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Abstract
A cerium (Ce) and terbium (Tb) co-doped silica optical fiber (CTDF) is fabricated by the powder-in-tube technique. Its luminescence and energy transfer characteristics are investigated. The experimental results show that a strong excitation peak at 252 nm appears for the Tb-doped fiber materials. Another excitation peak with a similar intensity at 305 nm appears for the CTDF rod. The emission of Tb3+ ions at 542 nm is efficiently excited at 305 nm after Ce3+ ions doping in the silica fiber core. Moreover, the photoluminescence decay of CTDF rod materials at 542 nm is longer than that of Tb-doped fiber materials, from 1709.21 to 2173.17 μs. These results indicate that an obvious energy transfer process from Ce3+ to Tb3+ ions are achieved. Then, an energy level scheme with a local structure model of CTDF is built. Furthermore, numerical simulation results indicate that dipole-dipole interaction is the most-likely energy transfer mechanism. It is significant for the fabrication of Ce/Tb co-doped green emitting fiber lasers and remote fiber-optic radiation dosimeters.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terbium (Tb) doped materials have recently demonstrated promise in the radiation detection and green laser fields. C. A. G. Kalnins et al. developed a fluoride phosphate glass fiber for X-rays and beta-rays (β-rays) radiation dosimetry based on optically stimulated luminescence and improved the materials sensitivity by doping with Tb3+ ions [1]. A. A. Jalali used high optical quality of potassium terbium fluoride, KTb3F10, crystals to get laser powers up to 400 W [2]. However, the Tb3+ ions exhibit strong absorption in the range 240-260 nm, which is difficult to access because of the short wavelength [3]. Thus, it can be advantageous to use Cerium (Ce) ions as sensitizers because their allowed 4f-5d transition occurs at 320 nm, which corresponds to a more accessible and convenient wavelength region [3]. There is an efficient energy transfer process from Ce3+ to Tb3+ ions, which can significantly enable a redshift of excitation wavelength and strengthen the emission intensity of Tb3+ ions [3–7]. Y. H. Hu et al. prepared a Ba2Mg(PO4)2:Ce3+,Tb3+ phosphor and analyzed its photoluminescence properties and energy transfer between Ce3+ and Tb3+ ions experimentally [8]. Theoretically, H. A. A. Seed Ahmed et al. researched the luminescence characteristic and energy transfer mechanism between Ce3+ and Tb3+ ions in different kinds of Ce/Tb co-doped material such as glasses and nanoparticles [3, 9, 10]. Compared with glasses and crystals materials, silica optical fiber materials have unique advantages, such as electromagnetic insensitiveness, radiotolerance and simpleness for coupling with commercial fibers [11]. However, there has been little research reported on Ce/Tb co-doped silica optical fiber (CTDF), and the energy transfer process between Ce3+ and Tb3+ ions in the silica optical fibers has not been investigated.
In this work, Tb-doped and Ce/Tb co-doped silica optical fiber materials were prepared by a PIT technique [12, 13] combined with the sol-gel method. The photoluminescence (PL) spectra, including emission, excitation and decay characteristics were measured and analyzed. Finally, a brief energy level scheme with a local structure model for CTDF was built, and the energy transfer characteristics from Ce3+ to Tb3+ ions were analyzed by numerical simulation.
