Recent progress in gradient metasurfaces

Abstract

Recent advances in metasurfaces, i.e., artificial arrays of engineered inclusions assembled over a thin surface, have opened promising venues to control electromagnetic waves in unique and unprecedented ways, by means of locally engineering their near-field wave–matter interactions. Gradient or locally nonperiodic metasurfaces are one of the most exciting recent advances in nano-optics, due to the promise of enabling ultimate light molding, in both the near-field and the far-field, with large efficiency and a minimal footprint. These artificial surfaces are characterized by a transverse variation of their surface properties and lack of local periodicity, distinguishing them from conventional frequency selective surfaces and optical gratings. In this paper, we review recent work in the area of gradient metasurfaces, aimed at arbitrary wave shaping. The significant recent progress and novel applications achieved through optical metasurfaces, including ultrathin invisibility cloaks and polarization-dependent light splitting, are discussed, outlining the typical challenges and their outstanding prospects in integrated nanophotonic devices. Following our discussion on metasurface design approaches, we then revisit the problem of controlling the distribution of energy between multiple diffraction orders by means of gradient metasurfaces. Our discussions reveal that Huygens-based designs hold the promise of overcoming the low conversion efficiency issues associated with other techniques.