Reflection and refraction problems for metasurfaces related to Monge-Amp\`ere equations

A metasurface is a surface, tipically a plane, on which a function called phase discontinuity is chosen so that the metasurface produces a desired reflection or refraction job. We derive the equations that the phase discontinuity function must satisfy, they are Monge-Amp`ere partial differential equations, and we prove existence of solutions.