Synthesis, theoretical analysis, and characterization of highly Er<sup>3+</sup> doped fluoroaluminate&#x2013;tellurite glass with 2.7 &#x03BC;m emission

Ying Tian; Xufeng Jing; Bingpeng Li; Pengcheng Li; Yinyan Li; Ruoshan Lei; Junjie Zhang; Shiqing Xu

doi:10.1364/OME.6.003274

1. Introduction

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 [4]. 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 [9] 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 [10]. However, the thermal stability is still worse than other glass systems, such as tellurite glass. In 1952, Stanworth [11] 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 ⁴I_11/2 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⁺: ⁴I_11/2 and ⁴I_13/2 levels in prepared fluoroaluminate–tellurite glass.

2. Experimental

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 [12]. The content of OH^- groups in the glass can be expressed by the OH^- absorption coefficient at 2.84μm, which can be given by α_OH- = 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) [13].Thus, the fluoride-based glass containing some TeO₂ possesses good mid-infrared transmission property, which is a potential candidate for mid-infrared laser materials.

Fig. 1 The transmission spectra of the samples. The inset is OH^- concentration at 2.84μm dependence on the content of TeO₂.

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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 T_g, onset crystallization temperature T_x, and the peak temperature of crystallization T_p. Based on the measured characteristic temperatures, the glass forming ability of these glasses was evaluated by two different parameters ΔT = (T_x−T_g) and S = (T_x−T_g)(T_p−T_x)/(T_g) [14,15]. However, the peak temperature of crystallization T_p 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.

Table 1. Characteristic temperatures (T_g,T_x,T_p) and ΔT, H of TE samples.

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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 [16] and comparable to tellurite glass [15]. In addition, a higher T_g 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 [17]. 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 ⁴I_15/2→⁴I_13/2 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 [18]. 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 ⁴I_15/2 $\to$ ²H_11/2 and ⁴I_15/2 $\to$ ⁴G_11/2 are notable with the increase of ErF₃ contents, which is defined as hypersensitive transitions (HSTs) [19]. 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.

Fig. 2 Absorption spectra of samples doped with ErF₃. The inset presents the maximum absorption coefficient value of the ⁴I_15/2→⁴I_13/2 transition in prepared glasses with various ErF₃ concentrations.

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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 [20]. 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.

Table 2. The J–O parameters of Er³⁺ in fluoroaluminate–tellurite glass.

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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 [23], 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 [24]. 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⁺:⁴I_15/2 $\to$ ²H_11/2 (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.

Table 3. Oscillator strength and wave number of Er³⁺ for selected transitions in samples

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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 [13]. 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.

Fig. 3 The relative fluorescence spectra of samples with Er³⁺ ions excited under 980LD. The inset shows relationship between the emission cross section peak intensity and the content of Er³⁺ in glass.

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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 σ_em is calculated and showed in Fig. 2. According to the emission spectrum and Füchtbauer–Ladenburg theory [25], the 2.7μm emission cross section σ_em, can be calculated by

σ_{e m} = \frac{λ^{4} A_{r a d}}{8 π c n^{2}} \times \frac{λ I (λ)}{\int λ I (λ) d λ} .

where A_rad represents the spontaneous radiative transition probability corresponding to the Er³⁺:⁴I_11/2→⁴I_13/2 transition, λ is the wavelength, c is the speed of light, n is the refractive index and I(λ) is the intensity of emission spectrum. It is worth mentioning that the maximum of the calculated emission cross section at 2.7μm of the TE3 in this work can reach to 7.63 × 10⁻²¹cm² which is significantly larger than those of, fluorophosphate glass (6.57 × 10⁻²¹cm²) [25], ZBLAN (5.4 × 10⁻²¹cm²) [26]. 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 ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 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 [27] and ZBAN glass [28], but to the best of our knowledge, it is firstly reported in fluoroaluminate–tellurite glass.

Fig. 4 Diode-pumped(980nm) spectra of prepared TE: (a) upconversion spectrum and (b) 1.52μm emission spectrum.

