Wavelength and polarization multiplexed metasurface for joint encryption in the near- and far-fields

In recent years, multi-foci metasurfaces and metasurface holograms have drawn much attention and become popular areas of research in fields such as encryption. However, most of the existing metasurfaces used for encryption mainly concentrate on hologram encryption in the far-field, overlooking the potential functions that can be achieved in the near-field. Under such a circumstance, this work has designed a single-layer dielectric metasurface. It can achieve joint near-field and far-field encryption by means of wavelength and polarization multiplexing. This metasurface consists of specific sub-metasurfaces for holograms and sub-metasurfaces for multi-foci. By combining the periodic arrangement method with the phase coupling method, introducing a T-state matrix to design the phase of the multi-foci metasurface, and using the Gerchberg-Saxton (G-S) algorithm to design holographic phases, it can display four corresponding Morse code dot matrices at 10 µm in the near-field. Also, it can show four holograms in the far-field under four combinations of wavelength and polarization. The designed metasurface improves spatial utilization and information storage capacity. Therefore, it is a promising optical storage medium and has broad applications in optical encryption as well as wavelength and polarization multiplexing.