Optical nanothermometer based on the calibration of the Stokes and upconverted green emissions of Er3+ ions in Y3Ga5O12 nano-garnets

Abstract

The temperature-dependent green luminescence of Y₃Ga₅O₁₂ nano-garnets doped with different concentrations of Er³⁺ ions has been measured from 300 to 850 K and, in more detail, in the biological range from 292 to 335 K. The green emissions were obtained by excitation under 488 nm blue or 800 nm near-infrared laser radiations. Both excitations give rise to bright green luminescence that can be seen by the naked eye, and which can be associated either with Stokes processes, i.e. multiphonon relaxations followed by green spontaneous emission, in the former case or with infrared-to-visible upconversion processes in the latter. The temperature-induced changes in the Er³⁺ green emissions have been calibrated for both excitations and results point to a strong dependence on the concentration of optically active Er³⁺ ions. The maximum value of the thermal sensitivity, 64 × 10⁻⁴ K⁻¹ at 547 K, has been obtained for the nano-garnets doped with the lowest concentration of Er³⁺ ions, which is one of the highest values found in the literature. These results allow to conclude that a relatively low concentration of optically active ions is advisable and the changes induced by temperature on the green emissions are independent of the laser excitation radiation used, which is necessary to calibrate the temperature of the immediate environment of the Er³⁺-doped Y₃Ga₅O₁₂ nano-garnets.