Subwavelength Optical Engineering with Metasurface Waves

As artificial 2D materials made of subwavelength inclusions, metasurfaces bear exotic optical and electromagnetic properties not obtainable in naturally occurring materials. Based on unique wave behaviors such as short effective wavelength and local field enhancement that stem from the strongly local light-matter interaction, metasurfaces have the ability to overcome many tough problems faced by traditional optical engineers. Here, the history, basic principles, practical applications, and recent advancements of metasurfaces are reviewed in three main aspects: the breaking of the diffraction limit, the generalized laws of refraction and reflection, as well as the localized enhancement of light absorption. These landmark achievements could open a door for the optical engineering at the subwavelength scale, i.e., for Engineering Optics 2.0, and provide alternatives to traditional approaches based on bulky optical components. Moreover, multifunctional metasurfaces have also been introduced which could simultaneously achieve several different functionalities with a single device. As a concluding remark, the major challenges faced by this developing field are discussed.