New absorbing boundary conditions and analytical model for multilayered mushroom-type metamaterials: Applications to wideband absorbers

An analytical model is presented for the analysis of multilayer wire media loaded with 2-D arrays of thin material terminations, characterized in general by a complex surface conductivity. This includes the cases of resistive, thin metal, or graphene patches and impedance ground planes. The model is based on the nonlocal homogenization of the wire media with additional boundary conditions (ABCs) at the connection of thin (resistive) material. Based on charge conservation, new ABCs are derived for the interface of two uniaxial wire mediums with thin imperfect conductors at the junction. To illustrate the application of the analytical model and to validate the new ABCs, we characterize the reflection properties of multilayer absorbing structures. It is shown that in such configurations the presence of vias results in the enhancement of the absorption bandwidth and an improvement in the absorptivity performance for increasing angles of an obliquely incident TM-polarized plane wave. The results obtained using the analytical model are validated against full-wave numerical simulations.NASA/MS Space Grant Consortium Research Infrastructure Program NG05GJ72HMinisterio de Ciencia e Innovación TEC2010-16948, CSD2008-00066Junta de Andalucía P09-TIC-459

Artificial Impedance Surfaces and Wire Media for Absorption and Cloaking

The main objective of this dissertation is to investigate the ability of utilizing artificial impedance surfaces and wire media for absorption and cloaking applications. The dissertation includes two parts which focus on the electromagnetic wave propagation in absorbers formed by stacked metasurfaces and structured wire media, and electromagnetic wave interaction with the cylindrical cloaking structures. In the first part, we propose a variety of physical systems that show multiband and wideband absorption properties in the microwave regime. For the multiband absorbers, we propose a simple analytical model to study the absorption properties. Further, using the same circuit model, the physical mechanisms of the observed behavior is clearly explained in terms of the open/coupled Fabry-Pérot resonators. To design wideband absorbers, we first analyze a single-layer wire medium loaded with an arbitrary material (a thin copper patch with finite bulk conductivity and a graphene patch characterized by its complex surface conductivity) at one end and a ground plane at the other. Based on the properties of the single-layer structure (which acts as a narrowband absorber), we next propose a novel multilayered mushroom structure with thin resistive patches at the wire-medium junctions for wideband absorption. To characterize the wideband properties, here, we derive new additional boundary conditions and solve the scattering problem using an analytical model developed particularly for the problem at hand. We also show a methodology to design these absorbers and explain the wideband absorption mechanisms. The second part focuses on the application of various metasurfaces for cloaking dielectric and conducting cylinders for plane-wave incidence and for line sources in close proximity. The cloaking mechanism is based on a mantle cloaking technique, wherein the scattered field produced by the object is cancelled by the cloak. The purpose of this work is to design the mantle cloaks using the metasurfaces, to render the objects invisible. The analysis here is carried out using a rigorous analytical model which employs the Lorenz Mie-scattering theory. Two-sided impedance boundary conditions are applied at the interface of the metasurfaces and analytical grid-impedance expressions derived for the planar cases have been successfully used in tailoring the reactances of the cylindrical surfaces. Further, the analytical results presented in the dissertation are verified using the numerical simulations

Electromagnetic Characterization of Metasurfaces

Electromagnetic characterization of metasurfaces (MSs), electrically/optically thin sheet metamaterials (MMs), is the subject of the current study. Briefly, a MM is a composite material with unusual electromagnetic properties offered by specific response of its constituents and their arrangement. The main goal in this work is to attribute some macroscopic characteristic parameters to MSs.  We first discuss the definitions and present a brief review of the electromagnetic characterization of MMs and MSs. We explain the failures of the traditional characterization approach when applied to MSs. We discuss two known approaches especially suggested for the characterization of MSs in 1990s-2000s.  We continue to introduce a heuristic homogenization model of MSs located on a dielectric interface. Indeed, we derive the general boundary conditions invariant on the polarization of the excitation field. Then, we present the most general algorithm to retrieve the characteristic macroscopic parameters through two-dimensional reflection and transmission dyadics.  We next present two explicit examples of MSs in order to prove the applicability of our theory. The first one is a periodic array of plasmonic nano-spheres while the second one is an array of coupled plasmonic nano-patches positioned in a disordered fashion on a flat surface. We show that our approach works for for both random and periodic MSs. Indeed, the restriction of our theory is a sufficiently small electrical/optical size of a unit cell (area per one particle).  We finally present the main results of the thesis through functional MSs. We theoretically reveal and discuss novel physical effects and various functionalities. We present some discussions on the intrinsically bianisotropic and intrinsically magnetic MSs operating in the visible range. We also discuss the microscopic effect of substrate-induced bianisotropy for a substrated array of plasmonic nano-spheres. Moreover, we reveal the magnetic response within the framework of our homogenization model; i.e., retrieving some magnetic parameters. Furthermore, we obtain the perfect absorbance conditions for different topologies and discuss them in this chapter. Finally, we present a model which explains the different behavior of electric and magnetic resonant modes of MSs in transition from periodic to amorphous arrangements

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

This work was funded in part by the Predoctoral Grant FPU18/01965 and in part by the financial support of BBVA Foundation through a project belonging to the 2021 Leonardo Grants for Researchers and Cultural Creators, BBVA Foundation. The BBVA Foundation accepts no responsibility for the opinions, statements, and contents included in the project and/or the results thereof, which are entirely the responsibility of the authors.Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques.Predoctoral Grant FPU18/01965BBVA Foundatio

Kinetic Monte Carlo Methods for Computing First Capture Time Distributions in Models of Diffusive Absorption

In this paper, we consider the capture dynamics of a particle undergoing a random walk above a sheet of absorbing traps. In particular, we seek to characterize the distribution in time from when the particle is released to when it is absorbed. This problem is motivated by the study of lymphocytes in the human blood stream; for a particle near the surface of a lymphocyte, how long will it take for the particle to be captured? We model this problem as a diffusive process with a mixture of reflecting and absorbing boundary conditions. The model is analyzed from two approaches. The first is a numerical simulation using a Kinetic Monte Carlo (KMC) method that exploits exact solutions to accelerate a particle-based simulation of the capture time. A notable advantage of KMC is that run time is independent of how far from the traps one begins. We compare our results to the second approach, which is asymptotic approximations of the FPT distribution for particles that start far from the traps. Our goal is to validate the efficacy of homogenizing the surface boundary conditions, replacing the reflecting (Neumann) and absorbing (Dirichlet) boundary conditions with a mixed (Robin) boundary condition

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic