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Conventionally, antennas are reciprocal devices, exhibiting the same properties in transmission and in reception. In order to break antenna reciprocity, in this letter, we exploit the properties of space-time varying metamaterials, whose dielectric properties are modulated in both space and time. We propose to surround the antenna by a space-time varying cloak that imparts a momentum bias to the electromagnetic field propagating through it. The propagating wave interacts with the space-time modulation only in one direction, breaking time-reversal symmetry and, thus, the reciprocity of the system. Here, we show that a properly designed space-time varying metamaterial cloak can be used to significantly change the realized gain exhibited by an antenna in its transmission and reception mode. Since the proposed concept is based on weak coupling between propagating modes, electrically long propagation distance in the cloak is required, increasing the overall dimension of the system. However, the cloak thickness can be controlled by modifying the modulation parameters. The proposed concept may enhance the performance of radio-frequency communication systems, reducing the in-band captured noise or interfering signals.

Recent years have witnessed an increasing interest in topological states of condensed matter systems, whose concepts have been also extended to wave phenomena. Especially at optical frequencies, several studies have reported applications of structured light exploiting topological transitions and exceptional points or lines, over which a field property of choice is undefined. Interesting properties of light beams with phase singularities (such as the creation, annihilation or motion of these topological points) have been observed in composite vortices consisting of superimposed light beams with different topological charges. Here, we discuss how these concepts may have a relevant impact on antenna technology at microwave frequencies. We obtain the superposition of vortex fields with different topological charges by simultaneously exciting different modes of a patch antenna. This can be useful to give a physical interpretation for the behavior of some structures, already proposed at microwave frequencies, which use superposition of different radiating modes to manipulate the radiation pattern of patch antennas. Moreover, this approach may open new strategies to design at will the directivity properties of a patch antenna with inherently robust responses, and it may find applications in the design of smart antenna systems, requiring pattern reconfigurability.

We extend the mantle cloaking technique to bi-layered geometries, in order to broaden the bandwidth of scattering suppression for applications in scattering reduction from passive and active radiating elements. In particular, we demonstrate a bi-layer cloak covering a finite-length conductive rod for multiband and wideband operation, offering a >5 dB (70%) scattering suppression over bandwidths that exceed any passive cloak demonstrated so far by over nine times. In addition to large bandwidths, we demonstrate also large scattering reduction at the design frequency of 3.3 GHz. The ease of fabrication of the proposed cloaks lends itself to rapid prototyping for various radio-frequency applications, and it requires no dielectric substrate, with advantages in terms of cost, weight and loss reduction.

In this paper, we apply the mantle cloaking method to reduce the mutual coupling between two closely spaced wideband antennas operating at microwave frequencies. We show that by introducing two dielectric elliptical shells around the antennas and covering them with two strongly coupled elliptically shaped metasurfaces comprised of I-shaped printed unit cells, it is possible to significantly increase the original bandwidth of the antennas and, at the same time, reduce the mutual electromagnetic interaction. This effect is observed both in a reduction of mutual S-parameters between the antennas and in the restoration of the original radiation patterns, as if the antennas were almost completely isolated from each other. Broadband metasurface/cloak designs are proposed for two geometries, with the free-standing strip monopole antennas placed over a ground plane and with the metasurface cloaks integrated with microstrip antennas.

In this contribution, we investigate the effect of an infinitely-extended reactive metasurface on the complex input impedance of an electrically small dipole placed in its close proximity. We consider, as a reference scenario, the variation of the input resistance and reactance of a vertical (V-) and horizontal (H-) electric dipole placed above a perfect electric conductor for different electrical distances. Then, the perfect electric conductor is replaced by an inductive and a capacitive metasurface. The complex input impedance of the electric dipole is affected by the presence of the metasurface differently compared to the reference scenario. Our preliminary results demonstrate that a control of the input resistance can be achieved by tuning the surface impedance of the metasurface.

Although cloaks are effective at suppressing the observability of static objects, they can be defeated when in motion. Here we discuss a general technique to cloak the motion of objects from static observers, based on compensating the Doppler shift associated with their motion with frequency conversion sustained by a spatiotemporally modulated cover. The concept is theoretically and numerically demonstrated in a system composed of a planar reflector covered by a spatiotemporally modulated slab. It is shown that, for properly selected modulation frequency, the composite system can appear to an external observer as stationary, even though it is actually moving. This concept may pave the way to the minimization of clutter produced by moving objects as well as to new directions in the science of cloaking.

In this contribution, we review our latest results about the unprecedented possibilities enabled by electromagnetic cloaking in antenna theory and applications. In particular, we show that, in the receiving regime, cloaking allows designing antennas exhibiting a minimum level of scattering for a given level of absorbed power, whereas, in the transmitting one, cloaking enables the co-sitting of different antennas in a confined region. Both full-wave and experimental results are presented to support the theoretical considerations.

