A system-by-design approach to the synthesis of mantle cloaks for large dielectric cylinders

Abstract

In the last few years, the possibility to engineer the properties of artificial Metamaterials at microwave, THz, and optical frequencies has represented one of the key factors enabling the study, development, and demonstration of effective electromagnetic cloaks [1]. Indeed, several devices have been proposed which are able to manipulate the wave propagation not only to reduce radar cross section, but also to mitigate mutual coupling in close radiators and to reduce antenna backscattering [1]. In this framework, “perfect cloaks” have been initially designed through Transformation Electromagnetics (TE) methodologies [1]. Such techniques are based on the formulation of the cloaking problem as a “geometry deformation” one, in which the target is made invisible by “bending” the wave around it [1]. Unfortunately, the effectiveness of TE-based devices, numerically demonstrated in several different scenarios [2–5], can be obtained only if inhomogeneous and anisotropic permittivity and permeability tensors can be synthesized. In order to overcome this issue, alternative cloaking strategies based on the Scattering Cancellation principle have been proposed [6–10]. In this case, the cloak is made of a set of patterned metasurfaces covering the scatterer, whose properties are not aimed at bending the wave, but rather at enforcing that the combined scattering contributions of the target and the mantle cancel out [6–10]. Despite their effectiveness and potentialities [6–10], standard scattering cancellation methods are usually applied to thin dielectric/metallic cylinders since the design of multi-layer cloaks, required for “large” targets, is a theoretically and computationally challenging task (also because of the arising mutual coupling among the layers). To address this challenge, an instance of the System-by-Design paradigm [11] will be proposed in this work. Towards this end, the design of multi-layer mantle cloaks based on patterned metasurfaces and able to reduce the radar cross section of large dielectric cylinders will be reformulated as a task-oriented SbD problem. More specifically, (i) the objective will be the minimization of the scattering coefficients of the overall structure (taking into account mutual coupling effects), which will be encoded in a suitable cost function, and (ii) the degrees-of-freedom will be the patterns of each metasurface comprised in the cloak. Thanks to the capability of the SbD paradigm to formulate the design problem in a modular way, the most proper methodology will be adopted for each one of the functional blocks constituting the synthesis process [11]. The effectiveness of the resulting SbD-enhanced devices will be demonstrated through selected numerical experiments.