Enhancing Thermoresponsive Upconversion of Erbium Sublattice Through Energy Looping for Ultra‐Low Temperature Sensing

ABSTRACT Highly sensitive cryogenic temperature probing is important for many frontier fields such as quantum computing and deep‐space exploration. However, it has remained a huge challenge to achieve high‐sensitivity thermometry at cryogenic temperatures for luminescent materials. Here, we propose an effective strategy to tune and enhance thermo‐responsive upconversion of Er sublattice at ultralow temperatures through Ho 3+ ‐mediated energy looping. We show that the design of energy looping between Er 3+ and Ho 3+ ions makes the populations of Er 3+ at its intermediate states much more sensitive on temperature, resulting in a better enhanced upconverted green emission at low temperatures. This further amplifies the spectral contrast between the non‐thermally coupled green and red emissions, and contributes to a high relative sensitivity of 7.97%·K −1 at 20 K, much higher than that of the pure Er sublattice (1.98%·K −1 ). Moreover, the sample also exhibits emission color changes from red to yellowish green with decreasing temperatures, allowing for the low‐temperature visual. These results demonstrate the great potential of Ho 3+ ‐modulated NaErF 4 :Ho@NaErF 4 nanoparticles for applications such as ultralow‐temperature sensing and information security.