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Abstract
We investigate the spectral characteristics of three kinds of home-made active silica fibers, namely, Er-doped fiber (EDF), Bi/Er co-doped fiber (BEDF), and Bi/Er/Yb co-doped fiber (BEYDF), and thus explore the influences of Bi and Yb ions on the absorption and emission performances in the EDF. Compared with the EDF, the absorption cross section at 980 nm and the fluorescent lifetime at 1535 nm in the BEDF are enhanced by 1.2 × 10−25 m2 and 0.68 ms, respectively. Found that the fluorescent intensity in the BEDF is higher, and the emission slope efficiency of Er ions in the BEDF is more than doubled. Moreover, compared with the BEDF, the absorption intensity and bandwidth around 980 nm in the BEYDF are significantly enhanced and broadened, and its fluorescent lifetime at 1535 nm is improved by 1.08 ms. Their fluorescent intensities are increased with the increase of the pump power. Also found that the emission slope efficiency of Er ions in the BEYDF is more than doubled. Furthermore, their local microstructural models are also built up, and their energy levels and excited state characteristics are analyzed based on the density functional theory. These results indicate that co-doping Bi and Yb ions into the EDF could improve the optical characteristics of Er ions, especially the emission efficiency, which is potentially applied in optical amplifier and laser systems, and so on.
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1. Introduction
Er-doped fibers (EDFs) have been developed as the main gain material for optical amplifiers and lasers since their emission at 1.53 µm coincides with the low loss window for fiber-optic communication systems [1–3]. However, the narrow absorption and emission cross sections of EDFs are not conducive to the development of wavelength division multiplexing technology. Bi-doped fibers (BDFs) are currently the most promising broadband emission materials, whose fluorescent emission is exhibited in a wide spectral range of 1100-1800nm [4–6]. With this characteristic of Bi ions, the emission spectra in EDFs can be broadened to 1000-1570 nm in Bi/Er co-doped fibers (BEDFs) [7]. In addition, the pump conversion efficiency of EDFs is undesirable because of the low concentration quenching threshold, the small absorption cross section at 980 nm, and the low fluorescence lifetime at 1.53 µm of Er ions, which directly deteriorates the performance of the optical amplifiers and lasers. One solution is to co-doping Yb ions into EDFs. As the typical sensitizer and dispersant, Yb ions could effectively suppress the concentration quenching of Er ions at a suitable concentration of Er and Yb ions [8,9]. And the wider and stronger absorption of Yb ions in the 980 nm band is beneficial to the energy transfer from Yb ions to Er ions, which could enhance the emission and fluorescent lifetime of Er ions [9–12].
The realization of the above spectral characteristics has driven the study into the mechanisms of interaction among Bi, Er, and Yb ions. In 2010, Dai et al investigated the energy transfer from Yb ions to Bi ions in Bi/Yb co-doped silicate glasses, which promoted the emission of Bi ions [13]. Since 2011, some researchers have paid considerable attention to the interactions of Er ions with bismuth active centers (BACs) in BEDFs [14–18]. In particular, Zhao et al demonstrated the energy transfer between BAC-Al and Er ions in the BEDF, which provided a promising strategy for improving spectral performance over the 1000-1600 nm range [16]. In 2015, Sathi et al reported the energy transfers from Yb ions to both Er ions and BAC-Al in Bi/Er/Yb co-doped fiber (BEYDF) to explain the impact of Yb ions on the improvement of emission [19]. In 2020, Luo et al designed a BEYDF with an ultrabroad bandwidth emission spectrum of 692 nm [20]. However, so far, how Bi and Yb ions affect and enhance the emission efficiency of Er ions has rarely been studied.
In this paper, we analyze the spectral characteristics of the EDF, BEDF, and BEYDF, which are fabricated by modified chemical vapor deposition (MCVD) combining with atomic layer deposition (ALD). Furthermore, their local microstructural models are also established, and then their structures and excited state characteristics are studied.
