Figure 1.

Schematics for the inverse electromagnetic approach for designing free-space metadevices. The desired optical functionality is defined by a set of input and output conditions at the boundaries of the design space. A polarization splitter (A) is a grating that converts normally incident plane waves of parallel and perpendicular polarizations into two different diffraction orders. A bending device (B) converts a normally incident plane wave into the same diffraction order. A flat metalens (C) is a device that converts a plane wave into a cylindrical wave converging to a chosen focal point.