Optical temperature sensing in β-NaLuF4:Yb3+/Er3+/Tm3+ based on thermal, quasi-thermal and non-thermal coupling levels

Three methods for optical temperature sensing are investigated in the NaLuF4:Yb3+/Er3+/Tm3+ phosphor, which is prepared by a hydrothermal method. The temperature-dependent luminescence is investigated under 980 nm excitation. Utilizing fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors are studied in the range of 300-600 K based on thermal coupling levels (H-2(11/2) and S-4(3/2) of Er3+), "quasi-thermal coupling levels" (F-4(7/2) of Er3+ and F-3(2) of Tm3+) and non-thermal coupling levels (D-1(2) and (1)G(4) of Tm3+), respectively. The maximum sensitivity is 604 x 10(-4) K-1 at 300 K, which is based on non-thermal coupling levels of Tm3+. Meanwhile, the temperature dependent emission colors are discussed, which are changed from bluish green to light blue with the temperature rising. These significant results, high sensitivity and the temperature dependent multicolor emissions, indicate that the NaLuF4:Yb3+/Er3+/Tm3+ phosphor is robust for optical temperature sensing.