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

High-resolution grayscale image hidden in a laser beam

Images perceived by human eyes or recorded by cameras are usually optical patterns with spatially varying intensity or color profiles. In addition to the intensity and color, the information of an image can be encoded in a spatially varying distribution of phase or polarization state. Interestingly, such images might not be able to be directly viewed by human eyes or cameras because they may exhibit highly uniform intensity profiles. Here, we propose and experimentally demonstrate an approach to hide a high-resolution grayscale image in a square laser beam with a size of less than half a millimeter. An image with a pixel size of 300 × 300 nm is encoded into the spatially variant polarization states of the laser beam, which can be revealed after passing through a linear polarizer. This unique technology for hiding grayscale images and polarization manipulation provides new opportunities for various applications, including encryption, imaging, optical communications, quantum science and fundamental physics

Multifunctional metasurface lens for imaging and Fourier transform

A metasurface can manipulate light in a desirable manner by imparting local and space-variant abrupt phase change. Benefiting from such an unprecedented capability, the conventional concept of what constitutes an optical lens continues to evolve. Ultrathin optical metasurface lenses have been demonstrated based on various nanoantennas such as V-shape structures, nanorods and nanoslits. A single device that can integrate two different types of lenses and polarities is desirable for system integration and device miniaturization. We experimentally demonstrate such an ultrathin metasurface lens that can function either as a spherical lens or a cylindrical lens, depending on the helicity of the incident light. Helicity-controllable focal line and focal point in the real focal plane, as well as imaging and 1D/2D Fourier transforms, are observed on the same lens. Our work provides a unique tool for polarization imaging, image processing and particle trapping

Optical Metasurface Generated Vector Beam for Anticounterfeiting

The potential in cost and energy savings by replacing a feed forward weather compensated control (WCC) controlled radiator system with a linear MPC controller is investigated in a Modelica-Python setup. It is shown that if the MPC is optimized for minimum energy consumption it can reduce the energy consumption by up to 12 %. It is also demonstrated how variable price signal can influence the heat demand profile, and thereby shift energy consumption away from peak hours. By introducing a peak load tariff, it is also possible to reduce the rapid changes and large peaks often caused by optimization-based controller

Research summary of cable channel fire extinguishing technology

Cable channel fire is an important factor affecting the safe operation and maintenance of power cables. This paper summarizes the current research situation of fire-extinguishing technology, analyzes the fire-extinguishing technology suitable for cable channel, and proposes to use S-type aerosol fire-extinguishing technology to extinguish the fire of cable channel.