Phonon side band spectroscopy of Eu3+ in binary Sb2O3-GeO2 glasses

Abstract

Binary glasses in the Sb${}_2$O${}_3$-GeO${}_2$ system have been prepared with europium doping for spectroscopic investigation of the rare-earth environment. Phonon side band analysis of the photoluminescence excitation spectra reveals an invariant phonon energy of 756 $\pm$8 cm⁻¹ for binary glasses with between 11–54 mol% Sb${}_2$O${}_3$. The electron–phonon coupling strength of the high frequency vibronic mode is between 0.005–0.006 for the series. These observations indicate the beneficial impact of Sb${}_2$O${}_3$ on the rare-earth spectroscopy is achieved in the germanate rich side of the composition space (Sb:Ge$<$1), where thermal properties (glass transition and thermal expansion) do not display significant compositional dependence according to previous studies. The combination of favorable spectroscopic properties and slowly varying thermal properties in the binary system with between 18–33 mol% Sb${}_2$O${}_3$ makes these glasses promising for core/clad active optical fiber applications. The phonon energy is lower and the electron–phonon coupling weaker in antimony germanates than in other heavy metal germanates containing lead or bismuth. Using Co${}^{2+}$ as a probe ion, the impurity ion sites appear to be coordinated with bridging oxygen offered by higher coordinated germanate or antimonite polyhedra. Analysis of the Eu${}^{3+}$ emission spectra shows a decrease in crystal field strength and increasing site symmetry as Sb${}_2$O${}_3$ replaces GeO${}_2$.