The synthesis, theoretical analysis, and optical characterizations of fluoroaluminate–tellurite glasses with high Er3+ concentration are reported. The 2.7 μm emission from Er3+ doped fluoroaluminate–tellurite glasses upon excitation of a conventional 980 nm LD is demonstrated with minimized concentration quenching. The prepared glass possesses high fluorescence lifetime of 4I11/2 level (1.343ms) and large calculated emission cross section (7.63 × 10−21 cm2). Besides, the radiative transfer microscopic parameters of 4I11/2 and 4I13/2 levels were theoretically analyzed. Hence, these results indicate that this Er3+ doped fluoroaluminate–tellurite glass has potential applications in 2.7 μm laser.
© 2016 Optical Society of America
In last decade, considerable effort has been devoted to the development of 2.7 μm laser activated by Er3+ ions due to the potential applications in remote sensing, military weapon, atmosphere pollution monitoring, and medical surgery area [1–3]. Since the development of stable single frequency fiber laser required the use of short cavity length and therefore high gain per unit length medium, in pursuit of efficient mid-infrared laser operation, high Er3+ concentration was required . Some studies of spectroscopic properties had been made using Er3+ highly-doped glasses, including fluoride, fluorophosphate, tellurite, phosphate, and silica glasses [5–8]. However, the concentration quenching and the difficulty in making stable high-doping content glasses were still the major obstacle in this area. Among various glasses, fluorozirconate glass had been regarded as an important host glass which can dope high content rare earth ions. But fluorozirconate glass required more complicated fabrication methods and had inferior thermal stability.
In 1991, Hu  reported the AlF3-MgF2-CaF2-SrF2-BaF2-YF3 (AMCSBY) glass. AMCSBY has an important advantage over fluorozirconate glass as a laser host because of its higher glass transition temperature, and more chemically and mechanically durable than those of fluorozirconate glass . However, the thermal stability is still worse than other glass systems, such as tellurite glass. In 1952, Stanworth  reported the tellurite glass firstly. Tellurite glass is a heavy metal oxide glass which has good chemical and thermal stability. To further improve mechanical stability and chemical durability, we introduced different concentrations of TeO2 into AMCSBY to get fluoroaluminate–tellurite glass (AlF3-MgF2-CaF2-SrF2-BaF2-YF3-TeO2). Compared with the AMCSBY or tellurite glass, fluoroaluminate–tellurite glass not only had lower phonon energy and a wider infrared transmission range, but also owned better thermal properties. The fluoroaluminate–tellurite glass is supposed to be a suitable host for 2.7μm laser.
In this paper, a series investigation on the fluorescence properties of Er3+ highly-doped fluoroaluminate–tellurite glass was reported. In order to understand the thermal properties of the Er3+ highly-doped fluoroaluminate–tellurite glass system, in this work, the characteristic temperatures of prepared samples had been measured by differential scanning calorimeter (DSC). The glass transition behaviors had been evaluated and compared with those of other glass systems. At the same time, the luminescence properties of Er3+ highly-doped fluoroaluminate–tellurite glass, up-conversion fluorescence, 1.5μm and 2.7μm emission were obtained by 980nm LD. The Judd-Ofelt parameters were calculated and discussed using Judd-Ofelt theory. The fluorescence lifetime of 4I11/2 level was measured and analyzed in fluoroaluminate–tellurite glass. Besides, the energy transfer microscopic parameters had been calculated and analyzed by the Förster-Dexter model to understand the energy transfer processes about Er3+: 4I11/2 and 4I13/2 levels in prepared fluoroaluminate–tellurite glass.
The fluoroaluminate–tellurite glass with molar compositions of (85-x)(40AlF3-10MgF2-15CaF2-10SrF2-10BaF2-15YF3)-15TeO2-xErF3 (X = 0,1,3,5,7.), denoted as AYF, TE1, TE3, TE5, TE7, were prepared by conventional melt quenching method. Keep the raw materials and the procedures dry during glass fabrication in order to minimize OH content and great tendency of fluorides to crystallization. Then the 20g mixture powder batches were melted at 1050°C for 30min in alumina crucible with a cover. The refined liquids were cast on a pre-heated stainless steel plate and annealed for 2h around glass transportation temperature. Finally the samples were cut and polished to the size of 10mm × 10mm × 1.5mm for test.