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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 ⁴I_11/2→⁴I_13/2 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 [29].

In fluorescence decay measurements of Er³⁺: ⁴I_11/2 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) [30], oxysulfide glasses(0.52ms) [31], and ferroelectric ceramics(0.28ms) [32]. 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.

Fig. 5 Room-temperature fluorescence lifetime at 2.7μm of fluoroaluminate–tellurite glass excited by 540nm LD.

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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.

Fig. 6 The energy transfer sketch of Er³⁺ in fluoroaluminate–tellurite glasses pumped at 980nm LD. In the figure, every number corresponds to the discussed processes in energy transfer mechanism part, respectively.

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(1) Ions of ⁴I_15/2 state are excited to ⁴I_11/2 level by ground state absorption (GSA:⁴I_15/2 + a photon→⁴I_11/2) when the sample is pumped by a 980 nm LD.
(2) On the one hand, ions in the ⁴I_11/2 level undergo the excited state absorption (ESA1: ⁴I_11/2 + a photon→⁴F_7/2) process, or the energy transfer up-conversion (ETU1: ⁴I_11/2 + ⁴I_11/2→⁴F_7/2 + ⁴I_15/2) to populate the ions in ⁴F_7/2 level.
(3) Then, the ions in ⁴F_7/2 level relax non-radiatively to the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 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⁺: ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 radiative transitions, respectively.
(5) On the other hand, ions in ⁴I_11/2 level relax radiatively or non-radiatively to the ⁴I_13/2 level, and radiative relaxation process generates the 2.7μm emission.
(6) Subsequently, the ions in ⁴I_13/2 level can relax radiatively to ground state and 1.52μm emission happens.
(7) Then followed by the ETU2 (⁴I_15/2 + ²H_11/2→⁴I_13/2 + ⁴I_9/2) process, the ions in ⁴I_9/2 and ⁴I_13/2 level are excited simultaneously. This process improves the population of ⁴I_11/2, which is beneficial to the 2.7μm emission.
(8) Also, some ions in ⁴I_13/2 level are populated to ⁴S_3/2 level by ESA2 process (⁴I_13/2 + a photon→⁴S_3/2), which not only makes a contribution to the up-conversion emission, but also is beneficial to the ⁴I_11/2→⁴I_13/2 transition.

In addition, a quantitative understanding of energy transfer processes about Er³⁺: ⁴I_11/2 and ⁴I_13/2 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 ⁴I_11/2→⁴I_11/2 process and strengthen ⁴I_13/2→⁴I_13/2 process. The energy transfer parameters of ⁴I_11/2→⁴I_11/2 and ⁴I_13/2→⁴I_13/2 processes can be evaluated by the calculation of the absorption and emission cross sections. According to Förster [33] and Dexter [34], the energy transfer probability between donor and acceptor can be estimated by [35,36].

W_{D - A} = (\frac{2 π}{ℏ}) {| H_{D A} |}^{2} S_{D A}^{N},

where $| H_{D A} |$ represents the perturbation Hamiltonian between initial and final states in energy transfer process. N is the total phonons in the transfer process (m + k = N).

S_{D A}^{N}

is the integral overlap between the m-phonon emission side band of donor ions and k-phonon absorption line shapes of acceptor ions. In this work, the donor and acceptor in ET1 process are ions on ⁴I_11/2 level. At the same time, in the ET2 process, the donor and acceptor are ions on ⁴I_13/2. In the case of weak electron–phonon coupling which is suitable for rare earth ions,

