Hyperspectral Imaging and Optical Trapping: Complementary Tools for Assessing Direction‐Dependent Polarized Emission from Single Upconverting LiYF4:Yb3+/Er3+ Microparticles

Single‐particle fluorescent probes with the capacity to infer specific intracellular conditions, for instance, have great application potential in the realm of biomedicine. Imaging techniques that improve our understanding of the fluorescence processes at a single‐particle level are thus instrumental in actualizing this potential. This study demonstrates the importance of implementing synergistic single‐particle spectroscopic techniques to gain a more comprehensive understanding of the optical anisotropy exhibited by upconverting erbium and ytterbium co‐doped lithium yttrium tetrafluoride (LiYF4:Yb3+/Er3+) microparticles. More specifically, optical trapping and single‐particle polarized emission spectroscopy is herein leveraged to provide a plausible explanation for the spatial emission intensity distribution variation exhibited by LiYF4:Yb3+/Er3+ microparticles during hyperspectral imaging. By probing the polarized emission stemming from a single, optically trapped LiYF4:Yb3+/Er3+ microparticle, it is possible to find evidence that the emission intensity anisotropy exhibited by the respective microparticles during hyperspectral imaging arises as a consequence of the selection rules governing the emission probability in rare‐earth (RE3+) ions doped into a uniaxially birefringent host matrix such as LiYF4. Hyperspectral imaging and optical trapping are highly synergistic tools to assess photoluminescence at a single‐particle level. In this work, optical trapping and single‐particle polarized emission spectroscopy are leveraged to demonstrate that direction‐dependent polarized emission is a likely source of the spatial emission intensity variation exhibited by upconverting LiYF4:Yb3+/Er3+ microparticles during hyperspectral imaging.