Ultrahigh-sensitive optical temperature sensing based on quasi-thermalized green emissions from Er:ZnO

Fluorescence intensity ratio (FIR) based optical temperature sensors employ the variation of intensity ratio between two peaks, which are very close to each other. Here, we prepare Er doped ZnO microrods by a hydrothermal route and exploit them for FIR based temperature sensing. The Er:ZnO shows the band-to-band UV emission and broad defect emissions with small Er³⁺ related peaks upon excitation with a 355 nm laser, while only peaks corresponding to Er³⁺ transitions are observed with 532 nm laser excitation. The green emissions (²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2) under 532 nm excitation are considered for temperature sensing performance as they possess a quasi-thermal equilibrium between them and the variation of intensity ratio with temperature follows a Boltzmann type distribution. The sensitivity obtained here is very high (3445/T² K^-1) compared to the reported Er³⁺ ion based temperature sensors and can be applied in a wide range of temperature (83-493 K). The value of sensitivity is found to be almost independent of the concentration of Er doping and is the highest reported for Er-doped materials.