2. Experimental
The Ce/Tb co-doped silica optical fiber materials were prepared by a sol-gel technique. The source materials were silicon tetraethoxide (TEOS), distilled water, hydrochloric acid (as a catalyst), cerium nitrate (Ce(NO3)3), terbium nitrate (Tb(NO3)3), ethanol (EtOH) and germanium tetraethoxide (TEOG). In order to research the luminescence characteristic of Ce/Tb co-doped silica optical fiber materials, and analyze the mechanism of energy transfer from Ce to Tb, seven samples with different dopant compositions were prepared. The emission of Ce3+ ions is quenched for a high doping concentration [14]. Therefore, Ce3+ ions were doped in samples at a concentration of 0.05 mol %. Samples 1 to 6 were precursors for fiber materials prepared by sol-gel method and processed by a slowly heating treatment. Sample 1 was a Ce-doped fiber material. Sample 2 was doped with Tb3+ ions a concentration of 4 mol %. Samples 3 to 6 were Ce3+ and Tb3+ co-doped samples with the concentration of Ce3+ ions fixed at 0.05 mol % and that of Tb3+ ions at 1, 2, 4 and 6 mol %, respectively. Sample 7 was a slice of CTDF rod prepared by sample 5. The compositions of all samples are summarized in Table. 1. The synthesis process used was described as follows. First, the TEOS and EtOH were mixed, and subsequently, stirred for approximately 60min. Second, the cerium nitrate and terbium nitrate solution were dropped into TEOS and EtOH. A few drops of the hydrochloric acid were added to catalyze the reaction. Finally, the mixed solution was stirred at room temperature for 120 min, and the resulting homogenous sol was stored in a drying oven at 30 °C for several days. The gel-to-glass transition was driven by slowly heating treatment at 750 °C in the furnace and maintaining that temperature for 2h. Finally, the xerogel powders were filled into a quartz tube and drawn into a silica optical fiber in a fiber drawing tower by the PIT technique. To suppress the oxidation of trivalent cerium ions, a nitrogen (N2) atmosphere was provided in the heating treatment. In addition, before the drawing process, the helium (He) was filled into quartz tube to exhaust gases in tube. Then, a vacuum state for the quartz tube was supplied by a vacuum pump. In the drawing process, a N2 (purity: 99.9999%) atmosphere was also provided in the quartz tube. The elemental compositions of the CTDF were analyzed using a scanning electron microscope (JSM-7500F field emission SEM, Japan) combined with an energy dispersive spectrometer (EDS) (SEM-EDS, MX80-EDS, OXFORD, England). As shown in Fig. 1, the elemental concentrations of Ce and Tb elements in the CTDF core are approximately 0.26% and 4.18%, respectively.
Table 1. Composition of samples
The PL spectra of these samples were measured by a fluorescence spectrometer (FLS980, Edinburgh Instruments) with xenon lamp excitation. The energy transfer mechanism from Ce3+ to Tb3+ ions was analyzed by numerical simulations based on theoretical models of non-radiative energy transfer.
3. Result and discussion
3.1 Photoluminescence spectra
For the Tb-doped fiber materials (sample 2, Tb: 4 mol %), there is a primary excitation peak at 252 nm, producing emission at a wavelength (λem) of 542 nm, as shown in Fig. 2. The emission spectrum consists of two parts. The first group consists of the emission peaks at 378, 414, 438 and 458 nm derived from the Tb3+ transitions 5D3→7FJ (J = 6, 5, 4, 3). These peaks are very weak for the high doping concentration because of non-radiative relaxation (NR) and cross-relaxation (CR). The second group has very strong peaks at 488, 542, 585 and 621 nm, from the transitions 5D4→7FJ (J = 6, 5, 4, 3). The weak emission spectrum is magnified in the Fig. 2. These results are basically consistent with other results reported [15, 16].
For the Ce/Tb co-doped fiber materials (sample 5, Ce: 0.05 mol %, Tb: 4 mol %), in addition to a primary excitation peak at 246 nm, there is second peak at 316 nm. These lead to emission at λem = 542 nm. The excitation peak at 316 nm appears related to the Ce3+ ions dopant.
For the CTDF rod, sample 7, the excitation peak at 305 nm is very strong, as shown in Fig. 3. This is because the Ce3+ ions interact with the silica network during the high-temperature drawing process. The excitation spectrum is blue-shifted with some Ce3+ oxidized to Ce4+ ions [17]. In addition, there is a strong emission peak at 413 nm for the CTDF rod, which differs from the emission peaks around 414 nm for the Tb-doped and Ce/Tb co-doped fiber materials, which are sharp and very weak. The PL decay and this new emission peak are discussed in the next section.
