Triple Space‐Polarization On–Off‐Key Modulation With AlN‐Based Tri‐Focal Metalens

ABSTRACT We designed and demonstrated a tri‐focal metalens (TF‐lens) made of high‐temperature‐annealed AlN (HT‐AlN) on sapphire. The HTA‐AlN film possesses an ultrawide bandgap ( E g  = 6.2 eV), atomic‐level smoothness, and single‐crystalline quality, making it an exceptional candidate for a metasurface in visible spectral wavelengths. By incorporating polarization‐insensitive zone‐plate null rings into a polarization‐selective bifocal metalens, the incident light will be focused into two or three foci on the focal plane, each with a distinct polarization state. The extinction ratio between the on and off peaks exceeds 1000 after subtracting the background signal, and the absolute value of S 3 is measured to be higher than 0.96 at the side foci. By manipulating the incident polarization state and the angle of the linear analyzer behind the TF‐lens, all combinations of the on–off keying at three distinct foci can be generated.