Fig. 1. Optical microscope images of in-plane oriented single microcrystals. (a) Single hexagonal NaYF₄∶‍5% ‍Sm³⁺ microcrystal; (b) single monoclinic BiPO₄∶5%Sm³⁺ microcrystal

Fig. 2. Schematic of single-particle polarization spectrum characterization system

Fig. 3. Polarized luminescence analysis of a single hexagonal NaYF₄∶Sm³⁺ microcrystal. (a) Photoluminescence spectra of a single NaYF₄∶Sm³⁺ microcrystal recorded at the six Poincaré sphere basis (inset: measured single-particle in-plane orientation, approximately 5 μm in length, 0° is the slit direction of the spectrometer); (b) S₁, S₂, and S₃ within the measured spectral range; (c) changes in intensity of four emission peaks with polarization angle and fitting curve graph

Fig. 4. Changes in luminescence peak intensity with polarization angle and fitting curve graph of two NaYF₄∶Sm³⁺ single-particle crystals with different orientations

Fig. 5. Polarized luminescence analysis of a single monoclinic BiPO₄∶Sm³⁺ microcrystal: (a) Photoluminescence spectra of a single BiPO₄∶Sm³⁺ microcrystal recorded at the six Poincaré sphere basis (inset: measured single-particle in-plane orientation, approximately 3 μm in length); (b) S₁, S₂, and S₃ within the measured spectral range; (c) changes in intensity of four emission peaks with polarization angle and fitting curve graph

Fig. 6. Polarized luminescence analysis of two single BiPO₄∶Sm³⁺ microcrystals. (a) (b) In-plane orientations and changes in intensity of four emission peaks with polarization angle and fitting curve graph; (c) (d) emission peak II fitted by two crystal-field transition peaks with different linear polarization angles; (e) (f) emission peak IV fitted by two crystal-field transition peaks with different linear polarization angles

Fig. 7. Reversal of monoclinic crystal results in the dipole mirror symmetry along the c-axis. (a) Reverse side of the crystal attached to the substrate surface; (b) front of the crystal attached to the substrate surface