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 $\pi$ out-of-phase. The resulting destructive interference prohibits its detection in measurements that probe the scattered light. Whereas at radio or microwave frequencies feasible designs have been proposed that allow to tune the operational frequency upon request, similar capabilities have not yet been explored in the visible. However, such ability is decisive to capitalize on the technique in many envisioned applications. Here, we solve the problem and study the use of small metallic nanoparticles with an ellipsoidal shape as the material from which the shell is made to build an isotropic geometry. Changing the aspect ratio of the ellipsoids allows to change the operational frequency. The basic functionality is explored with two complementary analytical approaches. Additionally, we present a powerful multiscattering algorithm that can be used to perform full wave simulations of clusters of arbitrary particles. We utilize this method to analyze the scattering of the presented designs numerically. Hereby we provide useful guidelines for the fabrication of this cloak with self-assembly methods by investigating the effects of disorder., Comment: 23 Pages, 7 Figures, Accepted at PRB

In this contribution, we present some important advancements in Doppler cloak technology, which is based on the frequency mixing property of linear momentum-biased metamaterial, whose artificial permittivity function is modulated in space and time. The importance of such a technology is given by the possibility to manipulate the Doppler effect and restore the invisibility of cloaked object in relativistic motion. Here, we review the main characteristics of a conventional Doppler cloak, consisting of a system composed of a planar reflector covered by a spatio-temporally modulated slab. We demonstrate that such a scheme presents some drawbacks in terms of efficiency and, therefore, an enhanced version based on a momentum-biased parallel-plate medium is presented and discussed.

International audience; In this contribution, we present our latest results concerning the design of microwave mantle cloaking devices and show some of their applications in different operative scenarios. We start by describing the analytical models of the anisotropic metasurfaces needed to achieve a scattering cross section (SCS) reduction for any polarization of the incident field and, then, we prove the effectiveness of these formulas in a representative antenna scenario. Finally, we show that the proposed formulas still hold in the case of multilayered cloaks that, being able to tailor different scattering modes excited by an object, can be successfully used to cloak non-electrically small objects and/or achieve a wideband cloaking effect of 2D objects.

In this contribution, we discuss some of our latest results about the use of mantle cloaking in different antenna scenarios. Due to the intrinsic characteristics of the scattering cancellation technique, and, in particular, the needed interaction between the external field and the object to hide, mantle cloaking approach can be profitably applied in both receiving and transmitting antenna configurations. In particular, we will show that by exploiting cloaking technologies it is possible to conceive new receiving sensors exhibiting an optimized balance between the scattered and the absorbed power, as well as invisible transmitting antennas that can be placed in close proximity to other radiators without affecting their electrical and radiative performances.

In this contribution, we report our results about the design of metamaterial-inspired particles that can be integrated directly inside horn antennas to perform reciprocal and nonreciprocal manipulation of the received/transmitted signals. Depending on the specific inclusion, we show that the proposed solutions perform different operations on the electromagnetic signal such as: band-pass or notch filtering, polarization transformation, and signal isolation.

In this work, we apply angular momentum-biased metamaterials to break the intrinsic reciprocity of antenna systems, with potential applications to satellite communications. We focus our attention on a conical horn antenna exhibiting nonreciprocal response for left-handed and right-handed circularly polarized fields. A metallic screen with an annular aperture is placed between the feeding waveguide and the horn. This inclusion is loaded with modulated capacitors to achieve a nonreciprocal transmittance for circularly polarized fields impinging from opposite sides. We present the analysis of the nonreciprocal annular inclusion and explain its nonreciprocal response, obtained through proper spatiotemporal modulation, with the help of an equivalent transmission-line representation. A possible practical implementation of the modulation circuit based on band-pass and band-stop filters is also discussed. The electrical and radiating performance of the conical horn is numerically evaluated, showing the ability of the proposed antenna to filter signals with same polarization in two separate bands, e.g., uplink and downlink bands of a satellite link, depending on the propagation direction of the signal. These results may pave the way to several interesting applications of nonreciprocal radiators in satellite and radar systems.

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.

In this contribution, we propose a novel approach for the design of scattering cancellation devices operating at optical frequencies and exhibiting enhanced performance compared to the state of the art. The cloaks, consisting of an arrangement of plasmonic nanoparticles, are modeled through a 2D analytical model able to assign an average surface reactance to the nanoparticles array. The availability of a rigorous model allows us demonstrating that the use of nanoparticles with high eccentricity is able to generate a scattering cancellation effect within the entire optical range, strongly relaxing the current requirements on the nanoparticles size. Full-wave simulations confirming the desired cloaking performance of the nanoparticle-based cloaks are provided.