Alessio Monti

Controlling scattering and absorption with metamaterial covers

We discuss the use of metasurfaces and plasmonic metamaterials to minimize the scattering from receiving antennas and sensors, with the goal of maximizing their absorption efficiency. We first analytically study and highlight the potential of these approaches to realize optimized sensors with the desired level of efficiency, being able to minimize the electrical presence of a receiving antenna for a chosen level of overall absorption. Realistic cloak designs, investigated using full-wave simulations, verify the behavior analytically predicted by Mie theory. These optimized cloaks offer a practical way to flexibly tailor the scattering of receiving antennas, with great benefits in the design and optimization of near-field sensors, remote communication systems, spoof targets and improved antenna blockage resiliency. Optimized covers may also provide other interesting features for the same receiving antenna by just tuning its resistive load, such as optimal wireless power harvesting or high-to-low tunable absorption efficiency.

Optical cloaking of cylindrical objects by using covers made of core-shell nanoparticles.

In this Letter, we propose an engineered design of optical cloaks based on the scattering cancellation technique and intended to reduce the observability of cylindrical objects. The cover, consisting of a periodic arrangement of core-shell nanospheres, is designed in such a way to exhibit near-zero values of the real part of the homogenized effective permittivity at optical frequencies. Full-wave numerical simulations, considering the measured data of the dielectric function of the plasmonic material composing the shell, show that the cloak is able to reduce by about 6 dB the scattering cross section of a finite-length cylinder at around 740 THz with a -3 dB fractional bandwidth of about 7%. We show also that this result is not significantly affected by the perturbation of the periodic alignment of the core-shell nanospheres, due to possible fabrication issues or to an amorphous arrangement.

Anisotropic Mantle Cloaks for TM and TE Scattering Reduction

We present the design and realization of anisotropic mantle cloaks operating, at the same frequency, for both TM and TE incident polarizations. Starting from the analytical model of metasurfaces available in the literature, we first explore the potentials and limitations of the most common metasurface geometries for their application as mantle cloaks. Then, we introduce new types of patterned surfaces aimed at improving their polarization response. We demonstrate that, only by using four metasurface topologies, it is possible to obtain all required combinations of positive and negative reactance values in order to design effective mantle cloaks for planar, cylindrical, and three-dimensional (3-D) objects. We also show that the accuracy of the analytical formulas commonly used to design such metasurfaces is not necessarily sufficient for cloaking purposes. Therefore, we introduce and validate a numerical procedure to refine the analytical design and optimize the cloak performance. The effectiveness of the designed covers is checked with full-wave simulations. Finally, some antenna applications of dual-polarized mantle cloaks are proposed and several experimental measurements conducted on fabricated prototypes are also provided.

Optical invisibility through metasurfaces made of plasmonic nanoparticles

In this paper, we investigate the application of the mantle cloaking technique to near-infrared and visible frequencies, analyzing and designing thin covers consisting of 2D arrays of plasmonic nanoparticles. First, we validate and generalize an analytical model recently appeared in the literature to describe a 2D array of plasmonic nanoparticles as a metasurface characterized by its homogenized surface reactance. We prove that the proposed model allows to efficiently design 2D mantle cloaks with an assigned surface reactance, enabling, thus, the extension of the mantle cloaking technique to optical frequencies. Then, we design realistic optical mantle cloaks made of 2D arrays of spheroidal plasmonic nanoparticles with a high eccentricity. We show that the proposed cloaks allow significant, moderately broadband cloaking effects at visible frequencies. In our designs, we consider realistic losses and non-critical nanoparticle dimensions to envision a practical realization of the proposed cloaks.

Multiband and Wideband Bilayer Mantle Cloaks

Using two suitably tailored concentric cloaking metasurfaces, we demonstrate several interesting features of these covers, including significant broadband and/or dual-band scattering reduction. We also show that nearly perfect cloaking for moderately sized targets is enabled by complementary bilayer pairings. We apply these concepts to realistic bilayer cloaks covering a finite-length conductive rod for multiband and wideband operation, offering a >5-dB (70%) scattering suppression over bandwidths that exceed any passive mantle cloak demonstrated so far by over nine times. Finally, we apply the same covers to more complex geometries, showing that moderate scattering suppression is maintained, despite the change in geometry and increased size.

Mantle cloaking for co-site radio-frequency antennas

We show that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopoles, allow tightly packing the same antennas together in a highly dense telecommunication platform. Our experimental demonstration is applied to the relevant example of two cylindrical monopole radiators operating for 3G and 4G mobile communications. The two antennas are placed in close proximity, separated by 1/10 of the shorter operational wavelength, and, after cloaking, are shown to remarkably operate as if isolated in free-space. This result paves the way to unprecedented co-siting strategies for multiple antennas handling different services and installed in overcrowded platforms, such as communication towers, satellite payloads, aircrafts, or ship trees. More broadly, this work presents a significant application of cloaking technology to improve the efficiency of modern communication systems.

Dual-Polarized Reduction of Dipole Antenna Blockage Using Mantle Cloaks

We theoretically and experimentally explore the use of mantle covers to cloak dipole antennas and reduce their blockage on nearby antennas. The proposed structures allow reducing the observability in a given frequency range and overall scattering of conventional dipole antennas operating in a different frequency band, ideally suited to reduce the mutual influence among antennas placed in close proximity to each other. We prove this concept in the case of a low-band (LB) dipole antenna placed in close proximity to a high-band dipole antenna, and study several thin cover designs that are shown to be effective in reducing the LB blockage, without disrupting the performance of both antennas in terms of radiation pattern and impedance matching. An optimized cover is proposed to strongly reduce the interference and shadowing over a large bandwidth targeted for one polarization, and we also experimentally demonstrate the operation of dual-polarized covers for near-field horn and log-periodic antenna excitation.

Exploiting the surface dispersion of nanoparticles to design optical-resistive sheets and Salisbury absorbers

In this Letter, we propose a method to implement resistive sheets exhibiting a desired value of the intrinsic surface resistance at optical frequencies. Considering the sheet made by arrays of plasmonic nanoparticles, the idea is to tailor the surface dispersion occurring when the dimensions of the nanoparticles are smaller than the mean free path of electrons in the constituent material. An analytical model of the surface resistance is proposed and its effectiveness assessed through full-wave simulations. Finally, the applicability of the proposed resistive sheets to implement optical Salisbury screens is discussed and validated through full-wave simulations.

Tunable scattering cancellation cloak with plasmonic ellipsoids in the visible

The scattering cancellation technique is a powerful tool to reduce the scattered field from electrically small objects in a specific frequency window. The technique relies on covering the object of interest with a shell that scatters light into the far field of equal strength as the object, but

Exploiting Intrinsic Dispersion of Metamaterials for Designing Broadband Aperture Antennas: Theory and Experimental Verification

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