Design and Analysis of Metasurfaces for Polarization Conversion of Electromagnetic Waves

Abstract

Control and manipulation of polarization state of electromagnetic waves has always been of interest in the scientific community due to its fundamental role in wide range of applications including contrast imaging microscopy, optical sensing, molecular biotechnology and optical and microwave communication. Although, conventional techniques are applied for polarization control using natural materials such as optical activity of crystals, Faraday-effect and solutions of chiral molecules such as sugar. However, such methods generally result in bulky volumes, narrow bandwidth and incidence angle dependent response which greatly limit their use for many practical applications. Therefore, scientists have explored the use of artificial structures in the form of ultra-thin metasurfaces to achieve miniaturized polarization control devices with wide bandwidth and angularly stable response. However, most of these designs achieve polarization conversion for normal incidence only, which practically becomes prohibitive, as incoming waves can have arbitrary incidence angles. Thus, metasurfaces with stable response for arbitrary incidence angles are highly desirable. In this perspective, there are two main objectives of this research thesis: firstly, to realize wideband metasurfaces achieving polarization conversion both for normal as well oblique incidence and secondly, to design metasurfaces which can achieve multiple functionalities through a single structure. The first three metasurface designs presented in Section I achieve angularly stable (maximum up to 60o ) wideband cross-polarization conversion in reflection mode. The cross-polarization conversion is achieved through anisotropy of the unit cell while the bandwidth is extended through multiple plasmonic resonances. Multifunctional metasurfaces are presented in Section II. These metasurfaces are extremely desirable in practical applications as they can replace multiple optical components, thus miniaturizing size and reducing the cost of the overall system. The first of the multifunctional metasurfaces presented in this thesis, not only transforms linear and circular polarization to their corresponding cross polarization, but also achieves linear-to-circular and circular-to-linear polarization conversion in different frequency regimes. Thus, it exhibits both half- and quarter-wave plate operations in different frequency bands using an ultra-thin bilayer anisotropic metasurface. The final design presented in this thesis is based on a flexible single layer anisotropic metasurface manifesting both quarter-wave plate and half-mirror (1:1 beam splitter) operation.