Yb³⁺ Mediated Multi-Band Responsive Full-Color Upconversion for Encrypted Optical Communication

Lanthanide-doped upconversion (UC) nanocrystals have emerged as pivotal optical transducers in advanced photonic systems, yet precise spectral control remains a critical challenge. This study pioneers a multi-shell engineering strategy to achieve full-color UC luminescence across multiple near-infrared (NIR) excitation bands. NaYF₄:Yb,Tm@NaYF₄:Yb@NaYF₄:Yb,Ho@NaYF₄ core–shell nanocrystals that exhibit distinct color-tunable UC luminescence characteristics, with dominant emission colors of white, blue, green, and red as the excitation wavelengths of 900, 980, 1150, and 1960 nm, are designed. The white-light emission at 900 nm originates from synergistic Tm³⁺-Ho³⁺-Yb³⁺ energy transfer, while 1960 nm excitation-induced red emission expands the spectral toolbox for optical communication. Through systematic dopant concentration and shell thickness optimization, the dual role of Yb³⁺ interlayers in mediating energy migration pathways and suppressing cross-relaxation losses is elucidated. Integration with MAPbI₃ perovskite photodetectors enables wavelength-selective photocurrent generation, forming the basis for an 8-bit binary encryption system. By exploiting four distinct NIR excitation bands beyond conventional telecommunication windows, this platform achieves enhanced security through spectral dimensionality expansion and interference immunity. This work establishes a paradigm for high-capacity encrypted optical communication while advancing fundamental understanding of lanthanide energy transfer dynamics.