decrease of from TZF5 to TZF30 indicates the lessening mechanical properties of glass samples. As for the value of it
is observed to decrease slightly with the increasing ZnF2 content, but it is still approximate to that of ZBLAN glass (1.52).4
According to the obtained J-O intensity parameters, radiative transition probabilities Arad, radiative lifetimes trad and branching ratios j for different samples were calculated and tabulated in Table II. Arad of Er3+: 4I11/2 ^ 4I13/2, 4I11/2 ^ 4I15/2, and 4I13/2 ^ 4I15/2 transitions depends largely on and because the reduced matrix elements U4 and U(6) are much larger than U2 in the mentioned transitions.13,17 In this case, the decrease of Arad with increasing ZnF2 content, as shown in Table II, is consistent with the drop tendency in and Additionally, Arad is also affected by the refractive index, which could account for the case that TZF5 with marginally larger
and still has a comparatively smaller Arad than TZF0. On the other hand, the calculated fluorescence branching ratio j of the 4I11/2 ^ 4I13/2 transition increases from 16.13% to 17.95% with the increment of ZnF2, which is favorable to the 2.7 /m fluorescence. The value of j (4I11/2 ^ 4I13/2) as large as 17.95% is greater than that in other glasses reported before.5-7 Moreover, radiative lifetimes for 4I11/2 and 4I13/2 are found to be prolonged from TZF0 to TZF30 due to the reducing of
TABLE II. Calculated transition probabilities of electric dipole (Aed) and magnetic dipole (Amd), total spontaneous radiative probabilities (Arad), branching ratios (fi), and radiative lifetimes (trad) of Er3+ in glass samples.
Sample Initial state Final state Aed (s-1) Amd (s 1) Arad (s 1) P (%) Trad (¡s)
4I13/2 4l15/2 213.31 84.09 297.40 100 3363
TZF0 4t 4l15/2 294.00 - 294.00 83.87 2853
I11/2 4l13/2 33.52 23.02 56.54 16.13
4I13/2 4l15/2 200.01 78.97 278.98 100 3584
TZF5 4t 4l15/2 275.60 - 275.60 83.85 3043
I11/2 4l13/2 31.45 21.62 53.07 16.15
4I13/2 4l15/2 188.79 75.91 264.70 100 3778
TZF10 4I 4l15/2 260.25 - 260.25 83.72 3218
i11/2 4l13/2 29.68 20.78 50.46 16.24
4I13/2 4l15/2 175.91 73.35 249.26 100 4012
TZF15 4 4l15/2 242.00 - 242.00 83.52 3451
I11/2 4l13/2 27.66 20.08 47.73 16.48
4I13/2 4l15/2 164.26 70.24 234.50 100 4264
TZF20 4I 4l15/2 224.39 - 224.39 83.30 3712
i11/2 4l13/2 25.77 19.22 44.99 16.70
4I13/2 4l15/2 141.29 67.02 208.31 100 4800
TZF25 4 4l15/2 190.10 - 190.10 82.47 4338
I11/2 4l13/2 22.07 18.35 40.42 17.53
4I13/2 4l15/2 131.52 64.17 195.69 1.00 5110
TZF30 4I 4l15/2 173.90 - 173.90 82.05 4719
i11/2 4l13/2 20.47 17.57 38.04 17.95
34567 34567
Wavelength (|jm) Wavelength (pm)
FIG. 3. FTIR transmission spectra of samples as a function of (a) ZnF2 content and (b) shielding atmosphere.
B. FTIR transmission spectra
Fig. 3 shows the FTIR transmission spectra of all samples. It is obviously displayed that oxyfluo-ride tellurite glasses, as attractive mid-infrared fiber materials, have excellent optical transparency up to 6 ¡m. As is intuitively observed in Fig. 3(a), the absorption bands become remarkably flatter with increasing ZnF2, implying the reduction of OH- content. The broad absorption bands are attributed to the various existing statuses of OH- in the glass matrix. More specifically, the stronger one around 3 ¡m is originated from the stretching vibration of free OH- and the weaker one around 4.4 ¡m is supposed to be associated with the presence of stronger hydrogen bond =Te-O-H... -O-Te=.18,19
047101-6 TABLE III.
