Cross Relaxation Controlled Ultralow Temperature Detection in High‐Level‐Doping Upconversion Thermometer

Abstract Achieving accurate and real‐time ratiometric luminescence thermometry is one important application of lanthanide‐based upconversion materials. Traditional sensoring mechanism relies on the thermal coupling between two closely linked energy levels (following Boltzmann's law), which locks the ability of these sensors to detection of relatively high temperatures (usually above 77 K). In this work, this limitation is broken by establishing a new type of upconversion thermometer with a lanthanide high‐level‐doping nanostructure, whose thermal sensitivity is primarily controlled by a cross‐relaxation process. This ratiometric thermometer works not only in a broad temperature range from relatively high to ultralow (300–15 K, 808 or 980 nm excitation are all available), but also with an unprecedented detecting sensitivity at 15 K (≈240% K −1 ) and high resolution of 0.01–0.02 K, which is about two orders of magnitude better than that of the traditional sensors. This result offers a new perspective for developing high‐quality cryogenic thermometers.