High-sensitivity optical thermometer enabled by Yb3+-Sensitized upconversion in tungsten bronze-type KCaAl2F9:Er3+/Ho3+ phosphors

Non-contact fluorescent temperature sensing has garnered significant attention in the fields of life sciences and medicine owing to its non-invasive operation with deep tissue penetration and minimal autofluorescence interference through near-infrared excitation with enhanced sensitivity. In this work, we report a tungsten bronze-type KCaAl₂F₉ (KCAF) host co-doped with Yb³⁺-Er³⁺/Ho³⁺ ions, which achieved intense visible upconversion luminescence (UCL) under 980 nm excitation. This arises from efficient energy transfer from Yb³⁺ sensitizers to Er³⁺/Ho³⁺ activators within the unique crystal field environment. Through systematic optimization of lanthanide ion concentrations, we engineered three distinct fluorescence intensity ratio thermometric channels: (1) FIR₁ (I_{521 nm}/I_{538 nm}, Er³⁺), (2) FIR₂ (I_{654 nm}/I_{536 nm}, Ho³⁺), and (3) FIR₃ (I_{654 nm}/I_{748 nm}, Ho³⁺). The optimized KCAF:Yb³⁺, Er³⁺ phosphor achieves a maximum relative sensitivity (S_r) of 1.0271 % K⁻¹ (FIR₁) at 298 K, while KCAF:Yb³⁺, Ho³⁺ demonstrates S_r of 0.4193 % K⁻¹ (FIR₂) and 1.4488 % K⁻¹ (FIR₃) at 298 K, surpassing over 50 % of the reported rare earth-based optical thermometers. Furthermore, power-dependent spectral evolution reveals tunable green emission purity through controlled cross-relaxation processes, enabling multifunctional applications in solid-state lighting technologies and high-performance optoelectronics. These results suggest the KCAF:Yb³⁺, Er³⁺/Ho³⁺ phosphor as a promising candidate for advanced optical sensors in precision medicine and industrial process monitoring.