Transparent dielectric metasurfaces for mode modulation and spatial multiplexing.

Expanding the use of physical degrees of freedom to employ spatial multiplexing of data in optical communication is considered the most disruptive and effective solution to meet the capacity demand of the growing information traffic. Development of space-division-multiplexing methods stimulated research on spatial modulation, detection and processing of data, attracting interest from various fields of science. A passive all-dielectric metasurface with near-unity transmission is used to engineer spatial mode profiles, potentially of arbitrary complexity. The broadband response of the metasurface covers S, C, and L bands of fibre communications. Unlike conventional phase plates, the metasurface allows for both phase and polarization conversion, providing full flexibility for mode engineering. We employ the metasurface for both mode modulation and mode multiplexing in free-space optical communication, and demonstrate that it is capable of mode multiplexing with an extinction ratio in excess of 20 dB over the whole C-band with negligible penalty even for 100 Gb/s DP-QPSK signals. These results merge two seemingly different fields, optical communication and metamaterials, and suggest a novel approach for ultimate miniaturisation of mode multiplexers and advanced LiFi technologies.