Origin of structural relaxation dependent spectroscopic features of bismuth-activated glasses

For the first time, we studied the effect of structural relaxation on the NIR spectroscopic properties of bismuth-activated germanium glasses below glass transition temperature. Interestingly, distinct change behavior of NIR luminescence is observed at two different heat-treatment temperature ranges corresponding to two different relaxation behavior of glass structure. Besides, when structural modified by partly substituting B(2)O(3) for GeO(2), a narrower and more thermal sensitive luminescence is observed, which is inexplicable by "inhomogeneous broadening" and we tentatively attribute it to a defect-involved reason. Fundamentally the results here not only provide us a deeper insight into the optical property of bismuth-activated materials but also increase our understanding of the glassy state, and practically it delivers some valuable guidance in designing bismuth-activated glasses with superior NIR optical properties.

Regulation of structure rigidity for improvement of the thermal stability of near-infrared luminescence in Bi-doped borate glasses

The effect of heat-treatment on the near-infrared (NIR) luminescence properties was studied in Bi-doped borate glasses. The luminescence intensity generally decreases with the increase of temperature, and the thermal stability can be improved by nearly 4.5 times with addition of 5 mol% La2O3. Collaborative studies by using steady photoluminescence (PL) and photoluminescence excitation (PLE) spectra, luminescence decay curve, differential thermal analysis (DTA), Raman spectra and X-ray diffraction (XRD) indicate that the luminescence decrement is associated with the agglomeration of Bi active centers during heat-treatment. The improvement of the thermal stability of NIR luminescence with the addition of La2O3 is benefited from the enhancement of structure rigidity due to the strong cationic field strength of La3+. The results not only provide valuable guidance for suppressing performance degradation of Bi-doped glass during fiber drawing process, but also present an effective way to control the luminescence properties of main group elements in glasses from the perspective of glass structure.

A new study on bismuth doped oxide glasses

Spectroscopic properties of bismuth doped borate, silicate and phosphate glasses have been reinvestigated in this work. It shows the typical decay time of Bi(3+) is around 500ns rather than 2.7-to-3.9 μs reported by Parke and Webb at room temperature. Introduction of higher content either alkali or alkali earth into borate glasses favors the Bi(3+) emission. As the contents increase excitation peak shifts regularly red while emission peak shows reverse trend. This, as revealed by Huang-Rhys factor, is due to the weakening of coupling between bismuth and glass host, and it can be interpreted within the frame of configurational coordinate diagrams. Differently, as bismuth concentration increases, both the excitation and emission shift red. The unknown origin of red emission from bismuth doped calcium or magnesium phosphate glass has been identified as Bi(2+) species on the basis of excitation spectrum and emission lifetime particularly after comparing with Bi(2+) doped materials. No near infrared (NIR) emission can be detected in these glasses within instrument limit.

Superbroad near-to-mid-infrared luminescence from Bi5(3+) in Bi5(AlCl4)3

Superbroad near-to-mid infrared (NIR-MIR) photoluminescence was observed from Bi5(AlCl4)3 at room temperature, spanning the spectral range of about 1000 to 4000 nm. On the basis of structural considerations and dynamic analyses, Bi5(3+) clusters were identified as the optically active species, inherently differing from the species which is typically believed to be active in NIR-emitting Bi-doped glasses. In comparison to most other NIR-luminescent Bi-doped materials, the MIR-part of the luminescence spectrum is still present at room temperature. Emission intensity and excited state lifetime were found to exhibit abnormal temperature dependence, where the former increases with temperature up to a critical value of about 150 K. This behavior is related to a temperature-dependent overlap between ground state and excited states. The observed stabilization of MIR photoemission at room temperature may be a starting point for the development of Bi-based NIR-MIR light sources with superbroad emission spectrum, where Bi5(3+) or similar polycationic species act as optical gain medium.

Broadband NIR photoluminescence from Bi-doped Ba2P2O7 crystals: insights into the nature of NIR-emitting Bismuth centers

We report on a novel type of Bi-doped crystal that exhibits ultrabroadband photoluminescence in the near infrared (NIR). Emission centers can be generated and degenerated reversibly by annealing the material in CO atmosphere and air, respectively, indicating that emission is related to the presence of Bi-species in low valence states. Correlating static and dynamic excitation and emission data with the size and charge of available lattice sites suggests that two types of Bi(0)-species, each located on one of the two available Ba(2+) lattice sites, are responsible for NIR photoemission. This is further confirmed by the absence of NIR emission in polycrystalline Ca(2)P(2)O(7):Bi and Sr(2)P(2)O(7):Bi. Excitation is assigned to transitions between the doubly degenerated ground state (4)S(3/2) and the degenerated excited levels (2)D(3/2), (2)D(5/2) and (2)P(1/2), respectively. NIR emission is attributed to (2)D(3/2)?(4)S(3/2). The NIR emission center can coexist with Bi(2+) species. Then, also Bi(2+) is accommodated on one of the two Ba(2+)-sites. Energy transfer between Bi(2+) ions occurs within a critical distance of 25.9 A.

Luminescence from Bi2+-activated alkali earth borophosphates for white LEDs

Bi(2+)-doped MBPO(5) (M = Ba(2+), Sr(2+), Ca(2+)), synthesized in air via solid state reaction, are considered as novel orange and red phosphors for white light emitting diodes with improved colour quality. Absorption of Bi(2+) due to (2)P(1/2)-->(2)S(1/2) and (2)P(1/2)-->(2)P(3/2) could be observed and quantified. Excitation to (2)P(3/2) is accompanied by vibronic sidebands, and corresponding emission behaviour is found. The electron-phonon coupling strength increases in the order M = Ba(2+)-->Sr(2+)-->Ca(2+). In the case of MBPO(5):Bi(2+), one-, two- and even three-phonon sidebands could clearly be observed. The crystal structure of all three compounds belongs to space group P3(1)21. Bi(2+) is incorporated on M(2+) sites, and reduction of Bi(3+) to Bi(2+) occurs for reasons of charge compensation. In accordance with crystallographic data, fluorescence decay behaviour indicates that only one type of Bi(2+)-emission centers is present.