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Thermal stability and broadband NIR luminescence of Pr3+ doped gallo-germanate glasses with BaF2 have been studied. The thermal factors are larger for glass samples with low BaF2 content exhibiting good thermal stability against devitrification. Luminescence due to 1D2 → 1G4 transition of Pr3+ was measured under 450 nm excitation. The 1D2 measured lifetimes depend critically on activator concentration, but remain nearly unchanged with BaF2 content. The emission linewidth, the emission cross-section, the figure of merit (FOM) and the σem x FWHM product are relatively large, suggesting that Pr3+-doped gallo-germanate glasses with presence of BaF2 are promising as gain media for broadband near-infrared amplifiers.
© 2016 Optical Society of America
1. Introduction
Rare earth - doped inorganic glasses have attracted much attention to develop broadband optical amplifiers operating within near-infrared (NIR) low-loss wavelength region. Emission linewidths and lifetimes belong to important spectroscopic parameters of rare earths, which are necessary to characterize material for broadband optical amplifiers. The near-infrared luminescence corresponding to 4I13/2 → 4I15/2 transition of Er3+ has been often used in optical telecommunication for signal amplification. The relatively broad NIR luminescence band and long lifetime of the metastable level are required for the high population inversion, which is a critical factor in the success of Er3+-doped fiber amplifiers in the optical communications. However, the commercial Er3+ doped fiber amplifiers (EDFA) based on silica glass exhibit a relatively narrow linewidth that limits broadband NIR transmission [1]. For that reason, it is necessary to search for a new glass host matrices with rare earths exhibiting relatively broader emission lines and longer lifetimes for signal amplification in optical telecommunication.
Among the rare earth ions, trivalent Pr3+ exhibits broadband near-infrared luminescence covering a wavelength range from 1.2 μm to 1.7 μm, which is important for optical fiber amplifiers operating at O-, E-, S-, C-, and L-band [2]. Several previously published works indicate that Pr3+ ions in various inorganic glasses are attractive to explore superbroadband luminescence sources in NIR low-loss transmission window and promising candidates for the optical amplifications [3–8 ]. Superbroadband NIR luminescence in low-loss wavelength region correspond to 1D2 → 1G4 transition of Pr3+. The Pr3+/Ln3+ co-doped systems, where Ln = Eu2+ [9] or Ce3+ [10] belong to the second group of NIR luminescent materials, which were developed as a potential solar spectral convertor for Si solar cells. In both cases, rigorous search is needed to find suitable host compositions with good thermal stability and unique spectroscopic properties for broadband NIR luminescence applications. Down-conversion processes in Pr3+/Yb3+ co-doped systems converting one blue photon into two near-infrared (NIR) photons have been also described. The emission of trivalent Yb3+ around 1000 nm can be absorbed by silicon solar cell without any losses due to its single 2F5/2 excited state, close to the silicon band gap (1.05eV). Owing to the broad absorption band of Pr3+ ions in the blue spectral region and possible resonant energy transfer to Yb3+, the Pr3+/Yb3+ couple is of special interest [11].
In this work, gallo-germanate glasses singly doped with Pr3+ ions have been studied for broadband luminescence. Luminescence spectra and their decays were examined as a function of BaF2 and activator concentration. Previous investigations were demonstrated that glass in BaO-Ga2O3-GeO2 chemical composition is known as a window for high energy laser HEL systems [12]. The near-infrared luminescence studies for germanate glasses doped with transition metal and/or rare earth ions are also well reported [13–16 ]. A special attention has been paid to barium gallo-germanate glass systems. Tm3+ doped barium gallo-germanate glass single-mode fibers have been investigated for application as NIR laser material at 2.0 μm [17]. The up-conversion luminescence phenomena of rare earth ions in barium gallo-germanate glasses and fibers were also presented and discussed in details [18, 19 ]. Only a few works are devoted to enhanced broadband NIR luminescence of transition metal/rare earth ions in mixed fluorogermanate glasses and glass-ceramics [20–22 ]. To the best of our knowledge, the NIR luminescence properties of Pr3+ ions in BaO-Ga2O3-GeO2 glasses modified by BaF2 were not frequently examined. The intention of our work was to prepare thermally stable Pr3+ doped gallo-germanate glasses and to characterize their NIR luminescence properties with BaF2 and activator concentration.
