From Two‐Step Excitation to Persistent Luminescence: Revisiting ZnGa₂O₄:Cr³⁺ Phosphor Through Upconversion Charging Approach

Abstract The ZnGa₂O₄:Cr³⁺ phosphor has emerged as a significant luminescent material due to its long‐lasting afterglow and near‐infrared emission, making it suitable for applications in bioimaging and night‐vision detection. However, the limited availability of excitation light sources poses a challenge for charging the phosphor. In this study, the charging capabilities of ZnGa₂O₄:Cr³⁺ using visible lasers and a white flashlight as excitation sources are explored. By absorbing two excitation photons, the high‐lying delocalized state of Cr³⁺ can be excited through a two‐step process, resulting in the filling of persistent luminescence traps and producing a long‐lasting emission peaking at 696 nm. The application of the white flashlight revealed a nonlinear excitation threshold for charging at 1.5 mW cm⁻². The findings also uncovered that the excitation mechanism involves excited‐state absorption and energy‐transfer upconversion. Moreover, taking advantage of the unique excitability of the near‐infrared persistent phosphor, the potential for charging persistent luminescent probes in vivo using chicken breast tissue as a representative model is showcased. The present upconversion charging approach may offer promising possibilities and introduce a novel excitation technique for ZnGa₂O₄:Cr³⁺ persistent phosphor.