Origin of near-infrared luminescence in Bismuth-doped silica and germanosilicate glass fibers: Crystal field modeling

Bismuth-doped silica-based glasses have already been proved to be promising materials for the fiber lasers and amplifiers operating in the near-infrared (NIR) spectral region. However, the development of such devices, which would have eligible efficiency, is impossible without a solid understanding of the NIR emitting center's nature. In the present work, based on the crystal field calculations we demonstrate that in silica and germanosilicate glasses this center is a univalent Bi ion affected by a rather strong local crystal field. According to our results, a transition from the first excited state to the ground state in Bi+ causes the famous NIR photoluminescence. The relatively high degree of reduction of the spin-orbit coupling constant indicates, most probably, the formation of weak covalent bonding of Bi+ with ions of the environment.