Spectroscopic properties of Bi-doped SrB4O7 glasses, sintered compounds, polycrystalline materials, and single crystals were investigated. Broadband near-infrared luminescence was realized in Bi-doped SrB4O7 glasses with basicity and polycrystalline materials with non-bridging oxygens. In Bi:SrB4O7 single crystals, only visible luminescence of Bi3+ and Bi2+ was observed, but no near-infrared. The rigid three-dimensional network of SrB4O7 crystal is proved to be unfavorable for accommodation of Bi+ ions.
©2009 Optical Society of America
The evolution of the wavelength division multiplexing (WDM) technology based on the rare-earth ion doped fiber amplifier promotes the development of optical communication during the past decades. Rare earth element doped fiber amplifiers, which can amplify the light signals directly without light→electricity→light conversion, have been widely applied in the present optical communication systems. Unfortunately, the bandwidth of rare-earth ion doped fiber amplifier is hardly to surpass 100 nm because the f–f transitions between 4f orbits are confined in the inner-shell, which are insensitive to local environmental . Therefore, expanding the response bandwidth of the fiber amplifier and the laser source in order to achieve more efficient WDM transmission network with higher capacity and faster bit rate has become a key and attractive aspect for the future development of the optical communication.
Recent studies have found that some of the bismuth ions in the glass substrate can have a center wavelength of light-emitting at 1.3 µm near fluorescent FWHM (Full Width at Half Maximum) up to about 300 nm, and the fluorescence of life is up to 600 µs [2–8]. The bismuth-doped glass has the potential to become the next great wide-band optical fiber amplifier. There are a large number of in-depth researches and explorations, which have made a great advance. However, the nature of the active center emitting in the near-infrared (NIR) is still unknown in the Bi-doped glass or fibers. In the previous publications, the emission has been attributed to electronic transition of Bi5+ [2,3], Bi2+ , Bi+ [5,6], Bi clusters  or color centers , but the origin is still controversial.
Obviously, it is much easier to interpret the spectroscopic properties and understanding the nature of the Bi-related active centers in crystal hosts than glasses or fibers, because of the ordered and rigid crystal lattice. Furthermore, the crystalline structure can also provide much higher quantum efficiency than glasses or fibers. Thus, study on the NIR luminescence properties of Bi-doped crystals is very attractive and promising to understand the nature of the luminescent centers and developing new all-solid-state broadly-tunable or ultrashort-pulse lasers. In fact, doping Bi ions into crystalline materials is relatively difficult because of the strong volatility of bismuth during crystal growth process. Up to now, only a few works [9–12] on the NIR luminescence properties of Bi-doped crystalline materials have been reported. In reference , the broadband NIR luminescence peaking at 1080 nm with the lifetime of 140 µs was observed in Bi-doped RbPb2Cl5 crystal and Bi+ ion was proposed to be the luminescence mechanism. The NIR luminescence of Bi-doped BaF2 crystals was reported by some of the current authors . More and more experiment results place weights on the assignment of Bi+ as the nature of NIR luminescence.
In this Letter, the spectroscopic properties of Bi ions in SrB4O7 glasses and crystalline materials were compared and analyzed. SrB4O7 was selected because it was reported to be the first example as an host crystal for realization of unique orange-red luminescence of Bi2+ . Moreover, SrB4O7 compound has a congruent melting point, which can be prepared with the forms of both glass and single crystal.
The bismuth-doped SrO-B2O3 glass (marked as Bi:SrBO) samples, with the same Sr/B ratio as the SrB4O7 crystal, were prepared by the conventional melting–quenching technique. High-purity reagents, SrCO3, H3BO3 and Bi2O3, were selected as the raw materials.100g batch corresponding to the glass composition (in mol%) of (100- x) SrCO3,4(100 -x) H3BO3,x/2 Bi2O3 (x=0.02, 0.05) was mixed homogenously in an mortar, and then melted at 1050 °C in a platinum crucible for 30~40 min in air. Consequently, the melt was chilled to solid.
