High sensitive ratiometric optical thermometer based on different multi-photon processes of upconversion luminescence in scheelite Ca1-xMgxWO4:Er3+/Yb3+ phosphors

The relative fluorescence intensity from different multi-photon processes is affected by the excitation light source fluctuation, which reduces the anti-interference and sensitivity of thermometers based on fluorescence intensity ratio (FIR) in upconversion (UC) luminescence. Ca1-xMgxWO4: Yb3+, Er3+ phosphors with scheelite structure were synthesized to investigate their anti-interference and higher temperature sensitivity. With x increasing from 0 to 1, the Bragg angle shifted toward higher angle, indicating that the larger disorders were introduced into the phosphors. The emissions assigned to Er3+ were enhanced with Mg2+ concentration increasing, attributed to oxygen vacancies according to the results of electron paramagnetic resonance. The photon numbers of upconversion luminescence assigned to H-2(11/2) -> I-4(15/2), S-4(3/2) -> I-4(15/2) and F-4(9/2) -> I-4(15/2) transitions of Er3+ were three- and two-photon processes, respectively. The ratio between the cube of S-4(3/2) and the square of H-2(11/2) emissions revealed higher temperature sensing performance and anti-interference on the excitation source power than the that between S-4(3/2) and H-2(11/2). The maximum relative sensitivity was 0.66 % K-1 at 300 K based on the common optical ratiometric thermometry. The maximum relative sensitivity based on the emissions assigned to different multi-photon processes was 7.2 % K-1 at 573 K. The dopants of Mg2+ composited of oxygen vacancies in CaWO4 crystals improved the luminous intensity and temperature sensing performance. This suggested the potential application of optical ratiometric thermometers based on different multi-photons of UC luminescence in temperature sensing field.