Ratiometric Luminescent Thermometry with Excellent Sensitivity over a Broad Temperature Range Utilizing Thermally‐Assisted and Multiphoton Upconversion in Triply‐Doped La₂O₃:Yb³⁺/Er³⁺/Nd³⁺

Abstract A ratiometric optical thermometer based on triply‐doped La₂O₃:Yb³⁺/Er³⁺/Nd³⁺ microcrystals is reported with a relative sensitivity above 1% K⁻¹ in the entire range from 300–700 K, and is between 1.8–0.7% K⁻¹ over the range 290–833 K. The 825 nm upconversion (UC) emission from the Nd^{3+ 4}F_5/2 level relies on thermally‐assisted energy transfer from Yb³⁺; thus, unusually, the near‐infrared emission increases with increasing temperature in the relevant range. More typically, the two‐photon 660 nm UC from Er^{3+ 4}F_9/2 level decreases in intensity with increasing temperature due to increasing non‐radiative rates. The variation of fluorescent intensity ratio between these emissions is amplified by their opposite responses to temperature change leading to excellent sensitivity. Concurrently, the different pathways for the temperature response in the two emitting ions enable the high sensitivity to be maintained over an atypically broad temperature range. The wide separation in wavelength means that a standard silicon‐based monochrome camera with broad (inexpensive) band pass filters is sufficient to use this phosphor for thermography. The concept of combining thermally‐activated UC with classical Stokes‐shifted emission is demonstrated to provide combined features of excellent and broad‐range sensitivity plus excellent repeatability. Materials based on this concept are very promising for optical thermometry.