Optical temperature sensing using upconversion emission of erbium-doped lanthanum oxysulfide powders

Abstract

Upconversion processes in rare-earth doped materials have led to a wide range of applications including thermal sensing at bulk or micro-nano scales. One of the requirements for the optical sensors based on upconversion is that the radiative probabilities of the emitting levels must be high enough to show relatively large emission intensities. Rare-earth oxysulfides (R2O2S) with relatively low phonon energy (∼400-500 cm-1), good chemical and thermal stabilities, and low toxicity are promising materials for upconversion applications. In this work we report a detailed spectroscopic study of the near-infrared to visible upconversion luminescence in La2O2S:Er3+ nanocrystalline phosphors following excitation into the 4I9/2 level. The analysis of the upconversion emission and excitation spectra as well as the decay curves indicates that energy transfer upconversion is the main mechanism responsible for the green (4S3/2) and red (4F9/2) upconversion luminescence. The fluorescence intensity ratio of the green upconverted emission from the two thermally coupled 2H11/2 and 4S3/2 levels and the temperature sensitivity have been evaluated between 230 K and 300 K for different Er3+ concentrations in order to determine the suitability of this material as a temperature sensor. A proof of concept which has been worked out by the authors demonstrates the feasibility of localized cooling by anti-Stokes IR pumping of this erbium-doped nanocrystalline powders.