Bi-doped SrB4O7 compounds were synthesized by solid-state reaction at 850 °C in N2 atmosphere for 5 hours. Platinum crucible was used. Polycrystalline Bi-doped SrB4O7 materials were prepared through cooling the melting in N2 atmosphere at the rate <5 °C/h. SrB4O7 crystals were grown in N2 atmosphere by the conventional Czochralski method. The powder was put into a platinum crucible 60mm in diameter and 80mm in height. Platinum wire was used as seed, and the pulling and rotation rates were 0.1–0.3 mm/h and 5–20 rpm.
The optical absorption spectra were recorded by a Jasco V-570 UV/VIS/NIR spectrophotometer. The visible and infrared luminescence spectra were obtained with a Fluorolog-3 (Jobin Yvon, France) spectrofluorometer upon exciting with a 450 W Xe lamp and 808 nm or 980 nm LD (Laser Diode). Emission decay curves were measured with a Tektronix TDS 3020 oscilloscope using an 808 nm LD as the excitation source. The measurements were performed at room temperature and all the emission spectra were corrected for the setup characteristic.
Raman spectra were recorded using a Dilor LabRam-1B Raman microscope (Dilor, France), operating at a resolution of 1 cm-1. A 3 mW He–Ne laser (of wavelength 632.81 nm) was focused with a 100× objective (0.8 NAOlympus, Olympus, Japan) to a diameter of approximately 0.7 mm. The spectra were collected in the conventional 90° scattering geometry.
3. Results and Discussion
It is interesting that the as-prepared 2.0 mol% Bi:SrB4O7 glass consisted of two parts with colorless and yellow color, but the 5.0 mol% Bi:SrB4O7 glass was thoroughly yellow. The concentrations of Bi element in the colorless and yellow SrB4O7 glasses were checked out 1.7 mol% and 2.2 mol%, respectively, by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The absorption spectra of the samples with different colors were shown in Fig. 1. The absorption peaks at 368 nm, 422 nm, 456 nm and 687 nm can be observed in the absorption spectrum of the yellow glass, while the peak at 368 nm dominates in that of colorless one.
The same as reported earlier , no infrared emission was observed in the colorless Bi-doped SrB4O7 glass under pumping in the visible absorption band, or with 808 nm/980 nm LD in this work. The cause was ascribed to the basicity of the glass by the authors in Ref . However, NIR broadband emission was appeared in the yellow glass under the same pumping conditions, as shown in Fig. 2. The emission band peaks at 1292 nm with FWHM of 202 nm. The emission decay curve at 1292 nm was shown in the inset of Fig. 2. The decay curve shows a good consistence with the first order exponential decay with the lifetime as long as 290 µs.
Raman scattering spectra were determined to investigate the structure difference between the colorless and yellow Bi:SrB4O7 glasses, as shown in Fig. 3. There are five Raman scattering peaks at 506, 662, 772, 955, and 1330 cm-1 (or 1417 cm-1). The first four Raman peaks can be attributed to the bending mode of free BO4 units, the stretching mode of metaborate rings group, the symmetric breathing mode in rings with one BO4 unity, and the boron-oxygen stretching mode of tetrahedrally coordinated borons, respectively, while the broad band in the region 1100–1600 cm-1 is due to the different types of boron-oxygen stretching vibrations in the so-called continuous random networks (CRN), associated with non-bridging oxygens (NBOs), i.e., oxygen atoms not involved in B-O-B linkages [15,16]. The proportion of CRN structure in the yellow 2.2 mol% Bi:SrBO glass is evidently higher than that of the colorless 1.7 mol% one, along with the center shifting from 1420 to 1330 cm-1. So, the NIR luminescent centers should be involved with the unit with vibration mode of 1330 cm-1 in the CRN structure.
Through heat annealing in air atmosphere at 600 °C, broadband NIR luminescence was also realized in the 1.7 mol% Bi:SrBO glass, as shown in Fig. 4 under 808 nm LD pumping. The emission spectrum is similar to that of the 2.2 mol% Bi:SrBO glass. The Raman spectrum of the annealed glass was also measured, as presented in Fig. 3. One can see that the proportion of CRN structure increased and the central frequency shifted from 1420 to 1330 cm-1 with heat annealing, similar to the changes induced by increasing Bi concentration from 1.7 mol% to 2.2 mol%. This experimental result re-confirmed the association of NIR luminescent centers with the vibration mode of 1330 cm-1. Since the centers were most probably due to Bi+, the unit structure with mode of 1330 cm-1 should be associated with Bi+-O-B groups.
