1. Introduction

In the past several decades, much attention has been paid on the development of mid-infrared (~3 μm) laser owing to its extensive applications such as medical surgery, remote sensing, light detection and ranging (LIDAR), and so forth [1–3]. Developing new materials for mid-infrared lasers operating in this wavelength region is becoming extremely urgent. It is known that two important factors, the active ions and the host materials, must be considered in developing more efficient optical devices based on rare-earth ions. Among various rare-earth ions, Ho³⁺, Dy³⁺, Er³⁺ can provide ~3 μm emission corresponding to ⁵I₆→⁵I₇, ⁶H_13/2→⁶H_15/2, ⁴I_11/2→⁴I_13/2 transition, respectively. Compared to Ho³⁺ or Dy³⁺, Er³⁺ is an ideal luminescent candidate, since Ho³⁺ and Dy³⁺ ions both lack an efficient, commonly commercial and high power excitation source, whereas the absorption bands of Er³⁺ ions match well with the commercially available and low-cost 808 nm or 980 nm laser diode (LD) [4–7]. The host materials for 2.7 μm emission which are expected to possess a minimal absorption coefficient in the typical H₂O absorption band around 3 μm, low nonradiative decay rates as well as high radiative emission rates have been primarily focused on glasses [8–10] and single-crystals [11–13]. However, both materials have their own weaknesses. On one hand, glass exhibits weak crystal field, which results in high probability of nonradiative transition and weak emission of rare-earth ions. On the other hand, although with excellent luminescence performance, single-crystals are very difficult to prepare quality and long single crystal fibers. Glass ceramic (GC), combining the strong crystal field of nanocrystals and excellent fiber-drawing ability of glass matrix, is considered as a promising medium for Er³⁺-doped fiber to produce intense 2.7 μm emission. Recently, several efforts have been made on the synthesis and mid-infrared optical properties of Er³⁺-doped oxyfluoride GCs [14–16]. However, to our best knowledge, no any study about the 2.7 μm emission in Er³⁺-doped tellurite GCs is reported.

TeO₂-based glasses are very interesting hosts due to their important properties that confer to these materials a large applicability in telecommunications, photonics, gas sensors, and so forth [17, 18]. It is known that tellurite glasses possess low phonon energy (~700 cm^-1), large refractive index (~2.0), high transmittance in near to middle infrared regions, high solubility for rare earth ions and good chemical durability and thermal stability, etc. [19-22]. The low phonon energy decreases the multi-phonon relaxation and non-radiative decay rate, especially for the mid-infrared emission. The large refractive index enhances both the absorption and emission cross-section, and improves the refractive index match between nanocrystals and glass matrix. The good chemical durability and thermal stability result in good environmental adaptation. Although tellurite glasses possess so many excellent properties, there is no any publication about the mid-infrared emission of tellurite GCs. Therefore, we try to prepare and study the optical properties of tellurite GCs. According to previous research, TeO₂, as a conditional glass former, requires the addition of other metal oxides to form stable glasses. In this view, PbO is an interesting component in glasses due to its dual characters: it functions as a network modifier at low concentrations when Pb-O is ionic, and it aids in glass network formation as a function of its increasing concentration owing to the formation of PbO4 structural units when Pb-O is covalent. It also results in a high capacity of vitrification and contributes to the further increase in the refractive index of modified tellurite glasses [23, 24].

Concerning all the reasons mentioned above, herein we choose TeO₂–PbO–Er₂O₃ system as the precursor glass to prepare Er³⁺-doped tellurite GCs containing PbTe₃O₇ nanocrystals. X-ray diffraction (XRD) and transmission electron microscope (TEM) characterizations of the GCs were used as combined techniques to understand the crystallization in this glass. Thereafter, we obtained intense 2.7 μm emission and discussed in detail the influence of the Er doping concentration and heat-treated temperature on the intensity of 2.7 μm emission. In addition, the absorption and emission cross-sections were calculated by using McCumber formula and Fuchtbauer-Ladenburg equation, respectively, and the gain cross section was obtained from absorption and emission cross-sections to survey the feasibility of the present tellurite GC as candidate for a new type of rare-earth-doped laser host material.

