Supernormal Temperature Sensing Performance Realized through the Blue Emitting Level of Er³⁺ along with Detection Ability in Deep Tissues

Fluorescence intensity ratio (FIR)-type optical thermometers based on thermally coupled energy levels (TCLs) of rare earth ions are suitable candidates for noncontact temperature detection in living organisms, microelectronics apparatus, and so forth. Therefore, the improvement of the thermometric sensitivity of TCL-based thermometers has become a research hotspot in recent years. Herein, ultrahigh sensitivity and outstanding resolution for temperature sensing have been realized in YNbO₄: Yb³⁺/Er³⁺. Unusually, the thermally coupled three-level system of Er³⁺: ⁴F_7/2/²H_11/2/⁴S_3/2 is first employed for optical thermometry based on FIR technology. A supernormal thermometric sensitivity of 2.67% K^-1 is obtained from the thermally coupled ⁴F_7/2 and ⁴S_3/2 states due to the large energy gap between them, significantly surpassing that of most temperature sensors in the same category. Furthermore, the existence of the intermediate level ²H_11/2 can effectively prevent the decoupling effect between ⁴F_7/2 and ⁴S_3/2. Additionally, the temperature sensing behavior realized by the Stark sublevels of the Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition, with a penetration depth of 8 mm, shows the potential of temperature measurement in deep biological tissues, benefiting from its excitation and emission wavelengths located in the biological window. All of the data reveal that YNbO₄: Yb³⁺/Er³⁺ is an ultrasensitive optical thermometer and exhibits the capacity of temperature detection in deep tissues.