An electromagnetic metasurface refers to a kind of artificial sheet material with sub-wavelength thickness. Metasurfaces can be either structured or unstructured with subwavelength-scaled patterns in the horizontal dimensions
In electromagnetic theory, metasurfaces modulate the behaviors of electromagnetic waves through specific boundary conditions, rather than constitutive parameters in three dimensional (3D) space, which is commonly exploited in natural materials and metamaterials. Metasurfaces may also refer to the two-dimensional counterparts of metamaterials.
One the most important applications of metasurfaces is to control a wavefront of electromagnetic waves by imparting local, gradient phase shifts to the incoming waves, which leads to a generalization of the ancient laws of reflection and refraction. In this way, a metasurface can be used as a planar lens, vortex generator, beam deflector, axicon and so on.