2. Experimental

Tellurite glasses with the composition of (75-x) TeO₂–25 PbO–x Er₂O₃ (x = 1, 3, 5,) in mol% were prepared by the conventional melt-quenching method. The starting materials are anhydrous powders of TeO₂ and Er₂O₃ with 5N purity, and PbO was introduced with 3N purity. Batches of 20 g were melted in covered corundum crucibles at 900°C for 30 min in an electrically heated furnace. Then melts were cast on a preheated copper plate, and the obtained glasses were cut to the size of 10 mm × 10 mm and heat-treated at 340°C-380°C for 8 h to achieve GCs. For further optical measurements, the samples were optically polished with a thickness of 1.5 mm.

XRD analysis was performed on a X’Pert PRO X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation to identify the crystalline phase and estimate the nanocrystal grain size. High-resolution transmission electron microscope (HRTEM, 2100F, JEOL, Japan) equipped with energy-dispersive spectrometer (EDS) system to analyze the microstructure and the elements distribution of GC samples. The absorption spectra were measured in the range of 500-3200 nm using a Perkin-Elmer Lambda 900/UV/VIS/NIR spectrophotometer. The emission spectra in the range of 2500-2850 nm were recorded by a computer controlled Triax 320 type spectrofluorimeter (Jobin-Yvon Corp.) with a lock-in amplifier upon excitation of a 980 nm LD. All the measurements were carried out at room temperature.

3. Results and discussions

The XRD characterizations of the as-prepared glasses and GCs are shown in Fig. 1. Apparently, the XRD spectra recorded with as-prepared samples show no peaks of crystallization, exhibiting a typical amorphous structure. After a heat-treated process above 360°C for 8 h, strong diffraction peaks appear, which are assigned to the cubic phase PbTe₃O₇ (JCPDF: 37-1392). With the increase of heat-treated temperature, the diffraction peaks become more evident and sharper, which indicates crystalline size increases and grows gradually. When the heat-treated temperature is elevated to 380°C, another crystalline phase α-TeO₂ occurs at 3% and 5% Er³⁺-doped GCs. However, compared with Fig. 1(a), when the amount of Er³⁺ reaches 3% and 5%, the diffraction peaks are much higher than that of 1% Er³⁺-doped tellurite glass matrix, even though the samples were heat-treated under the same temperature schedule. This phenomenon can also be explained by Fig. 1(d), which indicates the addition of Er³⁺ is beneficial to the PbTe₃O₇ nanocrystals growth in the glass matrix. Therefore, we can deduce that PbTe₃O₇ nanocrystals are probably nucleated on the surface of Er³⁺ ions as nucleating agent, which will be further confirmed by the following HRTEM and EDS results.

 

Fig. 1 (a)-(c) XRD patterns of 1%, 3%, 5% Er³⁺-doped as-prepared glasses and resulting GCs obtained by heat treatment at 340°C-380°C for 8 h. (d) XRD patterns of 1%, 3%, 5% Er³⁺-doped GCs heat-treated at 370°C for 8 h. The main crystalline phase is PbTe₃O₇, and the stark ‘*’ represents the TeO₂ phase.

