High‐Sensitivity Ratiometric Temperature Sensing and Information Encryption Based on Upconversion Emissions from Lanthanide Ions Doped BaTiO ₃ Phosphor

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Upconversion emissions from lanthanide ions have unparalleled advantages in the field of temperature sensing and information encryption. Despite extensive research on temperature sensing probes, developing highly sensitive temperature measurement applications still poses a significant challenge. This study utilizes lanthanide ions doped BaTiO₃ as the foundational material to develop a fiber optic probe with enhanced temperature sensitivity. The optical temperature‐sensing capabilities are assessed based on the intensity ratio of various energy levels in BaTiO₃: Yb/Tm/Er phosphors. Notably, the non‐thermal coupling energy level between Tm and Er demonstrates the highest sensitivity, achieving a maximum relative sensitivity of 2.70% K⁻¹ at 303 K. By leveraging the temperature‐dependent color change of the material, a large‐capacity photonic barcode for temperature information storage and encrypted transmission is developed, thereby expanding the potential applications of temperature monitoring.