Multi-mode optical thermometers via CaLaMgTaO6：Bi3+, Er3+ phosphors for fibre-optic temperature sensing

Abstract

Remote optical thermometry and multi-mode anti-counterfeiting demand luminescent materials with tunable dual-activator emissions and high sensitivity. However, achieving multi-functional platforms that integrate temperature sensing, anti-counterfeiting, and real-time device monitoring remains challenging. Bi³⁺/Er³⁺ co-doped CaLaMgTaO₆ phosphors were synthesized via a high-temperature solid-state reaction to address this gap. Optical thermometers were developed based on the different thermal stabilities of the Bi³⁺ and Er³⁺ emissions, achieving a relative and absolute sensitivities of 1.45% and 0.0028 K⁻¹, respectively, which serve as the first detection signal. Upon excitation at 980 nm, Er³⁺ exhibited up-conversion emissions at 533, 547 and 663 nm, enabling optical thermometry through fluorescence intensity ratio of ²H_11/2/⁴S_3/2 and ²H_11/2/⁴F_9/2, which serve as the second and third detection signals, respectively. The phosphors also exhibited a potential to be used in multi-mode anti-counterfeiting applications and were utilized to design a multi-level system with dynamic colour transitions. Finally, the phosphors were successfully used for fibre-optic temperature sensing by incorporating them in a platform that measures the temperature of rechargeable lithium batteries in real time. These results highlight the potential of Bi³⁺/Er³⁺ co-doped CaLaMgTaO₆ phosphors for advanced applications in remote temperature sensing and anti-counterfeiting.