Mechanisms of optical losses in the ⁵D₄ and ⁵D₃ levels in Tb³⁺ doped low silica calcium aluminosilicate glasses

Trivalent Tb-doped materials exhibit strong emission in the green and weak emission in the UV-blue levels. Usually, this behavior is attributed to the cross relaxation (CR) process. In this paper, the luminescence properties of Tb³⁺-doped low silica calcium aluminosilicate glasses are analyzed for UV (λ_exc = 325 nm) and visible (488 nm) excitations. Under 325 nm excitation, the intensity of green luminescence increases proportionally to Tb³⁺ concentration. However, the blue luminescence intensity is strongly reduced with the increase of concentration from 0.5–15.0 wt. %. In the case of 488 nm excitation, a saturation behavior of the green emission is observed at intensities two orders of magnitude smaller than expected for bleaching of the ground state population. Using a rate equation model, we showed that this behavior can be explained by an excited state absorption cross section two orders of magnitude larger than the ground state absorption. The blue emission is much weaker than expected from our rate equations (325 nm and 488 nm excitation). We concluded that only the CR process cannot explain the overall feature of measured luminescence quenching in the wide range of Tb³⁺ concentrations. Cooperative upconversion from a pair of excited ions (⁵D₃:⁵D₃ or ⁵D₃:⁵D₄) and other mechanisms involving upper lying states (4f5d, charge transfer, host matrix, defects, etc.) may play a significant role.