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Abstract
This research investigated the wideband near-infrared spectroscopy characteristics of 60SiO2-25Al2O3-10La2O3 glass doped with high levels of bismuth up to 5 mol%. The near-infrared radiation range was explored under excitation wavelengths of 488 nm, 532 nm, 808 nm, and 980 nm, resulting in near-infrared radiation spanning from 1000 nm to 1800nm with Full Width at Half Maximum (FWHM) values of 313.0 nm, 336.3 nm, 296.2 nm, and 262.9 nm, respectively. Notably, the sample exhibited a lifetime of 1.473 ms when pumped at 808 nm, corresponding to a stimulated cross-section of σe=3.35 × 10−21 cm2. Through an in-depth investigation of the luminescence properties, the underlying physical mechanism behind the near-infrared luminescence was revealed. The emissions observed at approximately 1150 nm and 1300 nm were attributed to the aluminum-related bismuth active center (BAC-Al) and the silicon-related bismuth active center (BAC-Si), respectively. Furthermore, it is postulated that the emission at the 1150 nm band originates from the 3P1, 3P2 →3P0 transition of Bi+ and the 2D3/2 → 4S3/2 transition of Bi°, while the emission at the 1300 nm band may be linked to mixed valence states of Bi3+. This work will find potential applications in broadband near-infrared optical devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The rapid advancement of computer networks and the exponential growth in data traffic necessitate the development of efficient and high-capacity optical communication technologies. Wave division multiplexing (WDM) systems have proven to be effective in expanding communication capacity [1,2]. However, the limited bandwidth of optical amplifiers quickly saturates the transmission capacity. Traditional rare earth fiber amplifiers (RDFAs) suffer from a narrow f-f electron energy level transition, resulting in a gain bandwidth of less than 100 nm. Furthermore, the conventional erbium-doped optical amplifier (EDFA) primarily operates within the C + L band and fails to meet the demands of high-speed and high-capacity communication [3,4]. Consequently, the pursuit of novel gain materials and amplification devices with wide bandwidth and high gain has become a prominent research area [5–8]. In 2001, Fujimoto and Nakatsuka made a seminal discovery regarding the ultra-wideband radiation phenomenon in bismuth-doped oxide materials. They successfully demonstrated fluorescence at 1250 nm with an extraordinary bandwidth of 300 nm [9]. Since then, such materials have garnered significant recognition as highly promising candidates for near-infrared fluorescence applications.
In recent years, there has been a burgeoning interest in the investigation of bismuth-doped materials exhibiting near-infrared (NIR) luminescence. Extensive research efforts have been devoted to exploring the potential of bismuth-doped optical fiber amplification devices [10–12], thin film materials [13], and glasses infused with various dopants [14–18], particularly in relation to their luminescent properties. The electronic configuration of a bismuth atom is denoted as [Xe]4f145d106s26p3. Notably, the presence of partially filled outer electron shells renders the outer electrons highly active, resulting in profound sensitivity of bismuth atoms to the surrounding host material as well as the facile formation of ions with diverse valence states, such as Bi° [15,16], Bi + [15–18], Bi2 + [19], Bi3 + [20] and Bi5 + [21]. In a comprehensive review by E.M. Dianov, it was documented that distinct fluorescence bands in the NIR region were exhibited by bismuth ions with varying valencies during stimulated emission [18]. Given the coexistence of multiple valence states, each contributing to different excited radiation bands, the collective effect leads to a broadening of the gain bandwidth. Hence, optimizing the composition system of bismuth-doped glasses and carefully controlling the distribution of ions with differing valence states hold paramount significance in advancing the development of NIR devices incorporating bismuth-doped materials.
