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Broadband near-infrared luminescence covering 900 to 1600 nm has been observed in Bi-doped oxyfluoride silicate glasses. The partial substitution of fluoride for oxide in Bi-doped silicate glasses leads to an increase of the intensity and lifetime of the near-infrared luminescence and blue-shift of the near-infrared emission peaks. Both Bi-doped silicate and oxyfluoride silicate glasses show visible luminescence with blue, green, orange and red emission bands when excited by ultra-violet light. Careful investigation on the luminescence properties indicates that the change of near-infrared luminescence is related to optical basicity, phonon energy of the glass matrix and crystal field around Bi active centers. These results offer a valuable way to control the luminescence properties of Bi-doped materials and may find some applications in fiber amplifier and fiber laser.
©2012 Optical Society of America
1. Introduction
The rapid development of telecommunication technology has shown great demand for optical fiber transmission with ultra high speed and ultra large capacity. For their super broad near-infrared (NIR) emission covering the whole optical fiber communication windows, Bi-doped NIR emission materials have been widely studied over the past decade, and NIR emission from many kinds of bulk glasses [1–5], glass fibers [6, 7] and films [8], ionic liquid [9], conventional and molecular crystals [10–18] have been reported. Significantly, efficient all-fiber optical amplifiers and lasers have been fabricated [6, 7]. However, there remain three primary issues for the potential applications of bismuth activated glasses:
1. Till now, much work has put weight on Bi ions with low valence state or clusters as the origin of NIR emission [1–5, 9–18]. The PL origin in bismuth doped glasses is still not clear, although the structure-property relationships of several bismuth NIR active centers have been discussed in detail [9, 15–18]. It is believed that thorough evaluation of bismuth active centers in glasses will serve to the design of high-quality glass systems containing the optimal bismuth species, which is important not only for the development of optical devices using such smart systems, but also is helpful to solve some basic issues.
2. The high drawing temperature of oxide optical fibers leads to depletion of Bi species resulting in lower residual concentration and inhomogeneous distribution of Bi across the fiber diameter [19, 20]. For example, in the germano-aluminosilicate fiber perform, the residual bismuth concentration was ~50 ppm, and there were two inhomogeneously distributed pure silica subnetwork and aluminosilicate subnetwork with bismuth active centers distributed unevenly across the fiber core [20].
3. Usually, Bi ions with different valence states or Bi clusters coexist in glasses. Up to now, except for the preparation of silica glass with a single Al-connected Bi-center [21], there is no effective way to prepare glasses doped with single Bi NIR active center.
For applications as fiber laser and amplifier medium, Bi-doped glasses should possess many good properties besides excellent NIR luminescence characteristic, such as low viscosity, low melting temperature, low fiber drawing temperature, high doping level, and good chemical stability etc.
Oxide glasses have good chemical and optical stability, but suffer from large phonon energy. On the contrary, fluoride glasses have low phonon energy, low melting temperature, high doping level, low viscosity, wide transparency from UV to IR, but they are very vulnerable to possible contamination by oxygen ions during melting and have low mechanical durability, thermal and chemical resistivity [22]. Oxyfluoride glasses can potentially comprise the best advantages of both oxide glasses and fluoride glasses. Due to these advantages, many investigations have already been carried out on rare-earth ions-doped oxyfluoride glasses, and the oxyfluoride silicate glass fibers [23]. With these materials, researchers have successfully manufactured optical fiber amplifiers [24], optical waveguides [25]. To date, however, there is no report on Bi-doped oxyfluoride silicate glasses.
In this investigation, Na2O and MgO were substituted by NaF and MgF2 in Bi-doped Na2O-MgO-Al2O3-SiO2 glass, and the luminescence properties of the silicate and oxyfluoride silicate glasses were compared. The intensity and lifetime of NIR luminescence increase and emission peaks show blue-shift for oxyfluoride silicate glass in comparison with silicate glass. Visible luminescence properties were also examined to reveal the mechanism of these changes. Blue, green, orange and red luminescence was observed in both glasses. Compared with that of silicate glass, the intensity of visible luminescence all decreases for oxyfluoride silicate glass. The physical mechanism of the luminescence behavior is discussed. The results are valuable for the design of Bi-doped materials with controllable luminescence and Bi-doped optical fibers with low drawing temperature.
2. Experimental
Glass samples with the compositions of 10Na2O-20MgO-20Al2O3-50SiO2-1.0Bi2O3 (marked as MO) and 10NaF-20MgF2-20Al2O3-50SiO2-1.0Bi2O3 (marked as MF) (in mol%) were prepared by the melt-quenching method using analytical grade reagents NaCO3, NaF, MgO, MgF2, Al2O3, SiO2 and Bi2O3 as raw materials. 20 g batches corresponding to each glass were mixed homogeneously in agate mortar and then melted in platinum crucibles at 1550 °C for 30 min in air. The melts were cast onto a stainless steel plate and then annealed at 350 °C for 5 h. The obtained glasses were cut and polished into pieces with the size of 5 × 5 × 2 mm3.
