Optical thermometry based on the thermally coupled energy levels of Er³⁺ in upconversion materials

Contactless thermometry with the requirements of high accuracy and high efficiency is an extremely acute need in many fields. Optical thermometers based on the fluorescence intensity ratio (FIR) of the thermally coupled energy levels of Er3+ have been demonstrated to be excellent candidates to afford that due to their advantages of high spatial resolution, rapid response, anti-jamming capability, etc. In this paper, we summarize the recent developments in optical thermometry based on the FIR of the electronic levels Er3+:2H11/2/4S3/2 and the Stark sublevels of Er3+:4F9/2 and Er3+:4I13/2 manifolds, including physical mechanism, improvement of thermometric sensitivity, biological application and so on. Moreover, the challenges in creating novel Er3+-based optical thermometers and potentially new research directions for future work are discussed in detail. Overall, the Er3+-based optical thermometers have exhibited outstanding advantages for non-contact temperature sensing, but great efforts are still needed to improve their key performance indicators for meeting the demands of practical applications.