S_{D A}^{N}

can be approximated by

S_{D A}^{N} \approx \sum e^{- (S_{0}^{D} + S_{0}^{A})} \times [\frac{{(S_{0}^{D} + S_{0}^{A})}^{N}}{N!}] S_{D A} (0, 0, E) δ (N, Δ E / ℏ w_{0}),

where

S_{0}^{D}

and

S_{0}^{A}

are the Huang-Rhys factors,

S_{D A} (0, 0, E)

is the overlap between the zero phonon lines shape of emission and absorption. Then the integral overlap in the case of m-phonon emission by the donor and no phonon involvement by the acceptor can be defined as:

\begin{array}{l} S_{D A} (m, 0, E) = \int g_{e m i s (m - p h o n o n)}^{D} (E) g_{a b s}^{A} (E) d E \\ = \frac{S_{0}^{m}}{m!} e^{- S_{0}} S_{D A} (0, 0, E) \\ = \int [\frac{S_{0}^{m}}{m!} e^{- S_{0}} \int g_{e m i s}^{D} (E - Δ E)] g_{a b s}^{A} (E) d E, \end{array}

where

Δ E = m ℏ ω_{0}

, T is temperature. The emission cross section (σ_emis) and absorption cross section (σ_abs) can be proposed as:

σ_{e m i s (m - p h o n o n)}^{D} = σ_{e m i s}^{D} (λ_{m}^{+}) \approx \frac{S_{0}^{m} e^{- S_{0}}}{m!} {(\bar{n} + 1)}^{m} σ_{e m i s}^{D} (E - E_{1}),

σ_{a b s (k - p h o n o n)}^{A} = σ_{a b s}^{A} (λ_{k}^{-}) \approx \frac{S_{0}^{k} e^{- S_{0}}}{k!} {(\bar{n})}^{k} σ_{a b s}^{A} (E + E_{2}),

where

E_{1} = m ℏ ω_{0}

,

E_{2} = k ℏ ω_{0}

, and

Δ E = E_{1} + E_{2}

. The

λ_{m}^{+}

and

λ_{k}^{-}

represent the translation of emission cross section spectra wavelength and the translation of absorption cross section spectra wavelength, respectively. The energy transfer microscopic parameter is then expressed by

W_{D A} (R) = \frac{6 c g_{l o w}^{D}}{{(2 π)}^{4} n^{2} R^{6} g_{u p}^{D}} \sum_{m = 0}^{\infty} e^{- (2 \bar{n} + 1) S_{0}} \frac{S_{0}^{m}}{m!} {(\bar{n} + 1)}^{m} \int σ_{e m i s}^{D} (λ_{m}^{+}) σ_{a b s}^{A} (λ) d λ = \frac{C_{D A}}{R^{6}}

where Cda and Cdd represent energy transfer coefficient from donor to acceptor(donor).

Energy transfer properties of ⁴I_11/2 level and ⁴I_13/2 level in present glass are calculated by Eqs. (2)–(7) and showed in Fig. 7. The energy transfer microscopic parameter of ⁴I_13/2→⁴I_13/2 process is dramatically larger than that of ⁴I_11/2→⁴I_11/2 process which indicates that ⁴I_13/2 level has more opportunity to transfer its ions to the same level nearby compared with ⁴I_11/2 level. Thus, the ⁴I_13/2→⁴I_13/2 process is much easier to realize than ⁴I_11/2→⁴I_11/2 process, which is beneficial to the population inversion of the ⁴I_11/2→⁴I_13/2 transition and efficient mid-infrared radiation in present glass.

Fig. 7 The sketch of calculated microscopic parameters for energy transfer process in TE samples. The left Y axis represents the energy transfer microscopic parameter of ⁴I_11/2 level(black line), and the right Y axis represents the energy transfer microscopic parameter of ⁴I_13/2 level(blue line).

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Figure 7 also shows the trend of calculated microscopic parameters for energy transfer processes. In the Fig. 7, the C_D-D of ⁴I_13/2→⁴I_13/2 process increases from 43.84 × 10⁻⁴⁰cm⁶/s to 58.06 × 10⁻⁴⁰cm⁶/s when the Er³⁺ is 3mol%, then the C_D-D starts to decrease. The C_D-D of ⁴I_11/2→⁴I_11/2 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 ⁴I_11/2 level and larger C_D-D of ⁴I_13/2 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].

