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

Geometric Phase Generated Optical Illusion

Abstract An optical illusion, such as “Rubin’s vase”, is caused by the information gathered by the eye, which is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. Metasurfaces are metamaterials of reduced dimensionality which have opened up new avenues for flat optics. The recent advancement in spin-controlled metasurface holograms has attracted considerate attention, providing a new method to realize optical illusions. We propose and experimentally demonstrate a metasurface device to generate an optical illusion. The metasurface device is designed to display two asymmetrically distributed off-axis images of “Rubin faces” with high fidelity, high efficiency and broadband operation that are interchangeable by controlling the helicity of the incident light. Upon the illumination of a linearly polarized light beam, the optical illusion of a ‘vase’ is perceived. Our result provides an intuitive demonstration of the figure-ground distinction that our brains make during the visual perception. The alliance between geometric metasurface and the optical illusion opens a pathway for new applications related to encryption, optical patterning, and information processing

Generation of Structured Light via Nano Structures and Applications

The generation of structured light by means of metasurfaces is presented and the applications in the characterizations of polarization rotation and Pancharatnam-Berry phase are discussed

Nonlinear Mid‐Infrared Metasurface based on a Phase‐Change Material

The mid-wave infrared (MWIR) spectral region (3-5 {\mu}m) is important to a vast variety of applications in imaging, sensing, spectroscopy, surgery, and optical communications. Efficient third-harmonic generation (THG), converting light from the MWIR range into the near-infrared, a region with mature optical detection and manipulation technologies, offers the opportunity to mitigate a commonly recognized limitation of current MWIR systems. In this work, we present the possibility of boosting THG in the MWIR through a metasurface design. Specifically, we demonstrate a 30-fold enhancement in a highly nonlinear phase change material Ge2Sb2Se4Te1 (GSST), by patterning arrays of subwavelength cylinders supporting a magnetic dipolar resonance. The unprecedented broadband transparency, large refractive index, and remarkably high nonlinear response, together with unique phase-change properties, make GSST-based metasurfaces an appealing solution for reconfigurable and ultra-compact nonlinear devices operating in the MWIR.Comment: 15 pages, 3 figure