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From the peak width of XRD pattern, the crystalline size of PbTe₃O₇ crystals in the GCs can be estimated by using Scherrer’s equation:

TEM permits the direct imaging of nanocrystals and provides information about their shape, size and size distribution. Figure 2(a) shows the TEM image of 5% Er³⁺-doped GC heat-treated at 360°C for 8h. It can be seen that the particles are quasi-spherical with an average size of ~14 nm, which is consistent with our calculated result based on the XRD data. The annular pattern appearing on the selected area electron diffraction (SAED) image, shown in Fig. 2(b), is another view of the crystallites in the glass. In Fig. 2(b), outward from the core, the diffraction rings can be indexed to the (111), (200), (220) and (311) plane diffraction of PbTe₃O₇ nanocrystal, respectively. The HRTEM image, as shown in Fig. 2(c), shows the resolved lattice fringes with a constant spacing of 0.33 nm, ascribing to the (111) plane of PbTe₃O₇ nanocrystal. To detect the distribution of Er³⁺ ions in the GCs, the EDS spectra taken from the nanocrystals-rich region and glass matrix are shown in Fig. 2(d). Te, O, Pb, and Er peaks are detected in the nanocrystals-rich region, while very weak Er signal are found in the glass matrix. These results indicate that Er³⁺ ions preferentially enter into the precipitated PbTe₃O₇ nanocrystals after crystallization. In addition, Er³⁺ ions also probably act as the nucleation centers of PbTe₃O₇ nanocrystals and enrich in the nanocrystals. The two-dimensional mapping distribution of Te, O, Pb, and Er image, illustrated in Fig. 2(e)-2(h), shows that the distribution region of Te, O, Pb, and Er are almost overlapping, which further confirms that Er³⁺ ions enrich in the precipitated PbTe₃O₇ nanocrystals.

 

Fig. 2 (a) TEM micrograph, (b) the corresponding SAED pattern, and (c) HRTEM image, (d) EDS spectra of 5% Er³⁺-doped GC sample heat-treated at 360°C for 8 h. (e)-(h) the two-dimensional mapping distribution images of Te, O, Pb, Er elements, respectively. The circle 1 and circle 2 in figure (a) indicate the nanocrystals-rich region and glass matrix, separately.

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The absorption spectra of 3% Er³⁺-doped tellurite glass and GC heat-treated at 370°C in the range from 500 to 3200 nm are shown in Fig. 3(a). Five absorption bands at 520 nm, 652 nm, 800 nm, 975 nm and 1532 nm are found and attributed to the transitions of Er³⁺ ions from the ground state ⁴I1_5/2 to the excited states ²H_11/2, ⁴F_9/2, ⁴I9/2, ⁴I_11/2 and ⁴I_13/2 respectively. The baseline of the absorption curve becomes precipitous toward to the short wavelength region when the temperature reaches 370°C. It can be ascribed to the transmittance loss causing by the Rayleigh scatting effect, which is resulted from the growth of PbTe₃O₇ crystallite. Figure 3(b-d) show the absorption spectra of the glass doped with different amounts of Er³⁺ heat-treated at 360°C, 370°C, and 380°C, respectively. From the figures it can be easily found that the absorption intensifies with the increase of Er³⁺-doped concentration by virtue of the incorporation of more Er³⁺ ions into the PbTe₃O₇ nanocrystals. The inset of Fig. 3(a) shows the transmittance spectra of 3% Er³⁺-doped as-prepared glass. The maximum transmittance around 3 μm due to the stretching vibrations of -OH group is ~70%. Further reduction of -OH contamination can be accomplished via purification during glass melting.

 

Fig. 3 . (a) Absorption spectra of 3% Er³⁺-doped as-prepared glass and GC heat-treated at 370°C for 8 h. (b)-(d) Absorption spectra of Er³⁺-doped GCs heat-treated at 360°C, 370°C, and 380°C for 8 h with the Er³⁺ concentration of 1%, 3%, and 5%. The inset in figure (a) shows the transmittance spectra of 3% Er³⁺-doped as-prepared glass.

