DESIGN AND ANALYSIS OF METASURFACES FOR VARIOUS APPLICATIONS

Abstract

Electromagnetic metamaterials, new type of artificially engineered materials, have been actively investigated in the past two decades and have gained significant attention due to their electromagnetic (EM) properties, which are not present in naturally occurring materials. These are the periodic structures with unit cell sizes of the order of quarter wavelength. The constitutive EM properties like negative permittivity/ permeability and anomalous refraction make them unique. The first theoretical concept of negative permittivity and permeability of metamaterial was given by Sir Veselago in 1967. After three decades, the first experimental demonstration of negative permittivity was first given by John B. Pendry. Since then, metamaterials have gained a great interest in various applications. The metamaterials are bulky in size and also very complex in design due their three dimensional geometry. Later, the planar devices with the similar properties came into existence named as metasurfaces. The metasurfaces, due to their lightweight, simple design, high efficiency, and ease of integration with other RF devices like antennas and RF components have attracted the attention of researchers all over the globe. In this regard various applications have been identified in RF, microwave, millimeter-wave (mm-wave), terahertz (THz) and optics domain. The present thesis deals with the design and analysis of metasurface based absorber, polarization converters, and beam shaping devices. Several approaches have been made to improve the performance of metasurfaces-based components. However, the improvements in the performance are required with the growing demand for high speed and point to point communication systems. Moreover, the designing of metasurfaces is a hit and trial method in EM simulator. Chapter 1 briefly introduces the application of metasurfaces along with classification based of EM properties of materials. Moreover, the motivation of the thesis is discussed. Along with this, the main objectives of the thesis are also listed. At the end of this chapter a chapter-wise organization of the thesis is included. A brief overview of metasurfaces and their properties are given at the start of Chapter 2. Further, a technical literature review of metasurfaces for various applications is presented in this chapter. The alternative materials being used for these applications have also been discussed. As the scarcity of bandwidth increases, the lower frequency band experiences congestion, posing challenges in enhancing data rates and latency for more efficient communication systems. The one solution is to move towards higher frequencies specifically in millimeter wave and THz frequency regime. Several approaches have been made to design the metasurfaces-based devices in the mid-THz frequency regime. However, the improvements in bandwidth and other performances are required. In Chapter 3, a metasurface based polarization insensitive broadband absorber is proposed. The frequency of operation of the absorber is in THz frequency range. The proposed structure is four fold symmetric, and hence it is insensitive to TE and TM mode of incident EM wave. Another application of metasurfaces is polarization converters. In Chapter 4, broadband, conformal, and multi-functional polarization converters are proposed. In the first stage of this chapter, a broadband linear to circular polarization converter working in Ka and U band is designed and experimentally verified. The conformal geometry of the polarization converter is also realized with experimental verification. As we are moving towards the higher frequency, requirement of other functioning devices are increasing. Switching the mode of operation of polarization converter is one of them. In the second stage of this chapter, a multi-functional polarization converter for mid-THz frequency is designed. The polarization converter is giving linear to linear polarization conversion in one mode and linear to circular polarization conversion in another mode over the same frequency band. The application of metasurfaces is not limited to absorbers and polarizers. In long distance and high speed communication systems, narrowing of beam and multiplexing of channel is required. The orbital angular momentum (OAM) RF beam can support multiple modes, but the beam is very divergent in nature. In Chapter 5, metasurface based lens is designed for converging the OAM beam originating from a uniform circular array (UCA). The UCA and lens are also fabricated for experimental verification. The simulation and experimental results are in agreement to each other. For various applications, different shape and size of unit cell of metasurface is required. Designing of the unit cell is a very tedious work as it is based on hit and trail method. In Chapter 6, CAD model of metasurface unit cell is proposed. Here, a unit cell of metasurface based polarization converter is taken as design model, but any other unit cell for different application can also be taken. Using the CAD model, linear to linear, linear to circular, linear to multiband polarization converters have been designed as case studies. Further, one case is verified with current distribution, polarization ellipse, and experimentally as well. At the end of this thesis, conclusions are drawn and are summarized in Chapter 7. Further, the future scopes are discussed in this chapter.