Qiu et al. [15] investigated the luminescence mechanism in bismuth-doped germanium aluminum-silicate glass under different excitation conditions. They observed a fluorescence lifetime of 473 µs when excited at 808 nm, and a fluorescence lifetime of 806 µs when excited at 980 nm. The authors attributed the luminescence of bismuth to the energy level transition of Bi+ and Bi° ions. Similarly, Meng et al. [17] explored the NIR broadband emission characteristics of bismuth-doped aluminum phosphate glass. Upon excitation with lasers at 405, 514, and 808 nm, three distinct infrared emission peaks were observed at wavelengths of 1210, 1173, and 1300 nm, respectively, with corresponding FWHM values of 235, 207, and 300 nm. The fluorescence lifetime was estimated to be 500 µs. It was postulated that the near-infrared luminescence in bismuth originates from energy level transitions between the 3P1 to 3P0 in Bi+ ion. While previous studies have confirmed the broadband luminescence of bismuth, achieving long lifetime and high intensity fluorescence through high doping has remained a challenge due to the inherent instability of the atomic structure.
To address these challenges, we studied the wideband NIR luminescence properties of SiO2-Al2O3-La2O3 glass materials doped with bismuth ions. The co-dopant Al2O3 was employed to enhance the solubility of bismuth materials [22]. Additionally, the inclusion of La2O3 was found to enhance the excitation lifetime of rare-earth ions and improve other optical properties of the glass [23–25]. By optimizing the composition and processing scheme, high concentration doping of bismuth ions up to 5 mol% was successfully achieved. Experimental measurements revealed a fluorescence lifetime of 1.473 ms and a maximum FWHM of 336.3 nm for the broadband luminescence. Comprehensive studies were conducted on the optical absorption, transmission, and refractive index of glasses with diverse component combinations. The luminescence mechanism of bismuth ions was also analyzed. By systematically comparing the luminescence under varying bismuth doping concentrations (1/3/5/10 mol%) and different excitation wavelengths (488/532/808/980 nm), we discussed the plausible energy level models that correlate with the observed behaviors in the sample. Furthermore, the individual luminescence contributions from BAC-Al, BAC-Si, Bi+, Bi° ions, and group luminescence were examined.
2. Experiment
Glass samples with composition xBi2O3-60SiO2-(30-x)Al2O3-10La2O3 (BSAL, where x = 1,3,5,10) were prepared using a conventional melting technique. Industrial powders of high purity SiO2 (99.99%), Al2O3 (99.9%), La2O3 (99.99%), and Bi2O3 (99.999%) were uniformly mixed in a mortar to prepare a 20 g sample. Subsequently, the sample was placed in a ceramic crucible and melted at 1650 °C for 1 hour. The molten glass was then cooled down to room temperature, resulting in the formation of bulk glass. Microscopic examination confirmed the transparency and absence of bubbles in the prepared glass samples. Following this, the glass samples were cut and polished into dimensions of 10 mm × 10 mm × 2 mm for further measurements. In order to investigate the luminescence mechanism of bismuth ions in the glass matrices, the NIR fluorescence and absorption properties of the BSAL glasses were comprehensively investigated. The density of the BSAL samples was determined using the Archimedes drainage method. Transmittance and absorption spectra were recorded over a wavelength range of 300-2000nm using a UV-Vis-NIR spectrometer (Lambda 950, PerkinElmer, Waltham, MA). Emission spectra and fluorescence decay curves were obtained by a fluorescence spectrophotometer (FLS1000, Edinburgh, UK). The single pulse mode is selected to excite the glass and was sampled by an oscilloscope. The fluorescence lifetime of the sample can be obtained by fitting the data. Fourier-transform infrared spectroscopy (FTIR) was employed to study the -OH groups. The refractive index of the sample was measured by an ellipsometer (J.A Woollam RC2).
3. Results and discussion
3.1 Physical properties
Figure 1 displays samples with different concentrations of bismuth doping. As the concentration increases, the color of the samples varies from light pink to a deeper wine red hue. This change in color is due to the reduction degree of the bismuth element during the preparation process [26]. During the preparation process, we strictly adhere to the molar ratios of the various materials, ensuring accuracy to three decimal places. However, due to uncontrolled self-reduction in the melting reactions at high temperatures, the number of active ions formed by the bismuth components, which influence NIR luminescence, remains uncertain. To address this, we conducted multiple repetitions of the melting process to verify the stability of the samples and ensure the reliability of the experimental data.
The density, refractive index, and bismuth particle concentration of the samples were measured and listed in Table 1. Figure 2 elucidates the relationship between doping concentration and density. It is evident that the density of the glasses monotonically increases from 3.340 g/cm3 to 3.702 g/cm3 with the increasing content of Bi2O3 in the glass components. This linear increment in the number of bismuth ions per cubic centimeter illustrates the effective doping of bismuth in the samples. Furthermore, the refractive index gradually increases with the rise in Bi2O3 doping concentration.
