Temporally Controlling Photochromic Upconversion Through Cooperative Cross Relaxation for Information Encryption and Security

Lanthanide‐doped materials hold great promise in broad frontier applications owing to their rich and editable emission properties. However, the research on the real‐time color‐switchable photochromic upconversion from a single emitter has remained a huge challenge. Here, we demonstrate a conceptual strategy to realize the red‐to‐green photochromic evolution of Er 3+ in nanoparticles through constructing cooperative cross relaxation in an Er 3+ /Tm 3+ /Yb 3+ tri‐doping system. We show that such cooperative cross relaxation between the Er 3+ emitter and the Tm 3+ ‐Yb 3+ couple has a temporal delay characteristic that is able to manipulate the decay dynamics of Er 3+ , resulting in a gradual emission color change from red to orange, yellow, and green under non‐steady‐state excitation. We demonstrate that the Yb 3+ ‐to‐Tm 3+ non‐resonant energy transfer plays a key role in delaying the occurrence of cooperative cross relaxation by slowing down the population of Tm 3+ at its initial 3 F 4 state. Moreover, a conceptual model for the red‐green‐blue primary colors is further established by constructing a rational core–shell nanostructure through integrating an additional blue light upconversion. These findings pave a new way for the development of smart luminescent materials toward advanced applications in full‐color volumetric display and information security.