Analysis and design of scattering cancellation based cloaking devices operating at microwave and optical frequencies

Abstract

The PhD Thesis "Analysis and design of scattering cancellation based cloaking devices operating at microwave and optical frequencies" has been developed as a commented collection of the main research results that I achieved during my PhD. The Thesis is organized in four chapters, corresponding to as many research areas, all related to the macro-topic focused on the design and the applications of the metamaterials and metasurfaces at microwave, infrared and optical frequencies. Chapter 1 is focused on the theoretical basis of the mantle cloaking at microwave frequencies. The main innovative results achieved in this context are: 1) the definition of a general procedure to design anisotropic mantle cloaks operating, at the same frequency, for both TM and TE incident polarizations. In particular, starting from the analytical model of metasurfaces available in the literature, I have first explored the potentials and limitations of the most common metasurface geometries for their application as mantle cloaks. Then, I have introduced new types of patterned surfaces aimed at improving their polarization response. It has been demonstrated that, only by using four metasurface topologies, it is possible to obtain all the required combinations of positive and negative reactance values in order to design effective mantle cloaks for planar, cylindrical and 3D objects; 2) The study of the interesting features of multilayered mantle cloaking devices, including, for instance, significant broadband and/or dual-band scattering reduction. Chapter 2 focus on the application of the cloaking technologies both in the transmitting and the in receiving operation mode of antenna systems. My results in this field can be summarized as follows: 1) definition of new-conception optimized receiving sensors with the desired level of absorption efficiency, being able to minimize the electrical presence of a receiving antenna for a chosen level of overall absorption; 2) use of the mantle cloaking to overcome the blockage effect affecting the over-crowed communication platforms, both for passive obstacle (e.g., the support structure of the platforms) and among different antennas; 3) definition of a procedure to design new-conception linear antennas and arrays specifically designed to properly work in a frequencies range and to be electromagnetically invisible in another one. These radiative systems provide a new degree of freedom in the design of modern communication platforms hosting an increasing number of radiators. In fact, the possibility to use invisible antenna systems that can be successfully placed in close proximity will allow to reduce both the space occupancy and the weight of the civil and military platforms as well as on of the satellite systems. Chapter 3 dealt with the topic of optical cloaking. The following results has been obtained: 1) possible implementation of optical epsilon-near-zero (ENZ) metamaterials based on the employment of an array of core-shell nano-spheres embedded in a dielectric medium. The core of the nano-spheres and the host medium are both made of silica whereas the shell is formed by a plasmonic material (i.e., silver). Using classical homogenization formulas, I show that it is possible to design the array in such a way to exhibit near-zero values of the effective real permittivity with relatively low-losses in the upper part of the optical spectrum; 2) investigation of the application of the mantle cloaking technique to near-infrared and visible frequencies, using thin covers consisting of 2D arrays of plasmonic non-spherical nanoparticles. Here, I focused in more detail in the plasmonic nanoparticles modeling and I generalized the literature results to demonstrate that the nanoparticles arrays can be characterized by a suitable average surface reactance. Then, I show that the use of two-dimensional model is more appropriate than the one based on the effective permittivity. This analysis has allowed to extend the metasurfaces-based cloaking up to optical frequencies and to engineer the shape of the plasmonic nanoparticles in order to dramatically improve the performance of the optical cloaking devices previously proposed. Finally, Chapter 4 collects my results in other research fields: 1) investigation of the use of non-Foster active circuits to increase the operation bandwidth of a split-ring resonator (SRR) for possible application in metamaterial-inspired components, In particular, I propose a possible realistic implementation of the active load, based on the employment of commercially available circuit elements. The obtained results (seven times improvement of the impedance bandwidth of the SRR-based monopole antenna) prove that non-Foster active loads can be successfully used to overcome the inherent narrow-band operation of SRR-based passive metamaterials and metamaterial-inspired components.; 2) definition of a simple and low-cost technique for realizing resistive 3 sheets. This approach, requiring only easy-to-find and inexpensive materials, consists in the deposition of graphite powder on ultrathin plastic sheet substrates. I show that, by controlling the deposition time and the mass of the deposited powder, it is possible to realize resistive sheets exhibiting a desired surface resistance in a wide range of values. All Chapters are preceded by a short introduction describing my publications related to that specific topic and summarizing the content of the various Section. In each Section, I have reported the state-of-the-art about the topic and my main findings, that are supported by a combination of theoretical, numerical, and, where available, experimental results.