Up-conversion charging dynamics exampled by Mn²⁺-activated persistent phosphor

Abstract

Persistent phosphor as a kind of light-emitting material can store excitation energy in the so-called traps, and then persistently release the energy in the form of light emission after the end of excitation. This emission is called persistent luminescence. Much attention has been paid to optimizing the emission performance of persistent phosphors, including emission wavelength and persistent time. However, research on the excitation for charging persistent phosphors is relatively lacking. To acquire the persistent luminescence effectively, the traps need to be filled typically by ionizing irradiation. That is, high-energy light (such as ultraviolet light) is a general requirement for charging the persistent phosphors. Taking into account the fact that low-energy illumination (e.g. visible or infrared light) is much more suitable and less harmful than ultraviolet light for some practical applications, taking advantage of the low-energy light excitation is therefore an urgent issue to be solved in the persistent luminescence area. Several low-energy excitation approaches have been reported, in which up-conversion charging (UCC) is a promising candidate for charging phosphors using low-energy excitation light sources. The definition of UCC is as follows: UCC is a non-linear excitation for storage phosphors, in which the traps are typically filled via a two-step ionization mechanism. Prior research on the UCC has focused primarily on the demonstration of two-step ionization and the associated trapping properties. Recently, researchers have realized that the excitation light may release some trapped electrons while filling the traps (i.e. excitation-light stimulated detrapping). Competition between the trapping and detrapping during the UCC has been roughly described on the assumption that the illumination dose is in a certain range and the effect of ambient-temperature stimulated detrapping is negligible. Despite the initial progress, the exact effect of detrapping on the UCC process needs to be further explored. Here we demonstrate the effect of detrapping on UCC dynamics by a rate equation approach. Accordingly, taking LaMgGa₁₁O₁₉:Mn²⁺ phosphor illuminated by a 450 nm laser for example, we measure its thermoluminescence. Our measurements reveal that the competition between the trapping and detrapping depends both on illumination power and on illumination duration. The experimental results are consistent well with the theoretical predictions, thereby offering a new insight into the understanding of UCC. In addition, the experimental demonstration on the LaMgGa₁₁O₁₉:Mn²⁺ phosphor allows us to explore the generality of the present UCC model. Accordingly, we expect some existing phosphors can now be revisited.