Stark sublevels of Er³⁺-Yb³⁺ codoped Gd₂(WO₄)₃ phosphor for enhancing the sensitivity of a luminescent thermometer

We report a strategy for enhancing the sensitivity of an optical thermometer by utilizing the Stark sublevels. Under 980 nm excitation, the upconversion emission originating from the Stark levels of Er³⁺ (⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2/⁴F_9/2(1)) is observed in a monoclinic phase Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor, which is synthesized through the co-precipitation method. The temperature sensing behavior is studied over the range of 296-620 K based on thermal coupling levels ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2, ²H_11/2/⁴S_3/2(1), ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) using fluorescence intensity ratio (FIR) technology. The sensitivity based on Stark sublevels ²H_11/2/⁴S_3/2(2) or ²H_11/2/⁴S_3/2(1) is almost twice more than that based on the traditional ²H_11/2/⁴S_3/2 levels. The obtained maximum sensitivity of the optical thermometer is 16.5 × 10^-3 K^-1 at 395 K (²H_11/2/⁴S_3/2(2)). These results suggest that the use of Stark levels is a promising approach for enhancing the sensitivity of optical thermometers.