2. Experimental samples
As a conventional deposition process, MCVD technique lacks uniformity, and the doping materials are easily to cluster, which have limited the excellent performance for the doped fibers’ fabrication. ALD is a chemical vapor deposition technique based on the sequential use of self-terminating gas-solid reactions, and that is a self-limiting surface reaction. By MCVD combining with ALD technology, the EDF, BEDF, and BEYDF with good uniformity and dispersion, precise refractive index distribution, and high doping concentrations are fabricated. The fabrication process in details was reported previously in Ref. [21]. The main elemental components of the fibers are measured by an electron probe micro-analyzer (EPMA-8050G, SHIMADZU, Japan), as shown in Table 1. The concentration of Er ions in the BEDF2 and BEYDF is similar, approximately 0.024 at%, which is higher than that in the EDF and BEDF1, where the concentration of Er ions is the same, approximately 0.014 at%. The concentration of Bi ions is almost the same among the BEDF1, BEDF2, and BEYDF, approximately 0.005 at%. The concentration of Yb ions in the BEYDF is approximately 0.037 at%, and the doping ratio of Yb and Er ions is approximately 3:2. Al ions are doped into the fiber cores to improve the doping concentration and dispersibility of Er ions. Doping with Ge and P ions could increase the refractive index of the fiber cores. And P ions could also promote the energy transfer efficiency from Yb ions to Er ions. In addition, the refractive index differences (RIDs: ncore - ncladding) of the EDF, BEDF1, BEDF2, and BEYDF are approximately 0.0041, 0.0056, 0.0043, and 0.0135, respectively. Their corresponding cut-off wavelengths are approximately 1133, 1370, 1200, and 2272 nm, respectively. For the BEYDF, the longer cut-off wavelength indicates that it is a few-mode fiber, and several higher-order modes may affect the emission of Er ions. However, its intensity of the output beam at 1550 nm excited by 980 nm has been detected to be still gaussian distribution, which means that the higher-order modes do not play a major role. Additionally, the corresponding diameter parameters of core and cladding layers of all fibers, and other detailed information are shown in Table 1.
Table 1. The geometry and elemental components of fiber samples
3. Experimental results and discussion
The absorption spectra are measured by the cut-back method using white light source and optical spectrum analyzer (OSA, Yokogawa AQ-6315A) in the 600-1700nm region, as shown in Fig. 1(a). The four typical absorption peaks of the four fibers are located at 651, 800, 980, and 1535 nm, which are derived from the 4I15/2→4F9/2, 4I9/2, 4I11/2, and 4I13/2 electronic transitions of Er ions, respectively [9]. Noted that the absorption intensities of the EDF and BEDF1 are similar due to the low concentration of Bi ions in the BEDF1, while the absorption intensity of the BEDF2 is higher than that of the BEDF1 due to the higher concentration of Er ions in the BEDF2. Also noted that the absorption intensity and bandwidth around 980 nm in the BEYDF are significantly enhanced and broadened due to the co-doping of Yb ions. And a new absorption peak at 916 nm is also observed, which is attributed to the 2F7/2→2F5/2 electronic transition of Yb ions [9].
Fig. 1. (a) Absorption spectra and (b) Absorption cross sections around 980 nm of different silica optical fibers
Based on the obtained absorption spectra, the absorption cross sections (${\sigma _a}$) around 980 nm could be calculated by Eq. (1) [22]:
Where L is the length of the test fibers, I0 and I are the input and output intensity of the fibers, respectively. Since there is no absorption of Bi ions around 980 nm [21–24], N is the concentration of Er ions in the EDF and BEDF. The fibers’ absorption cross sections are shown in Fig. 1(b). It is found that the absorption cross sections at 980 nm for the EDF, BEDF1, and BEDF2 are 9.9 × 10−25, 1.11 × 10−24, and 1.36 × 10−24 m2, respectively. The results indicate that co-doping Bi ions into the EDF enhances the pump absorption efficiency of 980 nm pump light in the BEDF1. For the BEYDF, although it is difficult to accurately calculate the absorption cross section of Er ions at 980 nm, the wider and stronger absorption in Fig. 1(a) still shows that its pump absorption efficiency is obviously enhanced.To investigate the effect of Bi ions on the fluorescent characteristics of the EDF, the fluorescence spectra of the EDF and BEDF1 (10 m) excited by 980 nm are detected by the forward pumping system, as shown in Fig. 2. With the increase of the pump power, the fluorescent intensities of the EDF and BEDF1 are clearly enhanced. The wavelength at 1510 nm is used to more clearly analyze the emission of Er ions. At the pump power of 11 mW, the emission intensity at 1510 nm in the BEDF1 (-64.05 dBm) is slightly higher than that in the EDF (-69.26 dBm). As the pump power is increased to 30 mW, the emission intensity in the BEDF1 is clearly increased to -51.21 dBm, an increase of 12.84 dBm, while that in the EDF is only enhanced to -60.81 dBm, an increase of just 8.45 dBm. The emission intensity in the BEDF1 is obviously 9.6 dBm higher than that in the EDF. By further increasing the pump power, the emission intensity at 1510 nm in the BEDF1 is increased to -44.57 dBm at 48 mW, and then the emission intensity is close to -41.81 dBm when the pump power is at 95 mW. The emission intensity is increased slowly and trends to be saturated. However, the emission intensity at 1510 nm in the EDF is only increased to -48.49 dBm when the pump power is at 95 mW, which is even lower than that in the BEDF1 with 48 mW pumping. Furthermore, the emission in the EDF is not nearly saturated until at 190 mW. These results indicate that the emission efficiency of Er ions in the BEDF1 is clearly larger than that in the EDF.
