Switching Yb/Tm Back‐Energy‐Transfer “On/Off” States Enables On‐Demand >573 K Temperature Sensing

ABSTRACT Despite substantial progress in nanothermometry typically operational at temperatures ≤573 K, fluorescencebased sensors for extremely high‐temperature environments such as aviation engines, nuclear reactors, and industrial processes remain quite inadequate due to the severe suppression of luminescence under such harsh conditions. Herein, we present a back‐energy‐transfer (BET) switching approach applied to Yb/Tm‐doped Y 2 O 3 nanostructures to selectively achieve either high relative sensitivity ( S r ) or an extended sensing range. When the BET channel is activated via uniform co‐doping (“on” state), the fluorescence intensity ratio (FIR) between the three‐photon (477 nm) and two‐photon (810 nm) emissions exhibits exceptional S r due to their dependencies on BET, yielding a maximum S r of 3.93% K −1 at 873 K, nearly quadruple the typical S r (∼1% K −1 ) of sensors operating at much lower temperatures. However, the activation of BET accelerates quenching of the three‐photon emission, making it unable to operate at higher temperatures. In comparison, spatially isolating Yb 3+ and Tm 3+ ions through core–shell engineering significantly inhibits BET (“off” state), mitigating thermal quenching and maintaining detectable luminescence up to 1173 K. This work establishes a fundamentally versatile approach to designing high‐temperature thermometers by strategically controlling BET pathways to meet tailored requirements for either high‐sensitivity detection or wide‐range monitoring.