The refractive index of samples was measured by the prism minimum deviation method and was in the range of 1.48-1.52. The characteristic temperatures of samples were determined using a NetzschSTA404PC differential scanning calorimetry at the same heating rate. The absorption spectra were recorded with a UV/VIS/NIR spectrophotometer in the range of 200-1800nm. The MIR emission spectra were measured using an Edinburgh FLS980 fluorescence spectrometer equipped with a liquid-nitrogen cooled steady state InSb detector. A semiconductor CW laser at 980nm was used as the excitation source. In fluorescence decay measurements, the glasses were excited with the 540nm laser modulated by a signal generator, producing a pulse with a width of 50μs and a repetition rate of 10Hz. The luminescence was detected by a liquid-nitrogen cooled InSb detector and the decay curves were recorded with a digital phosphor oscilloscope (TDS3000C).
In order to obtain comparable results, all the experimental conditions including the size, pump power and excitation beam position of each sample are consistently maintained. All the measurements were performed at room temperature.
3. Results and discussion
3.1 Mid-infrared transmittance
As we know, the residual OH- groups will participate in the energy transfer (ET) of rare-earth ions which affects the efficiency of the mid-infrared emissions . The content of OH- groups in the glass can be expressed by the OH- absorption coefficient at 2.84μm, which can be given by αOH- = ln(T/T0)/l where l is the thickness of the sample and T0, T are the transmitted and incident intensities, respectively. Therefore, the mid-infrared transmittance spectra of the prepared glasses, indicating the OH- content, are measured and showed in Fig. 1. It can be seen that the maximum transmittance reaches as high as 91% of the prepared glass. The 9% loss is mainly due to the Fresnel reflections. The OH- content of TE1 is 0.51cm−1 which is smaller than that of fluoroaluminate glass (2.78cm−1) .Thus, the fluoride-based glass containing some TeO2 possesses good mid-infrared transmission property, which is a potential candidate for mid-infrared laser materials.
3.2 Thermal stability
In order to analyze the effect of ErF3 contents on the thermal stability, Table 1 shows the characteristic temperatures of prepared samples, including the transition temperature Tg, onset crystallization temperature Tx, and the peak temperature of crystallization Tp. Based on the measured characteristic temperatures, the glass forming ability of these glasses was evaluated by two different parameters ΔT = (Tx−Tg) and S = (Tx−Tg)(Tp−Tx)/(Tg) [14,15]. However, the peak temperature of crystallization Tp is easily affected by heating rate. Thus, all the samples are prepared at the same heating rate to guarantee the accuracy. The thermal ability and forming ability can be estimated by the characteristic temperatures in Table 1.
It can be found from Table 1 that characteristic temperatures among these fluoroaluminate–tellurite glasses change slightly. With the increase of ErF3 content, the ΔT decreases slowly and the S also changes slightly. The results show the addition of ErF3 has a little influence on ΔT and S. It can be inferred that a high ratio of ErF3 will not affect the thermal performance fluoroaluminate–tellurite glass greatly. Besides, these TeO2 doped fluoroaluminate glasses have better resistance to devitrification and forming ability than ZBLAN  and comparable to tellurite glass . In addition, a higher Tg of fluoroaluminate–tellurite than that of other glasses in the Table 1 means that present glass has good thermal stability to resist thermal damage at high power densities . These results indicate that this kind glass doped with high ErF3 contents possesses good thermal stability,and will be an excellent glass host for laser.