3.2 Photoluminescence decay
To better understand the energy level configuration and energy transfer process in CTDF, a series of PL decay were measured. For the Ce-doped material (sample 1, Ce: 0.05 mol %), the lifetime monitored at 438 nm is 49.03 ns, which is in agreement with lifetime of Ce3+ ions between 30 and 80 ns [18–20]. For the Ce/Tb co-doped fiber material, sample 5, and CTDF rod, sample 7, the lifetime decrease to 44.37 and 39.71 ns, respectively, as shown in Fig. 4. Experimental results show that the lifetime for Ce3+ ions of samples 5 and 7 are shorter than that of sample 1, which indicates energy may transfer from Ce3+ to Tb3+ ions in these samples. In addition, compared with the lifetime for sample 5 (44.37 ns), lifetime for sample 7 decreases to 39.71 ns. According to the Forster-Dexter resonance energy transfer theory [21], the energy transfer efficiency (ηET) of sample 7 is greater than that of sample 5.
For the Tb-doped fiber material, sample 2, the PL decay monitored at 542 nm with 252 nm excitation is 1709.21 μs. The decay when Ce3+ ions are additionally doped in sample 5, for excitation at 246 nm and 316 nm, increase to 1790.49 and 1872.22 μs, respectively. For a fiber rod, sample 7, produced by the drawing process, the decay increases further to 1997.51 and 2173.17 μs, for excitation at 255 nm and 305 nm, respectively, as shown in Fig. 5 and Table. 2. Based on the experimental results of lifetime for Ce3+ ions, the energy transfer efficiency (ηET) of sample 7 is greater than that of sample 5, which is due to the increase of lifetime from sample 5 to 7 influenced by the high-temperature drawing process. Moreover, the non-radiative relaxation and cross-relaxation between Tb3+ ions may be suppressed by the dopant Ce3+ ions. The influence of drawing process on energy transfer efficiency is discussed in the next section.
Fig. 5 PL decay of (a) Tb-doped fiber materials, (b) Ce/Tb co-doped fiber materials and (c) CTDF rod.
Table 2. Decay time of samples
The new emission peak at 413 nm for the CTDF rod, excited at 252 nm, has a decay of 109.37 μs, which differs from the decay of Tb-doped fiber material (sample 2), 920.88 μs, generated by the transitions 5D3→7FJ of Tb3+ ions [22]. In addition, the decay of emission band around 438 nm generated from transition 5d→4f of Ce3+ ions is about 40 ns based on the results shown in Fig. 4, which is great difference between both lifetime. Besides, if germanium oxygen-deficient centers (Ge-ODCs) are present in the rod, the decay of the new emission peak would be consistent with their decay about 100 μs [23,24].
3.3 Energy transfer between Ce3+ and Tb3+ ions
In the CTDF rod material, sample 7, there is an obvious excitation peak at 305 nm. However, in the Tb-doped fiber material, sample 2, this excitation peak is almost lacking, as seen from Figs. 2 and 3. To explain this phenomenon, the PL spectra are analyzed using the energy level scheme of Ce3+ and Tb3+ ions, as shown in Fig. 6 (a). The broadband emission around 360-460 nm is assigned to allowed transitions from the 5d energy band to the 4f level of Ce3+ ions [25]. At the same time, Tb3+ ions can be excited at 375 nm from the 7F6 to the 5D2 level [26]. This excitation wavelength can be matched with the broadband emission. Therefore, energy transfer process from Ce3+ to Tb3+ ions may be occurring in the co-doped silica fiber.
Fig. 6 (a) Schematic energy levels of the Ce3+ and Tb3+ ions with the energy transfer process and a local structure model of Ce/Tb co-doped silica fiber materials. (b) The emission intensity tendency chart.
Moreover, for the CTDF rod material, the intensity of the emission peak at 542 nm is greatly enhanced, as shown in Fig. 6 (b). This may result from the extensive energy transfer between Ce3+ and Tb3+ ions. During the drawing process, the temperature is above 2000 °C. The clusters of doped ions are generated in the fiber materials and the size of clusters may further increase due to the high temperature role, which is consistent with some results reported [27, 28]. In addition, the distance between Ce3+ and Tb3+ ions in the materials may decrease after the heat treatment [29]. When the distance between Ce3+ and Tb3+ ions is less than the critical distance for resonance energy transfer, the efficiency of energy transfer will increase with decreasing distance. Besides, a possible local structure model of CTDF is built to understand the energy transfer process from Ce3+ to Tb3+ ions, as shown in Fig. 6 (a).