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Based on the absorption spectra, the Judd–Ofelt intensity parameters Ω_λ (λ = 2, 4, 6) can be determined and their values indicate the variation of the Er³⁺ environment from glass to GCs. The calculated Judd–Ofelt intensity parameters are presented in Table 1 . According to Judd-Ofelt theory, Ω₂ is sensitive to the environmental configuration symmetry of rare earth ions, and it decreases with the host changing from glass to GCs. The gradual decrease of Ω₂ value with the increase of heat-treated temperature indicates Er³⁺ ions are successfully incorporated into PbTe₃O₇ nanocrystals. On the other hand, the value of Ω₆, which is proportional to the rigidity of the host, increases with the increase of heat-treated temperature manifesting the mechanical properties of samples are improved gradually [25].

Table 1. J–O intensity parameters of Er³⁺ in 3% Er³⁺-doped samples

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To investigate the mid-infrared 2.7 μm emission characteristics of the Er³⁺-doped tellurite glasses and GCs, the emission spectra of the samples under a 980 nm LD excitation are shown in Fig. 4. Figure 4(b) shows the 2.7 μm emission spectra of 3% Er³⁺-doped as-prepared glass and GCs heat-treated at different temperature for 8 h. Obvious emission band centered at 2.7 μm corresponding to Er³⁺: ⁴I_11/2→⁴I_13/2 transition can be observed. For all samples, the locations of the emission peak are fixed, but the intensities of 2.7 μm emission peak vary to great extent. Weak 2.7 μm emission is found in the as-prepared glass. However, with the increase of heat-treated temperature, intense 2.7 μm emission of Er³⁺ ions can be observed in the GCs, indicating the enhancement of 2.7 μm emission originated mainly from the Er³⁺ ions in PbTe₃O₇ nanocrystals. It is well known that the intensity of 2.7 μm emission is sensitive to the crystal field effect which depends on the crystallinity of their matrix. According to the XRD pattern, the higher the heat-treated temperature is, the better the crystallinity of host matrix is, inducing the enhancement the crystal field effect. Due to the intensification of crystal field effect, the 2.7 μm emission of Er³⁺ in the PbTe₃O₇ nanocrystals will be stronger than that in the as-prepared glass. As a result, the 2.7 μm emission intensity of the GCs increased significantly with the increasing crystallization temperature.

 

Fig. 4 2.7 μm mid-infrared emission spectra of (a) 1% Er³⁺, (b) 3% Er³⁺, and (c) 5% Er³⁺-doped as-prepared glass and GCs heat-treated at different temperature for 8 h pumped by a 980 nm LD.

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Figures 4(a) and 4(c) show the 2.7 μm emission spectra of 1% and 5% Er³⁺-doped as-prepared glass and GCs heat-treated at different temperature for 8 h, respectively. It is found that with the Er³⁺ concentration increasing from 1 to 5 mol %, the 2.7 μm emission intensity increases when the temperature is below 350°C. It could be ascribed to the addition of Er³⁺ will increase both the luminescence activators and the number density of PbTe₃O₇ nanocrystals. However, when the temperature reaches above 370°C, the 2.7 μm emission intensity of 3% Er³⁺-doped GC is much stronger than that of 5% Er³⁺-doped GC, which is probably due to the concentration quenching effect. According to the previous results, the Er³⁺ ions preferentially enter into the precipitated PbTe₃O₇ nanocrystals after crystallization at higher temperature leading to a much higher concentration of Er³⁺ ions in PbTe₃O₇ nanocrystals higher than 5 mol%, and thus inducing the concentration quenching effect. In this factor, the probability of energy migration among Er³⁺ ions increases with the increase of Er³⁺ concentration resulting in their higher accessibility to the OH^- quenching center. Therefore, the 2.7 μm emission intensity decreases. Based on the above results, it can be concluded that tellurite glass doped with 3% Er³⁺ and heat-treated above 370°C is the optimal condition to produce an intensive 2.7 μm emission in Er³⁺-doped GCs.

