Overcoming the limitation: energy-gap tailoring of thermally coupled levels from Er3+ in fiber microspheres for optical thermometry over 213-823 K.

The use of a lanthanide-based ratiometric optical thermometer is a highly promising non-invasive method for detecting temperature. However, due to inherent bandgap limitations, the optimal temperature measurement range for specific elements is restricted to a specific range. In this work, we have developed a novel optical fiber microsphere fabricated by doping an Er3+/Yb3+ solution into an SiO2 optical fiber. With this microcavity, stable emissions corresponding to small, medium and large energy gaps between thermally coupled levels resulting from the splitting of the Er3+ energy levels (2H11/2 and 4S3/2) have been achieved. Owing to these tailored energy gaps, the effective temperature sensing range based on the fluorescence intensity ratio of Er3+ has been successfully extended from 295-503 K to 213-823 K. In terms of relative sensitivity (S r), a maximum value of 1.71% K-1 was obtained at 213 K for the small energy gap, while a considerable sensitivity of 0.20% K-1 was maintained even at 823 K for the large energy gap. The temperature uncertainty (δT) of the thermometer remains below 0.35 K within the commonly employed temperature measurement range of 200-500 K. Given its wide temperature measurement range, high relative sensitivity, and low temperature uncertainty, the easily fabricated Er3+/Yb3+ co-doped microsphere exhibits outstanding temperature sensing performance, showing great potential for applications in microscale thermometry.