Dynamic Upconversion Manipulation via Cross‐Relaxation Engineering for Optical Encryption

Lanthanide ions (Ln³⁺) doped upconversion originates from their diverse 4f 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 Er³⁺-Ln³⁺ (Ln³⁺ = Tm³⁺, Ho³⁺, Yb³⁺) doped NaYS₂ 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; Er³⁺ acts as the photon harvester, and Ln³⁺ 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 NaYS₂ 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.