Er³⁺/Ho³⁺ codoped nanogarnet as an optical FIR based thermometer for a wide range of high and low temperatures

In this work, a new strategy to increase the temperature working range of rare-earth based optical thermometers is demonstrated. Nanocrystalline yttrium gallium nanogarnets codoped with 0.1% Er³⁺ and 0.1% Ho³⁺ were synthesized. The codoped samples combine the high performance of erbium ions at high temperatures and that of holmium ions at low temperature, providing an overall relatively high sensor performance in a temperature range from 30 to 540 K, broader than that achievable by single Er³⁺ or Ho³⁺ doped nanogarnets. Measurements were carried out using 406 and 473 nm commercial diode lasers to excite the erbium and holmium ions, respectively. The emission spectra were analyzed as a function of temperature, using the fluorescence intensity ratio technique for each ion. Relative sensitivity and limit of detection were obtained in order to compare the efficiency of the sensor with optical thermometers reported up to date, capable of working in a similar range from cryogenic to high temperatures. The sensor presents a maximum relative sensitivity of about 1.3% K⁻¹ at 200 K when exciting the erbium ions and around 0.4% K⁻¹ at 200 K for the holmium ions. The limit of detection is under 0.5 K for most of the range from 30 K to 540 K using this combination of ions.