  We introduce the concept of metasurface spatial processor, whose transmission
is remotely and coherently controlled by the superposition of an incident wave
and a control wave through the metasurface. The conceptual operation of this
device is analogous to both that of a transistor and a Mach-Zehnder
interferometer, while offering much more diversity in terms of electromagnetic
transformations. We demonstrate two metasurfaces, that perform the operation of
electromagnetic switching and amplification.

Optics