Evaluated aoH- and Noh- of all samples.
Sample TZF0 TZF5 TZF10 TZF15 TZF20 TZF25 TZF30 TZF30-O2 TZF30-Ar
NOH-(x 1018cm-3) 39.602 36.169 27.611 20.157 17.214 14.455 12.163 4.806 3.028
aOH- (cm-1) 3.230 2.950 2.252 1.644 1.404 1.179 0.992 0.392 0.247
The absorption coefficient (aOH ) and concentration of OH (N0h ) in the glass network can be evaluated with the following two equations:
aoH- = —h (1)
NOH- = -aoH- (2)
where l is the thickness of the sample (cm), T0 is the maximum transmittance, T is the transmittance around 3 ¡m, NAV is the Avogadro constant (6.02 x 1023 mol-1), and e is the molar absorptivity corresponding to OH- in silicate glasses (49.1 x 103 cm2/mol).20
Table III shows the calculated aOH- and NOH- in different samples. A continuous diminution of residual OH- with increasing ZnF2 is clearly observed, which could be attributed to the presence of following processes:
f \ / \
— Te — OH + ZnF2 o — Te — O — Te —
1 1 1
-ZnO+lHFÎ
ZnF2 + H2O ^ ZnO + 2HF t (4)
The isoelectronic property and similarity in ionic size between OH- ion and F- ion make it effortless for F- ions to replace OH- ions during melting.21 Based on the results in Fig. 3(b) and Table III, it can be declared that the simultaneous utilization of ZnF2 and the shielding gas could bring about a better dehydration result, which can be associated with the depressed incorporation of environmental H2O and the facilitated evaporation of OH- from the melt into outside environment. Moreover, aOH- of TZF30-Ar is smaller than that of TZF30-O2, which suggests that the above reactions proceed more sufficiently in Ar atmosphere compared to O2 atmosphere.19 Actually, NOH-for TZF30-Ar (3.028 x 1018 cm-3) is approximately 13 times smaller than the value (39.602 x 1018cm-3) found in TZF0 glass. Besides, the minimal aOH- (0.247 cm-1) is much lower in comparison with other tellurite glasses reported before,22, 23 demonstrating that the simultaneous utilization of fluoride and shielding gas is a more effective method to extract OH- out of the mid-infrared laser glass during the fabrication process.
C. Raman spectra analysis
Fig. 4 exhibits the Raman spectrum of undoped TZF30 glass. The peaks located at around 411, 665, 775 cm-1 are typically assigned to the bending vibrations of Te-O-Te linkages, the Te-O stretching vibrations of TeO4 trigonal bipyramid (tbp) units, and the Te-O stretching vibrations of TeO3 trigonal pyramid (tp) units, respectively.20,24 As is well known, the maximum peak position in the Raman spectrum stands for the largest phonon energy (h«max) of the glass matrix. It is noted that theh«max in TZF30 glass is about 775 cm-1. Then, the multiphonon relaxation rate Wmp can be calculated from the following equation:
Wmp(T) = Cp exp(-a AE)[1 - exp(-^m^)]-p (5)
3500 3000
^ 2500 ■
^ 2000 >
» 1500
1000 500
200 400 600 800 1000 1200 Raman shift (cm1)
FIG. 4. Raman spectrum of undoped TZF30 glass.
2400 2500 2600 2700 2800 2900 3000 1400 1450 1500 1550 1600 1650 1700 Wavelength (nm) Wavelength (nm)
FIG. 5. Emission spectra of the Er3+ doped samples in the (a) 2.7 ßm and (b) 1.5 ßm region excited by 980 nm LD.
Excited by 532 nm laser
where p is the number of phonons required to bridge the energy gap AE and p = AE/hrnmcx, kB is Boltzmann's constant, and Tis the temperature. Constants a and Cp depending on the glass host are given in Ref. 25. The Wmp in TZF30 glass is calculated to be 207 s-1, which is slightly larger than fluoride glass (7.24 s-1),26 but significantly smaller than that of germanate glass (9.8 x 103 s-1)7 and sillicate glass (7.13 x 104 s-1).26 This result indicates that the phonon energy of oxyfluoride tellurite glasse is low enough to facilitate the radiative transition from 4I11/2 to 4I13/2 level.