Further, the energy transfer process and mechanism in the silica optical materials were analyzed by numerical simulation. The emission spectra of Ce/Tb co-doped fiber materials with different Tb3+ ions concentrations were measured for the 316 nm excitation. The emission intensity of Ce3+ ions decreases monotonously as the Tb3+ ions concentrations increase from 0 to 6 mol % with the Ce3+ ions concentration fixed at 0.05 mol %. However, the intensity from Tb3+ ions 5D4→7F5 transitions (peak at 542 nm) increases with increasing Tb3+ ions concentrations, up to a maximum occurring at 4 mol %, with intensity decreasing for higher concentrations as shown in Figs. 7. This may be an example of severe concentration quenching, resulting in decreased emission intensity.
Fig. 7 (a) The emission spectra of samples with different concentrations of Tb3+ ions and the enlarged emission spectra of (b) Ce3+ (peak at 438 nm) and (c) Tb3+ ions (peak at 542 nm).
The efficiency ηET of energy transfer from Ce3+ to Tb3+ ions can be expressed by [30]
where IS0 and IS are the emission intensities of a sensitizer (Ce3+ ions) in the absence and presence of an activator (Tb3+ ions), respectively. The dependence of ηET on the Tb3+ ions concentration is shown in Fig. 8, where ηET increases gradually with increasing Tb3+ ions concentration, reaching to 73.4% at 6 mol %. Although ηET increases with the concentration of Tb3+ ions, the emission intensity of Tb3+ ions has a maximum at 4 mol %. Therefore, sample 5 was selected as the precursor material for drawing into an optical fiber.Fig. 8 The emission intensity and efficiency of energy transfer (ηET), as functions of Tb3+ ions concentration.
To further analyze the energy transfer mechanism, the relationship between the emission intensity of a sensitizer and the doping concentration can be approximately expressed using Dexter’s formula of multipolar interaction with Reisfeld’s approximation [31]
where the factor C is the sum of the doping concentrations of Ce3+ and Tb3+ ions. In Eq. (2), n = 6, 8 and 10, correspond to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The curves of relationship between IS0/IS and Cn/3 are shown in Fig. 9. The R2 value of the fitting curve in Fig. 9 (a) is estimated to be the maximum value 0.98333 for n = 6, which indicates this curve is most similar to the linear relation between IS0/IS and C6/3. This result suggests that the ET from Ce3+ to Tb3+ ions is most likely to be a dipole-dipole mechanism in silica optical fiber materials, which is in agreement with other current research [9, 10].4. Conclusion
In this research, we fabricated a Ce/Tb co-doped optical silica fiber material by the PIT technique with sol-gel method and measured its photoluminescence spectra and decay. Compared with the Tb-doped fiber materials, a strong additional excitation peak at 305 nm appears for the CTDF rod. In addition, the intensity of the emission at 542 nm excited by 305 nm for the CTDF rod is greater than that for the Tb-doped fiber material. Moreover, its PL decay is increased from 1709.21 to 2173.17 μs which results from the energy transfer from Ce3+ to Tb3+ ions during the drawing process. Based on the PL spectra and decay, we developed an energy level scheme of CTDF with a local structure model. Theoretically, we further analyzed the energy transfer mechanism by numerical simulations method. As a result, the dipole-dipole interaction is thought to play an important role in the energy transfer in the silica optical fiber materials. This study demonstrates how the energy transfers between Ce3+ and Tb3+ ions in the silica optical fiber materials, and provides a possibility to enhance green emission at 542 nm of Tb3+ ions, using a shift in the excitation wavelength from 252 to 305 nm. In future research, we will apply the Ce/Tb co-doped silica optical fibers to sensors for X-rays and β-rays radiation, and to green fiber lasers.
Funding
National Natural Science Foundation of China (Grant Nos: 61475096, 61520106014, 61422507, 61635006, 61705126 and 11575108); Science and Technology Commission of Shanghai Municipality, China (15220721500).
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