Dynamic Upconversion Manipulation via Cross‐Relaxation Engineering for Optical Encryption

ABSTRACT Lanthanide ions (Ln ³⁺ ) doped upconversion originates from their diverse 4 f electron transitions. However, the direct manipulation of excited‐state electron populations for dynamic emission modulation remains challenging. In this study, a cross‐relaxation control paradigm, enabled by dual‐wavelength cooperative excitation, is developed for dynamic upconversion engineering. The Er ³⁺ ‐Ln ³⁺ (Ln ³⁺ = Tm ³⁺ , Ho ³⁺ , Yb ³⁺ ) doped NaYS ₂ platform precisely populates the dual‐target energy levels via cross‐relaxation pathways under cooperative excitation. This approach enables dynamic green‐to‐red luminescence switching while amplifying the red emission (∼20‐fold) by accelerating the cross‐relaxation kinetics; Er ³⁺ acts as the photon harvester, and Ln ³⁺ serves as the cross‐relaxation mediator to redirect population fluxes. The experimental and theoretical results demonstrate that this phenomenon originates from the unique properties of the low phonon energy, unique layered structure with a large interionic spacing, and high refractive index of the NaYS ₂ host. Finally, programmable upconversion logic gate arrays are implemented to develop a dynamic‐ random‐visual encryption system for optical information security. This study establishes a novel paradigm for upconversion manipulation with unprecedented capabilities for advanced information technologies.