Discrete Impedance Metasurfaces for 6G Wireless Communications in D-Band

Engineering and optimization of wireless propagation channels will be one of the key elements of future communication technologies. Metasurfaces may offer a wide spectrum of functionalities for passive and tunable reflecting devices, overcoming fundamental limits of commonly used conventional phase-gradient reflectarrays and metasurfaces. In this paper, we develop an efficient way for the design and implementation of metasurfaces with high-efficiency anomalous reflector functionalities. The developed numerical method provides accurate, fast, and simple metasurface designs, taking into account non-local near-field interactions between array elements. The design method is validated by manufacturing and experimental testing of highly efficient anomalous reflectors for the millimetre-wave band.Comment: 8 pages, 8 figure

Wire media for broadband enhancement of radiation and power transfer

Presented thesis is focused on the wire media applied for enhancement of the dipole radiation. Wire medium is a metamaterial that has seen a lot of interest due to the hyperbolic frequency dispersion and the strong spatial dispersion that does not vanish even in the quasi-static limit. A combination of these exotic properties observes in no one natural material and even in chemically synthesizable materials it is hardly possible to engineer. Meanwhile, creating new devices demands artificial materials with more and more unusual properties. Previously, it was proved theoretically and experimentally, that the evanescent spatial spectrum of the dipole source radiation efficiently converts into propagating waves if the dipole is placed inside the infinite wire medium. Thus, it is possible to extract efficiently the radiation from the separate inefficient sources or areas of distributed poorly radiating sources. However, the finite size for any physical object including any sample of a wire medium hinders the implementation of this enhancement for practical applications due to almost total internal reflection. This effect also hinders the use of wire media for broadband transfer of power. The implementation of the broadband enhancement of radiation and broadband power transfer becomes not so easy, it requires a modification of the wire medium samples and imply many scientific questions, which I tried to answer in this thesis. Among these questions, three are the main ones, corresponding to three different types of the finite-size wire-medium samples dedicated for the broadband enhancement of radiation and power transfer that we aim to achieve beyond any capacitive, inductive or galvanic coupling of the radiating dipoles with the wires. The first one is a wire medium-based hyperlens, which answers the question about the possibility to surpass drastically the blackbody limit for thermal radiation in a wide frequency range. I show how the divergence of metal wires and other design parameters of the hyperlens modifies the radiation of a dipole into free space offering its huge broadband enhancement. The second type of wire media are samples with parallel alignment of the wires. They are not suitable for the broadband enhancement of the embedded dipoles but answer the question is it possible to transfer the power over a wide frequency band across a wire-medium sample. It is possible if the interfaces of a wire-medium slab slightly submerge into two media or are inserted into two hollow waveguides. The third topic refers to a new type of wire metamaterial – irregular wire media – arrays with small but random tilt of the wires. This metamaterial allows a broadband enhancement of radiation, and, in contrast to a hyperlens, answers the question is it possible to achieve this effect without the enlargement of the radiating aperture

On the Integration of Reconfigurable Intelligent Surfaces in Real-World Environments

| openaire: EC/H2020/871464/EU//ARIADNEPeer reviewe

Unusual eigenmodes of wire-medium endoscopes

Wire-medium endoscopes represent a promising tool of THz sensing/imaging. Bending should not critically harm the endoscope operation and the issue of bending losses is that of key importance for any endoscope. In this paper we show that the frequency-averaged power transmittance of a wire-medium endoscope is weakly sensitive to the bending. However, the frequency dispersion of the power loss/transmittance of the endoscope is strongly oscillating. Frequency maxima of the loss factor result from unusual eigenmodes of an elongated wire medium sample. These modes comprise power vortices and their sensitivity to the sample bending seems to be a critical issue for the future of wire-medium endoscopes.Peer reviewe

Magnetic hyperbolic metamaterial of high-index nanowires

We show that the axial component of the magnetic permeability tensor is resonant for a wire medium consisting of high-index epsilon-positive nanowires, and its real part changes the sign at a certain frequency. At this frequency the medium experiences the topological phase transition between the elliptic and hyperbolic type of dispersion. We show that the transition regime is characterized by an extremely strong dependence of the permeability on the wave vector. This implies very high density of electromagnetic states that results in the filamentary pattern and noticeable Purcell factor for a transversely oriented magnetic dipole.Peer reviewe

Test case 2: Scattering properties of two anomalous reflectors

Anomalous reflectors scatter the incident electromagnetic wave not only in the specular direction but rather in the directions specified by their structure. Two such reflectors have been designed and manufactured at Aalto University by using two different designs, referred to as conventional phase gradient (CPG) and non-local implementation (NLI). The reflectors have been realized as MTM-FSS rectangular plates and experimentally tested in the frequency range between 6 GHz and 9 GHz at the premises of Aalto University and German Aerospace Center. Contributors to the test case are invited to compute the S parameters of the corresponding planar periodic arrays, near-field distributions and bistatic scattering cross sections of the reflectors

Multi-mode broadband power transfer through a wire medium slab

Peer reviewe

Comparative Experimental Characterization of Phase-gradient and Non-local Anomalous Reflectors

Results of experimental characterization of anomalous reflectors of two types (conventional phase-gradient metasurface and non-local metasurface) are presented. Measured results are compared with analytical and numerical models

Tunable localization of light using nested invisible metasurface cavities

Funding Information: Research funding: This work was supported in part by the Academy of Finland ( https://doi.org/10.13039/501100002341 ) under grant 330260 and by Nokia Foundation ( https://doi.org/10.13039/501100004181 ) under scholarship 20200224. Publisher Copyright: © 2023 the author(s), published by De Gruyter, Berlin/Boston 2023.An invisible cavity is an open resonant device that confines a localized field without producing any scattering outside of the device volume. By exploiting the scatter-less property of such device, it is possible to nest two invisible cavities, as the outer cavity would simply not notice the presence of the inner one, regardless of their relative position. As a result, the position of the inner cavity becomes a means to easily control the field localized inside the cavity and its quality factor. In this paper, we discuss the properties of nested invisible cavities as a simple method to achieve stronger localized fields and high tunable quality factor. Furthermore, we show that in optics, these cavities can be implemented using nanodisk-based dielectric metasurfaces that operate near their electric resonances.Peer reviewe