Polarized Upconversion\nof sub-100 nm Single Nanoparticles

Upconversion nanoparticles are popular as imaging probes\ndue to\ntheir advantages in photostability and controllable emission dimensions.\nHowever, upconversion polarization remains largely uncharted with\nprevious reports limited to microstructures. In this work, we report\nthe observation of polarized upconversion emissions from β-NaYF<sub>4</sub> single nanostructures below 100 nm. At the sub-100 nm scale,\nnanorods, nanodiscs, and nanoplates exhibit distinctive polarization\ndegrees despite the same doping concentrations of lanthanides. We\nfind this varied polarization degree results from the crystallographic\norientation of nanostructure in relation to the light field and can\nbe linked to the distinctive emission spectrum profile with varied\nStark splitting transition ratios from Er<sup>3+</sup>. Our findings\nprovide a comprehensive understanding of the polarization properties\nof upconversion nanoparticles, revealing a previously unexplored aspect\nof light emission. This discovery expands our knowledge of upconversion\nnanoparticles and also opens new possibilities for their use in future\nimaging and sensing applications, where polarization sensitivity is\ncrucial.