In order to better understand the mechanisms of NIR luminescence in Bi:SrB4O7 glass, the Bi-doped SrB4O7 sintered compounds, polycrystalline materials, and single crystals were also prepared, respectively. Broadband NIR luminescence was only observed in Bi-doped polycrystalline SrB4O7 samples, as shown in Fig. 5 with FWHM of 276 nm, but not in sintered compounds or single crystals. The emission lifetime was measured to be 180 µs. Raman spectra of Bi-doped SrB4O7 polycrystalline materials and single crystals were presented in Fig. 6, along with that of the 2.2 mol% Bi:SrBO glass. The significant difference between the polycrystalline materials and single crystals is that the vibration mode of 1330 cm-1 exists in the former but not in the latter, which should be due to the incomplete crystallization.
The emission spectra of Bi3+ and Bi2+ in Bi-doped SrB4O7 single crystal and glass were observed in visible region, as shown in Fig. 7 and 8, respectively, as well as corresponding excitation spectra. Bi3+ and Bi2+ were identified in accordance with the experimental results reported in Ref , , and . The emission spectrum of Bi3+ in Bi:SrB4O7 crystal peaks at 431 and 452 nm while that of Bi:SrB4O7 glass at 430 and 458 nm. The corresponding excitation spectra of the crystal and glass peak at ~370nm and ~340nm, respectively. The emission spectra of Bi2+ in Bi:SrB4O7 crystal and glass peak at 587 nm and 695nm respectively. The corresponding excitation spectrum of the emission at 587 nm in Bi:SrB4O7 crystal peaks at 470nm and 550 nm,while that of Bi:SrB4O7 glass peaks at 340 nm and 470 nm. As the band at 340 nm is due to Bi3+, it indicates that there exists the energy transfer between Bi3+ and Bi2+ in Bi:SrB4O7 glass, but not in the crystal. Therefore, it can be inferred that the distance between Bi ions in glass should be shorter than those in crystal and the three-dimensional [BO4] network of SrB4O7 crystal structure could prevent the interaction between Bi ions.
SrB4O7 forms a complex orthorhombic structure with the space group of Pmn21, where Sr2+ ions are coordinated only by [BO4] units. Then, the bismuth substituting Sr2+ is enclosed by the three-dimensional [BO4] network and hardly expected to be attacked by oxygen in the air even at high temperature and hence the divalent state of Bi ions will become very stable [13,19]. So, it was thought that NIR luminescence should be probably realized in Bi-doped SrB4O7 crystal if Bi+ is the origin. However, no NIR emission was observed in the Bi:SrB4O7 single crystal under exciting in the visible absorption band, or with 808 nm/980 nm LD. Furthermore, two kinds of usual reducing methods, H2 annealing and γ-irradiation, were carried out on Bi:SrB4O7 crystals, but the desired appearance of NIR luminescence wasn’t achieved yet. It can be inferred that the three-dimensional network of SrB4O7 crystal should be too rigid to accommodate Bi+ with radius of about 145 pm much larger than 112 pm of Sr2+.
In summary, the spectroscopic properties of Bi-doped SrB4O7 glasses, sintered compounds, polycrystalline materials, and single crystals were investigated. Broadband NIR luminescence was realized in Bi-doped SrB4O7 glasses with basicity and polycrystalline materials, with vibration mode at 1330 cm-1 associated with Bi+-O-B groups. Increasing the doping level of Bi or heat annealing can increase the proportion of Bi+-O-B groups in Bi:SrB4O7 glasses, thus modifying the connectivity of the glass structures. In Bi:SrB4O7 single crystals, only visible luminescence of Bi3+ and Bi2+ was observed, but no NIR. The rigid three-dimensional network of SrB4O7 crystal is proved to be unfavorable for accommodation of Bi+ ions.
The authors thank Prof. Jianrong Qiu, Dr. Guoping Dong and Bin Qian for meaningful discussions. The work was supported by the National Natural Science Foundation of China under the number of 60778036 and Shanghai National Natural Science Foundation under the number of 08ZR1421700.
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