3.3 Absorption spectra and Judd-Ofelt analysis
Figure 2 shows the absorption spectra of the fluoroaluminate–tellurite samples doped with different ErF3 concentrations. A variation of the 4I15/2→4I13/2 absorption coefficient against different ErF3 concentrations is presented in the inset of Fig. 1, which is a convenient approach to estimate the solubility of Er3+ ions . It shows good linearity, which means that the solubility of ErF3 can reach to 7mol% in this fluoroaluminate–tellurite glass. The shape and peak position of each transition in all samples are similar to the other Er3+ doped glasses, in spite of some slight changes compared to the other glasses which originate from the different ligand field strength of hosts. It is noted that two strong peaks corresponding to the transitions of 4I15/22H11/2 and 4I15/24G11/2 are notable with the increase of ErF3 contents, which is defined as hypersensitive transitions (HSTs) . They are sensitive to the local environment around rare earth ions. The intensity divergence among samples results from the different compositions of the fluoroaluminate–tellurite glasses and the changes in Er3+ surrounding local environment.
The Judd-Ofelt (J–O) parameters were determined from the least-squares fit to the value of measured and experimental oscillator strengths. Table 2 shows the J–O parameters of Er3+ in fluoroaluminate–tellurite glasses and other host materials. It is known that Ω2 generally increase with the degree of asymmetry of local structure and the covalency of the bond formed by rare earth ions . Therefore, we can deduce a lower covalency associated with rare earth ions and higher polyhedra asymmetry surrounding Er3+ in the fluoroaluminate–tellurite glasses with the increase of ErF3 content in Table 2. High Ω6 value represents ionicity enhancement between rare earth and ligand atoms. Compared with other samples shown in Table 2, the TE5 glass with the increase of ErF3 contents which has the high Ω6, indicates a higher ionicity associated with rare earth ions, coinciding with the indication of Ω2.
According to Judd-Ofelt theory [21,22], the magnitude of the measured oscillator strengths can be determined as an important spectroscopic parameter of Er3+ doped glasses. Details of the theory and method have been well described earlier , and only results are presented here. The experimental oscillator strengths of Er3+ in fluoroaluminate–tellurite glasses are listed in Table 3 and compared with other results reported. It can be seen from Table 3 that the measured oscillator strengths in present work are higher than those of Er3+ doped fluoride glasses but lower than those of Er3+ doped tellurite glasses . This behavior corresponds to the instinct rule of environment surrounding the rare-earth ions. It is worth mentioning that the oscillator strength corresponding to Er3+:4I15/22H11/2 (HST) is much higher than those of other transitions in this work. In addition, with the increase of ErF3, the oscillator strength of most transitions increase gradually, suggesting the additions of ErF3 have substantial influence on the local ligand.
3.4 Fluorescence spectra
The fluorescence spectra of prepared samples are measured and showed in Fig. 3. Besides, the inset of Fig. 3 is the fluorescence peak intensity upon the contents of Er3+ in TE samples excited under 980LD. For the Er3+-doped samples, there is no shift in the center wavelength of the absorption peaks but the peak intensity is far different. The inset of Fig. 3 shows the 2.7μm fluorescence peak intensity as a function of Er3+ concentrations. The maximum fluorescence peak intensity for fluoroaluminate–tellurite glass is observed around 3mol % ErF3. The reduction in the emission intensity as the ions concentration above a certain value has been attributed to concentration quenching and OH- absorption . However, it can be inferred that concentration quenching occurs slightly in highly-doped fluoroaluminate–tellurite glass from the slight drop of peak intensity when concentration reaches to 5mol %. Thus, this Er3+ highly-doped fluoroaluminate–tellurite glass maybe considered to be a hopeful host for a 2.7μm microchip laser and other optical laser applications.
As is showed in Fig. 2, 2.7μm emission spectrum pumped by 980nm LD from 2500nm to 3000nm can be observed. In order to examine the characteristics of 2.7μm emission for potential laser applications, the emission cross section σem is calculated and showed in Fig. 2. According to the emission spectrum and Füchtbauer–Ladenburg theory , the 2.7μm emission cross section σem, can be calculated by25], ZBLAN (5.4 × 10−21cm2) . The highly Er3+ doped fluoroaluminate–tellurite glass possesses large emission cross section, and it might be one of promising candidate materials applied in mid-infrared fiber lasers.