The excitation (PLE) and emission (PL) spectra and the fluorescence decay curves were all measured using an FLS920 fluorescence spectrophotometer (Edinburgh Instrument Ltd., U.K.). The spectral resolution and time resolution was 1.0 nm and 1.0 μs, respectively. The bandwidth of slit was 2 mm. The Absorption spectra were recorded using a UV3600 UV-Vis-NIR spectrophotometer (Shimadzu Corp., Japan). All the measurements were taken at room temperature.
3. Results and discussion
The broad NIR PL of Bi-doped glasses is utmost important for optical fiber communication and fiber laser applications. NIR PL spectra of MO and MF glasses excited by 680 nm are shown in Fig. 1(a) . We summarize the dependence of peak position and relative intensity of NIR PL on excitation wavelength for MO and MF glasses as shown in Fig. 1(b) and 1(c). Compared with MO, the emission peaks of MF show blue-shift and the emission intensity of MF is enhanced at each excitation wavelength. Recently, Bi+ and Bi0 have been proposed as the origin of NIR luminescence, and the emission peaks at 1100 and 1260 nm have been ascribed to Bi+ and Bi0 ions, respectively [3]. The model is quite reasonable to explain the observed phenomena though further investigations may be necessary to confirm the origins of the NIR emission. Here, we tentatively applied their explanations. The enhancement of the emission intensity indicates the increase of the concentration of Bi+ and Bi0 in MF glass compared with MO glass. Besides, the blue-shift of the peak position and the enhancement of emission intensity depend strongly on excitation wavelength. While excited at 800 nm, the emission peak has a blue-shift of 20 nm and the intensity increases by 1.1 times. By contrast, the emission peak excited by 680 nm shows a blue-shift for 100 nm and an enhancement of the intensity by 3 times. The decay curves and the lifetime with different excitation and emission wavelength of MO and MF glasses are presented in Fig. 1(d) and the inset, respectively. The mean lifetime is calculated by the equation:
where I(t) is the luminescence intensity at time t and Imax = I(t0) is the maximum of I(t). The curves show a bi-exponential decay with a lifetime of 114 μs for 1160 nm emission of MO glass and 415μs for 1080 nm emission of MF glass excited by 680 nm. We also estimated the lifetime using exponential fitting by software Origin and obtained similar values. The inset shows that the lifetime of Bi in MF glass is much longer than that in MO glass.
Fig. 1 (a) NIR PL spectra of MO and MF glasses excited by 680 nm. (b)-(c) Dependence of peak position and relative intensity of NIR PL on excitation wavelength for MO and MF glasses. (d) NIR PL decay curves for MO and MF glasses (excitation wavelength is 680 nm and the corresponding monitoring wavelength is 1160 and 1080 nm, respectively). The correlation coefficients for the fits by bi-expotential decay equation (MO: I = 0.35016e-t/286.8 + 0.60650e-t/21.8, MF: I = 0.55442e-t/643.4 + 0.43560e-t/133.0) are 0.996 for MO and 0.999 for MF. The inset is excitation wavelength dependent PL lifetime for MO and MF glasses (the excitation wavelength are 470, 560, 640, 680, 800 and 880 nm, the corresponding monitoring wavelength are as the peak position in Fig. 1(b)).
In order to reveal the mechanism of the change of NIR PL by the substitution of fluoride for oxide, we examined the visible luminescence. Figure 2 shows the PLE and PL spectra of samples MO and MF in the ultraviolet-visible region. Both the samples show blue and green luminescence of Bi3+ ions. The intensity of blue-green luminescence for MF glass is much lower than that of MO glass, indicating the reduction of Bi3+ ions in MF glass. The emission and excitation peaks show a blue-shift to 425 and 310 nm, respectively. The excitation peak of MO locating at 320 nm is due to the transition of Bi3+ from 1S0 to 3P1 [14]. The emission peak of MO locating at 440 nm is not a Gaussian peak. It has a side band in the green region which may be due to the existence of Bi3+ pairs or clusters whose luminescence band locates at lower energy side than that of isolated Bi3+ ions [26]. Moreover, the side band in the green region of the emission peak of MF glass is not as obvious as that of MO glass which may be due to the reduction of Bi3+. It indicates that the site symmetry around Bi3+ ions is higher in MF glass than that of MO glass.
Fig. 2 PLE spectra of MO glass monitored at 440 nm (curve a), and MF glass monitored at 425 nm (curve b). Visible PL spectra of MO excited by 320 nm (curve c), and MF excited by 310 nm (curve d).
