Simultaneous Second Harmonic Generation (SHG) and Upconversion Luminescence Enabling Fully Non‐Linear Optical Thermometry in Ferroelectric Ceramics

ABSTRACT Accurate and wide‐range temperature monitoring is crucial for advanced optoelectronic applications and photonics, where non‐linear optical effects play a substantial role. In this work, we investigated the second harmonic generation (SHG) and upconversion luminescence (UCL) behaviors of a series of rare‐earth‐doped ferroelectric ceramics and evaluated their potential for optical temperature sensing. Among the studied materials, Na 0.48 Bi 2.48 Nb 2 O 9 host activated with Er 3+ exhibits outstanding optical signal intensity, leading to great thermometric performance, enabling multi‐parameter ratiometric optical temperature sensing based on UCL and its combination with SHG effect. The use of fully non‐linear, non‐Boltzmann ratiometric thermometry based on UCL/SHG intensity ratios results in improved sensitivity of temperature readouts, compared to the traditional Boltzmann‐type approach based on Er 3+ thermally‐coupled levels (530/550 nm ratio). Such a strategy ensured monotonic and high‐sensitivity temperature‐dependent responses of the thermometric parameters used, which were determined in a wide T ‐range studied, starting from room temperature (∼300 K) up to around 900 K. These findings demonstrate the feasibility of non‐linear optical thermometry in polycrystalline ferroelectric ceramics, and provide guidance for the development of multifunctional materials for stable, non‐contact optical temperature sensing and related photonic devices.