Unlocking Thermally Enhanced NIR-II Luminescence in Er3+ via Integer Phonon-Matching for High-Sensitivity Ratiometric Thermometry

Luminescent thermometry in the second near-infrared (NIR-II) window holds significant promise for non-invasive. However, its practical application is severely hindered by thermal quenching at elevated temperatures, which limits the development of highly sensitive NIR-II thermometers capable of thermally enhanced emission. Here, we present a rationally designed integer phonon-matching strategy that enables NIR thermal enhancement specifically in singly Er 3+ -doped Ca 2 YNbO 6 double perovskite hosts. Through combined experimental investigations and rate equation modeling, we reveal that efficient multi-phonon relaxation (MPR) between the Er 3+ : 4 I 11/2 and 4 I 13/2 levels is activated when the electronic energy gap corresponds to an integer multiple ( n ≤ 5) of the dominant phonon energy. This resonant phonon-matching condition facilitates highly efficient MPR with increasing temperature, leading to a pronounced 1.66-fold enhancement in the Er 3+ NIR-II emission. Furthermore, we demonstrate the simultaneous occurrence of NIR thermal enhancement and ratiometric temperature sensing in Ca 2 YNbO 6 : Yb 3+ /Er 3+ /Ho 3+ /Pr 3+ crystals, achieving an exceptional relative sensitivity of 0.87% K −1 at 398 K. These findings not only circumvent the thermal quenching limitations of NIR-II phosphors but also establish phonon engineering as a versatile strategy for designing thermally enhanced luminescence, opening new avenues for flexible monitoring of pipeline integrity and organic compound leakage based on differential NIR-II luminescence absorption.