Evidence of the influence of phonon density on Tm³⁺upconversion luminescence in tellurite and germanate glasses

Spectral properties of blue upconversion luminescences in Tm3+doped tellurite (PWT, PbF2-WO3-TeO2) glasses and germanate (PWG, PbF2-WO3-GeO2) glasses pumped by a tunable dye laser were studied at room temperature. Two emission bands centered at 453 and 477 nm, corresponding to the1D2→3H4and1G4→3H6transitions of Tm3+ions respectively, were observed. The two-photon absorption mechanism responsible for the 477 nm luminescence was confirmed by a quadratic dependence of luminescent intensities on the excitation power. Tellurite glasses showed a weaker upconversion luminescence than germanate glasses. This observation was inconsistent with the prediction from the phonon sideband measurement. In this article, Raman spectroscopy and transmittance measurement were employed to investigate the origin of the difference in upconversion luminescences in the two glasses. Compared with phonon sideband spectroscopy, Raman spectroscopy extracts more information, including both phonon energy and phonon density, and therefore, is a more effective analytical tool for understanding upconversion luminescence. Our results showed that the phonon energy as well as phonon density of the host glass is important in determining the upconversion efficiency.