Diffractive- and metalenses are shown to be quite useful in recent years for many applications. They are both defined as thin structured layers on a surface which is usually flat, and the difference between them lies in the scale of the structure: sub-wavelength range in the case of metalenses, wavelength range for diffractive. The expected functionality of such components is to produce a change in the incidence wavefront phase. Additional electromagnetic phenomena, like amplitude and polarization changes, can and do occur, and they must be considered in the modeling so to evaluate the overall performance of the system. We will present a physical-optics-based approach to deal with the modeling of the whole diffractive-/metasurfaces, by decomposing the input field into numbers of sub-fields and studying the interaction of each sub-field with a local extension of the surface. Examples on meta-gratings and diffractive-/metalenses will be presented.
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