elements. Thus, realizing phase manipulation of EM waves at the nanoscale has become a key pursuit for the development of modern optics and nanophotonics.Metamaterials are 3D artificial nanostructures composed of periodic subwavelength unit cells that resonantly couple to the incident EM waves, exhibiting effective electric and magnetic responses not found in nature. [1][2][3] However, these promising potential applications are hindered in their applications due to the challenges of fabricating the required complex 3D nanostructures and the inherent metallic losses and strong dispersion of plasmonic elements at optical frequencies. Planar metamaterials, or so-called metasurfaces, can be fabricated using existing technologies, such as the lithography method and have attracted increasing attention due to their feasibility, low loss, and ease of fabrication. [4,5] The most prominent advantage of metasurfaces is that they can generate spatial phase discontinuities over the full 2π range with an optically thin interface; moreover, the resolution is less than one wavelength. Thus, wavefronts can be shaped with a distance of much less than the wavelength. With the increasing development of metasurfaces, the aforementioned limitations can be solved using various ultrathin optical devices, which have properties superior to their conventional counterparts. [6][7][8][9][10][11][12] Here, we concentrate on the new capabilities of metasurfaces in recent years in manipulating the phase and propagation behaviors of EM waves. In Section 2, we briefly introduce the underlying mechanisms of three types of phase discontinuities. In Section 3, we review the basic applications of phase modulation using metasurfaces. In Section 4, we review more complex and advanced information photonics that have emerged from metasurfaces. In the last section, we provide concluding remarks and an outlook on future development directions.
Three Basic Types of Phase Discontinuities Generated by Metasurfaces
Resonance PhaseThe pioneering approach to achieve phase discontinuities was to use the dispersion of various metallic nanoantennas, as shown in the left panel of Figure 1a. The optical energy is coupled to surface EM waves propagating back and forth along the antenna surface. Due to the localized surface plasmon resonance, these waves are accompanied by oscillating free electrons Relative to conventional phase-modulation optical elements, metasurfaces (i.e., 2D versions of metamaterials) have shown novel optical phenomena and promising functionalities with more compact platforms and more straightforward fabrication processes. With the ability to generate a spatial phase variation, optical wavefronts can be manipulated into arbitrary shapes at will, enabling new phenomena and integrated ultrathin optical devices to be explored. This review is focused on recent developments regarding phase manipulation of electromagnetic waves with metasurfaces. Starting from their underlying physics for realizing full 2π phase manipulation, an overview of the applica...