Significant Fluorescence Enhancement through Rapid Laser Annealing and Nonthermal Coupling Optical Temperature Sensing of Er-Doped Yttria Nanocrystals

Rare-earth-ion-doped nanocrystals have low quantum conversion efficiency due to their high surface-to-volume ratios and many surface defects, which induce fluorescence quenching. We propose a laser annealing strategy that uses a 974.5 nm laser with a high power density to heat Er3+-doped Y2O3 nanocrystals for several seconds. As a result, the upconversion of Er3+-doped Y2O3 nanocrystals is enhanced obviously after laser annealing: the green emission is enhanced by 90 times and red emission by 24 times. The main reason for the upconversion enhancement is the ultrahigh temperature caused by laser annealing, which increases the grain size and reduces the surface defects. It is an encouraging result, and laser annealing is suitable for various rare-earth-ion-doped oxide nanoparticles. Then, the strong upconversion is applied to measure temperature. After laser annealing, the highly improved fluorescence efficiency decreases the required excitation power density. The thermal effect of excitation light is negligible when the excitation power density is less than 0.13 W/mm2. The thermally coupled levels of I538/I563 and nonthermally coupled levels of I661/I800 are studied for thermometry. The experimental results show that I661/I800 has high absolute sensitivity than I538/I563 at a temperature below 400 K. Rate equations and multiphonon transitions are applied to explain the relationship between the nonthermally coupled fluorescence intensity ratio and temperature. It is instructive for the design of new temperature sensors.