Optical temperature sensing through the upconversion luminescence from Ho3+/Yb3+ codoped CaWO4

Under 980 nm diode laser excitation, efficient upconversion emissions from the Ho3+/Yb3+ codoped CaWO4 phosphor were obtained. The upconversion quantum efficiency was evaluated to be about 3.3% when the power density of the excitation laser was 47 W/cm2. Additionally, temperature dependent blue emissions from the 5F2,3/3K8 and 5G6/5F1 states of Ho3+ ions were studied in the range of 303–923 K. The result demonstrated that using the ratio between the blue luminescence intensities of Ho3+ ions, the sensitivity and the accuracy for optical thermometry achieved here are superior to the previously reported Er3+ green fluorescence based optical temperature sensors.