Optical temperature sensor based on upconversion luminescence of Er 3+ doped GdTaO 4 phosphors

For the development of optical temperature sensor, a series of GdTaO 4 phosphors with various Er 3+ ‐doping concentrations (0, 1, 5, 10, 25, 35, 50 mol%) were synthesized by a solid‐state reaction method. The monoclinic crystalline structure of the prepared samples was determined by X‐ray diffraction (XRD). Under excitations of 980 and 1550 nm lasers, the multi‐photon‐excited green and red upconversion (UC) luminescence emissions of Er 3+ were studied, and the critical quenching concentration of Er 3+ ‐doped GdTaO 4 phosphor was derived to be 25 mol%. By changing the pump power of laser, it was found that the two‐photon and three‐photon population processes happened for the UC emissions of Er 3+ ‐doped GdTaO 4 phosphors excited by 980 and 1550 nm lasers, respectively. Furthermore, based on the change of thermo‐responsive green UC luminescence intensity corresponding to the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions of Er 3+ with temperature, the optical temperature sensing properties of Er 3+ ‐doped GdTaO 4 phosphor were investigated under excitations of 980 and 1550 nm lasers by using the fluorescence intensity ratio (FIR) technique. It was obtained that the maximum absolute sensitivity ( S A ) and relative sensitivity ( S R ) of Er 3+ ‐doped GdTaO 4 phosphors are as high as 0.0041 K −1 at 475 K and 0.0112 K −1 at 293 K, respectively. These significant results suggest that the Er 3+ ‐doped GdTaO 4 phosphors are a promising candidate for optical temperature sensor.