Sensitivity of a Nanocrystalline Luminescent Thermometer in High and Low Excitation Density Regimes

Two figures of merit are typically taken into account studying rareearth-doped materials and processes suitable for optical nanothermometry, namely, temperature sensing range and the sensitivity. To optimize the composition of such phosphors and make quantitative comparison between different materials, other factors, such as excitation density and pulse duration, have to been included. Owing to the metastable character of lanthanide excited states, the excitation intensity has a critical importance but has been disregarded so far. Our studies show, based on a new nanocrystalline Er:LiYbP4O12 luminescent thermometer, the influence of the excitation power density on its suitability for temperature sensing. The highest sensitivity was reached for LiYbP4O12:0.1% Er nanocrystals upon pulsed excitation: 2.88%/K at average power below 25 mW/cm(2), while the same material displayed lower,similar to 0.5%/K sensitivity at higher 50-300 mW/cm(2) excitation intensities. The mechanism responsible for the observed sensitivity changes was discussed in terms of competition between thermalization and nonradiative depopulation processes.