Fig. 1. Schematic of the directional UC emission of crown-shaped NaYF₄:Yb³⁺/Er³⁺ under an excitation of 980 nm and the experimental and simulated UC luminescent patterns.

Fig. 2. (a–b) SEM images of NaYF₄:Yb³⁺/Er³⁺ microparticles, the cross-section of single microparticle shown in the inset image in (b). (c) Element mapping of a single NaYF₄:Yb³⁺/Er³⁺ microparticle. (d) XRD pattern of the particles and standard pattern of the hexagonal phases of NaYF₄.

Fig. 3. (a–b) UC fluorescence patterns and spectra obtained through change excitation position from the center to corner/edge. (c) Energy-level diagram and possible transitions/emission schemes of Yb³⁺ and Er³⁺.

Fig. 4. (a–e) UC luminescence patterns of total (a), green (b), and red (c) emissions, the simulation pattern (d), and possible propagation/emission mode (e) of single NaYF₄:Yb³⁺/Er³⁺ under excited conditions on the middle. (f–j) UC luminescence patterns of total (f), green (g), and red (h) emissions, the simulation pattern (i), and possible propagation/emission modes (j) of single NaYF₄:Yb³⁺/Er³⁺ under off-centered excitation.

Fig. 5. The directional emission of the luminescent waveguide with center (a–d) and off-center excitations (upward offset, e–h, and downward offset, i–l).(a, e, i) optical pattern on the image plane, in which the excitation and collection positions were marked with red and white circles, (b, f, g) experimental and (c, g, k) simulated Fourier images of the selected region, and (d, h, i) the angular intensity distribution of radiation on the Fourier plane taken along the direction, where θ has maximum intensity. The insert images show the φ distributions on the Fourier plane.