Ultrathin metasurface devices for phase and polarization control
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Metamaterials are artificial materials, which consist of periodic or non-periodic structures of manmade “atoms” with a typical size of subwavelength scale. Benefiting from the freedom to tailor the properties of electromagnetic waves that are unavailable in nature, metamaterials have brought new concepts and new discoveries to the world (e.g., negative refraction, super imaging and cloaking). However, the application of 3D metamaerials in the optical range is limited by current nanofabrication techniques. As 2D counterparts of metamaterials, metasurfaces, which are comprised of a single layer or a stack of several layers of 2D structures, can facilitate manufacturing process and manipulate light propagation in desirable manner. Metsurfaces are ultrathin and ultraflat, which has enabled ultrathin optical devices that can outperform the capabilities of traditional bulky optical elements. The unprecedented capabilities of metasurfaces in the manipulation of amplitude, phase and polarization have led to the development of novel, compact optical devices with specially designed functionalities. In this thesis, we simultaneously control the phase and polarization state of light with one single metasurface, enabling novel multifunctional devices that are not possible with conventional optical devices. To meet the growing requirement of device miniaturization and system integration, it is of great importance and interest to develop ultrathin optical devices that integrate multiple functionalities into one device while preserving their independent functionalities. To increase the functionality density, we develop metasurface devices that can dynamically control the superposition of laser beams with various orbital angular momentum (OAM). This unique approach can arbitrarily realize different functionalities in multiple channels based on a single plasmonic metasurface. As a proof of concept, we experimentally demonstrate an ultrathin optical device that can simultaneously realize polarization-controllable hologram and superposition of OAM beams in multiple channels, which is realized by controlling the polarization state of the incident light. Although human eyes or cameras are sensitive to spatially varying intensity or colour profiles, they are blind to polarization profiles with uniform intensity profiles. We propose and experimentally demonstrate an approach to hide a high-resolution grayscale image in a laser beam. The space-variant polarization profile originates from the superposition of two circularly polarized beams with opposite handedness that come from from a single metasurface device. Upon the illumination of a linearly polarized light beam, we experimentally demonstrate a metasurface device that can generate a light beam with inhomogeneous polarization profile for hiding a quick response (QR) code. The unique measurement technique used here holds great promise for anti-counterfeiting and encryption. This approach is extended to hide a color image through the local control of both polarization and color selectivity based on a high-efficiency transmissive dielectric metasurface, which consists of silicon nanoblocks that can precisely control brightness and contrast. This approach provides an extraordinary capability in additive color mixing and tailoring the polarization of light at the nanoscale. The unprecedented capabilities of optical metasurfaces in the local manipulation of the phase and polarization of light at the subwavelength scale, open a new window for the implementation of a plethora of novel optical components. The unique advantages of simplicity and robustness of our design, ease of fabrication, compactness and unusual functionalities of ultrathin metadevices render optical metasurfaces very attractive for new applications in free-space imaging, information processing, encryption, anticounterfeiting, optical communications, and fundamental physics. The plan for the future work in this amazing field is also discussed in the last chapter.