Metasurfaces for ultrathin optical devices with unusual functionalities
Metamaterials are artificial materials that are made from periodically arranged structures, showing properties that cannot be found in nature. The response of a metamaterial to the external field is defined by the geometry, orientation, and distribution of the artificial structures. Many groundbreaking discoveries, such as negative refraction, and super image resolution has been demonstrated based on metamaterials. Nevertheless, the difficulty in three-dimensional fabrication, especially when the operating band is located in the optical range, hinders their practical applications. As a two-dimensional counterpart, a metasurface consists of an array of planar optical antennas, which locally modify the properties of the scattered light. Metasurfaces do not require complicated three-dimensional nanofabrication techniques, and the complexity of the fabrication is greatly reduced. Also, the thickness of a metasurface can be deep subwavelength, making it possible to realize ultrathin devices. In this thesis, geometric metasurfaces are utilized to realize a series of optical devices with unusual functionalities. Phase gradient metasurface is used to split the incident light into left-handed polarized (LCP) and right-handed polarized (RCP) components, whose intensities can be used to determine the polarization state of the incident light. Then we propose a method to integrate two optical elements with different functionalities into a single metasurface device, and its overall performance is determined by the polarization of the incident light. After that, a helicity multiplexed metasurface hologram is demonstrated to reconstruct two images with high efficiency and broadband. The two images swap their positions with the helicity reversion of the incident light. Finally, a polarization rotator is presented, which can rotate the incident light to arbitrary polarization direction by using the non-chiral metasurface. The proposed metasurface devices may inspire the development of new optical devices, and expand the applications of metasurfaces in integrated optical systems.