Single-Particle Polarization Spectroscopy Reveals Energy Transfer Mechanism in Heavily Doped Rare-Earth Microcrystals

Highly doped rare-earth single micro- and nanocrystals exhibit extreme luminescence phenomena─including concentration quenching, photon avalanching, superradiance, and superfluorescence─characterized by dramatic changes in emission intensity, lifetime, and wavelength. These effects necessarily involve interactions and phase synchronization among the optical transition dipoles of rare-earth ions, which, in theory, should manifest as polarization-dependent luminescence at the single-particle level. Nevertheless, systematic investigations into the fundamental mechanisms of these photophysical processes using single-particle polarization spectroscopy remain almost entirely lacking. Here, we combine computational electromagnetics with quantitative single-particle polarization spectroscopy to elucidate the transition dipole orientations of rare-earth ions embedded in single microcrystals across a range of doping concentrations. Our results reveal that the dipole orientations remain remarkably stable, reflecting the invariance of point group symmetry. Crucially, this orientation stability suggests a constant dipole orientation factor during energy transfer, indicating that concentration quenching in highly doped systems arises solely from reduced interionic distances, which significantly enhance energy migration and nonradiative decay. These findings provide critical insights into dipole-orientation-dependent energy-transfer processes in rare-earth-doped materials. The intrinsic transition dipole orientations and polarized emission characteristics, governed by crystal lattice symmetry, endow rare-earth single particles with unique advantages for precision optical applications requiring high optical stability, and pave the way for the rational design of next-generation photonic materials.