2. Experimental
Gallo-germanate glasses with the following chemical composition: 60GeO2-(30-x)BaO-xBaF2-(10-y)Ga2O3-yPr2O3, where x = 0, 1, 3, 5 and 10; y = 0.1, 0.25 and 0.5 (in mol%), were prepared by mixing and melting appropriate amounts of metal oxides and BaF2 of high purity (99.99%, Aldrich Chemical Co.). In order to prepare the glass samples, appropriate amounts of all components were mixed homogeneously together. Due to the hygroscopicity of the fluoride component and, in order to minimize the adsorbed water content, batches were weighted and stored in glove box, in a protective atmosphere of dried argon. Then, they were melted at 1200°C for 0.45h. Transparent glassy plates of 10x10 mm dimension were obtained. Each glass sample of 2 mm in thickness was polished for optical measurements. The nature of the studied samples was identified using the X-ray diffraction analysis (X’Pert X-ray diffractometer). The characteristic temperatures of the received glasses were determined based on the measurement taken with a SETARAM Labsys thermal analyzer using the DSC method. The refractive index at a wavelength of 632.8 nm was determined using the Metricon 2010 prism coupler. Optical absorption spectra were recorded using a Varian 5000 UV–VIS–NIR spectrophotometer. Excitation and luminescence measurements were performed on a PTI QuantaMaster QM40 coupled with tunable pulsed optical parametric oscillator (OPO), pumped by a third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD). The luminescence was dispersed by double 200 mm monochromators. The luminescence spectra were recorded using a multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors controlled by a computer. All measurements were carried out at room temperature.
3. Results and discussion
3.1 Thermal characterization
Influence of BaF2 concentration on thermal behavior of Pr3+ doped gallo-germanate glasses has been investigated using differential scanning calorimetry (DSC). Figure 1(a) shows the DSC curves for Pr3+-doped gallo-germanate glasses measured under standard heating rate of 100C/min. From DSC curves glass transition temperature Tg, crystallization onset Tx and maximum of crystallization peak Tp were evaluated. Based on characteristic temperatures obtained for the studied glass samples, the thermal stability parameter ΔT was also calculated.
Fig. 1 DSC curves for gallo-germanate glasses containing Pr3+ ions (a) and the variation of glass transition temperature Tg (b) and thermal stability factor ΔT (c) with BaF2 content.
All characteristic temperatures and thermal stability parameters were determined with accuracy of 0.5°C. In general, the difference between the crystallization onset Tx and the glass transition temperature Tg (ΔT = Tx-Tg) is usually chosen as an approximate measure of glass formation ability. The larger value of ΔT gives a larger working range during operations for fiber drawing. If ΔT is higher than 100°C, glass can be considered as a glass with relatively good thermal ability. The variation of glass transition temperature Tg and thermal stability parameter ΔT with BaF2 content is presented in Figs. 1(b) and 1(c). Moreover, the Hruby parameter using the following relation H = (Tx-Tg)/Tg and the Saad-Poulain criterion referred as S = (Tx-Tg)(Tp-Tx)/Tg were applied to further evaluate the thermal glass stability. Thermal parameters for gallo-germanate glasses varying with BaF2 content are listed in Table 1 .
Table 1. Thermal parameters for gallo-germanate glasses with BaF2 content.
Zhang et al [23] suggest that only one type of phase evolution mechanism exist during heat treatment of glass samples with different BaF2 content, because only one exothermic peak was observed on each DSC curve. In our case, the systematic investigations using DSC method give interesting results. For glass without BaF2, only one nearly symmetrical exothermic peak corresponding to crystallization of the glassy matrix appears. The exothermic peak becomes unsymmetrical and shifts to lower-temperature region, when BaO was partially substituted by BaF2 in glass composition. On the DSC curve measured for glass sample with 10 mol% BaF2, the glass transition peak is followed by the additional well resolved crystallization peak, which could be probably assigned to the precipitation of BaF2 crystals. As the BaF2 content increases the values of glass transition temperature reduce rapidly suggesting that the BaF2 content acts as a glass network modifier. Furthermore, the thermal stability factors ΔT, H, S shown in Table 1 are larger for glass samples with low BaF2 content (up to 5 mol%) and their values are similar to that one obtained for Pr3+-doped heavy metal germanium tellurite glass [24] and fluorotelurite glass containing BaF2 [25]. It suggests that our low BaF2 concentrated glass samples exhibit good thermal stability against devitrification, giving a larger working range during operations for fiber drawing.
