Supernormal\nTemperature Sensing Performance Realized\nthrough the Blue Emitting Level of Er<sup>3+</sup> along with Detection\nAbility in Deep Tissues

Fluorescence intensity ratio (FIR)-type optical thermometers\nbased\non thermally coupled energy levels (TCLs) of rare earth ions are suitable\ncandidates for noncontact temperature detection in living organisms,\nmicroelectronics apparatus, and so forth. Therefore, the improvement\nof the thermometric sensitivity of TCL-based thermometers has become\na research hotspot in recent years. Herein, ultrahigh sensitivity\nand outstanding resolution for temperature sensing have been realized\nin YNbO<sub>4</sub>: Yb<sup>3+</sup>/Er<sup>3+</sup>. Unusually, the\nthermally coupled three-level system of Er<sup>3+</sup>: <sup>4</sup>F<sub>7/2</sub>/<sup>2</sup>H<sub>11/2</sub>/<sup>4</sup>S<sub>3/2</sub> is first employed for optical thermometry based on FIR technology.\nA supernormal thermometric sensitivity of 2.67% K<sup>–1</sup> is obtained from the thermally coupled <sup>4</sup>F<sub>7/2</sub> and <sup>4</sup>S<sub>3/2</sub> states due to the large energy gap\nbetween them, significantly surpassing that of most temperature sensors\nin the same category. Furthermore, the existence of the intermediate\nlevel <sup>2</sup>H<sub>11/2</sub> can effectively prevent the decoupling\neffect between <sup>4</sup>F<sub>7/2</sub> and <sup>4</sup>S<sub>3/2</sub>. Additionally, the temperature sensing behavior realized by the\nStark sublevels of the Er<sup>3+</sup>: <sup>4</sup>I<sub>13/2</sub> → <sup>4</sup>I<sub>15/2</sub> transition, with a penetration\ndepth of 8 mm, shows the potential of temperature measurement in deep\nbiological tissues, benefiting from its excitation and emission wavelengths\nlocated in the biological window. All of the data reveal that YNbO<sub>4</sub>: Yb<sup>3+</sup>/Er<sup>3+</sup> is an ultrasensitive optical\nthermometer and exhibits the capacity of temperature detection in\ndeep tissues.