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. NaYF 4 :Yb,Tm@NaYF 4 :Yb@NaYF 4 :Yb,Ho@NaYF 4 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 Tm 3+ ‐Ho 3+ ‐Yb 3+ 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 Yb 3+ interlayers in mediating energy migration pathways and suppressing cross‐relaxation losses is elucidated. Integration with MAPbI 3 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.