Metamaterials and plasmonics for novel components at microwave and optical frequencies

Abstract

In the last century, electromagnetic theory has been widely employed to deeply understand the interaction with matter at any scale, from cosmological down to atomic scale related phenomena. Classic electromagnetic theory is also the base of many practical applications which strongly influence everyday life. It seems very difficult, thus, to add something conceptually new to such wellestablished theories and technologies. However, around ten years ago, a new concept appeared in the universe of classical electromagnetic theory: metamaterials. What is a “metamaterial”? It is an artificially engineered material whose electromagnetic response is not readily available in nature. For instance, metamaterials can exhibit negative values of the electric permittivity of the magnetic permeability or both in a certain microwave frequency range. The unconventional response of these metamaterials is obtained through electrically small metallic inclusions or dielectric inhomogeneities within a dielectric host medium. The exotic properties of metamaterials can be predicted by using quasi-static approaches and considering the interaction among the different inclusions arranged in a 3D lattice. Although the interaction between electromagnetic waves and matter is a hot research topic in Physics, the related applications are widely studied in Electrical and Electronic Engineering. Therefore, metamaterials helped to bridge the gap between these two communities and now physicists and engineers work together on these topics. The current interest in metamaterials is driven by their potential to obtain unprecedented electromagnetic properties useful in several application fields. By using metamaterials, in fact, it is possible to tailor the wave properties almost at will. Increasing the frequency of the electromagnetic field, some unusual properties are already given by nature, though it is always possible to engineer them through proper nano-scale fabrication technologies. For example, at optical frequencies, nature gives us the plasmonic materials. The plasmonic materials are simply noble metals, such as gold or silver, that at optical frequencies allow the propagation of surface waves extremely confined at the metal-dielectric interface. The propagation is given by the collective oscillation of the electrons inside the metal that are excited by the external impinging field. In order to explain such anomalous behavior, metals are described as dielectric materials and it has been shown that such propagation takes place only if the effective permittivity, or optical permittivity, of metal is negative, just as the metamaterials at the microwaves. With this scenario, it is evident that metamaterials and plasmonic materials may open many challenges of interest to physicists and scientists in general. From the technological and engineering viewpoint, the interest in such materials is based on the possibility of designing devices and systems with new properties or functionalities, able to open up new fields of application or to improve existing ones In this dissertation, I would like to collect the results of my research on the employment of metamaterials and plasmonic materials for the design of novel components at microwave and optical frequencies. Chapter 1 develops the analytical models for the analysis of metasurfaces with electrically small inclusions. They have been used to realize Partially Reflective Surfaces (PRS) and High-Impedance Surfaces (HIS) for circular polarization applications in antenna field. Chapter 2 is focused on the design of a metamaterial-based filter element employed for the realization of novel compact guiding and radiating microwave components. Chapter 3 is devoted to the epsilon-near zero (ENZ) metamaterials and their applications in horn antennas. Finally, Chapter 4 is devoted to plasmonic materials exploited to design a nano-waveguide, acting as its microwave counterpart, and an innovative horn nano-antenna working at near-infrared and optical frequencies. In all chapters, I have reported the state-of-the-art on the particular topic of the chapter and included a combination of theoretical, numerical, and experimental results to the understanding of the behavior of metamaterials in the particular considered application. 1 Metasurfaces with electrically small circular inclusions In this chapter, I present the analysis and design of a metasurface consisting of an array of circular inclusions with a sub-wavelength periodicity. The proposed study is suitable for circular holes in a metal conducting sheet, as well as metallic circular patches printed on a electrically thin metal-backed dielectric substrate. The first configuration realize a Partially Reflective Surface (PRS) that introduces a leaky wave and beam-forming effect when placed in front of a grounded waveguide aperture, patch antenna, or any standard radiator. The gain and bandwidth depend on the reflection (amplitude and phase) from the PRS as well as the distance from the radiator itself. On the contrary, the second configuration is known as High-Impedance Surface (HIS) and it is placed back to the radiator. Since the sub-wavelength dimensions, the array and the metal-backed substrate can be described in terms of a lumped capacitance and a lumped inductance, respectively. Around the resonant frequency, the HIS reflects totally an incident electromagnetic wave with zero shift in phase. Due to this property, it is widely employed in antenna systems as compact back reflector with improved performances with respect to typical metal reflector. In order to validate the analytical results, I compare the analytical results to the ones resulting from full-wave numerical simulations and from other analytical methods available in the open technical literature. 