Metasurfaces are two-dimensional (2-D) metamaterials that can control scattering waves of a light beam. Their applications include thin-sheet polarizers, beam splitters, beam steerers and lenses. These structures can control and transform impinging waves based on the generalized reflection and refraction law (GSL; generalized Snell’s law and generalized reflection law), which states that small phase-shifting elements can control the directions of the reflected and transmitted waves.