Cloaking and magnet-less non-reciprocity through metamaterials

The term “metamaterial” was first introduced in electromagnetics to include a wide variety of structured materials properly engineered to exhibit novel electromagnetic properties, such as a negative refractive index. From the early theoretical studies, metamaterials have been applied both to improve the performances of standard components and to conceive unprecedented electromagnetic devices that, recently, are becoming commercially available. In the last years, we have witnessed to an important development of the metamaterial research in other scientific and technological fields, such as mechanics, acoustics, thermodynamics, quantum theory, etc. This is confirmed by the number of publications relevant to metamaterials indexed in the international databases. Scopus, for example, counts about 12000 publications on metamaterials in the last 5 years of which more than 2000 are mechanic or acoustic metamaterial papers. In all the aforementioned fields, one of the most investigated applications of metamaterials is invisibility cloaking. The purpose of a cloaking device is to hide an object, so that it cannot be detected by an external sensor. In addition to the obvious military applications, related to the reduction of the radar or sonar signature of a platform, invisibility cloaks can be also exploited in several civil scenarios. Just to cite an example, we have recently experimentally proven [1] that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopole antennas, allow to tightly packing the same antennas together without performance degradation that, otherwise, would be unavoidable. Similar solutions have been also proposed for different application fields. For instance, it has been recently proven that acoustic cloaks for ultrasound waves can be realized [2] and, therefore, they may be used to conceal an object from detection or to create a new class of acoustic sensors. Another fascinating possibility offered by metamaterials is the possibility to break the intrinsic reciprocity of electromagnetic and acoustic components paving the way to the design of lightweight and magnet-less non-reciprocal devices. As an example, in [3] we have proposed the design of a conical horn antenna able 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. Similar effect can be also obtained in acoustics [4]. The main goal of this contribution is to provide an overview about the last achievements in these two application fields of metamaterials with a particular emphasis on the duality between electromagnetic and acoustic phenomena.