Abnormal near-infrared luminescence property of bismuth doped calcium germanate glasses

Bismuth doped calcium germanate glasses were prepared, and the relationship between Bi near-infrared (NIR) luminescence and the glass structure was investigated. The NIR emission with 808 nm excitation covered the whole optical telecommunication window. The intensity of the NIR emission increased firstly and then declined with the increasing CaO content, which cannot be explained by the theory of optical basicity. The X-ray photoelectron spectroscopy (XPS) and the visible luminescence spectra showed that the trivalent Bi3+ could reduce to lower state of Bi in these glasses. The Fourier transform infrared (FT-IR) absorption spectra and Raman spectra indicated that the major structure units were [GeO4] tetrahedral, and the 4-membered [GeO4] rings converted into 3-membered [GeO4] rings with the increasing optical basicity. The nature of NIR emission and the reason of the abnormal phenomenon were discussed.