Besides the blue-green luminescence, orange-red luminescence was also observed. Figure 3(a) and 3(b) show the emission spectra of MO and MF glasses excited by 405 and 475 nm, respectively. When excited at 405 nm, two peaks are observed at 630 (marked as peak I) and 790 nm (marked as peak II) for MO glass. For MF glass, the emission intensity decreases and peak I and peak II show a blue-shift to 625 and 760 nm, respectively. In Fig. 3(b), peak I becomes the main peak. In contrast to NIR PL, the intensity of orange-red luminescence is lower for MF glass than that of MO glass. The change of the intensity is the same as that of blue-green luminescence. We examined the PLE spectra of MO glass as shown in Fig. 3(c). When monitored at 630 nm, two peaks at 325 and 475 nm are observed. Two excitation peaks at 320 and 405 nm are observed while monitored at 790 nm. It is reported that Bi2+ ions have orange-red luminescence in many crystals and glasses [2, 13]. Two peaks at 716 and 733 nm were observed in Bi-doped Ba2P2O7 crystal which is ascribed to Bi2+ ions occupying two Ba2+-sites with different Ba-O bond length [13]. So the unusual red luminescence may be ascribed to Bi2+ ions in two different sites.
Fig. 3 (a)-(b) Visible PL spectra of MO and MF glasses excited by 405 and 475 nm, respectively. (c) PLE spectra of MO glass monitored at 630 and 790 nm, respectively.
Absorption spectra of MO and MF glasses are presented in Fig. 4 . In contrast with that of MO glass, the absorption edge of MF glass shifts to short wavelength. Bi3+ ions have strong absorption in UV region [26]. Hence this blue shift implies that less Bi3+ ions exist in MF glass than those in MO glass which agrees well with the results of Fig. 2. An absorption band at 490 nm with a shoulder at 700 nm can be observed in the absorption spectra of MO glass. The absorption spectra of MF glass show two peaks at 490 and 700 nm with a shoulder around 800 nm. These absorption bands are ascribed to Bi+ and Bi0 [1, 27]. The absorption coefficient of MF glass is obviously larger than that of MO glass indicating that more Bi+ and Bi0 exist in MF glass than MO glass, which is consistent with the results of Fig. 1.
As can be seen from the above, the substitution of fluoride for oxide leads to more Bi+ and Bi0 with less Bi2+ and Bi3+ in the glass. It is reported that low optical basicity favors Bi ions with low valence state, such as Bi NIR active centers [1, 28]. The theoretical optical basicity (Λth) of glasses can be calculated using the Eq. (2):
where Λn are the optical basicity values of each composition of glass, and Xn is equivalent fractions based on the amount of oxygen or fluorine each contributing to the overall glass stoichiometry. The optical basicity values Λn are 1.15, 0.78, 0.60, 0.48, 1.19, 0.50, 0.34 for Na2O, MgO, Al2O3, SiO2, Bi2O3, NaF and MgF2, respectively [29, 30]. Thus the optical basicity valuesfor MO and MF glasses are 0.5941 and 0.4984, respectively. It is apparently that MF glass has lower optical basicity than MO glass which favors the formation of Bi+ and Bi0 rather than Bi3+ and Bi2+. So the intensity of visible luminescence in MF glass is lower than that of MO glass, but the intensity of NIR luminescence increases. Fluorides have lower phonon energy (~500cm−1) than many oxides such as silica based glasses (~1200cm−1) [31], the incorporation of fluorides will decrease non-radiative decay rates of multiphonon processes which also results in the increase of the intensity and lifetime of NIR luminescence. The blue shift of the emission peaks indicates the formation of Bi-F bond in MF glass. Since the optical transition of Bi active centers are based on p electrons which are in the outer layer of atoms and different from the f-f transition of rare-earth ions, multiple factors can affect the optical transition of Bi active centers, such as spin-orbit interaction, crystal field and the additional electrostatic interaction [2, 28, 32]. Fluorine anions have larger electronegativity than oxygen ions, the Bi-F bond is less covalent than Bi-O bond, resulting in longer average cation–ligand distance in MF glass than that in MO glass and weaker crystal field. All these changes cause the increase of the lowest excited energy level of Bi active centers leading to the blue-shift of all emission peaks as discussed above. For Bi+ cations are more electropositive than Bi0, they are much easier to form bond with F anions. So the dependence of the blue shift of emission peak position and the enhancement of emission intensity on excitation wavelength in Fig. 1 may be due to the different environment and different increase rate of the concentration for Bi+ and Bi0 in MF glass.4. Conclusion
In summary, enhanced NIR luminescence of Bi-doped oxyfluoride silicate glasses has been observed. And both Bi-doped silicate and oxyfluoride silicate glasses show blue, green, orange and red visible luminescence. The partial substitution of oxides by fluorides enhances the conversion of Bi3+ and Bi2+ to Bi+ and Bi0 and leads to lower optical basicity, lower phonon energy, and weaker crystal field which results in the reduction of the intensity of visible luminescence and an increase of the intensity and lifetime of NIR luminescence with the blue-shift of all the visible and NIR emission peaks. For the NIR luminescence, the different extent of the blue-shift of emission peak position and the enhancement of emission intensity at different excitation wavelength may be due to the different environment and different increase rate of the concentration for Bi+ and Bi0 in glasses. The results provide a valuable way to control the luminescence properties and reveal the mechanism of near-infrared luminescence of Bi-doped materials and may have potential applications in fiber optics.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51132004, 51072054 and 51102209), National Basic Research Program of China (2011CB808100).
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