3.2 Broadband near-infrared luminescence
Figure 2(a) presents absorption, excitation as well as luminescence spectra measured for Pr3+ ions gallo-germanate glasses in the visible ranges. All luminescent transitions of Pr3+ are also indicated on the energy level scheme. Absorption spectrum contains characteristic narrower bands, which correspond to transitions originating from the 3H4 ground state to the higher-lying 3P0, 1I6, 3P1, 3P0 and 1D2 states of Pr3+. The absorption bands located at visible region are also observed on the excitation spectrum, which was monitored at λem = 620 nm. In order to detect visible emission of Pr3+, the glass samples were excited at 450 nm. Several observed emission bands are due to 3P0 → 3H4, 3P0 → 3H5, 1D2 → 3H4, 3P0 → 3H6 and 3P0 → 3F2 transitions of Pr3+, respectively. Two of them, 3P0 → 3H4 (blue) and 1D2 → 3H4/3P0 → 3H6 (reddish orange) transitions are the most intense lines. Their relative integrated intensities depend strongly on activator concentration. Furthermore, the activator (Pr3+) concentration is a relevant factor that must be taken into account when self-absorption at the lower wavelength side of emission transition band, due to the superimposing of absorption and emission is quite possible. Considering the spectral overlap between the high-energy emission side and the low-energy absorption side of Pr3+: 3P0 ↔ 3H4, some energy in the higher sub-levels of Pr3+: 3P0 can be absorbed by a nearby Pr3+ in the ground state when they are in a close separation. It results in a depleting of the higher energy sub-levels of Pr3+: 3P0 [4]. Ferrari et al [26] suggest that the degree of self-absorption for the 3P0 → 3H4 transition of Pr3+ also varies with the penetration depth. Our previous experimental results indicate that these phenomena are related to the activator concentration. Details are given in [27]. In comparison to 3P0 → 3H4 transition, the intensity of luminescence originating from the 1D2 state is reduced significantly with increasing Pr3+ concentration. These phenomena related to luminescence quenching from the 1D2 state of Pr3+ in inorganic glasses [28], powders [29] and single crystals [30] were well presented and discussed in details.
Fig. 2 Typical absorption, excitation and visible emission for Pr3+-doped gallo-germanate glass (a). NIR luminescence spectra measured under 450 nm (3P2) and 590 nm (1D2) excitation correspond to 1D2 - 3F3,4 (I), 1G4 - 3H5 (II), 1D2 - 1G4 (III) and 3F3,4 - 3H4 (IV) transitions of Pr3+ (b). All transitions of Pr3+ are also indicated on the energy level scheme.
NIR luminescence of Pr3+ ions in gallo-germanate glasses presented on Fig. 2(b) was measured in the 950 - 1700 nm spectral ranges. Four NIR luminescence bands (I-IV) are well observed under 450 nm (3P2) or 590 nm (1D2) excitation. Based on energy level diagram and positions of the excited states of Pr3+, the first near-infrared luminescence band located at about 1050 nm can be assigned to 3P0 → 1G4 [6] or 1D2 → 3F3,3F4 [31] transition of Pr3+. Our investigations clearly indicate that the profiles of luminescence bands near 1050 nm measured under direct excitation of the lower-lying 1D2 state by 590 nm line are practically the same in comparison to the experimental results obtained for glass samples excited at 450 nm (3P2). It evidently suggests that the near-infrared luminescence band (I) is related to 1D2 → 3F3,3F4 transition of Pr3+. Moreover, broadband near-infrared spectra covering a wavelength range from 1300 nm to about 1650 nm consist of three less-resolved luminescent lines. The shoulder at about 1350 nm (II) is due to 1G4 → 3H5 transition of Pr3+, which is important for optical fiber amplifiers operating at the second telecommunication window [32, 33 ]. The main most intense NIR luminescence band centered at about 1500 nm (III) is assigned to 1D2 → 1G4 transition of Pr3+. Also, the broad near-infrared luminescence can contain a contribution from 3F3,3F4 → 3H4 transition of Pr3+ (IV) at about 1600 nm [34]. In comparison to both 1G4 → 3H5 (II) and 3F3,3F4 → 3H4 (IV) transitions of Pr3+, the intensity of NIR emission band originating from the 1D2 state of Pr3+ (III) is reduced with increasing activator concentration. It confirms the experimental results obtained from emission measurements in the visible spectral region.
