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

Abstract

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 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ were three- and two-photon processes, respectively. The ratio between the cube of 4S3/2 and the square of 2H11/2 emissions revealed higher temperature sensing performance and anti-interference on the excitation source power than the that between 4S3/2 and 2H11/2. 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.