4. Conclusions

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⁺:⁴I_11/2→⁴I_13/2 and longer fluorescence lifetime (1.343ms). Additionally, energy transfer microscopic parameter of the TE3: ⁴I_11/2 level is 4.03 × 10⁻⁴⁰cm⁶/s, and the energy transfer microscopic parameter of the TE3: ⁴I_13/2 level is 58.06 × 10⁻⁴⁰cm⁶/s. It indicates that the ⁴I_13/2→⁴I_13/2 process is much easier to realize than ⁴I_11/2→⁴I_11/2 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.

Funding

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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Glass	Tg(°C) ± 1	Tx(°C) ± 1	Tp(°C) ± 1	ΔT(°C) ± 2	S(K)
TE1	454	559	583	105	3.46
TE3	451	546	571	95	3.28
TE5	451	545	571	94	3.37
TE7	450	540	568	90	3.48
Tellurite [15]	340	377	407	34	3.24
ZBLAN [16]	265	329	344	64	1.78

Glasses	Intensity parameter( × 10⁻²⁰cm²)			δ( × 10⁻⁶)
Glasses	Ω₂	Ω₄	Ω₆	δ( × 10⁻⁶)
TE1	4.5 ± 0.01	1.47 ± 0.02	1.08 ± 0.01	0.31
TE3	3.54 ± 0.03	1.54 ± 0.01	1.14 ± 0.02	0.23
TE5	3.19 ± 0.01	1.39 ± 0.01	1.21 ± 0.01	0.24
TE7	2.87 ± 0.02	1.26 ± 0.04	1.14 ± 0.02	0.24
Fluoride [24]	2.91	1.27	1.11	0.23
Tellurite [24]	5.21	1.18	0.79	0.13

Transition	Wave number (cm⁻¹)
Transition	Wave number (cm⁻¹)	TE1	TE3	TE5	TE7	Tellurite [24]	Fluoride [24]
⁴I_15/2 $\to$ ⁴I_11/2	10225	0.58	0.58	0.59	0.53	0.89	0.55
⁴I_15/2 $\to$ ⁴I_9/2	12500	0.28	0.46	0.29	0.24	0.35	0.32
⁴I_{15/2 $\to$}⁴F_9/2	15360	1.90	1.90	1.90	1.70	3.21	2.14
⁴I_15/2 $\to$ ²H_11/2	19230	6.71	5.60	5.07	4.43	13.73	3.44

Glass	Tg(°C) ± 1	Tx(°C) ± 1	Tp(°C) ± 1	ΔT(°C) ± 2	S(K)
TE1	454	559	583	105	3.46
TE3	451	546	571	95	3.28
TE5	451	545	571	94	3.37
TE7	450	540	568	90	3.48
Tellurite [15]	340	377	407	34	3.24
ZBLAN [16]	265	329	344	64	1.78

Glasses	Intensity parameter( × 10⁻²⁰cm²)			δ( × 10⁻⁶)
Glasses	Ω₂	Ω₄	Ω₆	δ( × 10⁻⁶)
TE1	4.5 ± 0.01	1.47 ± 0.02	1.08 ± 0.01	0.31
TE3	3.54 ± 0.03	1.54 ± 0.01	1.14 ± 0.02	0.23
TE5	3.19 ± 0.01	1.39 ± 0.01	1.21 ± 0.01	0.24
TE7	2.87 ± 0.02	1.26 ± 0.04	1.14 ± 0.02	0.24
Fluoride [24]	2.91	1.27	1.11	0.23
Tellurite [24]	5.21	1.18	0.79	0.13

Synthesis, theoretical analysis, and characterization of highly Er³⁺ doped fluoroaluminate–tellurite glass with 2.7 μm emission

Abstract

1. Introduction

2. Experimental

3. Results and discussion

3.1 Mid-infrared transmittance

3.2 Thermal stability

3.3 Absorption spectra and Judd-Ofelt analysis

3.4 Fluorescence spectra

3.5 Fluorescence lifetime

3.6 Energy transfer mechanism and miscroparameters

4. Conclusions

Funding

References and links

Cited By

Figures (7)

Tables (3)

Equations (7)

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