Influence of activator concentration and presence of BaF2 on broadband near-infrared luminescence of Pr3+ ions in gallo-germanate glasses is presented on Figs. 3(a) and 3(b) . In order to compare luminescence linewidths, the spectra were normalized. There is evidently see that the relative intensities of NIR luminescence bands related to 1G4 → 3H5 (1350 nm), 1D2 → 1G4 (1500 nm) and 3F3,3F4 → 3H4 (1600 nm) transitions of Pr3+ ions are changed with activator concentration. For glass samples with 5 mol% BaF2, the intensities of main NIR luminescence lines due to 1D2 → 1G4 transition are reduced with increasing Pr3+ concentration from 0.1 to 0.25 and 0.5 mol %, when we compare to 1G4 → 3H5 and 3F3,3F4 → 3H4 transitions of Pr3+, respectively.
Fig. 3 Influence of activator concentration (a) and presence of BaF2 (b) on broadband near-infrared luminescence of Pr3+ ions in gallo-germanate glasses.
Non-radiative energy transfer processes among the Pr3+ ions in gallo-germanate glasses are responsible for the reduction of the luminescence intensity. Similar to Pr3+ doped silicate glasses examined by Choi et al [31], two cross-relaxation routes that depopulate the 1D2 state are proposed:
The inset of Fig. 3(a) shows these cross-relaxation routes, which contribute quite well to population of lower-lying 1G4 and 3F3,3F4 excited states of Pr3+. Further analysis indicates that intensity of near-infrared luminescence band due to 3F3,3F4 → 3H4 transition of Pr3+ is reduced with increasing BaF2 content, as presented in Fig. 3(b). For the studied glass samples with presence of BaF2, the concentration of Pr3+ was equal to 0.1 mol%. Moreover, the position of broadband NIR luminescence band associated to 1D2 → 1G4 transition of Pr3+ in gallo-germanate glass is shifted to shorter wavelengths (blue shift) with increasing BaF2 content. Similar phenomena have been observed for hypersensitive 3P0 → 3F2 transition of Pr3+ [see Inset of Fig. 3(b)]. Details are given in our previously published work [35]. Also, early investigations have well demonstrated that NIR luminescence at 1530 nm corresponding to main 4I13/2 → 4I15/2 laser transition of Er3+ ions in the same germanate glass host matrices depend on BaF2 content [36]. Luminescence linewidth defined as the full width at half maximum (FWHM) is changed significantly with substitution BaO by BaF2. Here, the linewidth for NIR luminescence of Pr3+ remains nearly unchanged. Its FWHM value was found to be 208.5 ± 1.5 nm, independently on BaF2 content. Furthermore, the luminescence decay curves for the 1D2 state of Pr3+ ions in gallo-germanate glasses containing 5 mol% BaF2 were carried out, as shown in Fig. 4 . The curves were measured under excitation of 3P2 state (λexc = 450 nm) and monitoring emission wavelength λem = 1500 nm, respectively.Luminescence decays from 1D2 state were examined as a function of Pr3+ concentration. Based on decays, luminescence lifetimes for 1D2 state of Pr3+ ions in gallo-germanate glasses were determined. Luminescence decay analysis clearly indicates that the measured lifetime starts to reduce from 110 μs (0.1% Pr3+) up to 32 μs (0.25% Pr3+) and 12 μs (0.5% Pr3+) for glass samples with higher activator concentration. For glass sample containing 0.1 mol% Pr3+, the measured lifetime close to τm = 110 ± 2 μs is nearly independent on BaF2 concentration. Similar results were obtained for phosphate glass, where the 1D2 experimental lifetime of Pr3+ (0.1%) is derived to be 107.9 μs [5]. The glass samples with relatively long 1D2 measured lifetime of Pr3+ (0.1 mol%) and improved thermal stability (up to 5 mol% BaF2) were selected for further spectroscopic analysis. The luminescence linewidths and lifetimes were applied to evaluate stimulated emission cross-section, which is an important spectroscopic parameter affecting the potential laser performance. The stimulated emission cross-section can be calculated using the following equation [37]:
where λp is the emission peak wavelength, c is the velocity of light, n is the refractive index which is close to 1.736 for gallo-germanate glass, Δλ is the luminescence linewidth of the emission band and τm is the measured lifetime. The results are summarized in Table 2 .Table 2. Spectroscopic parameters for Pr3+ ions in gallo-germanate glasses with BaF2 content.
