  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.
