Multipolar Resonance Dominated Intermediate‐State Population in Manipulating Upconversion Luminescence Under Cryogenic Condition

Abstract Multipolar resonance and cross‐relaxation (CR) are the primary mechanisms of non‐radiative energy transfer between rare‐earth ions. It is well established that upconversion luminescence is enhanced by suppressing CR‐mediated depopulation of high‐energy levels through cryogenic fields. However, the role of intermediate‐state population processes, particularly those dominated by multipolar resonance, has largely been overlooked. In this study, theoretical derivations and experimental results are combined to distinguish, for the first time, between two types of resonance transfer processes: phonon‐releasing transfer (RRT) and phonon‐absorbing transfer (ART), and their distinct sensitivities to low temperatures are investigated. In both co‐doped and Er 3+ ‐self‐sensitized systems, it is demonstrated that the RRT‐mediated intermediate state population process is the key factor in the cryogenic enhancement and modulation of luminescence rather than merely the inhibition of CR. Based on these findings, a novel phenomenon of wavelength‐dependent, orthogonal temperature‐responsive behavior is reported. This study challenges the current consensus and provides new insights into traditional energy transfer mechanisms.