Efficient Moderate-to-High Temperature Optical Thermometry Enabled by Valley-to-Peak Ratio and Lattice Negative...

The development of high-performance photothermal thermometers for high-temperature applications is critically needed. As a response, we propose an efficient optical thermometry strategy based on a valley-to-peak ratio (VPR) approach utilizing negative thermal expansion (NTE) lattice materials, as demonstrated in Y₂W₃O₁₂:18Yb/2Er (YWO:Yb/Er). The upconversion emission intensity of the ²H_11/2 level in YWO:Yb/Er powder was observed to progressively increase with temperature. Also, the temperature-dependent spectra revealed gradual broadening and merging of Stark sublevels due to enhanced electron-phonon coupling, accompanied by thermally induced peak shifts arising from lattice contraction and modifications to the crystal field. Furthermore, the VPR-based optical thermometer exhibits competitive sensitivity compared to its FIR-based counterpart, with an absolute sensitivity (S_a) of 0.0134 K^-1 and a relative sensitivity (S_r) of 1.186% K^-1, while the maximum values for the FIR-based one are only 0.0092 K^-1 and 0.626% K^-1, respectively. This work provides a pathway for developing efficient optical thermometers based on rare-earth-doped NTE materials utilizing the VPR approach for operation in moderate-to-high temperature environments.