Erbium ions have abundant energy levels. In order to understand the energy transfer mechanism, some up-conversion spectra are indispensable measured. Figure 4(a) shows the up-conversion fluorescence spectrum of the sample, pumped by 980 nm LD. Three strong emission bands centered around 522nm, 544nm and 655nm correspond to the 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transition, respectively. Moreover, the green emission spectrum could readily be seen with the naked eye. Figure 4(b) displays the 1.52μm emission spectrum of fluoroaluminate–tellurite in the wavelength of 1300–1650 nm pumped at 980 nm LD. It is interesting that fluorescence in the spectral region at 522nm, 544nm, 655nm, 1.52μm, 2.7μm can be observed simultaneously. The similar behavior was found in Er3+ singly doped Ge–Ga–As–S glass  and ZBAN glass , but to the best of our knowledge, it is firstly reported in fluoroaluminate–tellurite glass.
3.5 Fluorescence lifetime
A long fluorescence lifetime is another important factor in the success of 2.7μm fiber laser. Even though Er ions have been widely doped into different host materials, measured lifetime of the 4I11/2→4I13/2 transition was rarely reported in fluoroaluminate-tellurite glasses, which may be due to their extremely weak emission intensity beyond the detecting reach of current facilities .
In fluorescence decay measurements of Er3+: 4I11/2 level, the glasses were excited with the 540nm laser modulated by a signal generator, producing a pulse with a width of 50μs and a repetition rate of 10Hz. The luminescence was detected by a liquid-nitrogen cooled InSb detector and the decay curves were recorded with a digital phosphor oscilloscope (TDS3000C). The experimental lifetimes are determined by the procedure of single exponential fitting. Figure 5 shows the measured decay curve of samples and the fitted lifetime. It shows that the fluorescence decay characteristic at 2.7μm and the measured lifetime τ of TE1, TE3, TE5, TE7 was estimated to be 1.343ms, 0.569ms, 0.440ms, 0.439ms, respectively. The measured lifetimes of TE1 and TE3 in this paper are greatly larger than those of tellurite glass (0.215ms) , oxysulfide glasses(0.52ms) , and ferroelectric ceramics(0.28ms) . Moreover, the quantum efficiency of 2.7μm emission in these glasses can reach as high as 32-98%. Thus, this new kind of fluoroaluminate-tellurite glass is very suitable for 2.7μm fiber laser development.
3.6 Energy transfer mechanism and miscroparameters
To further understand the phenomenon above, the energy transfer sketch between adjacent Er3+ ions is displayed in Fig. 6. Based on the photoluminescence and decay performance, a reasonable energy transfer mechanism is proposed systematically as follows.
- (1) Ions of 4I15/2 state are excited to 4I11/2 level by ground state absorption (GSA:4I15/2 + a photon→4I11/2) when the sample is pumped by a 980 nm LD.
- (2) On the one hand, ions in the 4I11/2 level undergo the excited state absorption (ESA1: 4I11/2 + a photon→4F7/2) process, or the energy transfer up-conversion (ETU1: 4I11/2 + 4I11/2→4F7/2 + 4I15/2) to populate the ions in 4F7/2 level.
- (3) Then, the ions in 4F7/2 level relax non-radiatively to the 2H11/2, 4S3/2 and 4F9/2 levels owing to small energy gap among them.
- (4) Finally, the red and green emissions around at 522nm, 544nm and 655nm occur corresponding to Er3+: 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 radiative transitions, respectively.
- (5) On the other hand, ions in 4I11/2 level relax radiatively or non-radiatively to the 4I13/2 level, and radiative relaxation process generates the 2.7μm emission.
- (6) Subsequently, the ions in 4I13/2 level can relax radiatively to ground state and 1.52μm emission happens.
- (7) Then followed by the ETU2 (4I15/2 + 2H11/2→4I13/2 + 4I9/2) process, the ions in 4I9/2 and 4I13/2 level are excited simultaneously. This process improves the population of 4I11/2, which is beneficial to the 2.7μm emission.