2 Metamaterial-based filter element for waveguide applications In this chapter, I present the analytical model of the bi-omega particle consisting of two opposite-oriented and spaced omega resonators. Properly placing the two omega particles very close to each other, the coupling effect between them contributes to shift down the resonant frequency, achieving a simple and deeply electrically small inclusion working in the microwave frequency range. The proposed model, based on the small antenna theory, takes into account all the coupling effects that comes into play when the distance between the two omegas is very small. Then, two bi-omega particles are connected together, in order to obtain a transfer of energy from one bi-omega, used as receiving antenna, and the other, the transmitting antenna, when it is placed across a slit in a metallic screen. Due to its resonant behavior, the proposed structure can be successfully used as pass-band filter able to select the frequency of interest with high accuracy, which is strongly required in many waveguide and aperture antenna applications. Among them, I present the design and the experimental realization of innovative microwave components, such as waveguide filters, diplexers, powersplitters, modal filters, horn antennas, etc. The ideas are verified through proper fullwave numerical simulations and experimental results. 3 ENZ metamaterial lens for novel aperture antennas In this chapter, I present two novel designs of flat lens, made by a conventional material and an epsilon near-zero (ENZ) metamaterial or by loaded ENZ metamaterial, to plug up the aperture of aperture antennas, in order to achieve new radiation performances. First, I present a metamaterial lens for shortened aperture antennas that lets to achieve radiating performances similar to the ones of the corresponding optimum horn over a broad frequency range. Lens operation is based on the phase-compensation concept: phase-fronts of the field propagating along the short flare of the horn propagate with different phase velocities in the two lens materials, resulting in an uniform phase distribution on the aperture. A realistic version of the lens, realized with a wire-medium and exhibiting a near-zero real part of the effective permittivity in the frequency range of interest, is presented. Considering two examples working in the C-band, we show that the lens can be designed for both conical and pyramidal horn antennas. In both cases, the length of the horns is half the one of the corresponding optimum versions, while the obtained radiation performances are similar to those of the optimum horns over a broad frequency band. Then, I present the numerical results of an aperture horn antenna with steerable radiation diagram. The rotation of the maximum of the radiation diagram is obtained modifying locally the index of refraction of the ENZ lens in order to change locally the electrical path inside the lens. 4 Guiding and radiating components at IR and optical frequencies In this chapter, the design and the performances of an optical waveguide and horn antenna working at near-infrared frequencies are discussed. First, I focus the attention on the nano-waveguide, which should satisfy some important requirements to be a good nano-waveguide: on one hand side, it should effectively guide the light and, on the other hand, it should present a topology that is similar to its counterpart at microwave frequencies. I start the design considering a symmetric nano-transmission line constituted by a non-plasmonic material slab sandwiched between two plasmonic layers that may represent a suitable candidate for this purpose. Such a structure supports a fundamental forward mode confined between the two metallic layers having a strong electric field component orthogonal to them. Then, I consider the problem to effectively radiate the guided electromagnetic field because recently the antenna concept has been brought to the attention of physics and optics communities, due the possible applications in energy harvesting, smart lighting, and optical wireless communication links. In addition, the recent advancements in nanotechnology make the actual fabrication of such devices ever more attractive. Here, I present an horn nano-antenna obtained shaping at one of the ends of the nanowaveguide in such a way to increase the physical aperture and progressively match the wave-impedance of the waveguide to the one of free-space.