  Refraction represents one of the most fundamental operations that may be
performed by a metasurface. However, simple phasegradient metasurface designs
suffer from restricted angular deflection due to spurious diffraction orders.
It has been recently shown, using a circuit-based approach, that refraction
without spurious diffraction, or diffraction-free, can fortunately be achieved
by a transverse metasurface exhibiting either loss-gain or bianisotropy. Here,
we rederive these conditions using a medium-based - and hence more insightfull
- approach based on Generalized Sheet Transition Conditions (GSTCs) and surface
susceptibility tensors, and experimentally demonstrate two diffraction-free
refractive metasurfaces that are essentially lossless, passive, bianisotropic
and reciprocal.