- (8) Also, some ions in 4I13/2 level are populated to 4S3/2 level by ESA2 process (4I13/2 + a photon→4S3/2), which not only makes a contribution to the up-conversion emission, but also is beneficial to the 4I11/2→4I13/2 transition.
In addition, a quantitative understanding of energy transfer processes about Er3+: 4I11/2 and 4I13/2 levels in present fluoroaluminate–tellurite glass are presented. Energy transfer microscopic parameter is an important parameter to characterize 2.7μm fluorescence characteristics. As is shown in Fig. 6, in order to obtain excellent 2.7μm emission, it is necessary to weaken the 4I11/2→4I11/2 process and strengthen 4I13/2→4I13/2 process. The energy transfer parameters of 4I11/2→4I11/2 and 4I13/2→4I13/2 processes can be evaluated by the calculation of the absorption and emission cross sections. According to Förster  and Dexter , the energy transfer probability between donor and acceptor can be estimated by [35,36].
Energy transfer properties of 4I11/2 level and 4I13/2 level in present glass are calculated by Eqs. (2)–(7) and showed in Fig. 7. The energy transfer microscopic parameter of 4I13/2→4I13/2 process is dramatically larger than that of 4I11/2→4I11/2 process which indicates that 4I13/2 level has more opportunity to transfer its ions to the same level nearby compared with 4I11/2 level. Thus, the 4I13/2→4I13/2 process is much easier to realize than 4I11/2→4I11/2 process, which is beneficial to the population inversion of the 4I11/2→4I13/2 transition and efficient mid-infrared radiation in present glass.
Figure 7 also shows the trend of calculated microscopic parameters for energy transfer processes. In the Fig. 7, the CD-D of 4I13/2→4I13/2 process increases from 43.84 × 10−40cm6/s to 58.06 × 10−40cm6/s when the Er3+ is 3mol%, then the CD-D starts to decrease. The CD-D of 4I11/2→4I11/2 process drops to the lowest value (4.03 × 10−40cm6/s) when the Er3+ reaches 3mol%. It can be deduced that smaller energy microscopic parameter of 4I11/2 level and larger CD-D of 4I13/2 state is beneficial for population inversion and efficient 2.7μm emission. Thus, TE3 sample is supposed to get the strongest 2.7μm emission among the prepared samples theoretically, which agrees well with the experimental results shown in Fig. 3. Therefore, 3mol% Er3+ ions concentration is beneficial to get intense 2.7μm emission in present glass [37,38].
In conclusion, we have prepared fluoroaluminate–tellurite glass with different Er3+ contents. The thermal stabilities of prepared samples with the addition of Er3+ did not change greatly. The prepared glass possesses high emission cross section (7.63 × 10−21cm2) of Er3+:4I11/2→4I13/2 and longer fluorescence lifetime (1.343ms). Additionally, energy transfer microscopic parameter of the TE3: 4I11/2 level is 4.03 × 10−40cm6/s, and the energy transfer microscopic parameter of the TE3: 4I13/2 level is 58.06 × 10−40cm6/s. It indicates that the 4I13/2→4I13/2 process is much easier to realize than 4I11/2→4I11/2 process, and 2.7μm emission is obtained in Er3+ high doped fluoroaluminate–tellurite glass. The trend of energy transfer microscopic parameter agrees well with the change of fluorescence intensity. These results suggest that this fluoroaluminate–tellurite glass can be considered as a promising highly-doped material for 2.7μm laser.
Zhejiang Provincial Natural Science Foundation of China (Nos. LY15E020009, LY14B010004, and LR14E020003); National Natural Science Foundation of China (Nos. 61370049, 61308090, 61405182, 51172252, 51372235 and 51472225); the International Science & Technology Cooperation Program of China (Grant no. 2013DFE63070); Public Technical International Cooperation project of Science Technology Department of Zhejiang Province(2015c340009).
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