Boltzmann vs. Non-Boltzmann (non-Linear) Thermometry - Yb ³⁺-Er ³⁺ Activated Dual-Mode Thermometer and Phase Transition Sensor Via Second Harmonic Generation

Second harmonic generation (SHG) and upconversion luminescence (UCL), as second-order nonlinear optical (NLO) effects are crucial in modern optics and optoelectronics. Here, through the chemical engineering process we obtained a series of optically active BaTiO3: Er3+, Yb3+ perovskite materials, exhibiting temperature-dependent NLO responses. Upon 976 nm laser light irradiation, the micron-sized, polycrystalline samples show simultaneously intense UCL and SHG effects. Comparison of the thermometric features based on luminescence thermometry, i.e., band intensity ratio based on thermally-coupled levels of Er3+, and NLO thermometry, i.e., SHG/UCL intensity ratio, shows that the superior thermal sensitivity (≈ 4%/K, at ~400 K) and excellent temperature resolution (δT= 0.07 K) is achieved for the NLO thermometry approach. Furthermore, the combination of NLO spectroscopy with UCL allows rapid, more precise and clear phase transition detection (from tetragonal to cubic phase at around 386 K), compared to conventional structural methods, such as X-ray diffraction and Raman spectroscopy. Our work suggests that some non-Boltzmann, non-linear thermometers can provide much more effective temperature readouts (compared to the commonly studied Boltzmann thermometers), simultaneously working as accurate, rapid and non-invasive phase transition sensors.