  Space-time modulation induced non-reciprocity in EM metasurfaces is proposed
and numerically demonstrated using rigorous Generalized Sheet Transitions
Conditions (GSTCs) under oblique plane-wave incidence. It is phenomenologically
shown that the space-time modulation of surface susceptibilities create an
inherent asymmetry with respect to the directional perturbation on the
metasurface and the transverse wave momentum of the input wave, between forward
and backward propagations, resulting in non-reciprocal wave transmission.
Exploiting the periodicity of the surface susceptibilities in both time and
space, Floquet mode expansion method is used to rigorously compute the
scattered fields from, inherently dispersive metasurfaces, by solving GSTCs in
combination with causal Lorentzian surface susceptibilities. Various harmonic
solutions are shown and the non-reciprocal wave transmission has been confirmed
under oblique plane-wave incidence.
