Polarized upconversion luminescence from a single LiLuF4:Yb3+Er3+ microcrystal for orientation tracking

Polarized upconversion luminescence (UCL) of lanthanide-doped micro/nano-crystals has shown great promise in single-particle tracking and super-resolution bioimaging. However, because of the spectral line broadening and multiple sites of lanthanide in upconversion particles (UCPs), the crystal-field (CF) polarization components of UCL are usually undistinguishable. Herein, we report the linearly polarized UCL in LiLuF4:Yb3+/Er3+ single microcrystals with resolvable CF transition lines and a polarization degree up to 0.82. The CF levels and CF transition lines of Er3+, as well as their emission polarization anisotropy, are unraveled for the first time through low-temperature and high-resolution photoluminescence (PL) and UCL spectroscopies. By taking advantage of the well-resolved and highly-polarized CF transition lines of Er3+, we demonstrate the application of LiLuF4:Yb3+/Er3+ single microcrystals as anisotropic UCL probes for orientation tracking. These findings provide fundamental insights into the polarization anisotropy of UCL in lanthanide-doped single particles, thus laying a foundation for the future design of anisotropic luminescent probes towards versatile applications.