Fig. 2

a–b The evolution of reflection spectra by continuously varying the geometry asymmetry parameter δ under y-polarization a and x-polarization b. The vanishing point indicated by the red circle denotes the BIC mode. The green circle denotes the Mie resonance. c–d Electric field distributions at resonant wavelength 660 nm for y polarization, and 540 nm for x polarization. e Slant view SEM image of the resonant metasurface unloaded with UCNPs. f Top view SEM image of the resonant metasurface loaded with UCNPs. Scale bar: 500 nm. g The measured reflection spectra of the resonant metasurface under x- (green) and y- (red) polarizations, respectively