Fluorescence lifetime is an important parameter to estimate the emission properties of the excited level. The lifetimes of Er³⁺: ⁴I_11/2 level (980 nm) and ⁴I_13/2 level (1530 nm) excited by 808 and 980 nm LD shown in Figs. 5(a) and 5(b) exhibit as a function of the heat-treated temperature. With the increase of heat treatment temperature, the lifetime of ⁴I_11/2 and ⁴I_13/2 level increases from 0.20 to 1.08 ms and from 1.04 to 3.89 ms, respectively. These results indicate that more Er³⁺ ions have entered into PbTe₃O₇ nanocrystals and the nonraditive relaxation probability decreased by increasing the heat treatment temperature.

 

Fig. 5 (a) and (b) Fluorescence decay curves of Er³⁺: ⁴I_11/2 and Er³⁺: ⁴I_13/2 level pumped by 808 and 980 nm pulsed LD, respectively, in 3% Er³⁺-doped as-prepared glass and GCs heat-treated at 340°C-380°C for 8 h; (c) and (d) Er³⁺-doped concentrations and heat treatment temperature v.s. the lifetime of Er³⁺: ⁴I_11/2 and Er³⁺: ⁴I_13/2 level, respectively.

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Figures 5(c) and 5(d) show the contrast of lifetime at ⁴I_11/2 and ⁴I_13/2 level produced by Er³⁺-doped glasses with different heat treatment temperature and Er³⁺-doped concentrations. It is observed that with the increase of Er³⁺-doped concentration from 1 to 5 mol %, the lifetimes of ⁴I_13/2 levels decreases. As we know, shorter lifetimes of ⁴I_13/2 level are beneficial to the population inversion between upper ⁴I_11/2 level and lower ⁴I_13/2 level and can improve 2.7 μm emission. Hence, it is expected that 2.7 μm emission can be enhanced. However, when the temperature reaches above 360°C, the lifetimes of ⁴I_11/2 levels in 5% Er³⁺-doped GCs are shorter than that of 3% Er³⁺-doped GCs and the 2.7 μm emission intensity of 5% Er³⁺-doped GCs is much weaker than that of 3% Er³⁺-doped GCs, which is probably attributed to the concentration quenching effect.

It is widely acknowledged that both absorption cross section and emission cross section are very important factors to obtain efficient laser output. The larger their values are, the more efficiently laser generate. On the basis of discussion above, it can be concluded that tellurite glass doped with 3% Er³⁺ and heat-treated above 370°C possesses optimal luminescence properties. Herein, by selecting the GC sample doped with 3% Er³⁺ and heat-treated at 380°C, the absorption and emission cross sections were calculated to further estimate its possibility as mid-infrared laser gain material. The emission cross section (σem) can be calculated from the emission spectra by Fuchtbauer-Ladenburg equation [26]

The absorption cross section (σ_abs) was calculated from the emission cross section by using McCumber formula equation [27]

 

Fig. 6 (a) Absorption and stimulated emission cross sections, and (b) gain cross sections at 2.7 μm in 3% Er³⁺-doped GC heat-treated at 380°C for 8 h.

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Besides, the gain cross section (G) was calculated derived from (σ_abs) and (σ_em) to evaluate the mid-infrared gain properties. The room temperature gain cross section can be simply evaluated by [31]

4. Conclusion

In this work, Er³⁺-doped transparent tellurite GCs containing PbTe₃O₇ nanocrystals have been prepared. The XRD and TEM results confirmed that PbTe₃O₇ nanocrystals were precipitated in the glass matrix with a significant number of Er³⁺ ions incorporated after a proper heat-treatment. Owing to the weak absorption band around 3 μm and efficient crystal field, an intense 2.7 μm emission of Er³⁺: ⁴I_11/2→⁴I_13/2 transition was observed in the transparent GCs compared to that of as-prepared glasses and the optimal condition for 2.7 μm emission is 3% Er³⁺-doped tellurite glass heat-treated above 370°C. Furthermore, the prepared tellurite GC possesses high stimulated emission cross section (0.80 × 10^-20 cm²) and a low pumping threshold, which endows its potential applications in mid-infrared laser or amplifier.