Optical Properties and Bismuth Redox in Bi-Doped High-Silica Al–Si Glasses

Due to their unique optical properties, Bi-doped glasses have a great potential for the development of new tunable laser sources. Particular attention is given to high-silicate Bi-doped glasses as active media for fiber lasers and optical amplifiers, capable to operate at the second and the third near-infrared (NIR) transparency windows. Although significant practical advances have been achieved in this area, the origin of the NIR luminescence remains unclear. In the present paper, a set of glasses 95.5SiO2–4.5Al2O3–xBi2O3 (x = 0; 0.005; 0.01; 0.05; 0.1; 0.2) were synthesized using a conventional melt-quenching technique and investigated using a multiscale approach. Variation of the NIR emission was correlated with the change of other optical properties, structure and bismuth redox, supported by fluorescence/absorption, Raman, and XANES experimental studies, respectively. It was found that redox of bismuth shifted toward the formation of more reduced Bi species with the decrease of the dopant level. Based on the complementary experimental results, the two NIR optical centers observed were attributed to the presence of a Bi+ ion, which dominates at the lowest dopant level, and presumably a Bi+–Bi+ dimer, forming at higher Bi concentrations. The dimer center was found to participate in a Förster-type energy transfer, interpreted as energy migration. This knowledge complements previous findings and will help in rational engineering of Bi-doped optical materials.