Geometric optical metasurface for polarization control
Like amplitude and phase, polarization is one of the fundamental properties of light. Controlling polarization in a desirable manner is fundamental to science and technology. However, practical applications based on polarization manipulation are mainly hindered by the complexity of experimental system, bulky size and poor spatial resolution. In recent years, metasurfaces have drawn considerable attention in the scientific community due to their exotic electromagnetic properties and potential breakthrough for light manipulation. With the development of nanophotonics, the generation of arbitrary spatially-varying polarization from an input beam is achievable. The objective of this thesis is to develop metasurface approaches to control phase and polarization of light in subwavelength scale for novel applications, such as polarization-controlled hologram generation and structured beam generation. The emphasis of the thesis is placed on the polarization control using geometric plasmonic metasurfaces. We start by reviewing recent progress regarding novel planar optical components. After the introduction of mechanism of light-nanostructure interaction and the far-field scattering of metal nanostructure arrays based on Mie theory, we discuss the abrupt phase change emerging from rotated nanostrips and the generalized Snell’s law. To demonstrate the precise phase manipulation, we develop a metasurface approach for polarization-controlled hologram generation. Moreover, we propose and experimentally demonstrate a novel method to realise the superposition of orbital angular momentum states in multiple channels using a single device. Spring from the superposition of two opposite circular polarizations, two different approaches for polarization manipulation at nanoscale are developed and experimentally verified. Based on the first approach, a vector vortex beam with inhomogeneous polarization and phase distributions is demonstrated, which features the spin-rotation coupling and the superposition of two orthogonal circular components, i.e., the converted part with an additional phase pickup and the residual part without a phase change. The second approach is to control the phase of the two orthogonal circular components simultaneously to engineer the polarization profile. Furthermore, we adopt this approach to develop a compact metasurface device which can hide a high-resolution grayscale image in a laser beam. The compactness of metasurface approach in polarization manipulation renders this technology very attractive for diverse applications such as encryption, imaging, optical communications, quantum science, and fundamental physics.