Superscattering and Directive Antennas via Mode Superposition in Subwavelength Core-Shell Meta-Atoms

Designing a subwavelength structure with multiple degenerate resonances at the same frequency can vastly enhance its interaction with electromagnetic radiation, as well as define its directivity. In this work we demonstrate that such mode superposition or ‘stacking’ can be readily achieved through the careful structuring of a high-permittivity spherical shell, with either a metallic or a low permittivity dielectric (air) core. We examine the behaviour of these structures both as scatterers of plane wave radiation and as directive antennas. In the case where the core is metallic this leads to a superposition of the magnetic and electric modes of the same order, causing suppression of backscattering and unidirectional antenna emission. For an air core, an electric mode can superimpose with the next-highest order magnetic mode, the backscattered power is maximized and antenna emission is bidirectional. This is shown experimentally at microwave frequencies by observing the backscattering of core-shell spheres and we propose two antenna designs demonstrating different emission patterns defined by the superposition of multiple modes.

A solution to the complement of the generalized Luneburg lens problem

Lenses are of interest for the design of directive antennas and multi-optics instruments in the microwave, terahertz and optical domains. Here, we introduce an optical problem defined as the complement of the well-known generalized Luneburg lens problem. The spherically symmetric inhomogeneous lenses obtained as solutions of this problem transform a given sphere in the homogeneous region outside of the lens into a virtual conjugate sphere, forming a virtual image from a real source. An analytical solution is proposed for the equivalent geodesic lens using the analogy between classical mechanics and geometrical optics. The refractive index profile of the corresponding inhomogeneous lens is then obtained using transformation optics. The focusing properties of this family of lenses are validated using ray-tracing models, further corroborated with full-wave simulations. The numerical results agree well with the predictions over the analyzed frequency bandwidth (10–30 GHz). This virtual focusing property may further benefit from recent developments in the fields of metamaterials and transformation optics.

On the integration of reconfigurable intelligent surfaces in real-world environments

<div>The use of reconfigurable intelligent surfaces (RISs) for optimization of propagation channels is one of the most promising and revolutionizing techniques for improving the efficiency of the next generation of communications systems. In this work, we combine the physical optics approximation and the theory of diffraction gratings to study the scattering properties of finite-size metasurfaces mounted on partially reflecting walls and illuminated by directive antennas. We consider both reflective and refractive metasurfaces designed to control both reflection and transmission of waves. We start the analysis under the assumption of uniform, plane-wave illumination, and then discuss non-uniform illuminations by directive antennas.</div>

On the integration of reconfigurable intelligent surfaces in real-world environments

<div>The use of reconfigurable intelligent surfaces (RISs) for optimization of propagation channels is one of the most promising and revolutionizing techniques for improving the efficiency of the next generation of communications systems. In this work, we combine the physical optics approximation and the theory of diffraction gratings to study the scattering properties of finite-size metasurfaces mounted on partially reflecting walls and illuminated by directive antennas. We consider both reflective and refractive metasurfaces designed to control both reflection and transmission of waves. We start the analysis under the assumption of uniform, plane-wave illumination, and then discuss non-uniform illuminations by directive antennas.</div>

Design and Comparative Analysis of Ultra-wideband and High Directive Antennas for THz Applications

This work presents a comprehensive detailed comparative study of the three ultra-wideband and high directive antennas for the THz imaging, spectroscopy, and communication applications. Three different types of photoconductive antennas (log-spiral, Vivaldi, and bowtie antennas) are designed and simulated in the frequency range of 1 to 6 THz in the CST microwave studio (MWS). The enhanced directivity of the designed PCAs is achieved with the integration of the hemispherical silicon-based lens with the PCA gold electrode and quartz substrate of the proposed antennas. The performance of the designed PCAs is compared in terms of impedance and axial ratio bandwidths, directivity, and radiation efficiency of the proposed antennas. The reported log spiral, Vivaldi PCAs with added silicon lens exhibit the -10 dB impedance bandwidth of 6 THz, 3dB AR bandwidth of 5 THz, 6 THz, and 6 THz and peak total radiation efficiencies of 45%, 65%, and 95% respectively.