Fig. 2. Fluorescence spectra at different pump powers for the: (a) EDF and (b) BEDF1 excited by 980 nm
To further analyze the effect of Bi ions on the fluorescent characteristics of Er ions, the dependences of normalized fluorescence integrated intensity in the 1450-1650 nm region on the pump power for the EDF and BEDF1 are analyzed. Obviously, with the increase of the pump power, the integrals of Er ions’ emission in the EDF and BEDF1 are increased gradually. And the integral intensity in the BEDF1 is always higher than that in the EDF. Noted that the emission slope efficiency of Er ions in the BEDF1 (η1 ∼5.8%) is more than twice that in the EDF (η2 ∼2.4%). Meanwhile, the emission of Er ions in the BEDF1 is easily saturated at 95 mW, while Er ions in the EDF are insufficiently excited, as shown in Fig. 3, where the insets are their normalized fluorescence spectra. These results indicate that the emission intensity of Er ions in the BEDF1 is increased faster and more fluorescent photons in the BEDF1 could always be excited at the same pump power. The emission efficiency in the EDF by co-doping Bi ions could be obviously enhanced. There may be the reason that the co-doping of Bi and Er ions enhances the absorption of Er ions at 4I15/2 for 980 nm pump light, and thus effectively activates more Er ions at 4I13/2 to participate in the near-infrared (NIR) emission, which is also consistent with Fig. 1(b).
Fig. 3. Dependences of normalized fluorescence integrated intensity in the 1450-1650 nm region on the pump power for the EDF and BEDF1, where the insets are their normalized fluorescence spectra
To further study the effect of Yb ions on the fluorescent characteristics of the BEDF, the forward fluorescence spectra of the BEDF2 and BEYDF (10 m) excited by 980 nm are detected, as shown in Fig. 4. With the increase of the pump power, the fluorescent intensities of the BEDF2 and BEYDF are both enhanced. At the pump power of 190 mW, the emission intensity at 1510 nm in the BEYDF (-82.33 dBm) is 5.97 dBm lower than that in the BEDF2 (-76.36 dBm). As the pump power is increased to 578 mW, the emission intensity at 1510 nm in the BEYDF is increased to -66.62 dBm, an increase of 15.71 dBm, while that in the BEDF2 is increased to -63.69 dBm, an increase of 12.67 dBm. The increase of fluorescent intensity in the BEYDF is slightly higher than that in the BEDF2. Furthermore, when the pump power is increased up to 971 mW, the emission intensity at 1510 nm in the BEYDF is increased to -48.06 dBm, which has been significantly 7.93 dBm higher than that in the BEDF2 (-55.99 dBm). The fluorescent intensity in the BEYDF is increased obviously faster than that in the BEDF2. It is obviously seen that the co-doping of Yb ions improves the emission efficiency of Er ions in the BEYDF. In addition, there exists a complete absorption of Yb ions for 980 nm pump light and the strong emission of Yb ions (λem ∼1027 nm) in the BEYDF, while there is only partial absorption for 980 nm pump light in the BEDF2. Also found that the emissions of BAC-Al (λem ∼1143 nm) and BAC-P (λem ∼1310 nm) are excited in the BEDF2. The stronger emission of BAC-Al (λem ∼1195 nm) is also excited. These results illustrate that the co-doping of Yb ions promotes the emission of Er ions and BAC-Al.
Fig. 4. Fluorescence spectra at different pump powers for the: (a) BEDF2 and (b) BEYDF excited by 980 nm
To further investigate and explain the above phenomena, the dependences of normalized fluorescence integrated intensity in the 1450-1650 nm region on the pump power for the BEDF2 and BEYDF are demonstrated. With the increase of the pump power, it could be seen that the integrals of Er ions’ emission in the BEDF2 and BEYDF are both increased. The integral intensity in the BEYDF is weaker than that in the BEDF2 at the pump power below 776 mW. However, when the pump power is over 776 mW, the integral intensity in the BEYDF is stronger than that in the BEDF2. Noted that the emission slope efficiency of Er ions in the BEYDF (η3 ∼0.51%) is more than twice that in the BEDF2 (η4 ∼0.24%), as shown in Fig. 5, where the insets are their normalized fluorescence spectra. For the BEYDF, its larger emission slope efficiency indicates that co-doping Yb ions into the BEDF2 further improves the emission efficiency of Er ions. Although its concentration of Yb ions is relatively low, its detected excitation and emission spectra still show that there exists the energy transfer from Yb ions to Er ions and its emission of Er ions at 1535 nm is stronger than that in the BEDF2. Therefore, the enhanced emission efficiency of BEYDF with low concentration ratio of Yb and Er ions may result from the energy transfer from Yb ions to Er ions. This phenomenon has also been relatively reported in Refs. [25,26].
Fig. 5. Dependences of normalized fluorescence integrated intensity in the 1450-1650 nm region on the pump power for the BEDF2 and BEYDF, where the insets are their normalized fluorescence spectra.
In addition, the fluorescent lifetimes of Er ions at 1535 nm in four fibers are also analyzed under excitation at 980 nm using a fluorescence spectrometer (FLS980, Edinburgh, England). The fluorescent lifetime of Er ions in the BEDF1 (10.60 ms) is extended by 0.68 ms compared with that in the EDF (9.92 ms). And the fluorescent lifetime of Er ions in the BEYDF (11.87 ms) is 1.08 ms longer than that in the BEDF2 (10.79 ms). The results indicate that co-doping Bi and Yb ions into the EDF could lengthen the fluorescent lifetime of Er ions at 4I13/2. Therefore, the longer fluorescent lifetime of Er ions indicates that the co-doping of Bi and Yb ions could further increase the number of particles that remain at 4I13/2 energy level, and enhance the spontaneous emission probability of Er ions at 4I13/2, which contributes to more Er ions to participate in NIR emission and improve the emission efficiency of Er ions, which is consistent with the above change of emission slope efficiency. Additionally, the longer lifetime of Er ions in the BEYDF also reveals the existence of the energy transfer from Yb ions to Er ions, which may also be one of the important reasons for the increased emission slope efficiency of Er ions in the BEYDF. The relationship between energy transfer and fluorescent lifetime in EYDF was reported in Refs. [9,12].
4. Theoretical section
Amorphous silica is an important material for single-mode fibers. Its network microstructure contains a large number of n-membered rings (nMRs, n = 3,4,5…) and hybrid rings structure units [27,28], where n is the number of Si atoms in the silica rings and also determines the size of the rings. The doping microstructures around silica rings help to form stable binary silica structures among hybrid rings and network interstices [29]. Based on the silica rings, the doping microstructural characteristics of the EDF, EYDF, BDF, and BEDF have been studied [12,30–33]. Here, the 3MR is applied as the silica network structure for economic calculations. Noted that the valence electrons of Bi ions are easily affected by the coordination electric field to change their valence states. Previous experiments revealed the stable existence of Bi ions in trivalent state on silica fibers using ALD technology [21]. Therefore, Bi ions in trivalent state are used for theoretical calculation. Here, the local microstructural models of the EDF, BEDF, and BEYDF are established using Gaussian-09 and Gaussian view Software. Then their geometric structures and excited state characteristics, based on density functional theory (DFT) and time-dependent density functional theory (TDDFT), are investigated to further understand the effect of Bi and Yb ions on the spectral characteristics of the EDF.
Using DFT with the Becker-type three-parameter Lee-Yang-Parr (B3LYP) hybrid function [31,34], the geometric structures are first optimized, and the harmonic vibration frequencies are calculated to obtain the stable structure with the lowest energy, which is the ground state structure when the virtual frequency does not exist. Then the excited state calculation of the ground state structure is performed to study the energy levels and excited state characteristics based on TDDFT. GENECP denotes the combined base sets. For H, O, and Si atoms, the all-electron basis set 6-31 + G** is used. For Er and Yb atoms, the 11 valence electron RECPs (relativistic effective core potentials) for trivalent state are used [12,31]. For Bi atom, the Lanl2DZ basis set is used [30,32].
4.1 Doping 3MR microstructural models
Since the diameter of Bi, Er, and Yb atoms is larger than that of Si and O atoms, they are difficult to replace the Si atoms in the silica rings to join with the bridging oxygen atoms, but are more likely to join with the non-bridging oxygen atoms outside the silica rings to form the stable binary silica structures containing T-O-T (T: Er, Yb, or Bi) bonds [31]. Previously, a local microstructural model of the EDF was reported in Refs. [12,31]. By co-doping Bi ions into this microstructure, the four typical isomers for the local microstructural model of the BEDF are established, as shown in Fig. 6. The optimized energies for the models are -2104.9726, -2104.9782, -2104.9979, and -2104. 9901 (a.u.), respectively. It is seen that Ec < Ed < Eb < Ea. Due to the lowest energy, the C model is used as the ground state structure.
Similarly, four typical isomers for the local microstructural model of the BEYDF are established, as shown in Fig. 7. The optimized energies for the models are -2259.7181, -2295.7961, -2295.8304, and -2295.8386 (a.u.), respectively. It is seen that Ed < Ec < Eb < Ea. The D model with the lowest energy is determined as the ground state structure.
4.2 Excited state calculation
The excitation energy, excitation wavelength, and oscillator strength corresponding to each electronic transition are calculated using the optimized ground state local microstructural models of the EDF, BEDF, and BEYDF based on TDDFT theory, which are used to analyze the excited state characteristics of the models.
The excited state parameters for the local microstructural model of the EDF present four typical excitation wavelengths at 1510.36, 962.54, 788.28, and 634.89 nm with their corresponding oscillator strengths of 0.0011, 0.0079, 0.0052, and 0.0148, respectively, as shown in Table 2. It could be seen that the oscillator strength in the 634.89 nm band is strong, while that in the 962.54 nm band is relatively weak. Although the oscillator strength corresponding to the excitation energy at 0.8209 eV is only 0.0011, there still exists an excited state energy level in the 1510.36 nm band within the error range. These parameters exhibit a basically consistent trend with the experimental results.
Table 2. Excited state parameters for local microstructural model of the EDF
For the local microstructural model of the BEDF, the seven excitation wavelengths, that is the excited state parameters, are1637.59, 1078.10, 979.45, 887.76, 783.02, 694.64, and 638.53 nm. Their corresponding oscillator strengths are 0.0043, 0.0011, 0.0095, 0.0055, 0.0007, 0.0164, and 0.0172, respectively, as shown in Table 3. There exist four excitation bands in 638.53-694.64 nm, 783.02 nm, 887.76-1078.10 nm, and 1637.59 nm bands. The 979.45 nm corresponding to higher oscillator strength (0.0095) is the excitation peak in the 887.76-1078.10 nm band. And the wavelength of 1637.59 nm corresponding to the excitation energy at 0.7571 eV is relatively high but within the error range. Noted that there exist several new excitation wavelengths within the error range compared with Table 2. The wavelength of 694.64 nm corresponding to the new excitation energy at 1.7849 eV may reveal the contribution of Bi ions because of the strong absorption in approximately 500 and 700 nm bands in BDF [21–24].
Table 3. Excited state parameters for local microstructural model of the BEDF
Furthermore, the excited state parameters for the local microstructural model of the BEYDF are shown in Table 4. The excitation wavelengths are 1568.91, 1055.59, 989.71, 902.21, 864.04, 837.24, 802.46, 693.65, and 635.22 nm with their oscillator strengths of 0.0491, 0.0085, 0.0242, 0.0102, 0.0032, 0.0017, 0.0024, 0.0120, and 0.0329 are presented, respectively. There exist four excitation bands in 635.22-693.65 nm, 802.46 nm, 837.24-1055.59 nm, and 1568.91 nm bands. Compared with Table 2 and Table 3, the oscillator strengths for all Er-related excited wavelengths are overall stronger. In particular, the oscillator strength corresponding to the excitation energy at 0.7903 eV is up to 0.0491. There also exist several new excitation wavelengths within the error range. The new excitation wavelength at 693.65 nm may also result from the role of Bi ions. The oscillator strength corresponding to the excitation peak at 989.71 nm in the 837.24-1055.59 nm band is 0.0242, which is obviously stronger than that of the local microstructural models of the EDF and BEDF. These results reflect the contribution of Bi and Yb ions to the excited state characteristics of the local microstructural model of the BEYDF.
Table 4. Excited state parameters for local microstructural model of the BEYDF
Based on three sets of excited state characteristics, we have plotted the energy level structure diagrams for the corresponding local microstructural models of the EDF, BEDF, and BEYDF, as shown in Fig. 8. The left axis is the excitation energy, which corresponds to the excitation wavelength. The oscillator strength f represents the excitation intensity. Obviously, Fig. 8(a) shows the excited state characteristics of local microstructural model of the EDF. There exists the excited state energy level at 962 nm and transition state energy level at 1510 nm for ground state absorption (GSA). And the oscillator strength corresponding to the excitation wavelength at 962 nm is 0.0079. This energy level structure is similar to that of the EDF. For the local microstructural model of the BEDF, the new excited state energy levels at 887, 979, and 1078 nm are generated. And the oscillator strength of excitation peak at 979 nm is 0.0095, which is slightly higher than that at 962 nm in the local microstructural model of the EDF, as shown in Fig. 8(b). These results indicate that the local microstructural model of the BEDF has wider and stronger excited state in the 980 nm band compared with that of the EDF. Furthermore, the local microstructural model of the BEYDF presents more excited state energy levels at 837, 864, 902, 989, and 1056 nm, where the oscillator strength of excitation peak at 989 nm is up to 0.0242, which is significantly higher than that at 979 nm in the local microstructural model of the BEDF, as shown in Fig. 8(c). These results also indicate that the local microstructural model of the BEYDF has wider and stronger excited state in the 980 nm band compared with that of BEDF. Therefore, Fig. 8 clearly illustrates that the co-doping of Bi, Er, and Yb ions could improve the pump absorption efficiency in the 980 nm band, and thus excite more electrons at ground state for GSA to reach the excited state. This process contributes to enhance the emission efficiency of Er ions. Theoretical analysis also further expounds the above experimental results. The superior optical characteristics in the BEYDF reveal its potential applications for optical amplifiers and lasers.
5. Conclusion
In summary, we fabricate the EDF, BEDF, and BEYDF using MCVD combining with ALD technology, and accordingly investigate their spectral characteristics to illustrate the effect of Bi and Yb ions on the spectral characteristics of the EDF. The results show that co-doping Bi and Yb ions into the EDF could further improve the pump absorption efficiency at 980 nm, the fluorescent lifetime at 1535 nm, and the emission intensity and emission slope efficiency of Er ions. Furthermore, their local microstructural models are also established to calculate their energy levels and excited state characteristics based on DFT and TDDFT theories. It is found that co-doping Bi and Yb ions into the local microstructural model of the EDF could further broaden and enhance the excited state of Er ions in the 980 nm band. These results sufficiently illustrate that the co-doping of Bi, Er, and Yb ions could further improve the absorption and emission efficiencies of Er ions, and then enhance its optical performance. Next, we would further optimize the component parameters of Bi, Er, and Yb ions to fabricate the BEYDFs with superior performances by taking full advantage of the effects of Bi and Yb ions on the emission efficiency of the EDF, then apply them to optical amplifier and laser systems for the better spectral bandwidth, higher gain, greater laser efficiency, and so on.
Funding
111 Project (D20031); Shanghai professional technical public service platform of advanced optical waveguide intelligent manufacturing and testing (19DZ2294000); National Natural Science Foundation of China (61935002, 61975113); National Key Research and Development Program of China (2020YFB1805800).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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