Table 1. The physical properties of xBi2O3-60SiO2-(30-x)Al2O3-10La2O3 glass
3.2 Absorption and transmission properties
In the present section, an investigation into the absorption and transmission properties of the BSAL samples is detailed. The absorption and transmission spectra of the BSAL samples, measured within the range of 300-1000 nm, are showcased in Fig. 3 The obtained spectra manifest three distinct absorption peaks located at 490 nm, 708 nm, and 810 nm, which correspond to the excitational transitions of bismuth ions from their respective ground states to higher energy levels. Notably, this finding substantiates the previous assertion in the literature regarding the near-infrared emission center of Bi-doped fiber being likely attributed to Bi+ [21]. It is important to note that the Bi+ (6s26p2) ion experiences splitting into the ground state (3P0) and several excited states (1S0, 1D2, 3P2,1) due to the influence of spin-orbit coupling. Consequently, the observed absorption bands occurring around 500 nm, 700 nm, and 800 nm in the Bi-doped aluminosilicate glass samples can be ascribed to the transitions originating from the ground state to these three excited states [27,28]. This attribution is consistent with existing theoretical explanations in the field [29–32].
Fig. 3. (a) Absorption and (b) Transmittance spectra of BSAL samples with different bismuth concentrations.
3.3 Fluorescence properties
The NIR emission spectra of samples with various concentrations of Bi-doping are presented in Fig. 4, under excitation wavelengths of 488 nm, 532 nm, 808 nm, and 980 nm, respectively. Notably, all samples exhibit wideband NIR luminescence ranging from 1000 nm to 1650 nm, demonstrating excellent NIR luminescence characteristics. The corresponding excited near infrared fluorescence spectra exhibit maximum FWHM values of 313.0 nm, 336.3 nm, 296.2 nm, and 262.9 nm, respectively. Additionally, the sample with a Bi-doping concentration of 5 mol% demonstrates the highest luminescence intensity, pointing to an optimal doping concentration for the glass at approximately 9.19 × 1020 cm-3, as indicated in Table 1. Higher doping concentrations can lead to severe concentration quenching effects.
Fig. 4. NIR emission spectra of BSAL samples with various bismuth doped concentrations at the excitation bands of (a)488, (b) 532, (c) 808, (d)980 nm.
To better comprehend the nature of the active centers in the BSAL sample system, we discuss the BAC-Al and BAC-Si centers. Notably, the NIR fluorescence peaks exhibit a discernible red shift with increasing Bi-doping concentration, accompanied by a reduction in aluminum content. This outcome aligns with the existing theory, which posits that BAC-Al can excite NIR light at 1150 nm, while BAC-Si can excite NIR light at 1300 nm [15,17,27,34]. The NIR luminescence centers will be discussed in detail in Section 3.4. Moreover, this observation substantiates the overall NIR luminescence redshift phenomenon observed following the decrease in BAC-Al content. Importantly, no shift in fluorescence peak position is observed with changing concentration for the 808 nm excitation, as shown in Fig. 4(c) This suggests that the 808 nm pump light can only be absorbed by BAC-Si, thereby stimulating fluorescence at approximately 1300 nm, but it fails to induce the luminescence of BAC-Al. Additionally, the sample doped with 3 mol% exhibits the most pronounced NIR luminescence under 980 nm excitation, as shown in Fig. 4(d) This indicates that the material structure with a 3 mol% doping concentration is more effective in absorbing 980 nm pump light, thus promoting the excitation of BAC-Al. Hence, the experimental results provide valuable insights into the NIR luminescent characteristics of the studied BSAL samples, shedding light on the optimal Bi-doping concentration for enhanced luminescence and the correlation between the BAC-Al and BAC-Si active centers.