Large stimulated emission cross-section profile reveals that Pr3+ doped glasses exhibit fascinating prospects in rare-earth ion single-doped ultra-broadband gain-flatten near-infrared fiber amplifier [2]. In our case, the stimulated emission cross-section depends slightly on BaF2 content. The average stimulated emission cross-section seems to be 0.98 ± 0.02 (x10−20cm2). The value of σem obtained for the 1D2 → 1G4 transition of Pr3+ ranges between 0.9x10−20cm2 in fluorotellurite glass [4] and 1.14x10−20cm2 in phosphate glass [5], respectively. Finally, the figure of merit (FOM) given by σem x τm and the σem x FWHM product were determined. From literature it is well known that luminescent transition with large values of σem exhibits low threshold and high gain laser operation [38]. In our case, the values of σem and σem x τm given in Table 2 are relatively large, suggesting that Pr3+-doped gallo-germanate glasses are promising as gain media for broadband near-infrared amplifiers. Furthermore, the emission linewidth (FWHM) and the stimulated emission cross-section (σem) were applied to calculate the gain bandwidth. There are also important spectroscopic parameters to achieve broadband and high gain amplification. The gain bandwidth of an optical amplifier can be estimated from the σem × FWHM product. The earlier spectroscopic investigations demonstrate that quite large σem × FWHM products were obtained for the 1D2 → 1G4 transition of Pr3+ ions in fluorotellurite glass (174.6 x 10−27cm3) [4] and phosphate glass (128.8 x 10−27cm3) [5]. Here, the average σem × FWHM product of Pr3+ in gallo-germanate glasses is close to 203.5 ± 3.5 x10−27cm3 and it was found to be larger compared to the previously reported inorganic glasses.
4. Conclusions
Thermally stable gallo-germanate glasses singly doped with trivalent Pr3+ were synthesized and next studied for broadband near-infrared luminescence. The glass samples were examined as a function of activator concentration and presence of BaF2. Based on DSC measurements, the characteristic temperatures and thermal stability factors ΔT, H and S were obtained. The thermal stability factors are larger for glass samples with low BaF2 content (up to 5 mol%) suggesting good thermal stability against devitrification and giving a larger working range during operations for fiber drawing. Next, they were selected for further near-infrared luminescence investigations.
Broadband near-infrared luminescence centered at 1500 nm corresponding to 1D2 → 1G4 transition of Pr3+ was measured under 450 nm excitation. The broad NIR luminescence of Pr3+ can also contain a contribution from 1G4 → 3H5 transition at 1350 nm and 3F3,3F4 → 3H4 transition at 1600 nm. Several spectroscopic parameters of Pr3+ in gallo-germanate glasses were determined. Luminescence decays from 1D2 state of Pr3+ ions were measured. The 1D2 measured lifetimes depend critically on activator concentration, but remain nearly unchanged with BaF2 content. Furthermore, the luminescence linewidths and measured lifetimes were applied to evaluate the stimulated emission cross-section, the figure of merit (FOM) σem x τm and the σem x FWHM product, which are important spectroscopic parameters affecting the potential laser performance. The spectroscopic parameters for thermally stable glass samples (up to 5 mol% BaF2) containing optimal activator content (0.1 mol% Pr3+) were evaluated. The average values of FWHM, τm, σem, σem x τm and σem x FWHM are close to 208.5 ± 1.5 nm, 110 ± 2 μs, 0.98 ± 0.02 x10−20cm2, 107.5 ± 0.5 x10−26cm2s and 203.5 ± 3.5 x10−27cm3, suggesting that Pr3+-doped gallo-germanate glasses with presence of BaF2 are promising as gain media for broadband near-infrared amplifiers.
Acknowledgments
The authors would like to thank Dr Lidia Żur and Mrs Marta Sołtys for spectra measurements. The project was funded by the National Science Centre (Poland) granted on the basis of the decision No. 2011/03/B/ST7/01743.
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