A time-dependent luminescent phosphor of Na₂Ba₂Si₂O₇:Eu for multi-level encryption and dynamic information display

Photo-stimuli responsive materials show great potential in the fields of information encryption and storage due to their distinctive spatial or temporal color changes. However, the conventional single or multi-color static outputs by light stimulus difficulty meet practical requirements for high-security optical storage technologies. Here, a novel dynamic irradiation-responsive phosphor of Na₂Ba₂Si₂O₇:Eu is demonstrated, exhibiting high storage stability and convenient readout behaviors. The inherent Eu²⁺ luminescence can be dynamically tuned, instantly read out, and conveniently erased by controlling irradiation duration of a portable diode laser (365 nm). The modulation mechanism is unraveled by optically induced oxidation reactions of Eu²⁺→Eu³⁺ and defects as killer centers. The excellent luminescence modulation degree (ΔR_t = 89.5%) and the accompanying larger color contrast enable the creation of invisible optical codes with multi-level encryption in bright or dark field. These results indicate potential applications of Na₂Ba₂Si₂O₇-based materials in information encryption and invisible optical storage, and are expected to expand more investigations on optically induced PL modulation behaviors based on mixed valences and defects.