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

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.