Effect of Yb3+ concentration on Er3+ doped CaF2 single crystal for temperature sensor applications

Yb-Er co-doped CaF2 crystals with different Yb3+ ions concentrations are successfully grown by the temperature gradient technique (TGT) method. Up-conversion (UC) luminescence characteristics and optical temperature sensing behavior of all samples are thoroughly investigated in this work. Under the excitation of a 980 nm laser-diode, the up-conversion luminescence intensity firstly increases and then decreases. By adjusting the content of Yb3+ ions, multi-color visible light emissions of red, yellow and green could be easily obtained. The fluorescence intensity ratio from two thermally coupled 2H11/2 and 4S3/2 levels technology is used to evaluate the temperature sensing performance of Yb-Er co-doped CaF2 crystals. The concentration of Yb3+ ions shows a significant influence on the temperature sensing performance. The sensitivity reaches the maximum value of 2.24 x 10-3 K-1 at 548 K with a 5 at.% doping concentration of Yb3+ ions. The experiment also confirms that Yb-Er co-doped CaF2 crystal exhibit high stability as optical temperature sensors. In general, it is proved that Yb-Er co-doped CaF2 single crystals are a kind of promising materials used in the optical temperature sensor.