A new up-conversion charging concept for effectively charging persistent phosphors using low-energy visible-light laser diodes

It is general knowledge in persistent luminescence that high-energy illumination, mostly ultraviolet light, is usually necessary in order to effectively charge persistent phosphors. However, the need for high-energy ultraviolet light excitation compromises some applications. In his pioneering work on ruby (Al₂O₃:Cr³⁺) laser materials in 1960, Theodore Maiman observed an excited-state absorption phenomenon under the excitation of a high-intensity green-light flash tube. Inspired by Maiman's observation, here we propose a new two-photon up-conversion charging (UCC) concept to effectively charge Cr³⁺-activated near-infrared persistent phosphors using low-energy, high-intensity visible-light laser diodes. As an example, we demonstrate that a low-energy 635 nm laser diode can produce persistent luminescence in the LiGa₅O₈:Cr³⁺ persistent phosphor at the same magnitude as that produced by high-energy 335 nm ultraviolet light from a xenon arc lamp. Moreover, the UCC appears to be a common phenomenon in persistent phosphors containing other UCC-enabling activators such as rare-earth Pr³⁺ ions and transition metal Mn²⁺ ions. The UCC technique offers a new way to study persistent luminescence and utilize persistent phosphors; for instance, in bioimaging it makes effective in vivo charging persistent optical probes using tissue-friendly visible light possible.