D. Emission spectra, lifetime and cross section
Fig. 5 depicts the emission spectra of the samples around 2.7 and 1.5 ^.m pumped by 980nmLD. Noticeable enhancement in emission intensities at 2.7 and 1.5 fim is found with increasing ZnF2. Besides, TZF30-Ar and TZF30-02 release even more intense fluorescence than TZF30. As examined in Fig. 1, the stretching vibration energy of OH- falls in the wavenumber range of 2500-3600 cm-1,
FIG. 6. Luminescence decay curves monitored (a) at 1530 nm (4I13/2 level) pumped by 980 nm pulsed LD and that (b) at 980nm (4I1 1/2 level) pumped by 808 nm pulsed LD.
TABLE IV. Measured lifetimes of 4In/2 and 4I13/2 levels acquired from the decay curves for different samples.
Sample
t(4I13/2) (ms) T (4I11/2) (xs)
TZF0 TZF5 TZF10 TZF15 TZF20 TZF25 TZF30
2.71 2.94 3.40 3.97 4.46 5.12 5.87
149 165 197 238 309 395 593
while the energy gap between 4I13/2 and 4I15/2 level is about 6500 cm-1. Thus, two or three OH-vibrational quanta involved in the energy transfer process between Er3+ and OH- would easily cause the fluorescence quenching of 1.5 ¡m emission. Therefore, OH- is an dominant quenching center not only for 2.7 ¡xm emission but also for 1.5 ¡xm emission in the Er3+-doped glass. In this case, the diminution of OH- would depress the interaction between Er3+ and OH- vigorously, and finally strengthens both 2.7 and 1.5 ¡ m emission. Meanwhile, enhanced 2.7 ¡ m fluorescence favors the population accumulation of 4I13/2 level, which is also beneficial to the improvement of the 1.5 ¡m emission. Again, the high dehydration efficiency combining fluoride and Ar gas is reflected by the observation that the emission for TZF30-Ar is most intense among all the samples.
Fig. 6 reveals the luminescence decay curves at 1530 nm (Fig. 6(a)) and 980 nm (Fig. 6(b)) as a function of ZnF2 content, and the corresponding lifetimes are summarized in Table IV. Lifetimes of 4I13/2 level are obtained by fitting with the single-exponential law, while lifetimes of 4I11/2 level are determined from the first e-folding time of emission intensities in the decay curves due to their non-exponential character. It can be seen that the experimental lifetime of 4I13/2 level grows gradually from 2.71 to 5.87 ms with increasing ZnF2 content up to 30mol%, while that of 4I11/2 level rises from 149 to 593 ¡ s. This regular increase is well consistent with the trend predicted by the calculated radiative lifetimes and the depressed nonraditive relaxation resulted from the reduction of OH-. Notably, the measured lifetimes of 4I13/2 level in TZF20, TZF25, and TZF30 are larger than the calculated radiative lifetimes in Table II. This discrepancy has already been reported in a lot of host glasses.24,27,28 The reason for this phenomenon might be assumed to be the occurrence of Er3+: 4I9/2 + 4I15/2 ^ 4I13/2 + 4I13/2 cross-relaxation, which greatly promotes the population of 4I13/2 state.29 In addition, the error generated during the J-O calculation and self-absorption effects maybe also have an impact on this behavior.24 Nevertheless, the definite reasons are still unknown so far.
Based on the measured lifetimes (tm) in Table IV and calculated radiative lifetimes (trad) in Table II, the non-radiative probability Wnr of 4I11/2 level can be obtained:
Wnr =------(6)
Tm Trad
As shown in Table V, there is a systematic decrease in nonradiative rate for 4I11/2 level with increasing ZnF2 (from 6360 s-1 to 1475 s-1).
TABLE V. Calculated nonradiative probabilities for different samples.
Sample TZF0 TZF5 TZF10 TZF15 TZF20 TZF25 TZF30
Wnr (s-1) 6360 5733 4765 3912 2967 2301 1475
FIG. 7. Calculated (a) absorption and emission cross-section spectra and (b) gain cross-section spectra corresponding to the Er3+: 4111/2 ^ 4I13/2 transition in TZF30 glass.
