Multimodal emissions from Tb3+/Yb3+ co-doped lithium borate glass: Upconversion, downshifting and quantum cutting

Abstract

This paper reports the optical properties of Tb3+/Yb3+ co-doped lithium borate (LB) glass prepared by melt quench method. The absorption spectrum of the Yb3+ doped LB glass contains intense NIR band centered at 976nm due to 2F7/2→2F5/2 transition. The emission spectra of the prepared glasses have been monitored on excitation with 266, 355 and 976nm. The Yb3+ doped glass emits a broad NIR band centered at 976nm whereas the Tb3+ doped glass gives off visible bands on excitations with 266 and 355nm. When the Tb3+ and Yb3+ ions are co-doped together, the emission intensity in the visible region decreases whereas it increases in the NIR region significantly. The increase in the emission intensity in the NIR region is due to efficient cooperative energy transfer (CET) from Tb3+ to Yb3+ ions. The quantum cutting efficiency for Tb3+/Yb3+ co-doped glass has been calculated and compared for 266 and 355nm excitations. The quantum cutting efficiency is larger for 355nm excitation (137%). The Tb3+/Yb3+ co-doped LB glass also emits upconverted visible bands on excitation with 976nm. The mechanisms involved in the energy transfer have been discussed using schematic energy level diagram. The Tb3+/Yb3+ co-doped LB glass may be used in the optical devices and in solar cell for solar spectral conversion and behaves as a multi-modal photo-luminescent material.