Fig. 2: Characteristics of afterglow and up-conversion charging (UCC) in Gd₃Ga₅O₁₂:Cr³⁺ phosphor.

a Steady-state photoluminescence excitation and emission spectra. b Conventional afterglow excitation and emission spectra, with the excitation spectrum determined by measuring afterglow intensity as a function of charging wavelength and the emission spectrum obtained post-charging with 280 nm light at room temperature. c Afterglow emission spectra recorded at both room temperature (RT, 27 °C) and high temperature (HT, 267 °C) following a 10-s charging at room temperature with a 450 nm laser at a power density of 0.01 W·mm⁻². The laser output (left) corresponds closely with the photoluminescence excitation band of the phosphor. d Temporal evolution of thermally stimulated luminescence intensities (top), with detailed experimental procedures outlined below. The inset schematically depicts the UCC process involving two-step ionization and trap-filling. e Fluorescence decay curves for Cr³⁺ emission upon 450 nm excitation at −196 and 27 °C, exhibiting lifetimes of 1.37 and 0.17 ms, respectively. The inset displays the corresponding photoluminescence emission spectra. f Thermoluminescence curves post-UCC at low temperature (LT, −196 °C) and room temperature (RT, 27 °C) following a 10-s exposure to the 450 nm laser at a power density of 0.01 W·mm⁻²