New Strategy for Circumventing the Limitation of Thermally Linked States and Boosting the Relative Thermal Sensitivity of Luminescence Ratiometric Thermometry

Optical ratiometric thermometry, based on the luminescence of trivalent rare-earth ions, has attracted considerable attention because of its noncontact working pattern, short response time, and strong antidisturbance ability. However, conventional optical thermal detection, on the basis of thermally linked states, has a relatively low relative sensitivity. To circumvent the limitation, here, a new strategy for boosting the relative thermal sensitivity, especially at relatively high temperatures, is introduced. It depends on two totally different mechanisms which own the contrary temperature dependences. For one thing, when excited at 310 nm, the green luminescence originating from CaWO₄:Tb³⁺ phosphors increases monotonously because of the red shift of the charge-transfer band of the WO₄^2- group. For another, this luminescence shows decreasing tendency gradually upon being directly excited at 380 nm. Thus, the intensity ratio between the luminescence obtained at two different excitation conditions is exploited for optical thermometry with success. By using this strategy, the relative thermal sensitivity is as high as 0.71% K^-1 at 783 K, which is increased by more than fourfold compared with the conventional method that depends on the thermally coupled states of the Er³⁺ ion, which are the ²H_11/2 and ⁴S_3/2 states. Moreover, this value is also among the highest sensitivities reported so far for ratiometric thermal sensing. What is more, it has been demonstrated that the sensitivity can be adjusted easily by changing the excitation wavelength. Therefore, the work provides an effective strategy for boosting the thermal sensitivity for optical ratiometric thermometry, which contributes to highly sensitive thermal detection in the future.