Additionally, Fig. 4 also shows the presence of absorption at 1380 nm. This absorption may be attributed to impurity particles introduced during the material processing, specifically hydroxyl groups. The characterization of -OH groups or water in glass is typically accomplished by analyzing the -OH absorption peaks or transmission in the infrared (IR) spectrum. In the case of silicate glasses [15,16], these absorption peaks are commonly observed at 2.8 µm. Moreover, the presence of -OH groups in the samples was further confirmed by examining the transmission spectrum in the wavelength range of 2.5-6 µm. In Fig. 5, a pronounced decrease in transmission is evident at 2.8 µm, indicating significant absorption attributed to the presence of -OH groups.
3.4 Discussion on NIR luminescence centers
As mentioned above, the luminescence center wavelength is related to the doped concentration. It indicates that different concentrations will result in different luminous centers and valence states while forming the glasses. Theoretically, the higher doped concentration will lead to a stronger the NIR luminescence. However, the high doped concentration reduces the bismuth’s ionic distance, inducing adverse energy transfer. As a result, most of the ion clusters are quenched. By designing the composition ratio scheme, the effective doped concentration of bismuth has been increased to 5 mol% and the fluorescence performance well in the study. Exploring the internal structure change brought by the concentration increases is valuable for studying NIR emission mechanisms of Bi-doped materials. As shown in Fig. 4, there are a main emission peak centering at 1300 nm and an acromion at 1400 nm. Due to the unstable structure of the bismuth material, Bi ions show different valence states during the high-temperature firing process and influence the NIR luminescence, resulting in the fluorescence spectrum is formed by the superposition of multiple peaks [17,33]. Here, the NIR emission centers in Bi-doped samples are analyzed in detail.
Multiple active Bi-doped centers are superimposed to form the NIR emission spectrum. To analyze the spectrum, we selected the 808 nm excitation that conforms to Gaussian superposition most regularly. And the peak-splitting method is used to analyze the NIR fluorescence spectrum. By studying the center of the emission peak, the energy level transition mechanism of NIR fluorescence excitation is deduced. Meanwhile, the valence states of bismuth ions involved in excitation and their respective contributions are analyzed.
Figure 6 describes the peak splitting of the samples at 808 nm excitation. There are four Gaussian peaks at 1200 nm, 1300 nm, 1450 nm, and 1560 nm, respectively. The calculation reveals that the energy corresponding to all Gaussian curves continuously enhanced as the Bi-doped concentration increases. Here, 1300 nm corresponds to the transition radiation from 2D3/2 to 4S3/2 in Bi° [15,27,34]. While increasing the Bi concentration, more Bi° ions can be formed. From Fig. 6, we find that the contribution of Bi-doped active centers to NIR emission would change with the increase of the concentration, indicating that there are at least two Bi-doped active centers affecting luminescence at 1200 nm and 1300 nm. In the study on the luminescence mechanism of bismuth-doped oxide glass, the relevant particle model from the first-character principle has been established [10,11]. It is found that the bismuth-doped material is complex in the high-temperature environment, and the bismuth element reacts make it change from Bi3+ to Bi+ to Bi° and the other Bi clusters [17].
Fig. 6. Peak analysis diagram of samples at 808 nm excitation with Bi-doped (a) 1mol% (b) 3mol% (c) 5mol%
Similarly, other Gaussian sub-peaks also affect the NIR luminescence, and there are various bismuth particles of different forms in the samples. From the experimental data, we can find that another particle produced by our samples may transit from 3P1 to 3P0, corresponding to the Bi+, with the emission light of 1200 nm. According to the NIR emission spectra excited at 808 nm in Fig. 4(c), it is inferred that the Bi+ ion is insensitive to this light absorption.
In exploring the mechanism of bismuth valence state on near-infrared luminescence, we found that the sample color increased with the degree of reduction from shallow to deep through high-temperature reduction and different degrees of reduction of reducing agent phosphorus, which is consistent with Zhang’s result [10]. With the high concentration of bismuth, the color of the sample deepens. At the same time, under the low concentration of bismuth doping of 1mol%, there is 1200 nm leading NIR luminescence. With the concentration increasing to 3-10 mol%, 1300 nm becomes the main luminescence center. It is also confirmed the glowing valence should be Bi+ in 1200 nm, and the contribution to the 1300 nm should be contributed to Bi°. Among them, the peak around 1300 nm gradually becomes the main peak that mainly affects the NIR luminescence while increasing the concentration of Bi°. In the test, I(Bi°)/I(Bi+) (the ratio of the intensity of Bi° and Bi+) increases, indicating that the proportion of Bi° increases at high concentration, leading to enhanced the Bi° radiative transition. It shows that the Bi ion concentration increasement generates more clusters that affect the 1300 nm band, resulting in a wider band of NIR emission.
