  Recent advancement in digital coding metasurfaces incorporating spatial and
temporal modulation has enabled simultaneous control of electromagnetic (EM)
waves in both space and frequency domains by manipulating incident EM waves in
a transmissive or reflective fashion, resulting in time-reversal asymmetry.
Here we show in theory and experiment that a digitally space-time-coded
metamaterial (MTM) antenna with spatiotemporal modulation at its unit cell
level can be regarded as a radiating counterpart of such digital metasurface,
which will enable nonreciprocal EM wave transmission and reception via
surface-to-leaky-wave transformation and harmonic frequency generation.
Operating in the fast wave (radiation) region, the space-time-coded MTM antenna
is tailored in a way such that the propagation constant of each programmable
unit cell embedded with varactor diodes can toggle between positive and
negative phases, which is done through providing digital sequences by using a
field-programmable gate array (FPGA). Owing to the time-varying coding
sequence, harmonic frequencies are generated with different main beam
directions. Furthermore, the space time modulation of the digitally coded MTM
antenna allows for nonreciprocal transmission and reception of EM waves by
breaking the time-reversal symmetry, which may enable many applications, such
as simultaneous transmit and receive, unidirectional transmission, radar
sensing, and multiple-input and multiple-output (MIMO) beamformer.
