Excitation Mechanism Rearrangement in Yb³⁺-Introduced La₂O₂S:Er³⁺/Polyacrylonitrile Photon-Upconverted Nanofibers for Optical Temperature Sensors

(La_1-x-yEr_xYb_y)₂O₂S polyacrylonitrile (LOS-EY/PAN) nanofibers enable the possibility of optical temperature sensing in microregions by introducing a dual temperature feedback fluorescence intensity ratio approach. A ratiometric strategy for the excitation of Er³⁺ ions by a special upconversion (UC) process is proposed. The green and red UC emissions of Er³⁺ single doping are confirmed to be unusual three-photon excitation processes attributed to cross-relaxation (CR) of Er³⁺ ion pairs. When Yb³⁺ is introduced, the CR process is forced to interrupt and energy is absorbed monopolistically by Yb³⁺ due to the huge absorption cross section of Yb³⁺ compared to that of Er³⁺; therefore, energy transfer from Yb³⁺ to Er³⁺ plays a dominant role, and the excitation process is transformed into a conventional two-photon process. With the rearrangement of the excitation mechanism, the overall increase in Er³⁺ emission is accompanied by an enhancement in the total quantum yield from 8.31 × 10^-5 to 1.23 × 10^-3. Significant fluorescence sensitization improves the signal-to-noise ratio in practical applications and keeps the relative sensitivity at a high level of 1.29% K^-1, further demonstrating the excellent self-feedback nano-temperature sensing function of LOS-EY/PAN fibers promising as a flexible sensing element in fabrics and biomedical devices.