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

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.