In addition, there are two weaker Gaussian peaks at 1450 / 1560 nm, where the energy is only 0.7% of the 1300 nm peak. We believe that Bi+ ions are combined with the base material during the melting process, forming Si-O-Bi groups and a small amount of Si-Bi groups. These two clusters correspond to the excitation light near 1450 nm and 1560 nm [11,23]. Based on the discussions above, we deduce the simplified energy-level distribution of the NIR luminescence center of the samples, as shown in Fig. 7.
3.5 Fluorescence gain characteristics
Figure 8 illustrates the fluorescence decay behavior of the samples under the influence of lasers with different wavelength bands at room temperature. It is evident that the lifetime curves of the BASL samples manifest distinct characteristics, varying according to the concentration of bismuth dopant. Remarkably, when excited by an 808 nm laser, the fluorescence lifetime of the BASL sample doped with 5 mol% Bi was measured to be 1.473 ms. This value represents a substantial improvement, ranging from four to ten times greater compared to the findings reported in previous investigations concerning the fabrication and characterization of Bi-doped materials and devices [29–31]. Such a considerable enhancement is attributed to the judicious optimization of composition and ratio schemes employed in this study. Moreover, given the unpredictability of the reaction behavior of bismuth in a high-temperature environment, the heating and holding times were carefully calibrated to ensure the achievement of desirable effective deposition concentrations and fluorescence lifetimes. It is worth noting that an increase in the concentration of bismuth ions induces a bolstered energy transfer effect and an augmented radiation rate, ultimately resulting in a shortened fluorescence lifetime. Consequently, as the Bi-doped concentration escalates from 1 mol% to 10 mol%, a gradual reduction in fluorescence lifetime from 1.8 ms to 1.4 ms is observed.
Fig. 8. (a) Fluorescence decay curves of 5mol% Bi-doped sample at 488/532/808/980 nm excitation. (b) The fluorescence lifetimes with different Bi-doped concentrations and different band excitations.
The efficiency of a laser gain medium is determined by a critical parameter known as the product of the stimulated emission cross section and the emission lifetime (σem×τrad), which displays an inverse relationship with the laser threshold. The determination of the stimulated emission cross section (σem) can be achieved through the utilization of the Fuchtbaner-Landenburg formula [32]. In this work, the calculated value of σem amounts to 3.35 × 10−21 cm2.To facilitate a comprehensive analysis, an informative comparative overview has been compiled and presented in Table 2, illustrating the performance data of diverse bismuth-doped glasses. It is evident that our study presents a notable competitive advantage with the doping concentration of 5 mol% and a fluorescence lifetime of 1.473 ms compared to other research endeavors.
Table 2. Comparison of the optical properties with reported Bi-doped glass
4. Summary
In summary, this research focused on the investigation of the wideband near-infrared spectroscopy characteristics and luminescence mechanism of highly Bi-doped SiO2-Al2O3-La2O3 glasses. The glasses were synthesized using a melting technique, and the optical properties were explored under different excitation wavelengths. The results showed that the optimum Bi-doped concentration was 5 mol%, with a corresponding stimulated cross-section of 3.35 × 10−21 cm2. When pumped at 808 nm, the glass demonstrated a long fluorescence lifetime of 1.473 ms. Through a detailed analysis of the luminescence properties, it was determined that the emissions around 1150 nm and 1300 nm originated from BAC-Al and BAC-Si. These findings indicate the potential applications of highly Bi-doped SiO2-Al2O3-La2O3 glasses in broadband near-infrared optical devices. The wideband spectrum and long fluorescence lifetime make these glasses suitable for use as gain media in wideband active devices.
Funding
National Natural Science Foundation of China (62205381, 62205383).
Acknowledgments
This work was funded by National Natural Science Foundation of China (NSFC), grant number 62205383 and 62205381.
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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