Metasurface-based non-orthogonal tri-channel polarization multiplexing for optical encryption [Invited]

Orthogonal polarization multiplexing is typically required to avoid crosstalk between channels. However, for practical applications in optical information encryption, orthogonal polarization channels are vulnerable to decoding. Using non-orthogonal polarization channels can enhance information security. Here, we extend the conventional Jones matrix approach to achieve non-orthogonal tri-channel polarization multiplexing metasurfaces for optical encryption via optical holography. Using a supercell configuration with two pairs of α-Si meta-atoms, we experimentally demonstrate metasurface-based multiplexing under three linear polarization channels, including copolarized and non-copolarized channels. This design strategy is further extended to circular and elliptical polarization channels, which exhibits minimum crosstalk. As a proof-of-concept demonstration, we implement two optical encryption applications, i.e., image encryption and character encryption, based on non-orthogonal polarization-multiplexed metasurfaces. For image encryption, encrypted images are generated only in the correct polarization channels. While using seven different statuses of metasurface under various polarization channels, we also present a character encryption scheme. We envision that the non-orthogonal polarization multiplexing metasurface platform will open new possibilities in optical communication, optical encoding, and information security.