Comprehensive Study on the Combined Effect of Laser-Induced Heating and Laser Power Dependence on Luminescence...

Luminescence ratiometric thermometry, on the basis of nonthermally linked states of lanthanides, became a hot research issue recently because of its several attractive features. Here, the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ transitions of Ho³⁺ embedded in calcium tungstate host are taken as an example to show the influence of laser pump power on this temperature detection technology. The luminescence intensity ratio between the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ upconversion emission lines was found to respond monotonously to the temperature between 303 and 603 K and could be fitted well with the use of an empirical function. It suggested that this ratio might be suitable for temperature measurement. However, at 303 K, the temperature readout derived from this ratio decreased from 303 to 248 K on increasing the laser pump power from 35 to 205 mW (the irradiated spot's area: ca. 2 mm²). This uncommon phenomenon differs from the conventional laser-induced heating effect. With the help of the Boltzmann distribution based on the two Stark components of the ⁵F₅ state of Ho³⁺, the laser-induced heating was calculated to be ca. 20 K when the excitation power was 205 mW. Thus, this suggested that there should be a mechanism responsible for the gradually decreasing temperature readout. It was then confirmed that this mechanism was the different dependences for the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ transitions on laser pump power, which was much stronger than the laser-induced heating effect. A calibration method to eliminate the influence of laser power dependence on luminescence ratiometric thermometry was then proposed.