The absorption and emission cross sections, which are typically acquired from the lineshapes of absorption and emission spectra, are essential for the modeling of optical amplifiers and fiber lasers. The emission cross section (ae) can be calculated from the emission spectra by Ftichtbauer-Ladenburg equation:30
a k5 Kail (k)
°e ( 8n cn2f k l (k)d k
where k is the wavelength, l(k) is the intensity of emission spectra, and c is the velocity of light.
As the absorption cross section (aa) of Er3+: 4I13/2 ^ 4I11/2 transition is difficult to attain by direct measuring the precise absorption lineshape, McCumber procedure has been successfully used to derive the other when absorption cross section or emission cross section is known. The relationship between ae and aa is expressed by the following equation:31
Zi (E{) - h f\ ae(v) = aa(v)-i exp , ^ k (8)
where Zi and Zu are partition functions of the lower and upper manifolds, respectively; E0 is the energy gap between the lowest Stark levels of two manifolds; and h is the planck's constant.
To determine the parameters in Eq. (8), knowledge of the electronic structure of Er3+ should be provided, however, it is usually difficult to obtain. Fortunately, Miniscalco and Quimby proposed a simplified model to solve this problem.32 When applying the method to Er3+: 4I11/2 ^ 4I13/2 transition, the assumption is made that stark levels of4I11/2 and4I13/2 are split into 6 and 7 components respectively, and each manifold is equally spaced. The values of E0 and Zi/Zu in TZF30 are acquired to be 3676.5 cm-1 and 1.085 respectively. Accordingly, the calculated absorption and emission cross section of Er3+ are presented in Fig. 7(a) for TZF30 sample. The maximum values of a a and a e reach 0.44 x 10-20 cm2 and 0.45 x 10-20 cm2 at about 2.7 ¡m respectively. They are roughly amount to the values in LiYF433 and YAG34 crystals. Besides, the full-width at half-maximum (FWHM) of absorption and emission transition are approximately 166 nm and 175 nm.
The gain cross section spectra G(k) can be computed from the derived absorption and emission cross section by the following formula:
G(k) = P x ae(k) - (1 - P)aa(k) (9)
where P is the population inversion given by the ratio between the population of 4In/2 level and that of 4Ii3/2 level. A set of P values varying from 0 to 1 composes the calculated gain cross section spectra versus the wavelength as presented in Fig. 7(b). Obviously, the positive gain appears when P is around 0.5, indicating that the pump threshold to obtain 2.7 /xm laser in TZF30 is as low as in ZBLAN glass.4
IV. CONCLUSION
In summary, efficient 2.7 /xm emission from Er3+ doped oxyfluoride tellurite glasses has been obtained under 980 nm LD excitation. The influence of introducing fluoride and shielding gas on the spectroscopic and structural properties of TeO2-ZnO-ZnF2 glass system has been discussed in detail. It is found that the addition of ZnF2 can distinctly reduce the content of OH- in the glass matrix and the combination of fluoride and shielding gas of O2 or Ar can further lower it. Furthermore, the minimal content of OH- (3.028 x 1018 cm-3) is achieved when 30 mol% ZnF2 substituted sample is treated with Ar gas in the glass melting process. The low multiphonon relaxation rate (207 s-1) combined with the reduction of OH- facilitates the radiative transitions of Er3+ at 1.5 and 2.7 /m, which are coherently strengthened with increasing ZnF2. Along with that, the lifetimes of 4I11/2 and 4I13/2 levels are prolonged. On the other hand, the introduction of fluoride gives rise to a decrease in refractive index, density, and J-O parameters, which can be considered as the result of the larger electronegativity of F than O. The large absorption and emission cross section (0.44 x 10-20 and 0.45 x 10-20 cm2), and good gain cross section demonstrate that such oxyfluoride tellurite glass could be an eligible material for 2.7 /xm laser.
ACKNOWLEDGMENTS
Financial support from National Science Foundation of China (51125005, 51302086 and U0934001), Chinese Ministry of Education (20100172110012), and Department of Education of Guangdong Province (cxzd1011) are gratefully acknowledged.
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