Polarized Luminescence of Sm³⁺-Doped Single NaYF₄ and BiPO₄ Microcrystals

In this paper, the polarized luminescence of single hexagonal NaYF4:Sm3+ and monoclinic BiPO4: Sm3+ microcrystals are measured by using high-precision single-particle polarization- resolved spectroscopy. Experimental results are analyzed by applying Poincare ' sphere and polarization fitting methods, which show that the emissions from Sm3+ ions in hexagonal and monoclinic microcrystals are partially polarized. However, the linear polarization angles of emissions from hexagonal microcrystals are completely parallel and perpendicular to the crystalline c-axis, whereas the angles of monoclinic microcrystals are not. From the perspectives of point group theory and macroscopic crystal symmetry, the orthogonally linear polarization angles of hexagonal microcrystals are due to the rotational symmetry of optical transition dipoles around the c-axis. However, the transition dipoles in the monoclinic phase do not allow rotational symmetry, resulting in the random linear polarization angles of the emissions. This finding will facilitate the achievement of rare-earthion- doped single micro/nanocrystals with desirable polarization properties originating from their crystal structure design.